Spin Prediction Techniques

8/11/2019 Spin Prediction Techniques

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VOL. 20, NO. 2 J. AIRCRAFT FEBRUARY 1983

Spin Prediction Techniques

William Bihrle Jr.* and Billy Ba rnha r t tBihrle Applied Research, Inc., Jericho, New York

A rotary balance facility located in the Langley spin tunnel was developed to rapidly identify an airplane saerodynamic characteristics in a rotational flow environment. The generation of on-line rotary balance dataplots and predicted steady spin m odes concurrent with data acqu isition perm it the designer to develop, on site, aconfiguration highly resistant to spins, or one exhibiting desirable spin characteristics if it is to be used foraerobatics or trainin g. The rotary balance data are also used to com pute tim e histories of a spin s incipient,developed, and recovery phases. These spin analysis techniques, i.e., evaluation of rotary balance data,predicted steady spin equil ibrium, an d large angle, six-degree-of-freedom time history ca lcula t ions , a represented an d discussed.

g

pq

C DC LC /C mC nI X > I Y > I Z

R s

SwV

p

a

12

tob/2 V

Nomenclaturewing spanmean aerodynamic chordacceleration o f gravitymassbody-axes roll ratebody-axes pitch ratebody-axes yaw ratedrag coefficientlift coefficientrolling-moment coefficientpi tching-moment coefficientyawing-momen t coefficientmoment s of inertia about th e X, Y , and Z bodyaxis, respectively

spin radius measured from spin axis to airplanecenter of gravitywing areafreestream velocityangle of attackangle of sideslipair densityinclination of the flight path to vertical,

= angu lar velocity abou t spin axis= spin coefficient, positive fo r clockwise spin

Introduction

THE NASA Langley Research Center has responsibil i ty

fo r advancing the state-of-the-art in s ta l l / sp intechnology. This includes development an d validation ofexper imenta l and analytical techniques for the prediction ofstall/spin characteristics. As a par t o f this effort, a rotarybalance facility w as developed to rapid ly identify an air-plane's aerodynamic characterist ics in a rotational flowenv i ronmen t .1 The inf luence o f spin radius, angle o f at tack,sideslip angle, rate of rotation, and control sett ings on theaerodynamics can be measured with this apparatus . Usingthese data , on-l ine s teady spin modes m ay be analyticallypredicted by solving the moment equations for steadyequi l ibr ium conditions. The rotary balance data may sub-sequently be used to compute post-stall motions such as the

Presented as Paper 80-1564 at the AIAA Atmospher ic Fl ightMechanics Conference , Danvers , M ass ., Aug . 11-13, 1980; submittedOct . 3, 1980; revision received April 2, 1982. Copyright Amer i canIns t i tu te of Aeronaut ics and Astronautics, Inc., 1980. All r ightsreserved.

*Pres ident . Associa te Fel low A IAA.tChief Engineer,

97

incipient , developed, and recovery phases of a spin by em-ploying an appropriate, lar^e angle, six-degree-of-freedomcomputer p rogram. These procedures a re discussed here in .

Historical Background1915-1945

Th e need fo r steady-state rotational flow aerodynamics fo rspin analysis had been recognized since aviation's early days.For example, Glauert2 attempted in 1919 to calculate thesedata theoretically using strip theory. By the early 1920s, aroll ing balance w as used to obtain some of these data ex-perimentally. Gates and Bryant3 remarked in 1926, however,that this simple roll ing balance must be replaced with anapparatus that actually reproduces, in a tunn el, the conditions

of a fast spin. NACA investigators also reasoned that sub-jecting al l components of an airplane to a constant velocityvector during static force tests was unsatisfactory forsimulating flow conditions in a spin. This belief w as sup-ported by Irving and Batson,4 w ho noted, in 1928, that th euse of static force test data conduced to e rroneous c onclusionswhen applied to the spin.

Consequently, in 1933, A l lwork5 as well as Bamber andZ i m m e r m a n6 reported on rotary balances installed in theBritish N PL 7-ft wind tunnel and the NACA 5-ft vert ical windtunnel , respectively. The NACA appa ratus w as capable ofmeasuring the six components o f aerodynamic forces andmoments as a function of angle of attack, sideslip angle, androtation rate. A s igni f icant number o f w ing and airplaneconfig urations were tested w ith this appa ratus anddocumented in NACA publica tions cited in Ref . 7.

