Flightlab Ground School 10. Ground School 10. Spins ... corporate jets are the descendents of...
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Bill Crawford: WWW.FLIGHTLAB.NET 10.1
Flightlab Ground School10. Spins
Copyright Flight Emergency & Advanced Maneuvers Training, Inc. dba Flightlab, 2009. All rights reserved.For Training Purposes Only
A Short Sermon
Theres 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 protectiona feeling thatcomes from the sense of a buffer betweenthemselves and any dangers likely to occur. Itsbeen 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 dont 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 arent designed to makeyou feel comfortable about the notion of hittingthings. Theyre 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 wouldnt help. Theconclusion they draw is that flight trainingshould rely on stall avoidance as the way to spinavoidancethat stall avoidance is the buffer.Aerodynamically, theyre right: stall avoidanceis the way to spin avoidance. But spin training isitself the best way to produce unswerving loyaltyto stall avoidance, because its 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 shouldnt make apilot feel complacent while maneuvering anaircraftor, 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 hasnt experienced spin training mightfeel safe, but have less real ability to look aheadand refrain from doing the wrong thinglessability 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 worlds first spin recovery, on August25, 1912. Well 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 afterParkes Dive, as it came to be called,underscores the wariness that remained. UntilParkes nick-of-time revelation, pilots hadalways held rudder into the direction of a turn toprevent sideslipa 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 maneuversand then not, atleast not in the U.S. after 1949, once theregulators changed their minds. Well 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 contextsince 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- regime, wherecontrol was jeopardized by the swept wingstendency 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- behavior, and the horizontal stabilizerwas re-positioned away from the downwash ofthe high- wing wake to improve nose-downpitch authority.
Figure 1 shows that swept wings stall at higherangles of attack than straight wings. Theres
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 . Instead of rolling away from it, theaircraft could yaw toward the wing with thedown aileronthe resulting sideslip and roll dueto yaw rate sending it into a departure against theailerons. At high , 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
Figure 1 Lift Curve
Larger lift difference for a given difference produces more roll.
Angle of Attack,
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along. (Described farther on, inertia momentsoperate on the same principle as the propellergyroscopics youre 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 alwaysthere 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 Zlins departurecharacteristics are dominated by wing-root-firststall patterns at high patterns youll see whenwe stall a tufted wing. The tapered-wing SF260sstall pattern is shifted outboard, which makes theaircraft more susceptible to a sudden wing drop.Their inertial characteristics come fr