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Bill Crawford: WWW.FLIGHTLAB.NET 10.1 Flightlab Ground School 10. Spins Copyright Flight Emergency & Advanced Maneuvers Training, Inc. dba Flightlab, 2009. All rights reserved. For Training Purposes Only A Short Sermon There’s a distinction between feeling safe and being safe. Unfortunately, we can experience the former without actually achieving the latter. Most of the decisions people make about safety rely on the feeling of protection—a feeling that comes from the sense of a buffer between themselves and any dangers likely to occur. It’s been pointed out that the reason some people prefer sport utility vehicles, despite the poor accident record, is that being surrounded by all that metal and padding simply feels safer. The impression is that an accident will be survivable. The fact is that an SUV accident will also be more likely because of poor handling qualities. Drivers are less likely to get into accidents in smaller cars that are more easily maneuvered out of danger. In the SUV case, people apparently assume that accidents are inevitable and therefore seek a physical buffer. In the small-car case, people accept more responsibility for their own welfare. They don’t really feel safe in their smaller cars, but the absence of that feeling makes them drive more safely, anticipant potential trouble sooner, and so actually be safer in the end. Their skill is the buffer. The problem of feeling safe versus being safe obviously applies to flying in general and to spin training in particular. Airplanes, even giant ones, are like small cars. They aren’t designed to make you feel comfortable about the notion of hitting things. They’re meant to be maneuvered back to safety. People who argue against spin training usually do so by relying on statistics showing that most stall/spin accidents start too close to the ground for recovery. In such cases, knowing how to recover from a spin wouldn’t help. The conclusion they draw is that flight training should rely on stall avoidance as the way to spin avoidance—that stall avoidance is the buffer. Aerodynamically, they’re right: stall avoidance is the way to spin avoidance. But spin training is itself the best way to produce unswerving loyalty to stall avoidance, because it’s the only way for a pilot to experience what happens when you take the buffer away. Its real purpose is to reinforce the buffer and render emergency spin recoveries unnecessary. Spin training shouldn’t make a pilot feel complacent while maneuvering an aircraft—or, for that matter, feel safe. It should make him better at anticipating the trouble in store if airspeed gets low and the precursors of autorotation appear (perhaps during an emergency landing following engine failure). Not feeling safe is what motivates people to act safely. Training shows them when to be wary and how to behave. An otherwise well-schooled pilot who hasn’t experienced spin training might feel safe, but have less real ability to look ahead and refrain from doing the wrong thing—less ability to keep the buffer intact. We cover spin theory in this briefing, and examine some of the characteristic differences between aircraft types. A Little History Lieutenant Wilfred Parke, of the Royal Navy, made the world’s first spin recovery, on August 25, 1912. We’ll note here that while Parke was desperately improvising, Geoffery de Havilland was watching anxiously from the ground. Sometime between late April and late November of 1914, de Havilland became the first to enter an intentional spin, recovering using the technique Parke had discovered of rudder opposite the spin direction. That this planned attempt only occurred some two years after “Parke’s Dive,” as it came to be called, underscores the wariness that remained. Until Parke’s nick-of-time revelation, pilots had always held rudder into the direction of a turn to prevent sideslip—a practice mistakenly carried over into spins. 1 Parke died in the unexplained crash of a Handley Page monoplane soon after his pioneering spin recovery. Geoffery de 1 Wing Commander Norman Macmillan, articles on the early history of spins, Aeronautics magazine, July, December 1960, September 1961.

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Flightlab Ground School10. Spins

Copyright Flight Emergency & Advanced Maneuvers Training, Inc. dba Flightlab, 2009. All rights reserved.For Training Purposes Only

A Short Sermon

There’s a distinction between feeling safe andbeing safe. Unfortunately, we can experience theformer without actually achieving the latter.Most of the decisions people make about safetyrely on the feeling of protection—a feeling thatcomes from the sense of a buffer betweenthemselves and any dangers likely to occur. It’sbeen pointed out that the reason some peopleprefer sport utility vehicles, despite the pooraccident record, is that being surrounded by allthat metal and padding simply feels safer. Theimpression is that an accident will be survivable.The fact is that an SUV accident will also bemore likely because of poor handling qualities.Drivers are less likely to get into accidents insmaller cars that are more easily maneuvered outof danger. In the SUV case, people apparentlyassume that accidents are inevitable andtherefore seek a physical buffer. In the small-carcase, people accept more responsibility for theirown welfare. They don’t really feel safe in theirsmaller cars, but the absence of that feelingmakes them drive more safely, anticipantpotential trouble sooner, and so actually be saferin the end. Their skill is the buffer.

The problem of feeling safe versus being safeobviously applies to flying in general and to spintraining in particular. Airplanes, even giant ones,are like small cars. They aren’t designed to makeyou feel comfortable about the notion of hittingthings. They’re meant to be maneuvered back tosafety. People who argue against spin trainingusually do so by relying on statistics showingthat most stall/spin accidents start too close tothe ground for recovery. In such cases, knowinghow to recover from a spin wouldn’t help. Theconclusion they draw is that flight trainingshould rely on stall avoidance as the way to spinavoidance—that stall avoidance is the buffer.Aerodynamically, they’re right: stall avoidanceis the way to spin avoidance. But spin training isitself the best way to produce unswerving loyaltyto stall avoidance, because it’s the only way for apilot to experience what happens when you takethe buffer away. Its real purpose is to reinforce

the buffer and render emergency spin recoveriesunnecessary. Spin training shouldn’t make apilot feel complacent while maneuvering anaircraft—or, for that matter, feel safe. It shouldmake him better at anticipating the trouble instore if airspeed gets low and the precursors ofautorotation appear (perhaps during anemergency landing following engine failure).Not feeling safe is what motivates people to actsafely. Training shows them when to be waryand how to behave. An otherwise well-schooledpilot who hasn’t experienced spin training mightfeel safe, but have less real ability to look aheadand refrain from doing the wrong thing—lessability to keep the buffer intact.

We cover spin theory in this briefing, andexamine some of the characteristic differencesbetween aircraft types.

A Little History

Lieutenant Wilfred Parke, of the Royal Navy,made the world’s first spin recovery, on August25, 1912. We’ll note here that while Parke wasdesperately improvising, Geoffery de Havillandwas watching anxiously from the ground.Sometime between late April and late Novemberof 1914, de Havilland became the first to enteran intentional spin, recovering using thetechnique Parke had discovered of rudderopposite the spin direction. That this plannedattempt only occurred some two years after“Parke’s Dive,” as it came to be called,underscores the wariness that remained. UntilParke’s nick-of-time revelation, pilots hadalways held rudder into the direction of a turn toprevent sideslip—a practice mistakenly carriedover into spins. 1 Parke died in the unexplainedcrash of a Handley Page monoplane soon afterhis pioneering spin recovery. Geoffery de

1 Wing Commander Norman Macmillan, articleson the early history of spins, Aeronauticsmagazine, July, December 1960, September1961.

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Havilland went on to build the series ofpioneering aircraft that carry his name.

Of course, spins went on to become standardcivilian training maneuvers—and then not, atleast not in the U.S. after 1949, once theregulators changed their minds. We’ll jumpforward some decades after Parke and begin atthe point where “jet-age” spin history begins.This will help us place the spin characteristics ofour training aircraft in better context—since ouraircraft are pre-jet-age, if you will, at least in afunctional if not a chronological sense.

Wing Planform and Aircraft MassDistribution

After World War II, a new generation of jetfighter aircraft appeared with swept wings andwith their mass distributed more along thefuselage axis, and less along the span (fuselageloaded). This changed for the worse thegoverning aerodynamic and inertial relationshipsthat determined their stall/spin behavior. Moderncorporate jets are the descendents of these earlyfighters; just as modern jet transports are thedescendents of early swept-wing bombers.

Swept wings allowed the early jets to achievehigh speeds by delaying transonic drag rise. Butthe swept-wing solution for high-speed flightintroduced problems in the high-α regime, wherecontrol was jeopardized by the swept wing’stendency to stall first at the tips. This caused aforward shift in the center of lift and a pitch-up.It also destroyed aileron authority. If one swepttip stalled before the other, the resultingasymmetry could send the aircraft into adeparture leading to a spin.

The hefty-looking chord-wise stall fences yousee on early swept-wing fighters, like the KoreanWar MiG-15, were an attempt to control thespan-wise airflow that encourages tip stall.(Fences are still used to manage spanwiseairflow and stall pattern.) During itsdevelopment the rival North American F-86Sabre was given leading-edge slats to improveits high-α behavior, and the horizontal stabilizerwas re-positioned away from the downwash ofthe high-α wing wake to improve nose-downpitch authority.

Figure 1 shows that swept wings stall at higherangles of attack than straight wings. There’s

typically also a more gradual change in slopearound the peak of the lift curve, and so the stallis less defined. Figure 1 suggests that when aswept-wing aircraft stalls asymmetrically, itswings reaching different angles of attack, the liftdifference, and thus the autorotative rollingmoment, is small. But because induced drag risesquickly with angle of attack, asymmetrical yawmoment may be large. If directional stability isweak, this can produce mostly yaw accelerationon departure in swept-wing jets. (However, athin airfoil with a sudden, leading-to-trailing-edge stall pattern will typically accelerate in roll.The lift curve peak is sharp, and differences inwing contour or surface texture usually causeone wing to stall first. See “Two-DimensionalAerodynamics.”)

Departure yaw can come from aileron deflection.Adverse yaw was a big problem on the NorthAmerican F-100-series swept-wing jets.Deflecting the ailerons could cause roll reversalat high α. Instead of rolling away from it, theaircraft could yaw toward the wing with thedown aileron—the resulting sideslip and roll dueto yaw rate sending it into a departure against theailerons. At high α, aircraft had to be rolled withrudder to prevent aileron-induced “lateral controldeparture.”

The redistribution of mass toward the fuselagewas also important. Once departure occurs and aspin develops, in a fuselage-loaded aircraftinertial characteristics can cause the spin attitudeto flatten or tend toward oscillation. Anti-spinaerodynamic yawing moments generated byrudder deflection can be insufficient forrecovery. To break the spin, the pilot (or flightcontrol computer more recently) often needs todeflect the ailerons into the spin to generate anti-spin inertia yawing moments to help the rudder

Coe

ffic

ient

of L

ift, C

L

Swept wing

Figure 1 Lift Curve

Larger lift difference for a given α difference produces more roll.

Lft. Wing

Rt. Wing

Angle of Attack, α

Straight wing

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along. (Described farther on, inertia momentsoperate on the same principle as the propellergyroscopics you’re familiar with, but the rotatingmass is the aircraft itself.)

The piston-engine fighters of World War IIbehaved differently than the new jets. They hadstraight wings, with stall patterns often morefavorable for aerodynamic warning and lateralcontrol (but not always—there were manyaircraft that announced a stall by suddenlydropping a wing). The lift curves for straightwings typically have well-defined peaks (Figure1). This can help promote well-defined stalls andprompt recoveries. But there can be strongrolling moments if the wings stallasymmetrically, and therefore predominantly rollacceleration on spin departure. Usually the pistonfighters departed to the left because of enginetorque and local airflow differences produced bythe slipstream. You can observe departurecharacteristics by watching the old training films.(Sociologically entertaining, as well. Times weredifferent. For an informative collection ofvideos, see www.zenoswarbirdvideos.com/)

Our trainers exhibit straight-wing spin behavior.The rectangular-wing Zlin’s departurecharacteristics are dominated by wing-root-firststall patterns at high α—patterns you’ll see whenwe stall a tufted wing. The tapered-wing SF260’sstall pattern is shifted outboard, which makes theaircraft more susceptible to a sudden wing drop.Their inertial characteristics come from a fairlyequal distribution of mass in fuselage and wings(pitch inertia only a little higher than rollinertia—close to the Iyy/ Ixx =1.3 neutral valuediscussed later). They need a bit of yaw to shiftthe stall pattern asymmetrically, respondpositively in autorotative roll, and then pick upthe yaw rate as the spin gets organized. In thedeparture and early incipient stage, before theyaw rate begins to develop and while spinmomentum remains low, neutralizing thecontrols is often all that’s necessary to recoverlift symmetry and break autorotation.