Several years later, Bamber, House, and Z i m m e r m a n8 '9

presented a method fo r solving th e three moment equat ionsfo r s teady-state equ il ibriu m spin conditions using rotarybalance data. This analytical technique w as born of necessity.Solving s imul taneously the six equations o f mot ion and otherassociated m athem atical expressions before th e advent o fanalog and high-speed digital computers was a formidabletask requ iring step-by-step integra tion procedure s laborious lyperformed by hand.

Clearly, th e need fo r rotational flow aerodynam ic data w asnot only acknow ledged at an ea rly date, but the technique forit s measu remen t and its application to spin analysis h ad beendemonst ra ted pr ior to 1937.

1945-1975

The effectiveness of the first NACA rotary balance w aslimited by the relative sizes of the model and tunn el . Also , th es ix-component ba lance was m ounted external to the model ,was large relative to the model, and w as of dubious accuracy.


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98 W. BIHRLE JR. AND B. B A R N H A RT J. A I R C R A F T

These shor tcomings were eliminated with a new ro tarybalance installed in the Langley spin tunnel in 1945. Becauseof the l imited instrumentation then available, an inordinatea m o u n t of t ime and effort was needed to obtain a very smalla m o u n t of da ta , which in m a n y instances was not repeatable.Unders tandably, t he rotar y balance again fell into disuse afterobta in ing data for one airplane conf igura t ion , which wereapplied dur ing subsequent analytical s tudies.7'1 0'1 1

By the mid 1950s, th e mass of fighters had become heavilyconcentrated in the fuselage, which led to a concern for thevertical tail loads result ing from inert ial coupling duringuncoordinated roll ing pull-out maneuvers . For tunate ly,analog computers were in wide use by this t ime and manyengineers became familiar with large angle, six-degree-of-freedom calculations. Eventually, these inertial coup l ingstudies were extended beyond stall to include spins by simplyextending the conventional aerodynamic model, consist ing ofstatic data and dynamic derivatives, to h igher a. values. Theneed for ro tary balance data w as vir tually overlooked by theaerospace indus t ry in th is count ry f rom the mid-1950s to m id -1970s.

Although NASA upda ted the rotary balance in the late

1950s, i t was used fo r only one analytical s tudy1 2

dur ing th is20-yr period. This s tudy properly employed rotary balancedata and, therefore, s ignificantly cont r ibuted to the methodsavailable for spin analysis. Nevertheless, th e practice o f usingstatic data and dynamic derivatives to calculate spinningmot ions cont inued because matching some por t ion of a full-scale, h igh a t ime history could be demonstra ted . Thesedemonst ra t ions , however, a lways required arbi t rary ad-jus tm ents in the magni tude of some dynam ic der iva tives .

Consequent ly, NASA installed a rotary balance appa ra tusin the Langley full-scale tunnel1 3 and suppor ted large angle,six-degree-of-freedom computer studies in the early 1970s toestablish r igorously the a e rodynamic models requi red fo r spincalcula t ions . For example, several studies14 '1 5 s imula ted th emotions of a f ighter a i rp lane , using both a convent ionalae rodynamic model and one which included the effect of spinro ta t ion ra te . The computed t ime histories were comparedwith those obta ined wi th an unpowered , radio contro l led ,dynamical ly scaled model. I t was shown tha t th e developedspin was dupl ica ted using ro ta t ional flow data , whereas thiswas not the case when the conventional model w as used. Itw as indicated, therefore, tha t th e aerodynamic momentsgenera ted in the spin due to ro ta t ional f low, as measured by arotary ba lance , are indeed s ignificant. I t was also shown tha tthe cont r ibut ion of the dynamic derivatives, obta ined f romforced oscil lat ion tests , to the computed mot ion must belimited to the oscillatory componen t of the total angular bodyrates, p, q, and r . This, of course, could not be done when t heae rodynamic model did not include steady ro ta t ional flowda ta . Clearly, modeling only static data and dynamicderivatives during spin ca lcula t ions was a pseudotechnique .

