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' NASA Technical M e morand u m 84 2 93 U S AAVRADCOM 82-A- 17
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( NA S A-T M-8 q 293) D Y N A M I C S _ U CT U RA [ _ 83-16 34 9
A EE C E LA ST IC ST A BI L IT Y TESTING C F _H_ XV -|5
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Dy n amic StructuralAeroelastic , _
Stability Te sting of the XV - 15
Tilt Rotor Research Aircraft ,
Laurel G. Schroers
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NASA Technical Memorandum 84293 USAAVRADCOM 82 A 1 1
Dynamic StructuralAeroelastic
Stability T esting of the XV - 15
Tilt Rotor Research Aircraft
i
L a ure l G . Schro e rs , Ames Research Center, Moffett Field, California
N ct lon a I Aeron a ut i c s and Un ite d States A rm y
SDa c e Ad mm_ st r aho n Av i ation Research and
Rele u ch C A mte r Development C o m man d
M o fle tt F i e l d C, ahfo r n = a u 40 35 St Louis , Miss o uri 63 1 66
p _FF " ;_ ; 1B - 1
ORIGI N AL "" " "
POOR -<- - - n HALI ' I Y nVNA M I C STRU C TUR A L AE R OE L ASTIC ST A B I L I TY TEST I NG
OF
O F T HE XV-1 B T I L T RO TO RRES E ARC H AIRC R A FT b y L . G . S c hroers Ae r o m ech a n i c s L a bora t ory U . S. A r m y A v i a ti on R&D Command A m e s _ es e arch C e n t e r M off ett F i e l d, Calif o rnia 94 0 35, U.S . A .
A B ST R A CT F o r t h e pa s t Z O y e ar s, a s tgn t f t cah * e ffo rt h as been mad e t o un de r s t and an d pre dic t t he st ruc t u r al a e ro elast i c s tabi l it y cha r a c teri s ti c s , f : he tilt ro tor c o n ce pt. Begin ni ng with th e rotor - pyl o n o scilla- tion of the X V - 3 aircraft , t' ;e pro b l¢ , wa s id e nt if i e d and tnen su b j e ct e d to a s erie s of th e or e tical s tudi es, p l u s model and fu 11-s cale v lnd tunn e l t es t s . F ro m thi s data ba se , m e thod s w e re deve i o pe d to pr ed ict th e s tructural aeroela s tic s t ) b llity ch a ract e ri s tics of the XV - I5 Tilt R otor Research Aircraft. This paper examin e s th e pred i ct e d Aeroela s tic charact e ri s t i c s in light of th e ma j or parameter s e f f ectlng rotor - pylon - wl ng sta b ility ; desc rib es fl i ght t es t t e chniqu es used t o ob t a i n X V-I 5 a e roela s t l c s ta b ilit y ; pre se nt s a su mmary o_ flight te s t res ul t s ; co mpare s the f light te s t re s ult s t o the predicte d va l ue s ; a n d present s a limited comp_rl s on r , wind tunn e l r esul ts, flight te s t r e s ul ts, a nd their C o rrelation with predicted values.
I . 8_ ¢; C_RhUND - PROB L EM IDEN T ir!CATION The XV-3 Tilt Ootor Aircraft, showh _n _ig. l, identified a oroblem of po ss ible rotor-pylon-wlng instability during m a neuvers in the airplane . * ' -_de. D u ring the 19 6 2 NASA @_me s 40- by 8 0-Foot Wi n d Tunnel te s t o f the XV- 3 alr c r a ft, a su s tained rot o r-pylon oscillati o n wa S e n co unt e r ed . An ex te ns ive program o f a n a lys e s a nd m o d el te s ti n g wa s be q un t o i n ves tigat e th e Inw fr equ e ncy r o t o r -pyl o n osclll atl on ph eno m enon , and the re s ult s are reported in Ref s . l and 2. The objectives of these inve s tigations w e re t o provid e a physic a l understanding of rotor-py l on s tability, and to establish means of a ss uring s ta b le configur a tions Cor the X V- ] and future tilt rotor VTO L d es igns. Th e s u s tained o s cillation {decr e a sed da m ping) wa s g e n- erated by d es tabili z ing rotor force s that, at hlgh inflow ,.ngte s , cnu l d beco m e significant in det e rmining the co u p l ed rotor-pylon s tability. Figure 2 illustrat e s t : le fo-c_ , )ctlng on a rotor and pylon sys te m duri n g s t eady pitching motion. A C om p l ete description o f thi s phenom e no_ i s J _ ; c r l bed in Ref. 1, b u t, i n b r ief, the d es t ab ili z ing m o me n t is generate d by t h e H f o r c e s that a dd t o p r Cd, . ce a _.Jbihear f o r c e in the o : _.)ttl o n o • the p y lon pitching r a t e. T h e de s tabi l i z i n g m o me n t i s dir ec tly pr o p o rtl on a i t o b l a d e i ne rtia , t h e hu mbl e , of b l ad e s , m as t l en gt h, a ir s pe ed , a nd is invers el y p r opo rti o na l t o rot o r ra d iu s s q ua r ed . T h e r e s ult s of th e s e analytical and model te s ting program s d e fin e d th e major parameters that can a f fect rotor- pylon-wlng s tability. T h e s e ma j or parameter s, and th e ir affect s on aeroe las tlc _ , _bility, are outlin e d in % b l e 1.