Training: What’s Possible

The spin training available to civilian pilots islimited to aerobatic, straight-wing, piston-enginesingles, like ours. This raises the obviousquestion of how such training corresponds to thekind of spin behavior a pilot might experience inan aircraft with a different wing planform or

distribution of mass. (Corporate jets are oftenswept-wing and fuselage-loaded, for example.)Does a stick-pusher make the issue irrelevant, orcan it fail and suddenly reveal the reasons it wasinstalled in the first place? In addition, whenaircraft are not flight tested for spins, on theassumption that a spin is a very unlikely event intype, their behavior can only be predicted.Prediction is complicated because the relevantaerodynamic stability derivatives, which arelinear when the aircraft experiences smallperturbations (the engineer’s term) from steadyflight, become nonlinear when perturbations arelarge. They can’t be extrapolated from staticconditions in a wind tunnel; they depend toomuch on the history of the unsteady airflow thatprecedes them. Dynamic wind tunnel testing ispossible,2 but only flight tests can really confirmspin characteristics and recovery techniques.

As a result of training in aircraft possibly quitedifferent from their own, pilots taking spintraining have to think in general terms. This isreasonable, however, because generic departureawareness—focused on the general principles ofspin avoidance rather than on the peculiarities ofa specific aircraft—works across the board:Aircraft depart into spins when lateral and/ordirectional stability break down in the stallregion, the wings develop asymmetrical lift anddrag along the span, and the asymmetry drawsthe aircraft into autorotation. The solution toavoidance is to deprive this combination of itskey ingredient—operation in or near stall,especially in uncoordinated flight, when theaircraft takes on a yaw rate and sideslip that canprovoke lift/drag asymmetry.

In recovery from a spin departure, rudderapplication is also generic: Always, full rudderopposite the spin direction. You can learn this inany spin-approved aircraft. Following therudder, stick neutral or forward of neutral isnearly generic, although the timing, amount ofdeflection past neutral, and the elevator stickforce necessary can vary among airplanes. Thedetails should always be determined from thePOH/AFM before first-time spin practice. Foraircraft with lots of their mass in the wings, theelevator can actually be a more effective anti-yaw recovery control than the rudder, as we’llsee.

2 Visit www.bihrle.com

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Spin-recovery technique depends on the abilityof these two primary anti-spin control surfaces,rudder and elevator, to generate large enoughaerodynamic anti-spin moments. Sometimes theycan’t, and that’s when recovery procedures needto generate persuasive anti-spin inertia momentsto help the aerodynamics along. This iscomplicated stuff in the telling (and predictiverather than proven for aircraft not spin-tested)but it essentially boils down to how the pilothandles the ailerons. In addition to anti-spinrudder and elevator, fuselage-loaded aircraftmay require aileron into the direction of thespin.

Check the aileron instructions for your aircraft. Ifan aircraft has not been certified for spins,there’s no requirement to list a hypotheticalprocedure in the handbook.

Regulations and Recoveries

The flight testing of both military and civilaircraft is done according to intended use andlikely user—the user being a pilot who isassumed to have only average skills in aircrafttype and who might be slow to react or mightmisuse controls (the guy test pilots refer tosolicitously as “your average Joe-Bag-of-Donuts”). In other words, testing is for intendeduse with some abuse. For uniformity, both themilitary and the FAA prefer standardized spinrecovery techniques, but neither actually requiresa specific set of inputs.

FAR Part 25, for large aircraft, doesn’t includespin certification, because spins are not intendeduse and aircraft are expected to behave in amanner making them very unlikely. The militaryechoes this. Under the military acceptancestandards (MIL-STD), “All classes of airplanesshall be extremely resistant to departure fromcontrolled flight, post-stall gyrations, and spins.The airplane shall exhibit no uncommandedmotion which cannot be arrested promptly bysimple application of pilot control.”3 Onlytraining aircraft that might be intentionally spunand Class I and IV aircraft undergo spin tests.(Class I includes small airplanes such as lightutility, primary trainer, light observation. ClassIV includes high-maneuverability airplanes suchas fighters, interceptors, attack, and tacticalreconnaissance.) 3 MIL-F-8785C, 3.4.2.2.1

Appendix 3-D in the Airplane Upset RecoveryTraining Aid4 shows the α/β flight-testingenvelope for a number of transport aircraft. Youcan see the variation in tested parametersbetween aircraft. In no cases are high angle ofattack and high sideslip conditions exploredsimultaneously. Stall tests are done at zerosideslip to prevent spins.

Back to smaller aircraft: Our ground school text,“Certification Requirements,” contains the civilaircraft FAR Part 23.221 spin requirements. Inaddition, “The Flight Test Guide forCertification of Part 23 Airplanes” (FAAAdvisory Circular AC-23-8A)5 providesinterpretation and procedures. Together theydescribe the minimum acceptable spincharacteristics for each aircraft category. For thenormal category, in particular, meeting therequirements actually means leaving much aboutthe aircraft’s spin characteristics still unknown.The normal-category, one turn spin recoveryrequirement is intended to address recovery froman abused stall, meaning a stall in which controlsare held in the pro-spin position and recoveryinputs are delayed, not recovery from adeveloped state with higher angular (rotary)momentums needing greater aerodynamicmoments to counteract. Consequently, meetingthe requirement does not clear an aircraft forintentional spins.

According to the “The Flight Test Guide forCertification of Part 23 Airplanes,” for aircraftcertified for spins under FAR Part 23.221,“Recoveries should consist of throttle reduced toidle, ailerons neutralized, full opposite rudder,followed by forward elevator control as requiredto get the wing out of stall and recover to levelflight, unless the manufacturer determines theneed for another procedure.”

What the Part 23 flight test guidance has in mindare aircraft with approximately neutralwing/fuselage mass distributions, and enoughrudder and elevator authority to do the job. Theuseful PARE acronym for recovery inputs,promoted by flight instructor Rich Stowell(Power off, Ailerons neutral, Rudder opposite,Elevator forward) follows this preferred recoveryformat. But the acronym is also adaptable tofuselage-loaded aircraft, because the ailerons are

4, 5 Available on the Internet, search the title.

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still taken care of in the propersequence—although deflected in the spindirection (for upright spins) rather than setneutral.

For certification purposes, the recovery controlsdescribed above are applied after one turn (or athree-second spin, whichever takes longer) fornormal category aircraft, and after six turns forspin-approved utility or aerobatic category. Atthat stage, PARE-input usually produces thequickest recovery.

Although PARE input is never inappropriate incertified aircraft (unless the manufacturer saysotherwise), it’s by no means uniformly essentialin all aircraft at the very beginning of a spindeparture, when autorotation first takes effectand a wing begins to drop. At that early stage,forward pressure to break the stall on thedropping wing is usually sufficient to endautorotation, even if spin-provoking rudder andaileron are still being held. In reality, by the timea pilot not current in spins remembers the PAREacronym, a PARE-input recovery is probablynecessary. Beyond the initial wing drop, once areal yaw rate begins to develop, pushing the stickforward out of the PARE sequence canaccelerate the spin, for reasons we’ll describe.

Autorotation

Spins feed on autorotation, which can follow astall if for some reason (sideslip, yaw rate, windgust, inherent asymmetry in response, pilotinput) the wings begin to operate at differentangles of attack. Figure 2 shows what happens.During our practice spins, for example, if wekeep the ball centered as we slow down andsimultaneously increase α, both wings shouldarrive at their maximum coefficient of lift, CLmax,more or less together.

If we then press hard left rudder, the right wingwill begin to move faster than the left. Theairplane will roll to the left in response (roll dueto yaw rate, plus dihedral effect). Because thewing’s rolling motion adds a vector to therelative wind (Figure 3), the left wing will see anincrease in α as it descends, but a decrease inlift, since the wing is past CLmax. Because of theincrease in α there will also be an increase indrag (as you’ll note in Figure 5).

The right wing will see a decrease in α as it rises,but still more lift than the left wing. Because ofthe decrease in α there will also be a decrease inthe right wing’s drag.

As a result, the coefficients of lift and drag willvary inversely in relation to one another alongthe span. The outcome is a self-sustainingautorotation.

Wing going upproduces downwardwind component.

Wing going down producesupward wind component.

Figure 3Roll-induced Angle of Attack

Decreased effective angle of attackdue to free stream plus roll rate

Increased effective angle of attackdue to free stream plus roll rate

Freestream

Free stream

Figure 2Autorotation

CLmax

Symmetrical stall atCLmax, left ruddersends wings todifferent α/CL.Autorotation begins.

Ascending Rt. Wing

DescendingLft. wing

Angle of Attack, α

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Although yaw typically leads to roll and thus toautorotation, sometimes departure happens inroll initially, even if the aircraft is in coordinatedflight with zero yaw rate or sideslip. One wingmight be rigged differently than the other, andstall first. Because of their sensitivity to anyasymmetry along the span, wings that producesudden leading edge stalls—or that generatesudden trailing-to-leading-edge stalls—tend toroll off. The down-going wing receives anincrease in α, which leads to an increase in drag.The up-going wing gets the opposite. Theasymmetry in drag sets the airplane off in yaw,which in turn reinforces roll.

Autorotation is roll damping reversed (Figure 4).Consider a wing operating normally on the leftside of the CL/α curve, before the stall region. Ifthe wing goes down, perhaps because of a gustor an aileron deflection that the pilot thenremoves, it doesn’t continue to roll, but stops.The downward rolling motion adds a vector tothe relative wind, which produces a geometricalincrease in α.The resulting increase in CLopposes the roll. This damping, rolling momentdue to roll rate, Clp, subsides as the roll ratereturns to zero.

If a wing operates on the right side of the CL/αcurve, past stall, a downward roll still producesan increase in α, but now accompanied by adecrease in CL, as we’ve seen. There’s nodamping effect. Just the reverse—the wingcontinues to fall. Roll damping turns unstablewhen the slope of the CL/α curve turns negativepast CLmax. A rolling motion kicks off a self-sustaining rolling moment.

Again consider a wing operating on the left sideof the CL/α curve, below the stall region, butnow rolling upward. An upward rolling motionwill induce a decrease in α (Figure 4) and a lossof lift—therefore generating roll damping. If thatwing were operating on the post-stall, right sideof the CL/α curve, an upward roll would stillinduce a decrease in α, but an increase in CL.The wing would continue to rise—again nodamping.

For autorotation to occur, at least one wing hasto operate on the right side of the curve, past themaximum coefficient of lift. Watching wing tuftsin a departure, you’ll typically see completeairflow separation on the inside wing (indicatinglow lift and high drag), while the outboard tufts

on the outside wing remain attached (more lift,less drag). Once autorotation gets going, inertialdynamics can take both wings to post-stallangles of attack, as they drive the nose up andthe spin attitude flattens. Spin recovery involvesgetting both wings back into the roll-dampingregion on the left side of the CL/α curve.

(If you remain a glutton for complication, notethat the derivative yaw-due-to-roll, Cnp,reverses sign at autorotation.)

Figure 4Roll Damping, Clp

Descending wingincreases α,decreases lift.

Ascendingwing decreasesα and lift.

Descendingwing increasesα and lift.

Ascending wingdecreases α,increases lift.

Damping No Damping

Angle of Attack, α

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Figures 5 and 6 show how the yaw component ofautorotation is generated by the rise in thecoefficient of drag, CD, as α increases. As wingsget shorter (smaller aspect ratio), or wing sweepincreases, the slope of the lift curve decreases.This reduces the divergence in lift coefficientwhen the left and right wings operate at differentα. In such cases, CL might not vary much overwide values of α, but CD will. Asymmetric dragthen dominates autorotation. That means moreyaw. One consequence is that spin attitude has atendency to go flat (nose up), especially in afuselage-loaded aircraft with flat-spin inertialcharacteristics.