1975-Present

In the mid-1970s, NASA modernized the spin tunnel rotarybalance by incorpora t ing the latest state-of-the-art in-s t rumen ta t i on and compu te r technology. Valid data were nowobta ined rapid ly and econom ical ly. Ins ta l la t ion of new ro tarybalances in France1 6 and I taly1 7 were also repor ted dur ing thisper iod. Langley tests demonst ra ted1 '1 3 the pronouncednonl inear dependency of the aerodynamic moments onrota t ional rate a t a cons tant a , and once again i l lustrated th efut i l i ty o f comput ing sp in equi l ibr ium conditions o r t imehistories w i thou t ro tary ba lance da t a . Consequent ly, 20

conf igura t ions representa t ive of genera l av ia t ion , f ighters ,and t ra iners h ave been tested in the spin tunnel faci l i ty dur ingth e past five years .

Data An a lys isThe rapid and economica l acquis i t ion o f ro tary ba lance

d i t th d l t f ifi f i t i

tai lored to achieve desired characterist ics. Conf igura t ionshighly resistant to spins or exhibit ing desirable spincharacterist ics, if they are to be used for aerobatics ort raining, can be specified. This is accomplished in thefollowing manner : F i rs t , component bu i l dup tests a re con-ducted and the data examined to determine the contributionof th e body, w ing , hor izonta l tai l , etc. to the momentcharacterist ics. Then th e ro ta t ional flow aerodynamics areobta ined for the comple te conf igura t ion with var ious cont ro lsett ings. Again , examining these data , in most instances,indicates a. reg ions in which the configuration is spinsusceptible. As m ent ioned previously, steady developed spinscan be predicted co ncurrent ly with wind tunnel tests, using asteady-state analysis technique. Consequently, if steady spinmodes a re predic ted , suff icient insight into the causes wil lhave been established to permit the selection of judic iousconf igura t ion or control modif ica t ions . Th e effectiveness ofthese modif ica t ions in m eet ing the desired design objectives isthen verified wi th additional rotary ba lance tests.

Steady-State Spin An alysisPredicting steady spin modes is not only requ ired durin g the

conf igura t ion defini t ion phase , as indicated above, bu t alsoappreciably reduces subsequent spin mode l testing t ime. Theon-line ro tary balance spin prediction capabil i ty identifies allpossible steady spin modes fo r specified airplane inertias,center-of-gravity location, control set t ings, and alt i tude. Thecalculated spin modes ' sensitivity to these variables can, ofcourse, also be de termined.

Method

Analyses3 as early as the m id- 1920s indic ate an un-ders tanding of the basic equations of m otion as applied to thespin . Figure 1 i l lustrates th e a i rp lane ' s o r ien ta t ion and theforces acting upon it in a spin. A steady spin implies tha t allaccelerations are zero along and about the body axes. For

pract ica l purposes , therefore , th e force equat ions reduce toth e relationships that side force is zero, drag equals weight ,and lift is balanced by centrifugal force:

mg= /2 P V2SwC D

The three body axes mo men t equations reduce to

1)

(2 )

3)

Fi 1 I l l i f i l i d i


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F E B R U A RY 1983 SPIN PREDICTION TECHNIQUES 99

4)

5)

The angular rates, p , q, and r, are clearly a function of thespin ra te , 12. Various authors6'18 22 have defined this

relationship in different , t hough generally equivalent, te rmsdepending u p o n their definit ion of the airplane's orientationand the order of the axes ro ta t ions .

Bamber, Z imm erman , and House8'9 used a notation that isconvenient because th e axes rotations are defined in terms ofa and 0. First, the airplane is considered to be yawed t h roughan angle, = -(/3 + a), where a is the inclination of the flightpath from th e vert ical , defined as

Th e airplane is then pitched from th e vertical by the angle a.This yields th e following for the body axis angular rates:

/? = ftcosacos(j(3 + a) (7)

(8)

(9)

Substi tuting Eqs. (7-9) into Eqs. (3-5) and solving for theaerodynamic coefficients:

C m = (4/pS wcb2) (tib/2V) 2 (I x-I z)sm2acos

2 (o+p) (10)

C /=(4/pS w b3 )(ttb/2V) 2 (I z-I Y)sinasin2(a + P) (11)

C n=(4/pS wb3)(tib/2V) 2 (I Y-I x)cosasin2(


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100 W. BIHRLE JR . AND B. BARN HART J. AIRCRAFT

.04

.02

04

A E R O D Y N A M I C C O E F F I C I E N T S

I N E R T I A LT E R M S

10 5. 0 5

SIDESLIP A N G L E , D E G10

Fig. 4 Rolling-mom ent coefficients an d inertial roll terms vs 3 forselected a s.

