HAROLDHOLMES'

COCKPITBy Harold Holmes (EAA 220238), CFI

1038 Inverrary LaneDeerfield, IL 60015

AERODYNAMICS OF STALLS

O,'NE OF THE most important pri-mary maneuvers for stall recognitionis flight at critical airspeeds and slowflight. Slow flight and flight at criti-cally slow airspeeds - just above thepoint of stalling - is done to develop asense of feel and ability to use controlscorrectly and to perform maneuversat airspeeds just above stall speed. Itis important to recognize changes inthe airplane flight characteristicswhile maneuvering close to the stallin various attitudes and flight config-uration such as no flaps, partial orfull flaps, gear up or down, climbing,gliding, turning or straight and level.Most important is to be able to recog-nize imminent and full stalls and toaccomplish prompt, positive and effec-tive recoveries in all normally antici-pated situations. In flight trainingand evaluation, I'll have the studentor applicant perform a 360 degreeturn flying with the stall warninghorn blaring. This type of maneuverwill require excellent control and itshould be done without abrupt controlmovements and no steep banks.

I recently had the pleasure of work-ing with Brad Alspach, flight instruc-tor and project director for George J.Priester Aviation Service, Inc.,Wheeling, IL in a Piper Warrior IIequipped with a NASA approvedtufted wing. In Photo A you can seewhat the tufts of yarn look like inlevel flight. Note that the wing is cov-ered with hundreds of pieces of yarnwhich are susceptible to even theslightest change in laminar flow orrelative wind. Priester Aviation ismaking this unique inflight demon-stration available to all pilots in thearea who are interested.30 NOVEMBER 1984

Note: Elevators control angle of attack.

Diagram A

Diagram C,Loss of Lift Results ina Stall (Imminent Stall) Diagram D1 The Complete Stall

Photo A - Level Flight

Photo B - Shows the root of wing stalling first

Photo C

The most important aerodynamicprinciple of the stall is understandingangle of attack. The "angle of attack"is the angle formed by the relativewind and the chord line of an airfoil(see Diagram A). At lesser angles ofattack, there is less lift than atgreater angles of attack. During thedemonstration flight, I had theunique opportunity to witness whathappens when the angle of attack isincreased to the point where the airdoes not smoothly flow over the uppersurface of the wing and actually be-gins to separate from the wing (seePhoto C - the Imminent Stall). In Pho-to C you can see where the tufts areuneven and indicate the points wherethe smooth flow of air over the wingis disturbed. In Photo D the wing hasbegun to lose almost all of its lift -this is recognized as the "stall". Thestall is caused by exceeding the criti-cal angle of attack. Also note in PhotoB that the wing roots will exceed thecritical angle before the wing tips -the roots will stall first. Another im-portant observation can be noted inPhoto C showing the tufts of yarnbeing affected all along the aileron.This is the reason we stress the use ofrudder instead of aileron as the maincontrol to raise the low wing in a stallrecovery. Photo C(a) shows the resultsof using aileron alone to raise the lowwing in a stall condition during a go-around. Use of too much aileron willonly aggravate the stall recovery.

An airplane cannot stall at a lowangle of attack, therefore, by reducingthe angle of attack (yoke forward) isa most important recovery procedure.We mentioned that maximum allow-able power will be needed. The needhere for power is to recover the lostaltitude in a climb. In the event of astall occurring near the ground, thiscould be critical. While practicingstalls, once the recovery has been ac-complished, and the airplane is flyingagain, start a climb back to the al-titude at which the stall was firststarted. In modern airplanes, all thatis necessary is to release back pres-sure on the yoke immediately as thestall occurs, using the elevator to con-trol the angle of attack, with coordi-nated flight (ball centered) and powercontrols. The main thing is to allowthe wing to regain lift. Maximum al-lowable power must always be usedin stall recoveries. The most impor-tant point to learn in stall practice isthat wherever the nose is pointing,the only way one can recover from astall is with releasing back pressure(forward yoke) immediately as thestall occurs (see Diagram E). It is easyto visualize why an inexperiencedpilot might panic in a stall close tothe ground - turning base to final, forinstance - and pull back on the yokeas a last resort. Remember, yoke for-

SPORT AVIATION 31

ward, recover from stall, full power,climb out. A sequence like this shouldbecome a habit each time stalls arepracticed.