Figure 6Swept-WingLift & Drag

Drag Curve

Drag differencebetween wings

Lift differencebetween wings

Angle of Attack, α

Descendingwing

Ascendingwing

Figure 5Straight-WingLift &Drag

Drag Curve

Lift differencebetween wingspromoting rollmoment.

AscendingWing

Descending wingproduces more drag thanascending wing.

Drag differencebetween wingspromoting yawmoment.

Angle of Attack, α

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Spin Phases

Spins are typically described as passing throughphases: departure, post-stall gyration, incipientspin, developed spin, and recovery. Thedeveloped spin may achieve steady rates ofrotation and a consistent nose angle against thehorizon, or the rates may oscillate—often withthe nose bobbing up and down accompanied byfluctuations in roll and yaw. The notion thatspins pass through identifiable phases is more astudied analytical observation than a factimmediately gladdening to pilots. If you’re newto spins, or new to the quirks of a particularaircraft, one moment can blur awfully quicklyinto another as a spin revs up; the chief sensationbeing that things are simply getting worse. Untilthe developed state, spin phases themselves aretransitional in nature, with uncommandedchanges in roll, pitch, yaw, and sideslip—oftengoing on all at once and difficult to sort intoseparate components. This is especially soduring the incipient phase, which ends quitedifferently than it begins. Some aircraft will passthrough the phases quickly, particularly duringintentional spins if control deflections generatestrong aerodynamic pro-spin forces and there’snot much inertia to overcome (strongaerodynamics and weak inertias are also theformula for good recovery characteristics).Others take longer to get going and finallystabilize, if indeed they do stabilize.

Departure

The military uses the term “departure” in thesense of a boundary between controlled anduncontrolled states, a boundary between linearand nonlinear aerodynamics. Within thisdefinition, an aircraft might depart and enter apost-stall gyration or a deep stall, but notnecessarily a spin. In initial spin training, we usepro-spin control inputs to bring the aircraftquickly through departure and “shape” the post-stall gyration so that the aircraft immediatelyenters the incipient phase. In a training situation,the pilot knows (or quickly learns) what theaircraft is doing. However, accidental departurescan come as a surprise, and the pilot might havedifficulty tracking aircraft motion. The militarytrains its student pilots to return the controlsquickly to neutral (and reduce power asappropriate) to try to prevent the aircraft frompassing beyond departure and into a developing

autorotation. If the aircraft has sufficient anti-spin stability characteristics it may then end upin an unusual attitude, but not in a spin. If a spindoes develop, the military pilot uses instrumentreferences (altimeter, AOA indicator, airspeed,turn needle) to determine the spin type and thecorrect recovery input. The military teaches“heads-in” recovery, it’s suspicious of outsidevisual references.6

Early in our Wide-Envelope training flights,you’ll observe directional stability and lateralstability (dihedral effect). You’ll evaluate thedeterioration of control effectiveness as αincreases, and you’ll find yourself introducingcorrective rudder inputs as the aircraft’sdirectional stability diminishes. You’ll see thetransformation of airflow over the tufted wingand the disappearance of roll damping asautorotation begins. These are lessons in thecomponents of departure.

Static directional stability usually decreases asaircraft angle of attack increases and the airflowover the tail slows down and becomes disruptedby the fuselage wake. As a result, anything thatcauses a disturbance around the aircraft’s z-axis

6 Naval Air Training Command, “Out-of-ControlFlight, T-34C,” 2006.

Recovery Controls

Developed Phase

Recovery

Autorotation begins.

Figure 7 Spin Phases

Incipient Phase

Departure/post-stall gyration

Spin axis

Helical path of aircraft cg

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can start a yaw that may be slow to correct, thusallowing the aircraft to go to a higher sideslipangle, β, and remain there longer. If dihedraleffect is present, the aircraft will tend to rollaway from the sideslip. The rolling motionimposed on wings at high α can send them intothe angle of attack disparity necessary forautorotation.

With American-turning engines, propeller effectsyaw an aircraft to the left as α rises and speeddecreases. Even if the pilot dutifully arrests theyaw rate with right rudder and keeps the ballcentered, the spiraling slipstream willnevertheless tend to increase the angle of attackon the left wing and decrease it on the right. Asaircraft angle of attack goes up, the left wingtherefore stalls first and the aircraft departsaccordingly.

Some aircraft will depart due to aileron adverseyaw. (We mentioned swept-wing fightersearlier.) The phenomenon is referred to as lateralcontrol divergence, or simply “aileron reversal.”It can happen when adverse yaw introduces asideslip that in turn produces a rolling momentopposite to and greater than the momentgenerated by aileron deflection. The airplanethen yaws and rolls toward the down aileron, notaway.

All the factors that lead to lateral controldivergence increase with α. The disturbed,lower-energy air generated by the fuselageand/or wing wake causes directional stability togo down, which allows a larger sideslip angle.And adverse yaw goes up, which promotes thatsideslip angle. Dihedral effect also goes up, atleast to stall, although more for swept than forstraight wings. The only thing that goes down isthe ability of the ailerons to generate an opposingroll rate.

Pilot lore often attributes a departure caused byaileron reversal to an increase in local angle ofattack as the aileron goes down. The idea is thatif you lower an aileron the angle between thewing chord line (as drawn from leading totrailing edge) and the relative wind increases.

This sudden increase in angle of attack issupposed to produce a local, sudden stall—thedecrease in lift causing the wing to go down. Inground school, you’ll see a wind tunnel film thatshows what can happen when a control surface isdeflected down on a wing already operating at ahigh angle of attack. There can indeed be asudden separation if airflow is unable to followthe abrupt change in camber. The effect dependsin part on the shape of the hinge line. Airflowtends to stay attached if the hinge design allowsa smooth curve. If the change is abrupt (pianohinge joining the top of the aileron to the top ofthe wing, for example), flow may separatesooner. That separation is accompanied by alarge increase in profile drag, as our film reveals.

Figure 8 uses a constant chord-line reference forangle of attack and shows how deflecting anaileron down shifts the lift curve to the left—andcan indeed bring a wing past stall angle of attack.But the lift curve also rises, and the lift of thestalled section actually increases (as does drag,even more). Notice the effect of a 20 deg. ailerondeflection at 14 deg. α: An aileron deflecteddown places the corresponding wing area outside

No ailerondeflection

20 deg.up

20 deg. down

30 deg.aileron down

0

1

2

Angle of Attack

Figure 8. Aileron deflection shiftslift curve. (LS(1)-0417 wing)

-4 0 4 8 12 16 20

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the region of roll damping. The lift of the areaincreases, but its contribution to damping—itsresistance to autorotation—drops out!

It’s usually difficult to persuade most airplanesto play along and depart into a down aileron bysuddenly deflecting that aileron just before astall. Typically, the aircraft has to start yawingdue to aileron-provoked adverse yaw (drag) first;a coupled roll leading to autorotation in thedirection of the down aileron then follows. If youuse active rudder inputs to counter theasymmetric drag and to prevent a yaw rate fromdeveloping, an aircraft with a well-manneredtrailing-to-leading-edge, root-to-tip stall typicallywon’t depart, no matter where the ailerons are.

Planforms (wing shape as viewed from above)that tend to stall initially outboard over theailerons, and that lack a compensating washout,might more easily misbehave following ailerondeflection (bad design). The trailing wing in asideslip is also, in effect, swept with regard tothe freestream. This could cause a thickening ofthe boundary layer outboard, which in turnencourages separation. By generating lowerpressures outboard, and creating a suction, adown aileron can definitely increase the rate ofstall propagation from root to tip on the insidewing during a cross-controlled skidding-turn-to-final. (We’ll show you this with a tufted wing inflight; so don’t worry if you can’t quite picturethings now.)

When we practice intentional spins, we force thedeparture issue by pressing the rudder in theintended spin direction. We deliberately producea yaw rate that leads to a rolling moment inresponse to the outside wing moving faster thanthe inside wing, and in response to dihedraleffect. This rolling moment sets up theconditions for autorotation.

You’ll notice that in all our upright spins,however we provoke them, the aircraft willalways depart in the direction opposite the ball inthe turn indicator or coordinator. The airplanefalls “into the hole,” as the arrow belowindicates. So “step on the ball to prevent thefall.”

The ball tells us the general direction of therelative wind or velocity vector (from/to theright, as illustrated above), and therefore of the

presence of a sideslip angle (see Figure 4 in theground school text “Rolling Dynamics”). Anaircraft that’s rigged fairly symmetrically (noneis perfect except by accident), that doesn’t sufferfrom extreme prop effects, and that tends to stallstraight ahead without dropping a wing won’tdepart into a spin if the ball is centered (zero β)and the velocity vector is thus on the plane ofsymmetry. The adventure starts when high α andhigh β combine. (Actually, an aircraft can be in asideslip even when the ball is centered. A twinon one engine is in a sideslip when the pilot usescorrective rudder but keeps the wings level. Thispoor technique creates more drag than when thepilot reduces the sideslip by banking a fewdegrees into the good engine. A power-on stall ina single-engine aircraft may involve a slightsideslip. Propeller slipstream and p-factorusually require right rudder to prevent yaw.Zeroing the yaw rate puts the aircraft in asideslip to the left. The ball will be centered ifthe wings are level.)

The displaced ball is predictive. It tells youwhich direction a departure will go. During aspin it’s an unreliable indicator of spindirection—unlike a turn needle, which is reliableupright or inverted. The aircraft symbol in a turncoordinator is reliable only in upright spins.

Since geometric dihedral causes an angle ofattack change in a sideslip—the angle of attackgoing up on the wing toward the slip—whydoesn’t dihedral cause that wing to stall and dropfirst during a sideslip (into the ball rather thaninto the hole)? It’s because the aircraft rolls awayfrom the slip as dihedral takes effect, the rollcausing a decrease in α on the upwind, up-goingwing. Because it rolls to lower α, it doesn’t stall.The down-going wing rolls past stalling α,however.

In our Zlin aircraft, if you’re carrying power andsimply hold the rudder neutral and continue tohold the stick full back after the stall, P-factorand spiraling slipstream will set up the necessaryyaw for a departure to the left. If you’re at idlepower (and depending on rudder trim, c.g., orturbulence), usually the airplane will oscillatearound its axes (in a post-stall gyration, seebelow) until it eventually trips into a divergentroll and autorotation takes over. Very polite,elevator-limited, directionally and laterally stableaircraft often won’t spin if you do nothing morethan hold the stick back with the rudder neutral

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or free, because they can’t generate the necessarycombinations of angle of attack and yaw withoutpilot intervention.

Post-Stall Gyration

A post-stall gyration is defined as anuncontrolled motion about one or more axesfollowing departure. The motions can becompletely random, and the angle of attack canwander significantly, as well. The militaryincludes snap rolls and tumbles as uncontrolledpost-stall gyrations. The term post-stall gyrationhas particular application to the behavior ofaircraft with the characteristics of fuselage-loaded, swept-wing fighters, as described at thestart. In a ready-and-willing straight-wingtrainer, if you do a standard entry, with stickback and full rudder at or just before stall in theintended spin direction, autorotation beginsimmediately and no post-stall gyrations may beevident. The aircraft goes directly to the incipientstate.

Incipient Spin

The rate at which an aircraft decelerates into astall is important. Certification flight-testing todetermine stall speed (and thus a collection ofnumbers derived from stall speed) is done at aone-knot-per-second rate of deceleration. If youincrease the rate of deceleration just a bit bybringing the stick back faster, you can oftendrive the stall speed down by virtue of the lag inthe change of pressure distribution over thewings. If you overdo it however, and start to pullg, the load factor goes up and stall speedincreases. You can also decrease stall speed byentering the stall nose-high, power on.