30 45 60 7 5

A N G L E O F AT TA C K D E G

Fig . 5 Sideslip angles required fo r pitch an d roll equil ibrium vs a

0.8

Ob2V

0 2

15 30 45 60

A N G L E O F AT TA C K D E G

90

0.02

I N E RT I A L T E R M

A E R O D Y N A M I C C O E F F I C I E N T

15 30 45 60

A N G L E OF AT TA C K DE G

75 90

Fig . 6 Q6 /2 V va lues required fo r pi tch an d roll equ i l ib r ium vs a .

Fig. 7 Yaw ing-mom ent coefficient an d inertial ya w term vs a curvesidentifying spin equil ibrium a s.

Th e inertial term in the yaw Eq. (12) is calculated,therefore, at each tested angle of a t tack, using the /5 andQb/2V values from Figs. 5 and 6; the aerodynamic yawing-rrioment coefficient is , l ikewise, extracted at these same

values . Plots of these inert ial and aerodynamic values vs angleof at tack are shown in Fig. 7.Intersections of the aerodynamic and inertial curves

identify angles of attack at which al l three mom ent equationsare in equi l ibr ium. Because of the mechanism of the spin,only the intersections where the aerodynamic curve has anegative slope (indicated by squares) can result in spins. Thisoccurs because a longitudinally stable airplane cannotmainta in p i tch equi l ibr ium a t a somewhat h igher o r lower athan th e predicted va lue wi thout a corresponding increase o rdecrease in 0/7/2V (Fig. 6). At the indicated intersections,however, th is Sib/2V change is prohibited by the presence ofrestoring lateral-directional mome nts abou t th e spin axis.

Th e spin tib/2V an d 3 value is then obtained from Figs. 6and 5, respectively, at the spin a. identif ied in Fig. 7. The dragand lift co efficients a re then extracted f rom th e ro tary balancedata fo r these equil ibrium a, ft and Qb/2V values. Equat ions(1 ) and (2) can then be solved for the descent velocity and spinr ad ius .

Significance of Predicted Steady Spin Modes

Over th e past fe w years, it has been observed tha t fo r eachpredicted steady spin m o d e , three possibilities exist in the real w o r l d ' ' : the steady spin will be duplicated, oscil latoryabou t the steady spin values, or nonexistent. The lat ter tw oconditions reflect th e under ly ing spin mode ' s level of in-stabil i ty. In no ins tance , howeve r, has a spin occurred i n flightfo r which a steady spin m ode has not been calculated.

An indication of recoverabil i ty f rom a predicted spin canalso be obta ined f rom these equi l ibr ium ca lcula t ions . Forexample , if a spin is predicted for a pro-spin cont ro l set t ingand none fo r neut ra l or recovery cont ro l set t ings, th e spin isassumed to be recoverable, al though i ts acceptabil i ty c anno tbe fore to ld . However, an examinat ion of the ro ta t ionalae rodynamic m o m e n t s obta ined wi th recovery cont ro ls cangive an ins ight in to th e ease of recovery. This is accompl ishedby de termining the existence o f damping or au torota t ivem o m e n t s and their var ia t ion wi th a and Qb/2V below th epredicted spin a.

In 1981, spins obta ined with sp in tunnel f ree-sp inningmode l s and full-scale a i rp lanes were cor re la ted with t hosepredicted ana ly t ica l ly.2 3 I t was found tha t th e s teady-stateequi l ibr ium spins were in excellent agreement wi th the spin

tunnel and f l ight resu l t s . Poore r cor re la t ion wi th f l ight testw as no t ed , however, for genera l aviat ion, c o n f i g u r a t i o n sexhibit ing very s teep spin modes . This was a t t r ibu ted to asignif icant Reynolds n u m b e r effect relat ive to the w i n gae rodynamics .