TAB LE _ . W A J O R PARAMETERS AFF E C T ING RO TOR -PYLON-WING AE RnELAS T IC S T A B ILITY P a ra me ter A f f e c t C o mm en t s Hig h p y lon St a b _ll z l n g In c rea s i n g the p y lon s tiffne ss _n c r e a s e s t h e fr eq u ency o f mo unt i n g th e p yIo n OSclIl at lon s o t ha t t h e rot O r c a nn o t f ol l ow, and )realne s s t he rotor mo ' _ of oscill at ion r e m a i n s h i gh l y d amp ed .
S w as h b l ate / Destabi l i z ing R o tor con tr ols m u st be i sol ate d f r o m p ylon m o tion to prevent p y lon c ou p l l n g de s ta b ili z ing for c e s that ar e ge ne rat ed w hen t he rotor p l a ne iS di s t u rb ed .
D e lt a t hre e h e stabi l i z l ng T he usa o f ne g a tive de lt a thre e con tro l r educes m ane uv e ring c on tr ol i ndu c ed ro t o r f la pp in g , b u t h as a des t abil i z in g effe c t on rotor-py l on-wlng st a bi l ity O o t o r el a s ti c Stabilizing Spring restrai n t on rotor f l a p pi n g pr oduc e s a s tabi l i zln g f l aP b l n g eff ec t .
restraint _i n g mo de D e st a bilizing W in g b ea m and torsi o na l d e gr e e s - of- f reedo m produce e de - e ff ec ts sta bil i zin g e ff ec t b y l o w e ri n g t h e p ylon s tiff ness a nd consequen t l y the p ylon n a t u r al f r e quency.
I nc re as i n g O es t a blll z i ng Inc rei sln g a irspe ed is d e s tabi l l z ing b e c a us e it is a ccom - a irspe ed pin l ed b y i ncr easi n g d estabi l izing rot o r f o rce s at hi gh i n-flo w i ng le s .
I nc r eas i ng S t ab ilizing I nc r eas i ng r oto r t h ru s t h is a st ib i l l z ln g ef f ec t b e cause it r ot o r thrust h as t h e eff ec t of incre a sing py l on s tiffness .
Inc r e a sin g D e st a bi l izing I ncreaS i ng r ot or rm i s d es t a bilizin g b e c ause t he incre a se rotor rp m in rotor a ngular m o m en t u m produce s an incr eas e in p r oces- s lona l m o ments resu l ting in grea t e r rotor desta b i l izin g f orces.
ORIGI N AL PAGE IS
1d - 2 OF POOR QUALI TY .
2. P RE D IC T ED X V -1 5 S TRU C TURAL A E RO ELA ST IC S T ABI L I T Y The tec hnolo gy b a s e d uri v e d fr o m the e arlier ana_ y tlcal a n d m odel testi n g p rograms ma de it p os sib l e to predlct the str u ct u ra l aeroe las tlc st a bility o f the XV-15 R o t o r R esearch A ircraft with a hlgh d egree of confidence. The va]idlty o f th e se predicti o ns w e re th e n e va l uate d by a d diti o nal m o d e l a n d fu 1 1-scale t e sts. T h e re s ult s o f the s e t e sts are prese n ted a nd d i s cus sed in a lat e r secti o n o f t h is pape r .
The XV-1 5 pre d ictlons pr od uced b y t h e Bell H e l ic o pter C o mpany were base d o n a linear analysis IBHC P r_protor Stabilit y Analysis, DYN4 ) , and a n o nli n ear anal ys is (B H C Pro g r o t o r Aer ol astic Analysls, DYN b ) t e chniq u es. T h e DYN4 a nd DYN5 anal y sis technl qu e s are d e s cribe d i n Ref . 3 . The DYN 5 p r o gram is an e x - p a n d ed versi o n o f a math m o del and c o mputer program d eve lo ped f o r the Air Force Flig h t Dynamic L ab o rat o r y a ,=d is de s c r ibed in Ref. 4 .