Stall speed affects the ballistic track of theincipient phase. Higher speeds mean that theaircraft travels a longer path over the groundbefore the spin axis becomes vertical, and issubject to higher aerodynamic forces at thebeginning of the phase. The forces createoscillations in roll, pitch, and yaw as the aircraftchanges orientation to the flight path, as shownin Figure 9.

Figure 9Incipient PhasePitching and Yawing Momentsdue to Ballistic Track

At 1-1/2 turns, relative windapplies a nose-down pitchingmoment.

At 1/2 turn, the horizontalcomponent of relative windapplies a nose-up pitchingmoment toward the horizon.

Decreasing horizontal component ofrelative wind as spin axis becomesvertical

At 1 turn, relative wind applies nose-uppitching moment.

At 3/4 turns, relative wind applies nose-upyawing moment.

As yaw rate builds, aircraftalso experiences anincreasing nose-up inertialcouple. See Figure 15.

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For example, after an aircraft departs and nearsinverted at 1/2 turn, the nose tends to stopfalling, even if the stick is held back, because therelative wind hits the bottom of the horizontalstabilizer. As the aircraft approaches the 3/4-turnpoint, the nose may yaw upward, the relativewind having shifted to the vertical tail. At theone-turn point, the horizontal stabilizer againtakes over, tending to hold the nose up. As theaircraft continues to the 1-1/2-turn point, thenose pitches down as the relative wind hits thestabilizer from beneath. These gyrationstypically decrease in intensity as entry stall speedgoes down and the horizontal wind componentbecomes less.

This, essentially weathervane, behavior is onlypart of the story. During the incipient phase theairflow can detach and reattach to the fuselage,wings, and tail surfaces, creating varyingmoments around the aircraft’s axes. This isparticularly evident on the outside wing on ourtrainers, as the wing tufts will show.

Judging from the training materials, the P-51grabbed a pilot’s attention during the incipientstage: “Upon entry in a power-off spin, the planesnaps 1/2 turn in the direction of the spin. Nosedrops nearly vertical. After one turn, the noserises to or above the horizon and spin almoststops. Snaps 1/2 turn again and nose drops 50 to60 degrees below horizon. Upon application ofcontrols for recovery, nose drops to near verticaland spin speeds up, then stops in one to 1-1/4turns. Approximately 1000 feet altitude lost perturn.”

Note that in half a turn the nose went from down“nearly vertical” up to “above the horizon.”That’s pretty frisky, but not unique. Note alsothat initially the “spin speeds up” after theapplication of recovery controls. That’s verytypical, for reasons we’ll see.

An aircraft’s mass distribution and its resultinginertial characteristics play an important roll inincipient spin behavior. In the case of the P-51,the interactions going on between propellereffects, inertias, and nonlinear aerodynamicswould challenge simulation even today. Ingeneral, an aircraft with low moments of inertiaaround its axes will be more susceptible to thechanging aerodynamic forces and easier toinfluence in the manner described above.

An aircraft with higher inertias might initially beharder to boss around aerodynamically. Once itstarts rotating, however, inertial characteristicsbecome increasingly influential as the spin axissettles down to vertical. Inertia moments (whichare not the same as moments of inertia!) can thenstart to define spin behavior. We’ll see this later.

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Developed Spin

In a steady, developed spin, aerodynamic andinertia forces come into balance. Yaw, roll andpitch rates settle down to constant values. Angleof attack, descent rate, and pitch attitude do thesame. In the case of oscillatory developed spins,which never settle down, the rates may fluctuatearound average values, with aerodynamicmoments in ascendance at one instant, inertiamoments at another. Dynamic equilibrium in adeveloped spin can take longer to reach thanmany realize. The aerobatic certificationrequirement of six turns before recovery inputsare applied doesn’t guarantee the aircraft hasreached equilibrium.

Spin Attitudes

Spins consist primarily of roll and yaw, with theairplane center of gravity following a helical patharound, and displaced from, the spin axis, asshown in Figures 7, 10, and 11. If the wings aretilted, relative to the helical path (Figure 10,bottom) the wing tilt angle introduces acomponent of pitch.

Flat spins are mostly yaw, while steep spins aremostly roll. Spins at 45-degrees nose down areequal parts roll and yaw. You can see why this isso by holding a model aircraft wings-level at a45-degree nose down angle and yawing it aroundits z-body axis. The nose is 45-degrees above thehorizon after half a turn. You need to roll as youyaw in order to keep the nose down. If you playwith other angles, you’ll see how roll and yawinteract. If you can figure out how to move themodel on a helical path and tilt the wing asillustrated, you’ll discover the need for a pitchrate, as well.

From the cockpit, spins often appear as mostlyyaw, even past the 45-degree nose-down angle,when roll rate is actually taking over. As thepitch attitude becomes steeper, roll rate increasesand roll perception starts to dominate. With thenose down it can be difficult for the pilot torecognize that he’s actually entered a spin andnot a vertical roll, or that he’s still in a spin witha yaw rate that has yet to be stopped.

Velocity vector

Figure 10 Spin Helix

Local wind axis (aircraft velocity vector) is tangent to helix at any moment. In this drawing the aircraft’s plane of symmetry and the velocity vector are coincident. Spin consists of roll and yaw.

Aircraft viewed from the bottom. Spin axis is some distance behind.

x axis

Velocity vector

Wing tilt angle introduces a pitch rate, also a sideslip.

Helix angle, γ: Angle between velocity vector and spin axis

Reference line perpendicular to velocity vector

y axis

y axis

Helix

Spin axis

CG follows helical path.

Spin axis

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Figure 11 shows some of the characteristics offlat versus steep spins. In the example shown,for simplification the spin consists of roll andyaw only—no pitch rate. In an equilibrium state,the aerodynamic pitching moments, which arenose down, are opposite to and equal the noseup inertia moments (more about this later).

As the angle of attack, α, increases and the spinbecomes flatter, the coefficient of drag, CD,increases. Because of the drag rise, the descentrate decreases. Lift goes down. The distance, r,from the aircraft center of gravity—riding thehelix—to the spin axis also decreases.

The figure shows the balances of forces in asteady spin. The resultant aerodynamic force(vector sum of lift and drag) balances theresultant of weight (the acceleration of a massby gravity) and centrifugal force. As aircraftangle of attack increases, and lift consequentlydecreases, the aerodynamic resultant tilts moretoward the vertical, or clockwise in theillustration. The resultant of weight andcentrifugal force tilts clockwise as well. Sinceweight stays the same, this means centrifugalforce decreases. As it does, the radius of thehelix, r, around the spin axis decreases. As theaircraft c.g. moves closer to the spin axis, spinrate, ω, increases. In an aircraft with the c.g.behind the cockpit, the axis can pass behind thepilot; the spin accelerating and becoming“eyeballs out.” Not fun, says those who havebeen there.

Whether the spin is steep or flat will depend onthe attitude necessary to balance themoments—aerodynamic versus inertial—aroundthe aircraft’s axes. As we’ll see, an aircraft withits mass predominately in its fuselage will tendto spin more nose-up. An aircraft with its masspredominately in its wings will spin more nose-down.

Flatter Spin:Higher αMore yaw than rollSmaller spin radiusHigher drag meanslower descent rate.

Weight = Drag

Drag = Weight

Figure 11Spin Attitudes,Steady Spin Balance ofForces

Spin axis rotation, ωFlatter attitude =faster rotation

Spin rate

Roll rate

Yaw rate

ω

Inertial pitchingmoment

Aerodynamicpitching moment

Resultant

Spin axis

Minimum spin rotationrate happens at 45-degree pitch attitudewhen the spin axis isvertical.

Resultant

Lift = mrω2Centrifugal Force, mrω2 = Lift

Spinradius, r

Steeper Spin:Lower αMore roll than yawGreater spin radiusLess drag meanshigher descent rate.

ω

ωω

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Spin Practice

Spin practice should build anti-spin and spin-recovery responses that will stick with youthroughout your flying career. A good flightinstructor lays the groundwork by unravelingspins in stages you can absorb, not with asudden, multiple-turn baptism. (Teaching spinsis not for instructors with lingering personalityissues.) Since you will likely be in survivalmode, concentrating on the plan for recovery,there’s a limit to how much real motioninformation you’ll be able to take in the first fewtimes. The initial blur factor is high, and spinsbecome increasingly difficult to follow as therotations accelerate and your tracking reflexesbreak down.

When practicing spins with an instructor the firsttime, READ THE AIRCRAFT Pilot’s OperatingHandbook or AFM. Don’t go flying until youhave. If you’re experienced with spins, butspinning an unfamiliar aircraft for the first time,READ THE POH/AFM. Don’t makeassumptions based on other aircraft. Assumptionsfail. When it comes to spins, the voice of cautionshould be the one in charge of the plans. Or youcan listen to the voice of Murphy, The Lawgiver,who actually was a flight-test engineer, “If it cango wrong, it will.”

Always run a weight-and-balance on anunfamiliar aircraft or a suspicious loading. A c.g.shifted aft of the approved envelope can cause aspin to flatten out due to diminished nose-downelevator authority. A c.g. shift measured ininches may not seem like much as a percentageof the distance (or arm) between the c.g. and theaerodynamic center of the elevator, but that’s notthe distance that matters. It’s the distancebetween the c.g. and the aircraft’s neutral pointthat makes the difference. (See “LongitudinalStatic Stability.”)

As you do before any aerobatic flight, clear thecockpit of all foreign objects. (Things hide—thewriter, now more vigilant, once brained hisaerobatic instructor with a quart-size can of fueladditive.) Search the manual for and askknowledgeable pilots about any characteristicsdifferent from those you’ve experienced. Andcheck for stretch in the elevator cables: Havesomeone hold the stick full forward while youlook for play by pulling up on the trailing edgeof the elevator. Do you have full down elevatorfor recovery?

Some aircraft require power to enter a spin. Ifyou’re accustomed to entering practice spinspower-off, you’ll have to remember to pull thepower once autorotation begins, both inconsideration of propeller gyroscopic effects andfor speed control during recovery. Simple detailslike the settings for trim and mixture control areeasy to forget, but can be important. Rememberthat an aircraft reluctant to enter a spin can alsohave limited aerodynamic authority for recovery.The aircraft may be asking you not to force theissue. “Spin-proof” aircraft can enter spins ifimproperly rigged and their control surfacesexceed design deflection, but might not berecoverable.

Practice Spin Entry

Unless an aircraft is reluctant to spin, andrequires the encouragement of a specialdeparture technique endorsed by themanufacturer, the following is typical for apractice spin.

Altitude. (Adequate if planned recovery isdelayed? The FAA says recovery from anintentional spin must be able to occur nolower than 1,500 AGL.)

Cockpit check. (Seatbelts, loose objects?Trim, engine controls, fuel valves set asspecified in the POH/AFM?)

Clear Airspace.

Power to idle. (But some, like the Cessna150 and 172 series, may need aid from theslipstream to enter a spin.)

Stick back for standard 1-knot-per-second deceleration. (This is thedeceleration rate used in certification todetermine the “book” stall speed. Just bringup the nose as necessary to hold altitude orclimb slightly as airspeed bleeds.)

Ailerons Neutral. (Although some aircraftmay require that the stick be held oppositethe intended spin direction. In that case theaileron deflected down contributes anadditional yawing moment—from adverseyaw—in the same direction as the rudder.)

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Rudder deflected fully toward intendedspin direction just before the stall. (Theaircraft might not enter a spin, or entrymight be delayed if a stall break occurs andthe aircraft dumps the necessary angle ofattack before the rudder is applied.Smoothest entries come from applying therudder first.)

Hold full rudder deflection.

Stick full back as aircraft departs.

Spin or Spiral?