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FEBRUARY 1983 SPIN PREDICTION TECHNIQUES 101

Large Angle, Six-Degree-of-Freedom An alysisSix-degree-of-freedom computer solut ions identify th e

degree of stability o r instability of developed spins, the ease ordifficulty of entering and recovering from a spin, as well ashigh a. departure, roll reversal, and post^stall gyrationmot ions . Unfor tunate ly, this analytical technique does no tlend itself to configuration analysis dur ing ro tary balancedata acquisit ion because a large vo lume of data mus t bemodeled and dynamic der ivat ives must be obtained pr ior torunn ing t h e p r o g r a m .

Method

Six-degree-of-freedom spin analysis utilizes standard largeangle, nonl inear ized equat ions of mot ion .2 4 The aerodynamicmodel, as mentioned previously, incorporates dynamicderivatives and rotary balance data. It is necessary to u se thedata in the s ame manner in which it was measured. Rotarybalance da ta , for example, is measured with the airplanemodel performing a steady rotation abou t a n axis aligned withthe freestream velocity vector. Therefore the Q used to extractrotary balance data is defined as the roll rate abou t th e windaxis, filtered to r emove nonsteady components . The dynamicderivatives, on the other hand, are measured or calculated forsmall oscillations abou t each of the body axes and are,consequently, multiplied only by the oscillatory componentsof the body axes rates, as mentioned previously. Theseoscillatory components are calculated as the total body ratesless the steady rota t ional com ponents abou t each axis.

o se P ~ (14)

(15)

(16)

High-angle-of-attack s imulat ions using this ae rodynamicmodel ing technique may not completely match a ll phases o f aspin t ime history for several reasons . Fo r example , Reynoldsn u m b e r differences b etween the model- a nd full-scale airplanecan lead to s ignificant ae rodynamic differences in the im-mediate post-stall a range, and thereby alter a very steep spinm o d e o r modify th e incipient spin t ime history. Ano the rpossible source o f error may be the dynamic derivatives,which are normal ly measured whi le forcing a mode l tooscillate abou t a nonrotat ing reference. A more properrepresentation for spin analysis would be to superimpose theforced oscillation onto a s teady rota t ing motion. Dynamicderivatives, estimated fo r this latter condi t ion, have indicatedthat values so derived can differ from the conventionallyobtained va lues . These differences may be s ignificant becausecertain dynamic derivatives, specifically C m and C, , havebeen shown1 4'1 5'2 5 to significantly affect the stability7 of thespin modes. Unfor tunate ly, no appara tus currently exists inthis country t ha t is capable of measur ing dynamic derivativesin this m a n n e r.

AcknowledgmentThe techniques discussed herein were developed under

N A S A Langley Research Center Contracts NAS1-14849 andNAS1-16205.

References1 Bihr le , W. Jr. and B o w m a n , J.S. J r. , Inf luence of Wi n g ,

Fuselage, and Tail Design on Rotat ional Flow Aerodynamics BeyondM a x i m u m L i f t , Journal of Aircraft, Vo l . 18 , Nov. 1981, pp. 920-925.

2Glauer t , H . , The Investigation of the Spin of an Aerop lane ,R& M 618, British A.R.C., 1919.

3Gates , S .B. and Bryan t , L . W. , The Spinning o f Aeroplanes,R& M 1001, British A.R.C., 1926.

4I rv ing , H . B . and Batson, A.S. , Exp eriments on a Mode l o f aSingle Seater Fighter Airplane in Connection with Sp inn ing , R& M1184, British A.R.C., 1928.

5Allwork , P.H., A Continuous Rotation Balance for theMeasurement of Yawing and Rolling Moments in a CompletelyRepresented Spin, R& M 1579, B ritish A.R.C. , 1933.

6B a m b e r , M.J. a nd Z i m m e r m a n , C.H., The Aerodynamic Forcesand Moments Exerted on a Spinning M odel of the 'NY-1' Airplane a sMeasured by the Spinning Balance, NACA TR 456, 1933.

7Stone , R . W. J r. , B u r k , S . M . Jr., and Bihrle, W . Jr., TheAerodynamic Forces and Moments on a 1/10-Scale Model of aFighter Airplane in Spinning Attitudes as Measured on a RotaryBalance in the Langley 20-Foot Free Spinning Tunnel, NACA TN2181,1950.