The X V -15 p re d icti o n s p r o d u ce d b y th e N ASA- Ame s R ese a rch C e nt e r are pr e sente d in R e f . 5 , a n d up d at ed predlctian s are prese n te d in Ref. 6 .
Th e pr e dicted r o t or -pyl o n-wing stabilit y c h ara c teristic s of t h e XV-£S in a ir p lan e m ode are pre s en t ed i n the r oo t l o c u st f o rmat In F igs. 3, 4 , S, a n d 6 . Bell predi,t! ons for the s y mmetric an d a sy mmetric mo d es are presente d in Fig. 3 an d 4 , res p ectively. The N A SA-Ame s pre d icti o ns f o r the s y_etric and . a sy mmetrlc m_des are presented In F ig s . 5 a n d 6 , re sp ectively. T h ey both s h o w: • I. Lo w freq u enc y , highly damped rotor m o des .
2 . High freq u ency, lig h tly damped pyl o n m o des.
3. Low freq u ency, lightl y dampe d wing _ d es.
The s e predlctinn s are a;S O c o mpare d t o flig h t t(st results a s a function of damping ratio ( _) and a l r- speed.
Diff e rence s in the predicte d damping le v els for t h e variou s m od e s ma y result fr o m diff e rences in • the ana l ysl s te c hniq u e s . T h e s e d iffere nc es a r e ll s ted In T able 2.
TABLE 2. ANALYSIS DI FF ERENCES Be ll H e l i co pt e r (l l near analy s iS) _overnment Wi n g m ot i o n DisCrete ma sse s, _ ne rtl as an d _vrlng s N A STRAN _ o de shap e s (a l l s l x c o m po nev : t s) w h ich are c ou p l e d t o match t h e 6 fun- d am e nt a l wlng mo des a n d py l on pltch and yaw m o des R O t Or b l ad e l ag mot io n Pu r e l y ] n p iane, r i g i d bo d y r o tation Co u pl ed e n p l ane / o ut-of-plane be n d l ng m od es o f ab o ut of f se t h_ n ge wi t h sprlng t h at el as tic blade r eprese n ts fir s t In-p l ane cyc 1 _c mo d e R o tor aer od y n amics A na ly t ical in te gra t i o n O ver r OtO r O lsk , N u _rl ca l _n t egr at _ o n o ve r di s k, u s _ n9 ]If t - us ing s_ng I e l lft-curve s l o p e v alue cur v e s lop e b ased on lo cal a ng le - o f-attack a nd ( c o rrect e d f o r co m pr ess ibility) Mat h n um O er (I d eally o wl |t ed blad e i 3 / 4 ra di u s ) A xi al f l o w a nd high inf l o w o nl y A ppl icable t o c o nversl on an d he l _c o pter m ode f l lgh t al s o R oto r d y n a mi cs _ 0 b l a d e t o rsl o n dyn amtcs C ouple d bl a d e b en ding an d t o rsi o n P itc h / l ag . oupl i n g c alc u l ate d f r_ Pit ch / la g coupl _ n g c a lc u l at e d a u t o m a ti c a l l y s e par a te anal ys t s ]. AI R C RAFT DE S C R I P TI O N T h e XV- I S air c raft is po wered by tw o L y c o mlng T - 5 3 t u rboshaft engines, w h i c h ha v e bee n u prat ed a nd mo d ified f or both v ert ic al a n d h o r i z on t al oper at io n . Th e t_re e- b l ad e pr oprO t O r S a_ " ?.62 m ( 25 ft ) in d ia m et e r, a n d the b lade t w ist is 45" from r oot t O ti p. T h e r Otor S are g i m b al . m ount e , _ to t he hu b wi th a n e la s to _ l r t c s p r t n _ f o r f l a ppi ng r _s t re in t. The _i nq s p a n t s _ . 7 5 m ( S Z f t) fr o m sptm er _ o s p inn er , and :he a irc r a f_ t s 12 . 8 m (4Z f t) lon g, ex c l u d tn 9 t h e i ns tr u _en t a t i on bo_ . Ai rcr af t d ma nsion s e r e sho w n o n t he t hree - vi ew dra w in g i n F tg. ?. Wt n _ toa dtng ts 3 687 n / m _ (7 7 lbs / ft_), and dts c biding a t th e des i gn g ro s s w ei gh t o f 1 3 , 0 00 lbs , i s 6 3 2 n / m " ( 1 3 . 2 lb s / f t _). T he XV-1 5 carr i e s 669 k g (1 , ¢75 lb s ) of f uel , w h ic h a llo ws a rese a rc h f l ig ht o f a b out I _ ou r . It i s e qu ip ped wit h LW - S B r o ck et s e a t s w h ich p rovide a O - al ti t u d e / O -air speed r ecov er y ca Pa b ility f or t h e c re w.