Spin-reluctant aircraft will often reward you witha spiral departure, until you figure out the trickof getting them to spin (rudder timing, blast ofpower, rapid deceleration leading to a higherangle of attack). You’ll immediately recognize aspiral departure, because the roll-off happensslowly. A spin departure will roll you faster.Opposite aileron will recover a spiral, but theresulting adverse yaw may aggravate a spindeparture. Airspeed and z-axis load factor willincrease in a spiral, and the ball will respond toyour feet in the normal ways, remaining centeredif your feet are off the rudders.

Recovery Controls

Neutral rudder and aileron, and neutralelevator (or perhaps stick forward of neutral)are all that’s required for recovery from theimmediate departure stage in a typical trainer.Responding quickly to a wing drop in thismanner is usually enough to breakautorotation. PARE-sequence recovery controlsbecome more critical as angular momentumstarts to grow and the spin heads toward adeveloped state. To recover from an upright spin:

Power to idle. (Reduces propellergyroscopic effects and slipstream-inducedyaw.)

Ailerons neutral. (Removes any inadvertentdeflection that may delay recovery. In afuselage-loaded aircraft the ailerons gotoward the yaw direction—toward the turnneedle whether upright or inverted—toproduce anti-spin inertia moment in yaw.)

Rudder opposite yaw direction. (Providesanti-spin aerodynamic yaw moment.)

Elevator forward to neutral or pastneutral. (Unstalls the wings; for wing-loaded aircraft generates anti-spin inertiamoment in yaw.)

When the spin stops, pull out with therudder neutral. (If the recovery rudder isstill deflected and you pull too hard, aircraftcan snap roll into a spin in the oppositedirection.)

In the following, we refer to inertia moments andgyroscopic effects that depend on how the massof the aircraft is distributed around its three axes.For simplicity in presentation we’ll just makereference to them now; try to explain them later.

Power

Power goes back to idle to reduce gyroscopicand slipstream effects. In an aircraft with aclockwise turning propeller as seen from thecockpit, power in an upright spin to the left tendsto raise the nose due to the gyroscopic forcesillustrated in Figure 14. Spiraling slipstreamtends to increase in-spin yaw moment. In a spinto the right, gyroscopics can bring the nose downand the slipstream can supply anti-spin yawmoment. Properly timed, power application in aright-hand spin can help damp the oscillations ofthe incipient phase shown in Figure 9. But that’san advanced spin technique; just bring the powerto idle in an emergency recovery.

In jets, power to idle is typically recommended.Although modern fighters designed to operate athigh angles of attack have capable fuelcontrollers, earlier jets or less robust designshave trouble handling the unsteady inlet flowsaccompanying spin departures. Compressorstalls and flameouts are common. In spin tests ofthe T-38 supersonic trainer, there were threeflameouts of both engines and eighteen singleflameouts in twenty-one upright spins. Someonegot good at restarts!

Note that power often isn’t mentioned in theaircraft handbook recoveries reproduced fartheron.

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Ailerons

Aileron deflection works in the normal rollingsense during a spin, even if both wings arestalled. At spin attitudes, aileron deflectioncauses a span-wise difference in drag. Thisproduces a component of roll as well as yaw. Aglance back at Figure 11 shows how the dragvector tilts toward the aircraft’s z-axis and thustoward roll effectiveness in a spin. Out-spin stickdeflection lowers the inside aileron. In a typicalspin trainer, this tends to drag the inside wing up,and bring up the nose, causing the spin to flatten.

Ailerons go to neutral for a recovery in anaircraft with broadly neutral mass distribution.Sometimes ailerons are hard to hold in neutral,because they have a tendency to float with thespin. Aircraft that recover more slowly if theailerons are deflected often have a line paintedon the instrument panel. The pilot uses this as anaim point to be sure that the ailerons are centeredwhen the stick goes forward.

As we’ll explain later, a roll moment canproduce a yaw moment, depending on theaircraft mass distribution. In a fuselage-loadedaircraft, aileron deflection into the spintypically produces an anti-spin yaw inertiamoment that helps recovery.

This anti-spin yaw moment generated by in-spinaileron in a fuselage-loaded aircraft tends to raisethe outer wing and increase wing tilt. Greaterwing tilt increases the nose-up pitch rate. This inturn causes an increase in both anti-spin rollingand anti-spin yawing inertia moments. (Relax.You’re not supposed to understand this yet.)

The standard recommendation for aircraft thatrequire aileron into the spin is aileron first, thenrudder and elevator.

Aileron deflection could make it difficult tofigure out when the spin has stopped, however. Ifheld, aileron will cause the aircraft to continue toroll after it ceases autorotation. Look at the F-4recovery procedure given later. You’ll see thatthe pilot is instructed to “Neutralize controlswhen rotation stops.” You have to wonder if itwould stop. Since the purpose of ailerondeflection is the creation of anti-spin yawmoment, and since the nose would go down asyaw rate decreases, pitch attitude would likely bean important cue in an aileron-assisted recovery.

In a wing-loaded aircraft (Ixx>Iyy) aileron into thespin can increase spin yaw rate and bring up thenose. Aileron against the spin will produce anti-spin yaw moment, but a pro-spin, acceleratingmoment in roll, the dramatic opposite of what thepilot’s instincts are suggesting. Since wing-loaded aircraft generally spin nose down, with ahigh apparent roll rate seen from the cockpit, thetemptation to use aileron against the spin can bestrong. Neutral ailerons are usuallyrecommended for spin recovery in wing-loadedaircraft.

Rudder

Stopping a spin requires slowing down therotation in yaw to the point where angle of attackcan be decreased, the wings returned to the pre-stall side of the lift curve, and roll dampingreestablished. The primary anti-spin recoverycontrol in most aircraft is opposite rudder. Fullrudder opposite the spin direction is alwaysappropriate.

Opposite rudder decreases the yaw rate, which inturn decreases the inertial couple driving thenose-up pitching moment. (Figure 15 willexplain this nose-up couple.) The rudder’seffectiveness will depend on the surface areaexposed to the relative wind at spin attitudes(perhaps the airflow to the rudder is partlyblocked by the horizontal stabilizer andelevator), by the additional available yawdamping effect of the fuselage, and by theposition of the center of gravity. Aft c.g. reducesthe arm and thus the available anti-spinaerodynamic moment.

Take a look at the wing tilt angle illustrated atthe bottom of Figure 10. During spin recovery,with recovery rudder deflected, the outside wingtends to rise. This increases the wing tilt angle,which reduces any outward, pro-spin sideslip,and introduces a positive, nose-up pitch rate. Infuselage-loaded aircraft, the pitch rate precessesto an anti-spin yaw inertia moment, assistingrecovery.

Elevator

After applying opposite rudder, apply forwardstick. (In the case of an inverted spin, the stickcomes back.) The sequence of rudder-then-elevator is important, since, once the yaw rate

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begins to develop, leading with the elevator canaccelerate the spin rate gyroscopically, makingthe rudder’s task more difficult. As justmentioned, elevator deflected down may alsodecrease the rudder surface exposed to therelative wind, limiting its effectiveness. In manyinstances, even aircraft designed for spins maynot recover if anti-spin rudder is applied whilethe stick remains too far forward (for example,after an instructor demonstrates an acceleratedspin but then fails to bring the stick all the wayback in a Pitts). Recovery from a developed spinshould start with full back elevator.

Once the yaw rate begins to build andgyroscopics come into play, a nose-down pitchproduces a pro-spin inertia moment in roll.You’ll feel the roll rate increase as the nosecomes down. This is always the case, once theaircraft begins to develop angular momentumabout the yaw axis, regardless of wing/fuselagemass distribution or spin direction. With thecontrols applied in the proper opposite-rudder,elevator-forward sequence, the roll accelerationmeans that recovery is on the way, but it’s adisconcerting sensation. It appears that the spinis getting ready to become nasty, when it’sactually getting ready to stop. The gyroscopicmechanism at work is a pitching moment thatprecesses 90 degrees around the yaw axis,producing a rolling moment. The initial anti-spinrudder will slow down the yaw rate (z-axisangular momentum) so that when the stick thencomes forward less spin-direction rollacceleration will occur.

Down-elevator can be the most effectiverecovery control with an aircraft with a wing-loaded mass distribution, because it produces ananti-spin inertia moment in yaw. That could becrucial if the rudder is too weak to produceenough aerodynamic anti-spin yawing momenton its own. The amount of forward sticknecessary may increase at aft c.g. loadings.

Our trainers actually behave as if they werewing-loaded as the spin just gets started. Theirroll acceleration is initially high; their yaw ratepicks up more slowly. As a result, angularmomentum is greater initially in roll than in yaw.Pushing the stick forward causes gyroscopicprecession around the roll axis that leads to ananti-spin moment in yaw. Plus, pushing gets theangle of attack back down, out of autorotation.But once the aircraft’s yaw rate and angularmomentum about the yaw axis has begun to

build, forward stick will cause the momentaryacceleration described above, even when itfollows the rudder in proper sequence.

If you hold pro-spin rudder while holding thecontrol forward, the aircraft typically will stay inan accelerated spin. Decelerate the spin bybringing the control all the way back, and thenrecover in the normal way: use full anti-spinrudder followed by forward stick.

Flaps

Flap recommendations are inconsistent, as youcan see in the pilot-handbook recoveries listedfarther on. With the exception of flaps usedduring practice slow-flight, flap deploymentimplies an aircraft flying close to the ground. Inthat case, unless the manufacturer directs,emphasis should be on the primary anti-spincontrols. During flight testing under Part23.221(a) iv, an aircraft is required todemonstrate spin recovery in the flaps-extendedcondition: “…the flaps may be retracted duringthe recovery but not before rotation has ceased.”Thus, by design, flaps at very least cannot delayrecovery beyond the maximum turns allowed bycertification. Retraction after rotation stops willreduce the chance of overstressing the wingswhile returning to level flight. Limit load usuallydrops to 2g when the flaps are fully deployed.

Pull Out

Spin students sometimes remind instructors thatthe game isn’t over once the spin stops. Groundrush, the sensation that the closure rate with theplanet is accelerating, can lead inexperiencedpilots to start yanking. The aircraft then goes intoa heavy buffet, and the pilot loses the nose-uppitch authority he’s desperate for. Pilots can alsofind themselves holding recovery rudder. If thepilot pulls too aggressively while holding rudderdeflection, the aircraft can depart into a rapidrotation in the direction opposite the originalspin. This is one of those scenarios in which anaircraft answers a pilot’s fearful response bygiving him even more to worry about. Thesolution is to neutralize the rudder andmomentarily decrease the aft pressure on thestick.

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Inverted Spins

Inverted spins present unusual motion clues.Pilots are accustomed to seeing aircraft yaw androll in the same direction, a visual paringreinforced whenever entering an ordinary turn.But in an inverted spin the aircraft appears to rollone way while yawing the opposite. (If you weresitting upright on the belly of the invertedaircraft, however, the yaw/roll relationshipwould appear normal.) The aircraft’s motion pathrelative to the landmarks below also takes gettingused to. Until you’ve had some practice, it’s hardto count the turns in an inverted spin.

In upright and inverted spins, one thing remainsthe same: In a standard intentional entry, ruddercauses the corresponding wingtip to fall.Imagine an aircraft beginning an intentionalupright spin to the left. The left wing falls towardthe earth when you press the left rudder withyour left foot. The same thing happens flyinginverted when you press your left foot during anintentional inverted spin entry: the left wing fallstoward the earth. Anti-spin rudder application isidentical in both the intentional upright andinverted cases. Use the foot opposite the fallingwing. If you press the left rudder to enter aninverted spin, the spin will be to the right as seenfrom outside. But the outside direction onlymatters in aerobatic competitions. Think of yourintroductory inverted spins in simple cockpitterms, as left- or right-footed.

Recovery from an inverted spin follows the samePARE sequence as recovery from upright, exceptfor the direction of elevator deflection. The stickcomes back in an inverted spin recovery.