The purpose of an Approach toLanding Stall (see Diagram Ej) is tobe able to recognize an approach stallthat may be encountered in the turnfrom base to final if the nose is al-lowed to come up inadvertently. Herewe establish a landing configurationwith full flaps, power reduced to idle(see manufacturer's recs) and theairspeed at 1.3 times VSO - approxi-mately 70 knots. This stall is enteredfrom a gentle 15 degree bank turneither right or left by applying exces-sive back yoke pressure until the stalloccurs. This stall should be practicedat least 2000 feet above the ground(AGL). Since the Approach to Land-ing Stall could occur close to theground, I would like to suggest aseries of steps during the recoveryphase.

The objective is to recover promptlyby setting up a climb to clear an ob-stacle (VX), then a standard rate ofclimb (VY) followed by a level off aftergaining at least 300 feet above thealtitude at which the stall occurred.Let's consider the recovery phase as astepwise procedure like everythingelse we do in flying:

1. First, at the indication of a stall,the pitch attitude and angle of attackmust be decreased positively and im-mediately - break the stall with for-ward yoke and apply power.

2. Second, dump about half of yourflaps so that you can initiate a climb.

3. Third, establish a climb at thebest angle of climb speed (VX - 60knots). Remove more flaps to 10 de-grees of flaps and climb until clear ofall obstacles - simulated.

4. Fourth, lower the nose toachieve the best rate of climbairspeed, dumping the remainder offlaps and continue climb to 300 feetabove initial altitude.

5. Fifth, straight and level flightshould be gained with coordinated useof all controls. Accelerate to cruiseairspeed, and reduce power to the ap-propriate cruise power setting.

The above five steps used for recov-ery from an approach to landing stall,from a safety standpoint, is probablyone of the most important safety se-quences in flying. Note that it is iden-tical to the go-around procedure (seeDiagram F). The five step recoveryshould be practiced to perfectionin that the same steps will be usedduring go-arounds. As a safe pilot,you will want to be able to performthe steps instinctively and preciselyshould you have the unfortunate ex-perience of stalling at a low altitudesuch as when turning from base tofinal.32 NOVEMBER 1984

Photo D

Diagram DStall Recovery Procedure

Diagram E

STALL RECOVERY

Recovcy should be accomplished by reducing the angleof attack immediately with forward pressure on yoke

Use opposite rudder to raise low wing with mm. use ofaileron.

Next, I would like to discuss thepower-on stall or the Take-Off andDeparture Stall. This is likely to bethe most abrupt of all of the stall ma-neuvers. This is true in that withpower the nose has to be raised to ahigher angle of attack for the stall tooccur; consequently, the pitch-downin the full stall break is much more

extreme. In relating this stall to aflight maneuver, it will most likelyresult when turning to the crosswindleg on take-off while using excessiveback pressure. It is usually practicedin a 15 degree turn to the crosswindleg in either left or right turn. TheVideo Training Aids, Inc. tapes vis-ualize and describe the Take-Off andDeparture Stall as follows:

Diagram E,

APPROACH TO LANDING STAL1PURPOSE OF MANEUVERTo be at>ie to recognize ap-proach to stall thai may beencountered in th« landingconfiguration a n d executeproper recovery if stall shouldoccur.

Use same altitudeas other stalls

STALL ENTRYPower-oft stalls are entered straight ahead or inmoderately banked gliding turns At landing ap-proach speed (approximately 70 knots).Perform both rmminent and lull stalls m nose-highturning and level fright

Diagram F

Go-Around ProcedureAfter descent has been stopped, landing (laps mayb« partially retracted or placed in takeotf position

• Wine Flaps to 20*(SecMf|n RccV)

• Establish best angle of climb (VX) — 60 knots(Check manufacturer's recommendation)

• Clear obstacleFlaps to 10

• At 200 feet, best rate ol climb (VY)• Flaps to 0'Retract landing gear after a positive rale of climbis established.