T h e key d e st g fl f eitures a nd t he reas o n f o r selec ti on in th e XV- 1 5 design ar e listed tn Tab l e 3.
T h e X V . I 5 f lt g h t con t rol syste m i_c l u d es e x ci ter act uator s _ fl the ri ght-hand fIape r o n and r i g h t .hand c ollec ti v e c On tr ol sy st e m s t o ex cit e th e mod_s s ho w n _n F ig . 8. I n ' l ig h t str uc t u r al ae roe la s t ic stabi l i ty _n ve stlga tlon s used the f l a pe ron e xcit e r 4C t ul t o r t o exc i te the w ing be a m : _ t O t S ; ori e l s y mm e t r ic al, and )
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18 - 3 _ABLE 3 . KEY XV-15 DESIGN F EA T URES i De s ig n F e atu. Rea son f o r S el ecti on T o rsi on ally stiff wi n g a nd stiff Amp l e stab l lity margin at l o w te c hnical rls k I pyl o n-t o -wing attachment Forward-swept wing planf_rm Ample clearance I12 degrees) for flapping in severe maneuvers and gust encounters Gimbaled, stiff-inplane, over- Proprotor loads not sensitive to flapping mass-balanced pr o protor Air and gr o und resonance problems av ol ded Blade pltch-flap-lag instabilities and stall f lu tter p r obl em s a vo i d e d Large tall v ol ume, H c o nfigure- Goo d damping of D u t c h r oll a n d sh o rt-p e rlod tion f l ight m odes a s y mmetri c a l be nd i n g m odes . T he c ol le c tive e x citer a c tuat o r was u s ed t o e xci t e t h e wi n g ch o r d s y mm e tr ic an d a s ymmetric bending m o des. Infllght use of these e x c i ter actuat o r s are sho w n and di s cus s e d in the f o ll o wing secti o n.
4 . FLIGHT TEST T E C HNIOUES Structural a e r o e l astic stability f l ight test ev al uati o ns were c o n d ucte d a t the contractor's Flight _e s earch ra c lllty i n Arll n gton, T ex a s, an d at the _A S A Dr y den Flight R e s e arch C e nter a t Edw a r ds AFB .
California. These tests were conducted within the limits listed below: I Design g r o s s weig h t of 5900 kg _13,000 Ibs) and a neutral C.S. locati o n.
2. At density altltudes of 1,500, 3,000, and 4,600 m e ters 15,000, IO,O00, and 15,000 feet).
3. In airplane m o de (P y lons d o wn and l o cked) w l thln the true a l rspeed range of 170 t o 29 6 kn o ts.
4. At tw o r o t o r speeds o f 9 8% (589 rpm) and 86% (517 rpm).
The XV-IS aircraft was predicted t o have l o w freguenc y , lightly damped wing beam, ch o r d and t o rsi o n bending modes. The three tec,n}ques used t o excite these mode s are: I. Atm o spheric turbulence.
2. Exciter frequenc y sweeps.
3 . Exciter freq u e ncy dwe l l / d eca y .
Straln g a ges, m O unt ed on th e left and right , Ing, m_a s u r ed th e be a m , c h o r d , a n d t o rsi o nal b e n d ing r esp o nse of the wing t o t h e e x citing f o rce. The left _ r _ght gages w e re c o mbined In a s u m / d iffere nc e n e tw o r k t O separate th e s y mmet r ic an d as_metrlc m od es .
In th e fir s t t e cB n l q u e , t h e a ir c raft wa s f l ow n _n n c d e rate t u rb ulo nce t ha t pr o v id e d a br oad ban d ex c itati o n f o rce. C o ntinu o u s tim e h ist or y r ec ord s o f th e wi n g gage s wer e ta ken w_i l e th e , I rcraft wa S f lo w n I n trimmed l e v e l f l ig,t I n t u rb ulenc e. T h e _I g lta l time his to r y o f the wi n g be a m, c, or d , a nd torsi on a l b end i n g da t a w e re the n ana l y z ed "3 d e t e rmi ne th e n a t u ra l ( o r r eso nant) f re que ncies G f th e wing str uc t u ra l m odes, a nd t o ca lcul ate th e as soc iated st r uct u ral da m p i n g r ati o f or e ac h mo de . Th e m e t hod u sed t o ana lyze th is da ta ts th e Rando m D ec r e m en t Sig n at u re s ( RA N DO t IOE C ) p r o gra m desc rib ed in R e f . 7, In th e ; eco nd t e c h niq u e , the aircra f t was _ 1o w n In tr_mme d le v e l flight while a co ntta n t am pl itu d e automatic frequency swe e p from I tc 10 H_. wa s performe d with either the flaperon or the collective exciter. Again , continu o u s time nlstory recor d s were taken during the frequency sweeps . T he data were analyze d o ff- llne using the R ANDOMOEC pro g ram and / or a modal analysl, technique developed by the Grum ma n Corp o ration.