An intentional inverted spin entry may lookweird from the cockpit at first, yet with practiceit’s easy to initiate or react to correctly, if youremember your feet. But an intensifying invertedspin entered by surprise is a different matter. Ifyour thoughts were elsewhere, or if the spin ishighly coupled and oscillatory or unexpectedlytransitions from upright to inverted, determiningyaw direction can take a while. Since spinrecovery technique depends on yaw direction,recognition is critical. Again, a turn needle willshow you the correct yaw direction, whetherupright or inverted. A turn coordinator, however,only works upright. In the absence of a turnneedle, look directly over the cowling to pick upthe yaw direction. Peripheral cues tend to bedistracting since they correspond more strongly

to roll. Looking back, behind the spin axis, givesyou a false yaw direction.

Inverted spins usually respond quickly to anti-spin rudder. With conventional tails (as opposedto T-tails), more unshielded rudder area isexposed to the relative wind in an inverted spinthan when the aircraft is upright. The aircrafthandbook tells you how far back to bring thestick after applying opposite rudder.

The possible difficulty of recognizing spin typeand direction is reflected in military recoveryprocedures. Check the procedures for the T-34Cand F-4, farther on. When there’s confusion, theturn needle and angle of attack indicator dictatepilot response, not the view outside. (Althoughanti-spin control inputs can be easier todetermine from instrument reference, identifyingthe recovery and regaining orientation is mucheasier using external cues.)

During spin recovery, less experienced aerobaticpilots sometimes unintentionally tuck-underfrom an upright into an inverted spin by pushingtoo aggressively while holding recovery rudder.As viewed from outside, the spin directionremains constant as the aircraft switches toinverted. The transition is hard for the pilot tosee. Another way to tuck into an inverted spin isby applying too much forward stick during theyawing transition to the descending line of ahammerhead. Aerobatic spin training doesn’thave to exhaust the complete spin matrix, butdual instruction that cautiously points to suchdangers is invaluable.

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Müller-Beggs Recovery

With the above in mind, aerobatic pilots shouldlearn the Müller-Beggs method, used forrecovery from unintended spin departures whenthe direction and mode is unclear.

The steps are:

• Power to idle.• Let go of the stick.• Establish the yaw direction by looking

directly over the nose (or at the turn needle).• Apply opposite rudder, and recover from the

dive when the rotation stops.

Look at the rudder pedals if you can’t figure outspin direction. In an aircraft with reversiblecontrols, the rudder will float trailing-edge-toward the spin. This will set the rudder pedalsso that the recovery rudder will be the one closerto you when the cables are taut. In a dim cockpitthe pedals might be hard to see. The recoverypedal is the one that offers the most resistancewhen you press.

This release-the-stick technique works well withcertain aerobatic aircraft, but not always withothers, a point that usually unleashes claims,counter-claims, and a certain amount ofposturing when aerobatic pilots start makingcomparisons. Spins are very sensitive to massdistribution, to c.g. location, and to the timehistories of complex, nonlinear airflows.Different examples of even the same aircraftmodel can behave in different ways, dependingon the inevitable differences in rigging. Pilots dothings differently without knowing it. Thereaction of an aircraft to control inputs candepend on how far a spin has developed and onits current oscillatory phase. So take thesuggestions of experienced pilots concerningMüller-Beggs seriously, but beware those whomake messianic pronouncements unsupported byevidence. You might be listening to theaforementioned Joe-Bag-of-Donuts. Neither theDecathlon nor the Zlin 242L nor the DeHavilland Chipmunk should be consideredMüller-Beggs recoverable.

The procedure of letting the stick go assumes thepilot is too confused to perform the aircraftmanufacturer’s recommended recovery, andcan’t identify the spin mode as upright orinverted. It releases any impulsive out-spin stickdeflection that might tend to flatten the spin, but

it usually also accelerates the roll rate throughinertial effects generated when the nose pitchesdown. The roll acceleration could delay recoverycompared to the recommended procedure.

Training in the Müller-Beggs technique isessential if you plan to fly or instruct aerobaticsin aircraft that depart quickly and can quicklyshift between upright and inverted modes ifmishandled—characteristics that can make visualtracking difficult. Fly with a qualified instructorin an aircraft with a known, positive Müller-Beggs response. Start with upright spin entries ataltitudes allowing delayed recovery. When yourelease the stick, watch where it goes in responseto the float angles of elevator and ailerons. Notethe force required to move it. The stick can feelalarmingly stiff. (In a tandem trainer, you mightthink the other pilot has frozen the control.)

Practice and familiarization are important so thatdifferences in aircraft motion and recovery timebetween the recommended and Müller-Beggsprocedures don’t come as a surprise during anemergency. If they do, you may be tempted tochange control inputs before they have time towork, delaying recovery even more. Duringtraining, be prepared if necessary to return thecontrols to the initial full stick-back, in-spin-rudder, spin entry position before beginning themanufacturer’s recommended recovery. Thisshould return the aircraft to a known recoverablestate.

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Handbook Recoveries

The following are directly from the respectiveaircraft handbooks, unless indicated. For foreignaircraft, the translation is from the manufacturer,including the original spelling and syntacticalcharm. Format follows the original as much aspossible.

All specify full rudder against the spin, asexpected. Elevator recommendation varies fromneutral to forward of neutral. Except for theFalcon and the F-4, ailerons remain neutral.Most remind the pilot to pull out of the dive, justin case:

Falcon 20 Business Jet

Intentional spins are prohibited. This aircraft hasnot been spin tested in flight. However, results ofwind tunnel tests have shown that the followingprocedure should be applied:

Configuration CleanRoll Same direction of rotationYaw Opposite direction to spin

rotationElevator Neutral

AT-6C (Army) and SNJ-4 Navy Trainers

Spins should not be made intentionally withflaps and landing gear down. Should aninadvertent spin occur, recovery can be effectedafter 1-1/2 or 2 turns by first applying fullopposite rudder and then pushing the controlstick forward to neutral. The ailerons are held inthe neutral position. Centralize the rudder assoon as the airplane is in a straight dive toprevent a spin in the opposite direction. Bringthe airplane out of the dive and return the controlstick to neutral.

Zlin 242L Single-engine Trainer

Mixture Max richThrottle IdlingRudder Full deflection

opposite to directionof rotation.

Elevator Immediately after fullcounteraction ofrudder push smoothlycontrol stickminimally to half ofthe travel betweenneutral and fullforward within 1-2sec. Ailerons inneutral position.

After rotation is stopped:Rudder Neutral positionElevator Pull steadily control

stick to recoveraircraft from diving.

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National Advisory Committee for AeronauticsTechnical Note No. 555, “Piloting Technique forRecovery From Spins,” W. H. McAvoy, 1936.

The recommended operation of the controls forrecovery from a spin, which presupposes that theailerons are held in neutral throughout therecovery, is as follows:

1. Briskly move the rudder to a positionfull against the spin.

2. After the lapse of appreciable time, sayafter at least one-half additional turn hasbeen made, briskly move the elevator toapproximately the full down position.

3. Hold these positions of the controlsuntil recovery is effected.

(The recommended delay in applying elevatoraddressed fact that in many aircraft the elevators,when deflected down, reduced the efficiency ofthe rudder by blocking its airflow.)

PZL M-26 Iskierka (Air Wolf)

Be sure of the direction of the aircraft’s rotation.Rudder Set vigorously

opposite the self-rotation.

Elevator Slightly forward,beyond neutralposition.

Ailerons Neutral PositionAfter stopping rotation:Rudder NeutralFlaps UpControl stick Smoothly backward

(recover the aircraftfrom dive withoutexceeding theairspeed and loadlimits).

Engine power Increase smoothly

Cessna Model 172P

1. Retard throttle to idle position.2. Place ailerons in neutral position.3. Apply and hold full rudder opposite

to the direction of rotation.4. After the rudder reaches the stop, move

the control wheel briskly forward farenough to break the stall. Full downelevator may be required at aft center ofgravity loadings to assure optimumrecoveries.

5. Hold these control inputs until rotationstops. Premature relaxation of thecontrol inputs may extend the recovery.

6. As rotation stops, neutralize the rudder,and make a smooth recovery from theresulting dive.

F-4 Phantom

TA-4F/J NATOPS Flight Manual

• Neutralize flight controls and physicallyhold the stick centered (visually checkposition of the stick).

• Retard throttle to idle.• Determine type and direction of spin.• Apply and maintain recovery controls.

Aileron: Full with turn needle if spin iserect. Full opposite turn needle if spin isinverted.Rudder: Full opposite turn needledeflection.Stick: Neutral to slightly aft.

• Neutralize controls when rotation stops andrecover from the ensuing dive at a maximumof 18 to 20 units angle of attack.

• PSG recovery procedures: Neutralize allcontrols.

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T-34C Turboprop Trainer

Naval Air Training Command, “Flight TrainingInstruction, T-34C Out-of-Control Flight,” Q-2A-0017, 1993.

1. Landing gear and flaps – Check “up”2. Verify spin indications by checking

AOA, airspeed and turn needle.Warning – Application of spin recovery controlswhen not in a steady state spin (as verified byAOA, airspeed and turn needle) MAY furtheraggravate the out-of-control flight condition.

3. Apply full rudder OPPOSITE the turnneedle.

4. Position stick forward of neutral(ailerons neutral).

Warning – “Popping” down elevator CAN resultin the spin going inverted in some airplanes. A“smooth” forward movement of the stick is bestfor most light aircraft during spin recovery.

5. Neutralize controls as rotation stops.6. Recover from the ensuing unusual

attitude.

(Note that the recovery procedures areinstrument-based in the two military examplesabove. Angle of attack indicator and airspeedverify the spin. The turn needle determinesrecovery rudder and aileron deflection if calledfor. The recovery procedure for the T-37Btrainer—a side-by-side jet twin built byCessna— reproduced in part below, takes anunusual approach. The pilot first attemptsrecovery from an inverted spin. If that doesn’twork, he tries to recover from an upright spin.)

T-37B Trainer

One procedure which will recover the aircraftfrom any spin under all conditions:

1. THROTTLES – IDLE.2. RUDDERS AND AILERONS –

NEUTRAL.3. STICK – ABRUPTLY FULL AFT AND

HOLD.a. If the spin is inverted, a rapid and

positive recovery will be affected[sic] within one turn.

b. If the spinning stops, neutralizecontrols and recover from theensuing dive.

c. If spinning continues, the aircraft mustbe in an erect normal spin (it cannotspin inverted or accelerated if thecontrols are moved abruptly to thisposition). Determine the direction ofrotation using the turn needle andoutside references before proceeding tothe following steps.

4. RUDDER – ABRUPTLY APPLY FULLRUDDER OPPOSITE SPIN DIRECTION(OPPOSITE TURN NEEDLE) ANDHOLD.

5. STICK - ABRUPTLY FULL FORWARDONE TURN AFTER APPLYINGRUDDER.

6. CONTROLS – NEUTRAL AFTERSPINNING STOPS AND RECOVERFROM DIVE.

L-39 Albatross Jet Trainer

Note

Spin character is stabile during the first turn,with increasing instabilities typical for jet aircraftin continuous turning. Unregular [sic]longitudinal oscillations develop with increasingamplitude and shudder. Rudder bounces andincreasing varying pedal forces must beovercomed [sic]. Unadequate [sic] control inputscan lead to inverted spin development (especiallywith extreme forward stick position), or to thechange in turning sense (ailerons against spinturning).

Upright Spin Recovery

Recovery is initiated with rudder and elevatorcentering. The aircraft stops turning and passesto steep dive with maximal one turn overturning.Ailerons remain held in neutral position. Moreaggressive turning stop can be initiated withrudder deflected against turning first, withsubsequent all controls centering.