8B a m b e r , M . J . a n d Z immerman , C.H., Spinning Characteristicsof W ings. I. -Rectangular Clark Y Monoplane Wi n g , NACA TR 519,1935.

9B a m b e r , M.J. and House , R .O. , Spinning Characteristics of theXN2Y-1 Airplane, NACA TR 607, 1937.

10Scher, S.H., An Analytical Investigation of Airplane SpinRecovery Mot ion by Use of Rotary-Balance Aerodynamic Data,NACA TN 3188, 1954.

11 Burk , S.M. Jr., Analytical Determinat ion of the Mechanism ofan Airplane Spin Recovery wi th Different Applied Yawing Momentsby Use of Rotary-Balance Data, NACA TN 3321, 1954.

1 2Angl in , E . L . and Scher, ' S.H., Analyt ical Study of Aircraf tDeveloped Spins and Determinat ion of M o m e n t s Required forSatisfactory Spin R e c o v e ry, N A S A T N D-2181, 1964.

1 3G r a f t o n , S .B. and Ang l in , E.L., Static and D y n a m i cAerodynamic Characteristics of a 0.15-Scale Model of the YF-17Airplane a t Spin Angles o f A t t ackCoord . No . AF-AM-407 (U),NASATMSX-3217, 1975.

14 Bihr l e , W . Jr. and Barnha r t , B. , Effects of Several Factors o nTheoretical Predictions of Airplane Spin Ch aracteristics, NASA CR132521,1974.

15 Bihrle , W . Jr., Correlation Study of Theoret ical and Ex-perimental Resul ts for Spin Tests of a 1/10-Scale Radio ControlModel, NASA CR 144995, 1976.

16 Ve r b r u g g e , R. , Balance Rota t ive De L ' I . M . F. Lille etTechn iques Expe r imen ta l e s Associees, I . M . F. Lille N o . 79/63, 1979.

17Bazzocchi , E. , Stall Behavior and Spin Est imat ion Me thod ByU se of Rotary Balance M easurem en t s , AG ARD-CP-199 , 1975.

18 Babister, A . W. , Aircraft Stability an d Control, Pergamon Press ,New Yo r k , 1961.

1 9C r o w, J.H., An E l emen ta ry S tudy of the Spin, AircraftEngineering, Vo l . XI, Feb. 1939, pp. 39-43; March 1939, pp. 111-114;Apri l 1939, pp. 158-160; May 1939; pp. 203-208; Ju ly 1939, pp. 273-278.

20Bazzocch i , E . , E tude Analyt ique de la Vrille en Ut i l i sant de s

Donnees a la Soufflerie Horizontale e t Methode d' Essai des Modelesa 1 Air Libre, Proceedings of the Second European AeronauticalCongress, Scheven ingen, T he Ne the r l ands , 1956, pp. 5.1-5.43.

21 Tischler, M . B . and Ba r low, J .B. , Applicat ion of theEqu i l i b r ium Spin Technique t o a Typ ica l Low-Wing Gene ra l Av ia t i onDesign, AIAA Paper 79-1625, Aug. 1979.

2 2M c C o r m i c k , B . W. , E q u i l i b r i u m Sp inn ing o f a Typical Single-Engine Low-Wing Light Aircraf t , Journal of Aircraft, Vol . 18 ,March 1981, pp. 192-199.

23 Bihr le , W . Jr. , Predict ion of High Alpha Flight CharacteristicsUtilizing Rotary Balance Data, Proceedings of the 13th 1C ASCongress I AIAA Aircraft Systems and Technology Conference,Seattle, Wash., Aug. 1982.

2 4E t k i n , B ., Dynamics of Flight, J o h n Wiley and Sons, I nc . , N ewYo r k , 1966.

25 K l i n a r, W. a n d G r a n t h a m , W ., I nves t iga t ion of the S tabi l i ty o f

Very Flat Spins and Analysis of Effec ts of App ly ing Var ious MomentsUti l iz ing th e Three M o m e n t E q u a t i o n s o f M o t i o n , N A S A M e m o 5 -25-59L, 1959.

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Spin Prediction Techniques