The third meth o d used t,e fr e qu e ncy d well / decay technlque. In this t e chnique , the pil o t fl e w the aircraft in trim m e d level f,lght or descen d in g win d -milling (p o wer off) flight, an d the copilot tun ed the selecte d e x citer to the desire d fr e quency and amplitude as dict a te d by the o n-line m o nitoring in t he ground co n t ro l room. Once the exciter was tuned to the desired wing ben d ing mo d e, It was turned on an d the mo de excite d at a constant amplitude an d const a nt frequency. On ce the desir ed mod e was e x cit ed, t he exciter was turned of f , and the excitation decay was quallt l tlve l y evaluated in t h e control r oo m befor e t h e test was repeat ed. These decays were later ana l y z ed off-line using an interactive comput e r pr o gram to obtain fr eque ncy an d d amping values . This interactive progra_ i( discussed in Ref. 8 and CW l scrl be d (n detail in Bell Helic o pter Company R ep o rt 29g-ogg-_gB .
Figures g th ro ugh 12 present axe)plea of the _ell and decay techrlque for the s_m e tr(c an d aS y m- metric m odes , wit h a nd wit ho ut the sum a nd dlffer e nc e o n -l l n e analysi s te c hni que . Fo r example, F ig . g pr e s e nt s a f re qu en c y dwe l l a t ] . ) HI ., an d a dec a y re s p on se o f the s) ame trlc wing b ea m bendl n9 m ode w ith - out usin g t h e s u m-a n d -d lff e r e nc@ t ecfl nl q u e. AS in o wn, b o th the r ig h t a nd l e ft be a m b e n din g m ode s ar e
ORIGINAL P / :, C7 I 3
1_-4 O F POOR Q U ALITY
excit ed , T h e right wing beam bending l o ad I s hig her t ha n th e l ef t, be ca u s e t h e fiaperon exciter i s ope r - ating on the right wing only . from these traces , it is difficult to det er mine if the s y mmetric or asym- metric beam b e ndlnq mod e is excited .
Figure no is the same frequ e ncy d well / deca y record using the su m and difference technique. Comparison of th e amplitud e of th e two traces makes it apparent that the symm e tric wing mod e has been excited . The positive damping of the symmetric wing beam bending mode is easily recogni z ed by the shape of the decay envelope _n Fig . g or no . The sum and difference was only used to identify the wing bending mode. T he dw e ll-a nd - de ca y tech n i que w o r ked v e r y w e ll on the b eam b e ndi n g m ode f o r t hree r e aso ns. First , th e da m pin t le vel is p os iti ve, bu t l o w , m a king i t eas y t o ex c ite th e l oad. S e c ond, t he am b i en t no i se l evel w a s l ow ( no nt u rbul e nt flight c ond iti on s), a n d th e s ign a l-t o -nolse r ati o is h i gh with o ut abusing th e structure w l th exc es sively high exciter input forces . Third, the s y mmetric natural frequency of 3. 4 HZ . was sufficiently separated from th e a s ymmetric natural frequency of 6.7 H z . to prevent coupling of the t_ modes .
A , iexample of coupled symmetric and as y mmetric response is shown in Figs• II and 1 2 , Figure II pre s ents a frequency dwell / decay record of th e symmetrical wing torsion mo d e . Both the left and right wing loads have a "beat" type r e spon s e caus e d by the coup]Ing of the symmetric and as y mmetric modes which . h av e a n a tur a l fre qu enc y o f 7 . 7 an d 8.2, r e spectivel y , an d are very cl o se t o the l per r evo l u ti o n freq ue ncy o f the r o t o r w h ich is 8 .6 H Z . F igure 1 4 pre se nt s t he same dwell / decay rec o r d using the sum and difference technique. Again, the s u m and difference t e chnique Is use d t o i d e ntify which m o de is 6xcit e d, but t h e damp ing lev e l i s n o t e a sil y _ec o g n i z ed b eca u s e o f the "beat" t yp e re s p o ns e that s til l exists in the " s um" trace.
T he dwell an d d e ca y techniq u e was the primary t oo l u s e d t o m e asure the aer oe lastic s t a bilit y o f the XV-15 aircraft. It s ad va n t a ge s a t* .
I. It pr ov i d es a p o int-by-p o lnt e v a lu ati on o f the aer o e l astic mo d es.
2. It provides, in most cases, the opportunity to qua l itatlvely evaluate the damping leve l at each point.
3. Final calculation s of natura l frequency and damping are relatively easy using the analysis tech- n iq u e d e s c rib ed in Ref. 8 .