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Aircraft Gyroscopic InertialCharacteristics

We’ve referred to pro-spin and anti-spin inertiamoments without defining what we mean ordescribing how they work. The following maynot be entirely easy going, but give it someeffort—especially if you’re headed for your CFI.You won’t go into (or remember) this muchdetail with your primary students, but you doneed to get a sufficient handle on things toanswer some tough questions without leadingyour students too far astray. You should takeyour eventual CFI students to as high a level ofunderstanding as you both can manage. Don’tshirk your responsibility! And get a toygyroscope to play with. It will help you figurethings out.

Some definitions:

A moment causes rotation about an axis.

The moment of inertia, I, of a rigid bodyabout a given axis is a measure of itsrotational inertia, or resistance to change inrate of rotation. It equals the sum, ∑, of thebody’s various masses, m, multiplied by thesquares of their respective distances, r, from thataxis:

Moment of inertia, I = ∑ (mr2)

The greater its moment of inertia about an axis,the greater the applied moment or torque (forceapplied x lever arm) needed to change therotational motion of the body around that axis.

Aircraft have moments of inertia around eachinertial, or principal axis, which are normallyclose to the body axes.

IxxRoll Moment

of InertiaA

Predominantly the massof the wings.

IyyPitch Moment

of InertiaB

Predominantly the massof the fuselage.

IzzYaw Moment

of InertiaC

Mass of wings andfuselage.

Izz is always greatest.

If Ixx > Iyy, then wing mass > fuselage mass.

If Ixx < Iyy, then wing mass < fuselage mass.

(A, B, C are symbols used in Great Britain.)

Since it comprises all the aircraft’s mass, yawmoment of inertia, Izz, is always greatest. Pitch orroll inertia are greater, respectively, dependingon the aircraft’s distribution of mass betweenfuselage and wing. An aircraft with puny wingsand a heavy fuselage, so that Ixx < Iyy, has rollinertia < pitch inertia, for example.

The pitch/roll, Iyy/ Ixx inertial ratio is important tothe character of an aircraft’s spin and recovery.Iyy/ Ixx =1.3 is approximately the neutral value.Above that number, when Iyy/ Ixx >1.3, aircraftare considered fuselage-loaded in their behavior.Below that number, when Iyy/ Ixx < 1.3, aircraftare wing-loaded in behavior.

An aircraft’s external shape determines itsaerodynamics. Pilots are accustomed to lookingat the external shape and anticipating aircraftbehavior (often unsuccessfully). But aircraft alsohave an “internal shape,” as determined by thedistribution of mass. Aircraft stability andcontrol is a contest between the two. That’salways so, and especially so in spins, when theinternal shape starts to take on angularmomentum.

Pitch Inertia Iyy

Yaw Inertia Izz

Roll Inertia Ixx

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Angular Momentum

Momentum is inertia in motion. A rotatingbody’s angular momentum, Iω, is its momentof inertia, ∑ (mr2), about a given axis times itsangular velocity, ω, around that axis.

Angular momentum = ∑ (mr2 ) ωor

Angular momentum = Iω

Angular momentum is also referred to as rotarymomentum.

Angular velocity

Angular velocity, ω, just referred to, is the rateof change of angular displacement. Consider aflat, rotating disk with a line drawn from centerto circumference, as below. The axis of rotationis perpendicular to the page. As the disk rotatesthrough an interval of time, the displacedreference line forms an angle with its originalposition. Angular velocity is typically stated inradians-per-second (and in aerodynamicsdenoted by the symbols p, q, and r, for roll,pitch, and yaw).

(Inpounds)

Moments of Inertia (in slug-ft2) Source:www.jsbsim.sourceforge.net/MassProps.html

Aircraft Weight IxxRoll

IyyPitch

IzzYaw

F-16C(BLK30)

18,588 6,702 59,143 63,137

F-4Phantomw/2 AIM-7, 20%fuel

33,196 23,668 117,500 133,726

C-5w/220,000lb. Cargo,20% fuel

580,723 19,100,000 31,300,000 47,000,000

X-15w/zerofuel

15,560 3,650 80,000 82,000

CherokeePA-28

2,100 1,070 1,249 2,312

C172Skyhawk 1,700 948 1,346 1,967

LearjetEmptyweight

7,252 3,000 19,000 50,000

B-727Emptyweight

88,000 920,000 3,000,000 3,800,000

B-747 636,600 18,200,000 33,100,000 49,700,000

T-38Emptyweight

7,370 14,800 28,000 29,000

F-15 26,000 22,000 182,000 200,000

SchweizerSailplane 874 1,014 641 1,633

Twin Otter 9,150 15,680 22,106 33,149

Change in angle, θ ω = change in time

S = arc length

r

θ

Every point on the line has the same angular velocity. Its tangential velocity is proportional to its distance from the axis.

θ = s/r θ = Angular displacement in radians per second 1 radian = 57.3 deg.

ω = angular velocity

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Gyroscopes

A spinning aircraft is a system of gyroscopes.The rotating mass of a gyroscope has twoimportant features. The first is rigidity inspace—as the spin rate of the rotor increasesaround its axis, it takes increasing force to tilt theaxis in a new direction.

The second feature is precession: An applied“input” will precess (go forward) and generate an“output,” 90 degrees ahead in the direction ofrotation. This is shown in the simplest andprobably easiest-to-visualize way in the drawingon the left, in Figure 12.

The same input is occurring in Figure 12, right,but presented in terms of moments. Thegyroscopic rotation is around the z-axis. If youapply a moment (or torque) around the y-axis, asshown, it will precess, causing a resulting inertiamoment around the x-axis. The inertia momentis our output.

Think of the drawing in Figure 12, right, in termsof the axis system of an airplane. If the aircraft isyawing around the z-axis (in a spin, perhaps) andyou apply a pitching (y-axis) moment withelevator, you’ll end up getting a rolling inertiamoment (x-axis), as well.

Because inertia moment = angular momentum xapplied angular velocity, the higher the appliedpitching angular velocity (i.e., greater the appliedpitching moment), the greater the resultingrolling inertia moment.

Aircraft Gyroscopics

The basic gyroscopic relationships in a spinningaircraft are easy to imagine once you get thetrick. If an aircraft is already rotating around anaxis, and thus acting like the rotor of a gyroscopearound that axis, a moment applied around asecond axis results in a moment produced aroundthe remaining third. Therefore:

Pitching into a yaw rotation gives a roll.

Pitching into a roll rotation gives a yaw.

Yawing into a pitch rotation gives a roll.

Yawing into a roll rotation gives a pitch.

Rolling into a pitch rotation gives a yaw.

Pitch Precession is 90 deg. in direction of rotation.

Input

Direction of gyroscopic rotation

Input push

Output tilt

Gyroscopic precession is the result of the conservation of angular momentum.

Figure 12 Gyroscopic Precession Direction of gyroscopic

rotation: Z-axis has angular momentum, Iω (moment of inertia times angular velocity).

Resulting precessed inertia moment

Applied moment (torque) produces an applied angular velocity, ωY.

X-axis

Z-axis

Y-axis

Inertia moment = Angular Momentum x Applied Angular Velocity (Moments of inertia and inertia moments are two different things. The former resist changes in angular velocity, the latter produce changes in angular velocity as the result of gyroscopic effects.)

Yaw

Roll

Output

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Rolling into yaw rotation gives a pitch.

Somewhere before the end of the list, youprobably got the point. It’s not so simple inpractice, however, because in an actual spinningaircraft a moment around one axis precessesaround two “gyroscopes” simultaneously. Forinstance, the applied angular velocity following apilot’s pitch input works both the yaw-axis androll-axis gyroscopes. The resulting inertiamoments generated depend on the relativeangular momentums (angular momentum =moment of inertia x angular velocity) aroundthose axes. Thus for a given applied angularvelocity around the first axis, the second axiswith the highest angular momentum will“precess” the highest inertia moment into thethird.

Take, for example, pitch inertia moment.Imagine an aircraft rolling and yawing in a spinto the right, as in Figure 13, top. It has an angularmomentum in roll (equal to Ixx times angularvelocity in roll, p). Since it’s also yawing to theright, it has an applied angular velocity aroundthe yaw axis. The yaw precesses 90 degrees inthe direction of roll, and produces a nose downpitching moment. This is an anti-spin moment.

At the same time, the aircraft also has angularmomentum in yaw (equal to Izz times the angularvelocity in yaw, r), as in the bottom drawing. Inthis case the applied angular velocity is providedby the roll rate. The applied roll precesses 90degrees in the direction of yaw, and produces anose-up pitching moment. This is a pro-spinmoment.

This nose-up moment will be larger than thenose-down moment already described, becausethe moment of inertia around the z-axis, Izz, isalways greater than the moment of inertia aroundthe roll axis, Ixx. The net effect of the gyroscopicinterplay between roll and yaw is always a nose-up, thus pro-spin, pitch inertia moment.

What if the pilot steps in and imposes a pitch rateaerodynamically, by moving the elevator?Because yaw moment of inertia, Izz, is alwayshigher than roll moment of inertia, Ixx, roll ismore likely to be affected than yaw. The rollinginertia moment generated will be higher, and themass it has to accelerate will be less. In a spin, apitch down always produces a pro-spin rollinertia moment. A pitch up always produces ananti-spin roll inertia moment. If a spin oscillates

in pitch, its roll rate will vary as it pitches up anddown.

OK, you deserve a break.

x-axis roll direction

y axis

z-axis yaw direction

y axis

A roll to the right precesses through yaw 90 deg. into a pitch up inertia moment.

Right yaw input (an applied angular velocity around the z axis)

Right roll input (an applied angular velocity around the x axis)

Spin is vector sum of roll and yaw

Pitch down output

Pitch up output

Net inertia pitch moment is up (pro-spin), because Izz is always greatest.

Figure 13 Pitch Inertia Moment

Yaw Gyro

x axis

z axis Roll Gyro

A yaw to the right precesses through roll 90 deg. into a pitch down inertia moment.

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Propeller Gyroscopics

There’s a fourth gyroscope to consider, and it’sthe one you actually learned about first inprimary ground school—the propeller.

The effects of propeller gyroscopics are mostevident in aircraft with heavy props and ratherlow directional and longitudinal stabilities,whenever the aircraft yaws or pitches rapidly athigh prop rpm and low airspeed. It also helps ifthe prop is a substantial distance from the aircraftcenter of gravity and can exert some leverage.High rpm gets the gyroscope’s angularmomentum going, and low airspeed reduces theaircraft’s stabilizing aerodynamic moments tothe point where prop gyroscopic effects canbecome apparent. Gyroscopic precession (notjust prop but also aircraft mass) is the essentialdriving force behind the aerobatic tumblingmaneuvers derived from the Mother of Tumbles,the lomcovak.

Gyroscopic effects acting through the propellercause yaw to produce a secondary pitchresponse, and pitch to produce a secondary yawresponse, as Figure 14 describes. With aclockwise prop, yawing to the left causes thenose to precess up. A spin to the left can go flatwith power, and possibly refuse to budge untilpower is reduced and the prop’s angularmomentum decreased.

In a spin to the right, power increases thegyroscopic tendency to bring the nose down andalso generates an anti-spin slipstream effect overthe tail. That may assist recovery (the gyroscopicpart may also increase the tendency for a spin togo inverted if the pilot applies forward stick tooaggressively). Many flight manuals—as well asthe usual generic recovery procedures—specifypower off at the start of spin recovery regardlessof direction. The idea is to prevent the prop fromcontributing anything harmful during the ensuingconfusion, and to prevent excessive airspeedduring the recovery pull out.

Unlike other propeller-induced effects,gyroscopic precession occurs only in thepresence of pitch or yaw rates, and depends ontheir magnitude. Precession tends to hold anaircraft’s nose up in a turn to the left and to forceit down in a turn to the right. Precession occursthroughout looping maneuvers and actuallydecreases the amount of rudder input thatcompensating for p-factor and spiraling

slipstream would otherwise require (right rudderin positive maneuvers).