Enroute CHmb• Airspeed — 00 knots (Se* manufacturer's

recotn mendalion)• Power — maximum allowable power• Mixture — full rich

CtwckiM:Flaps upFuel tanks checkedFuel pump — "off"

Diagram ISTALL RECOVERY

Diagram G

TAKE OFF ANDDEPARTURE STALL

f OO NOT•TORCH

[ CLEARING ,TURNS

2000 tt -_--_

PURPOSE OF MANEUVER• To be able to 'fcognue

approach to stall that maybe encountered after eitreme nose up attitudeafter T 0 and executeproper recovery it stallshould occur

• Perform both iniaiiiea.1,and Ml stalls

Stalls should be performedat altitudes no lower than2000 feet above groundlevel—recovery completedby 1500 feet above (roundlevel

EHTRr

• Establish tskeolf contiguration and airspeed at approximalely lift off speed

Throttle at climb power

Yoke bach-ease nose up above hoojon high angle ofattack (Stall should also be practiced m 20' to 30'banks )

Diagram HAPPROACHING STALLRelative Wind. Angle of Mack «-1«-Correct tor P factor by using right rudder to main-

i directional cooiroi

When performing turning stalls a constant pitchand bank attitude should be maintained until stalloccurs.

Check attitude of airplane visually.Listen tor special sounds, e.g, engine noise.Sense any changes in direction, or speed, ormotionNotice that control pressures are lessened Immi-nent stalls increase angle of attack until the firstbuffeting or decay of control effectiveness isnoted Recoveries from imminent stalls enteredfrom straight flight and from turning flight withpower-on or power-offStall is caused by exceeding critical angle ofattack — wing rools will exceed critical angle be-fore wing tips — roots will stall first.

Reduce the angle of attack by releasing backpressure on yoke immediately as stall occurs anduse elevators to control angle of attack, with co-ordinated flight and power controls. Allow wingto regain lift.

Apply maximumallowable power.

"All of the principles which applyto stalls without power also apply tostalls entered with power; however,there are some important differences.First of all, the pitching of theairplane resulting from a full stallcondition with at least 65 percentpower (2300 rpm) is much more steepand rapid and the airplane is harderto handle than during power-offstalls. During the Take-Off and De-parture Stall there is a tendency forthe airplane to fall off on one wing orthe other more readily than in apower-off stall."

The VTA tape instructions con-tinue to point out the fact that if theball is in the center, both wings willbreak simultaneously. In performingthe stall from a right climbing turn,there is a natural tendency not to useenough right rudder causing the leftwing to break first. This can be a verydangerous situation because an over-the-top spin can result here.

Again, let's present the entry andrecovery phases using the stepwiseprocedure (see Diagram G).

1. The first step is to reduceairspeed to approximately lift-offspeed, say, 60 knots in a take-off con-figuration.

2. Second, bring in power and starta gentle turn (simulating left or rightturn to the crosswind leg), keepingthe ball centered, and easing the noseabove the horizon at a high angle ofattack.

3. Third, when approaching thestall, correct for P factor by usingright rudder (see Diagram H). Notethat the airspeed will fall off promptlydespite the power and a good stallbreak should occur.

4. Fourth, recover from the stall byreducing the angle of attack - releas-ing back pressure on the yoke im-mediately as the stall occurs - andsimultaneously applying maximumallowable power. (Prompt action isneeded here to prevent a prolongedstalled condition - see Diagram I.)

5. Return to straight and levelflight at cruise power.

As I mentioned in the beginning ofthe article, there are two reasons forperfecting stalls. First, they instillrecognition of imminent stalls, so thatan actual stall might be preventedprior to its occurence. Another impor-tant reason is to learn safe stall recov-ery, in the event that one does occur.It should also be noted that stalls areamong the safest of maneuvers pro-viding they are performed correctly.While in the stall break itself, theaircraft is not subjected to any un-usual G forces and it is not reallyflying. A prompt and safe recoverywill even prevent the entry into asteep dive which could place someunusual forces on the airplane.

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