4. It is eas y t o ab o rt a test (turn o ff ex c iter) if a pr o blem Is encountered.
Its d is adv a n t a g e s are' l . It is time c on s u ming t o d o a p o lnt-by - polnt ev a l u a ti on .
2. It r e q u ir e s n o nt u rbulent at mosp h e ric conditions , L 3. It re q uire s extensive grnun d -to-alr - to-ground coordin)tlon.
4. It w as difficult to e x cit e the d es ir e d s ) n me tric o r a sy mm e tric m od e s beca u s e the flaperon a nd coll e c tive e xc iter act u at o rs were m o unte d onl y o n t h e rig h t wi n g and right rotor. In the future, t he e xclters S h ould _ e Incorp o r a te d on b o th r o tors a nd w ln gs .
Data obtained by flying in moderate tur b ulence using the RANDOMDEC analysis met h od compared very well with data from the dwell / decay technique a s shown in Ref. 8. The advantages of thls method are: l. Tests can be conducted in turbulent alr.
2. It is time efficient in t h at data for all modes are collected slmu I taneou s ly.
3. Very little grOun d -to - elf-to - ground coor d ination IS required.
4. It m ay identif y a n over look ed r esonan t fr e q u e ncy• ( Its di)adv a flt lge S are :
/
I. It d oes not pr ov i de a n on- li ne p o l n t -by -p o l n t e val u a ti o n o f i nd ivi du al a e ro ells tlc modes.
2. With o ut t h is p o l n t . by-polnt evalu ati on c a p abi l it y , it is n o t a S ea sy t o de te c t st a bilit y aug e lnt a tion / alrframe coupling as was encountered Jurlng evaluations of the as y mm e tric wing b e em ben d in g mode. (This prOb l em is discus s e d in Te s t Results sectlon of this paper.)
• If a pr o blem is encountered, (t is more _ifficult to ab o rt the test, as it i s harder t o "turn off" the turbulence than It is to turn off the e x citer in the dwe l l / decry technique.
4. It is difflcu l t to get t h e right a mount of _urbuience at the higher altitudes.
5 . T h e d e te is more difficult to analy z e, because of the m u ltiple mode c o ntent of the d i r e , r Th e au t Oe i l tlc fr e q uen c y swe ep tec nn i q u e w a s onl y u sed o c cas i on al l y during t hese t e sts. Olt e c O t i t ned ; ou t wei g he d t h e a dva nt a ges. Its a dv a n tag es Ir e .
I. It c a n help t O i den ti f y o ve r look e d , 'es on a flt f r eq u e nc i es i n t he r an g e of th e fre q ue n c) s w eep, 1 t O I0 H z , _si n g t he P ANI ) O # eDE C enil ysi$ c o m p a rl l l f avo r a bly with othe r d a t a, but th e dts ad v l nt l g e _ of t he t ec h nique { Z . T e St S c a n b e a bo r t e d eesl l y if a pro b lem i S e n c ounte re d .
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Its d i sad v antage s ar e : : 1. ' e st s must be f lo w n In n on t u r bu l en t a t mosph eri c co nditi ons .
? . It Is ti m e c o n su m i n g, b e c ause tt r equires a p o i n t -by- po int da ta c ollec t io n pr o ce ss.
3. [ t does n o t provid e a g oo d p oin t - by -po i n t e va l uat ion o f In d iv i du a l mod e s.
4 Co ntro l sys t em / a i r f ra me coupl i ng i s no t eas i l y r e c ognize d .
S . [t r eq ui re s con si de rab l e groun d - t o- a ir- t o-ground coor d in at ion.
6. T h e d a ta t s d if fi c ul t t o ana l y z e be c a u se o f t h e m ul t i p le mode conten t of t he di t i.
S . Flig HT T ES T RESULTS The r e su l ts o f the s tr uc t u ra l a e r oelas tlc stability test s c onduc te¢ _ I th th t X l -I S Y l l t Rotor Re s ( , arc fl Aircra f t are s_arl z ed In Fi g. 1 3. Natura l f requen c y and damp i ng ratio qata Js _ I t ) tteo a s i , fu nct i on of c a l ibr a t ed airs peed .