In jets, the rotating masses of compressors andturbines supply the fourth gyroscope. Aclockwise rotating engine, as seen from behind,produces a faster spin to the right. Remember themovie The Right Stuff, when Chuck Yeager tookthe rocket boosted F-104 above 100,000 feet andlost control? Aerodynamic damping wasinsignificant at that altitude. The air-breathing jetengine had been throttled back, but its rotationstill produced a significant yaw couple.According to a well-placed authority, this tookYeager by surprise, because he hadn’t flown theflight profile in the simulator like everybody wassupposed to and was therefore late getting on theyaw thrusters.

Aircraft yaws right.

Pilot pitches up.

Pilot yaws right.

Aircraft pitches down. Pilot pitches down.

Aircraft yaws left.

Pilot yaws left.

Propeller rotation as seen from the cockpit

Pilot’s input

Precession causes output 90o in direction of rotation.

Figure 14 Prop-induced Gyroscopic Response

Aircraft pitches up.

Propeller rotation

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Dumbbells

Figure 15 shows the fuselage and wing of anaircraft represented as dumbbells possessing theequivalent inertial characteristics. Rotationproduces centrifugal forces that tend to drive themasses apart. In this situation, equal andopposite forces, acting along different lines,produce a rotary inertial couple.

By itself the fuselage couple tends to drive thenose up, flattening the spin attitude (a pro-spinpitch couple).

The fuselage couple also tends to yaw the noseopposite the spin direction (anti-spin yawcouple), as in the bottom drawing. Thedumbbells in the wings, however, tend to yawthe aircraft in the pro-spin direction. Theultimate inertial couple in yaw depends on theaircraft’s mass distribution. Yaw couple is pro-spin when the wings are the dominant mass (Ixx >Iyy). Yaw couple is anti-spin when the fuselage isthe dominant mass (Ixx < Iyy).

Fuel load affects spin behavior by shifting thebalance. Filling the outboard or tip tanks isusually prohibited before intentional spins inaerobatic aircraft that have them, partly forweight concerns and partly because the buildupof greater angular momentum in roll and thegreater pro-spin yaw couple have to be overcomeduring recovery.

Note that the rotary inertial couples have thesame effect (the same positive or negative sign)as the gyroscopic inertia moments we’ve beendiscussing.

Figure 15 Inertial Couples in Pitch and Yaw

Anti-spin yaw inertial couple dominates.

Pro-spin yaw inertial couple dominates.

Nose-up, pro-spin pitch couple

Spin to the left Aircraft is seen from the top.

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Axis InertiaMoment

AngularVelocity

Momentof Inertia

AircraftComponent

xRoll

Li(positive

right,negative

left)

Roll rate, p(positive right,negative left)

IxxRoll

Inertia

Predominantlythe mass of the

wings.

yPitch

Mi(positive

up,negativedown)

Pitch rate, q(positive up,

negativedown)

IyyPitch

Inertia

Predominantlythe mass of the

fuselage.

zYaw

Ni(positive

right,negative

left)

Yaw rate, r(positive right,negative left)

IzzYaw

Inertia(alwaysgreatest)

Mass of wingsand fuselage.

Positives and Negatives

In the three simplified moment equations on thefollowing page, the relative magnitudes of Ixx,Iyy, and Izz determine whether an inertia momentgenerated by precession will be pro-spin or anti-spin. This is key to understanding how thedistribution of mass in an aircraft affects spinrate and attitude, and how control inputs affectrecovery.

Remember that in the aerodynamics sign systempositive values are up and/or to the right,negative values are down and/or to the left.(Remember from algebra that a negative times anegative equals a positive; a negative times apositive equals a negative; and a positive times apositive equals a positive.)

In the case of the inertia moment in pitch, Mi,imagine an airplane is spinning to the positiveright. Roll, p, and yaw, r, are both positive in thiscase. Since Izz is always the greatest moment ofinertia, (Izz - Ixx ) is also positive. Since a positivetimes a positive times a positive equals apositive, the inertia moment in pitch, Mi, ispositive. That means nose up, pro-spin. Thesame goes for spinning to the negative left, sincea negative times a negative (-p times -r) againequals a positive.

Look at inertia moment in roll, Li: (Iyy - Izz) isalways negative, because Izz is always greatest.In a spin to the left, a negative (Iyy - Izz) times anegative yaw, r, times a positive or zero7 pitch, q,equals a positive. When spinning to the negativeleft, a positive (rightward) inertia moment in rollis anti-spin. In a spin to the positive right, theinertia moment in roll would be negative, againanti-spin.

In the last equation, the direction of the inertiamoment in yaw, Ni, may be anti- or pro-spindepending on which is greatest, Ixx or Iyy. Inother words, on whether the aircraft carries moreof its mass in the wings (Ixx > Iyy), or in thefuselage (Ixx < Iyy).

7 Zero is neither negative nor positive.

Figure 16 Iyy / Ixx Ratio

Ixx > Iyy Roll moment of inertia greater than pitch moment

Ixx < Iyy Roll moment of inertia less than pitch moment

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If the aircraft is wing loaded, and thus Ixx isgreater than Iyy, the value in the brackets will bepositive. In a spin to the positive right, p ispositive, q is positive or zero, and so the appliedinertia moment in yaw will be positive and pro-spin. In a spin to the negative left, p is negative,q is positive or zero. A negative times a positivetimes a positive is a negative, thus again causingpro-spin yaw. (Note how this also corresponds tothe dumbbell illustration.)

Wing-loaded, Ixx > Iyy, aircraft generate pro-spinyaw inertia moments.

If the aircraft is fuselage loaded, and thus Ixx isless than Iyy, the value in the brackets will benegative. In a spin to the negative left, p isnegative, q is positive or zero. The result of allthree is positive, thus anti-spin in a spin to thenegative left. It’s also anti-spin in a spin to thepositive right.

Fuselage-loaded, Ixx < Iyy, aircraft generate anti-spin inertia yaw moments.

Aircraft become spin resistant, in terms of theirinertia moments, roughly when the ratio Iyy/ Ixx =1.3 or greater; in other words, when they leantoward the fuselage-loaded, with more pitchmoment of inertia than roll moment of inertia.

Once they get going, however, spins in fuselage-loaded aircraft may be more oscillatory or have atendency to go flat, due in part to their powerfulinertia moment in pitch, Mi. Anti-spinaerodynamic moments generated by the ruddermay not be sufficient for recovery against theangular momentum built up in yaw. Soadditional anti-spin yaw inertia moments, Ni,may be necessary for rescue. In this case, togenerate those moments, recovery may requireaileron into the spin direction in addition to out-spin rudder. This accelerates the roll rate, p,which precesses into a resulting anti-spin yawinertia moment (when Ixx < Iyy). The adverse yawprovoked by the aileron going down on theoutside wing might also provide some helpfulanti-spin aerodynamic yaw moment (onecertainly easier to visualize and understand thanconvoluted gyroscopic moments—and aperfectly good way to remember the recoveryaction), but generating that anti-spin yaw inertiamoment is the main idea.

Note from the third formula that when massshifts to the wings and roll moment of inertiabecomes higher than pitch moment of inertia, thesign in the brackets becomes positive. Aileroninto the spin now produces a pro-spin moment inyaw.

After this, is your head spinning? It’s not easy.

Ixx < Iyy Inertia moment in pitch = (yaw axis moment of inertia – roll axis moment of inertia) times roll rate times yaw rate .

( )prIIM xxzzi −=

Inertia moment in pitch, Mi, is nose-up, pro-spin. (Izz always > Ixx; sign in brackets always positive) Inertia moment in roll = (pitch axis moment of inertia – yaw axis moment of inertia) times pitch rate times yaw rate

( )qrIIL zzyyi −=

Inertia moment in roll, Li, is anti-spin. (Iyy always < Izz; sign in brackets always negative)

Inertia moment in yaw = (roll axis moment of inertia – pitch axis moment of inertia) times roll rate times pitch rate

( )pqIIN yyxxi −=

Inertia moment in yaw, Ni, may be pro-spin if roll inertia, Ixx , is greater than pitch inertia, Iyy (Ixx > Iyy; sign in brackets positive), or anti-spin if roll inertia is less than pitch inertia, (Ixx < Iyy; sign in brackets negative).

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Moments in Balance

As already noted, in a steady spin, rotary andgyroscopic inertia moments about the aircraft’saxes and aerodynamic moments about those axesbalance (or sum to zero, since they have oppositesigns). Take the example, in Figure 17, of theinertia moments in pitch, which are always nose-up. An aerodynamic, nose down moment,generated mostly by the horizontal stabilizer,balances this. (Even if you’re holding back-stickin a practice spin, the aerodynamic moment isnose down.)

Likewise, the inertia moment in roll, which isalways anti-spin, will be balanced by the pro-spin aerodynamic roll moments generated bysideslip and autorotation.

The situation around the yaw axis is morecomplicated, since the inertia moments in yaware affected by the ratio Iyy/ Ixx. They tend to bepro-spin in wing-loaded aircraft, as describedabove. In that case the balancing aerodynamicmoment is the damping moment generated bythe fuselage and tail. If the aircraft is fuselage-loaded, the inertia moments are anti-spin andbalanced by the aerodynamic moments drivingthe spin (pro-spin yaw moments generated byautorotation and by rudder deflection if the spinis intentional).

Nose up inertial moments

Nose down aerodynamic moment

Relative wind

Figure 17 Moments in Balance

=

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Inertia Pitching Moment in Particular

Figure 18a shows a curve of inertia pitchingmoment plotted against angle of attack for adeveloped steady spin. It’s always greatest at 45degrees for a given angular velocity, andincreases overall with angular velocity, ω (spinrate), as shown. Although inertia pitchingmoment is always positive (nose up), in this caseit’s plotted in the negative direction. Note thatthe curves each represent a different but constantangular velocity.

Figure 18b shows aerodynamic pitchingmoment, which is always nose down, plottedagainst angle of attack. Note the increase inaerodynamic moment as angle of attackincreases and/or as the stick moves from aft toforward.

Both moments appear in Figure 18c. The pointof intersection, where inertia pitching momentand aerodynamic pitching moment are equal andopposite, shows the angle of attack at which astabilized spin can occur.

Finally, Figure 18d shows what happens whenthe stick is moved from aft to the forwardposition. Angle of attack decreases as theincreased aerodynamic moment forces the nosedown. At the new angle of attack, the line ofincreased aerodynamic moment intersects a newinertia moment curve. The aircraft stabilizes at afaster spin rate.

The figures underscore the importance of stickposition and timing during spin recovery.Applying forward stick, before opposite rudder,can accelerate a spin rate. This increases thenose up inertia moment and actually makes theelevators less effective in reducing the angle ofattack and breaking the spin—since they havemore to work against. Holding the stick backdecreases the aerodynamic moment and thereforethe inertia moment as the system restoresbalance. Opposite rudder applied first allows thespin rate to decelerate to the point where forwardstick can be applied and the elevator will havesufficient authority to decrease the angle ofattack and break the spin. In practice, forwardstick following opposite rudder typically leads toa momentary acceleration, but anti-spinaerodynamic moments quickly prevail.

Angle of Attack

Iner

tial M

omen

t

(

Nos

e-up

tow

ard

arro

w)

0o 45o 90o

Angle of Attack

Aer

odyn

amic

Mom

ent

(N

ose -

dow

n to

war

d ar

row

)

0o 45o 90o

Angle of Attack

Aer

odyn

amic

Mom

ent (

Nos

e-do

wn)

. In

ertia

l Mom

ent

(Nos

e -up

)

0o 45o 90o

Angle of Attack

Aer

odyn

amic

Mom

ent (

Nos

e-do

wn)

. In

ertia

l Mom

ent

(Nos

e-up

)

0o 45o 90o

Increasing spin rate, ω

Aft Stick

Steady state: moments are equal and opposite.

Forward Stick

Neutral Stick

Aft Stick

Forward stick decreases α, equilibrium reestablishes at a higher spin rate.

b

d

c

a

Figure 18 Pitching Moments

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