The predicted natural freque nc ies of t he si x prim a ry wing bend i ng mo de s J gree ve ry w el l wit h those meas u r e d i n f l ig h t a s sho w n t n T ab le 4. Bo t h B e ll Hel i cop ter Co m p a n y an 1 NASA Am es use d t h e NASA N A ST R A N pr o g r am t o pr e dict mo d e n a tura l f_ eque n c l es . NASA A_ an d B ell H ellcop t er : _r ed I c ted cu r ves Of a e ro el a s t !c stru c tur al d amp in g l e vels I as a * unc ti on of _i r sp e e d ) _ r e al so pr e s en te d i n Ft g. 1 3 . Th e ! a r ges t dis crepan c y b e tw een t h e tw o pr e di c ti on te c h ni q ues i s seen t n th e s_etr Jc a nd a s )_tr ic wi ng be a m b en d i n g mo d e s.
Th e NASA Ames pr ed i c tion ap p ea rs to b e c o rre l at e d w l t h t he s _etr l c bea m be ndin g m od e . , fle r eJs t he 8ell prediction h i s better c orre la tion with th e as)_ nm etri c b e am be n di ng mo de. Bu t th e point o f greatest intere s t in th es e predicti o ns is the air s pee d where the dampln_ rmt L o approach e x z e r o : n e ither of them e pr e dlctlon technique s have b een t es t ed in this area as alrsoeed s to date have net approached t h e stability _undary • limits. Dat a pre s ented in Fla . l) repre s ent s Pate up to the _axlmum sPe_ obtalnaOle in leve l flight wit h ma x imum continuous p o w er at 861 (Sl7 r : x _) _tor s pee d , TABL£ 4. CO_I g ARISON OF PR6OICTED ANO MEA S UR E D (V -I S PING MOD E _ AT URA L fREQUENCI E S Nat u ra l F requ ency _# Z b i i n g S e n di n g Mope: _r edicted _ * as u red Sym m etri c be a m ben O l n g 3 .I 3.3 t o 3. 4 As y mmet ric b e a m b en Olng S.7 6.1 to 6.7 S_ e t rlC ch or d be ndin g S .) 6.3 t O 7 ._ A S y mm e tri C ch o rd be n di n g S. , / P.I ? S + n _. . _ _ y l mle tr_c t o rsion al be n d ln q _. _ ". S t o _ . _ 4$_tric t o r s i ona l b end i ng ?,' , I ' I t o ; _ .3 "r lrst N ASTRAN _ d ei dld not I nc i u d ( . _ w ln q/ fu se l a ge sn ee r tie me mb er. I nclus i on of t)_i$m_l)er I n _re l sed sti f f ne ss a nd fre quen c y.
The n ex t po int Of int e rest I$ th e larg e vari a tio n In me asu red *llll ip lflg ratiOS for a giv en _ d e a n d f l ight cond i tion . Th e s _tric win g bea m b end i n g , _ G d_ h a s th e le as t a moun t of st utt er . .' _Is is c ause d by tw o Fa c tors . rl r st , it h is l ow d e e d i n g le v e l a n d Is e as il y e xc ited bY th e fla oe ro n. Se con o , (i s n a tur al _ r eq uenc y ( 3 . 4 q z . ) i s sig n if ic a ntly lo w er th an the ot he r _ o d es, a n d t he ab s e n ce o f _ d e Coup i ):n • l k es it e as i e r t o a nal y ze ( see ;t a t. g and 10) . Ot _e r _od es. spe c i f i call y th e sy _ m e t rl c wi ng chord be nd.
I n g mode, h i v e a hi gh d t moi ng l evel at the a irs p eed s t es ted , and t he mo d e s a r e d iff i cul t t O exc i te wit h o nl y a rlg ht -h lnd exc it e r s y ste m . Th e g rea ter th e s ca t t w) r i_ t he d a t a, the more dif fi cu l t _ t _s to d e t ect tr e n d s in th e dlt l .
Th e third point of int e rest o n thi s s_ m _ e ry p l ot _S t_e couplln q of the ?_,11 st ab ility and c o ntrol a u g m e nta tion sy ste m ?SC A S ) witN the a s y m m et r i c win g D e i g n ben d i n g mo de. ( ' n upl i n g of th e r ol l S CAS c a us ed the os cil l a tio n t o COntin ue a f ter t h e f tap eron exc: ter ,is t u r n e d off. g i v i n g t h e a) pee r a nce o f low I d_In q, s ee F i g . 14 . C h e cks mo d e w ith th e _i I ' _C A S turn e d o ff prod u,, a <l sl_Ific l nt ly _i g n e r level s o f ) ; _ ) dam p e n 1 . I ts p e_,me n e n t s olu t ion _$ the inc o r_r a tl o n o f a 'n ot c he O" _l l ter in th e ro ll _C AS t o : ) re v e ri e Coupli n g at t he ni tQ r al f r equenc y of 6 . 0 XZ .
6. C_ I I IS OR OI r W| NI) TU IINIrL l rl . l_ T T EST _ ES U LT S _ f lyer t he N St Z G Fea rs, a st (j p i fi c a nt th eore t i c al and mo d el t es t l n e effo r t _ I S b een m ad e t o u nd e r - i $_ , e nd and t o p redict t he s t r uc tu r el on e , elas t i c stc b illty ch a r ec t e ristics o f the tilt r oto r Conc e pt.
Us i n g onl y on e _ *. t he sy mm et r ic ui n g b e _ be ndi ng mo de, a n atte m pt _1 ma d e t o s f _ o w cor r ela ti o n Oet w ee n g _ und and fl i g h t t e st r esul t : . _his mo d e _s sel e c t ed bec a u s e it _ad a l o w p redicte d d_Ing l ev el, a n_ the r e fo r e , it is used _ o s t of t en by t_e c o nJuctln q mo d e l tes t s t o evaluat e pr ed icti o n m e t hO ds. F igure 1S t$ ¢ c ollo $ i te p_ ' _t_ d r e g fl ;flo w l n,) f o u r _ 4 )o r ) r ou fl¢ _ tests c ond u c ted D r to r t o t he f l i gh t t ests. Th ese t ests a r e:
; _-6 oR IGt _ k l- pA GE | S
Fig . l S A - , tndt un n e l test o f t, , 1 / S sc , l e s ,. tspan . tng O F p OO R QU / _L_ i' C _ F ig . 158 - W tnd t un n e l te s t o f t h e f ull s c a l e se m_ , in wing Fi g. I S C - Wi nd tunn e l tests of t he 1 / S sc a! e XV-1 S ai r c r a f t Ft g . 1SD - W tnd t unnel te st of t he XV-I S a ir c ra f t F i g u re 1 6 presents d ata f rom ea c h of th ese t ests wtt h a c omp ar ison t o fl tght t es t r esul t s. I n g e ne ra l, t here ap pe ar s to be f a trl y good egreenmn t be t wee n grou nd a nd f llgh t t es t s r esul t s , w ith t he model te st s t er : dln g t o b e o p t im is t i c . Ftgu r e 17 presents t he same dat e on a s in g le p lot a nd i n c ludes 8e l l H el ic op t e r C o mp6 n y a nd NA S A Anmsp r edi ct ion c urves, Th e g ro u nd t es t r esul t s te nd t o c onf irm t he Be l l pr ed ict ions, whe rea s t he f l i gh t test r esu lt s t end t o conft rm t he NAS AA _ s pr ed i c t ions. I t m ust, however, b e pointed out a g a tn t h a t tt is t h t s mo de. t h e w tng beam mo d e , w he re t he g res t ent d i f f erence was no t ed b et wee n t he t wo pr ed i c t io n t echn i qu e s. C o apar tson w t t h t h e _rou nd t es t s re su lt s to t he B e l l pr ed i ction curve t n d t° ) ' cites t h at th e Bell p redic t ion met hods ire co n '; r v a tlve. Fl t qht c es t r e sul t s have no t been c ond u cted a t htgh e no u g h speeds t o de t erm i ne tf t h e NA SA A _ s c u r ve t s al so c on s e r vative.
7. CON CLL ' S IONS 1 . Wi t hin t h e air s peed s t es t ed , the XV-1 S is f re e of s tr uc t u ra l a e roe l as ttc I ns tabi li ti e s .
2. Resonant f r equ en c ie s ca n b e re l iab ly p r e dic te d ust n g t h e P U _ STRAN m ethod.
3 , The aeroo_a s t tc t asti ng In dic ati n g t ha t the t heore t i c al a nd mo d e l ta s t ing e f fort r esul t e( I tn p re di cti on me thods t h at a re. t n genera l, cons e rv ati v e an d adequate f or t 'u tu r e developmen t o f t he t i lt ro tor conc e pt.
4 . F li gh t t e st t echni qu es need t o D e r e fi ned t o l o _ r t he ris k t O t he 4 1rcr e w . d ec r e ase t he tim e re- q uired for da ta c o llec t i on, and pe ml t bett e r exc i t a tion o f se l ec ted s t ru c tura l m od e s. ( [x ctt e r s s ho u ld b e t nst al le(: on both wi ng s a n d roto r s.)
S . Pos t f ltg ht off- ]t n e d ata an aly s is method s ho ul d b e r e f t n e d , and i f pos s ible, _ ve d t o Ono li ne d at a pro cess i n g sys tem.
ACKNO W LEDG EM ENT S T h e a ut h o r wi s h es to express h ts a pprec i ati on t o M ess r s. J . S t l g e r a nd R . _ e rr of t he Bel l 14e l lc o p t e r C om pa n y fo r t h e e ff o r t s i n t h e colle ct ion a nd a nal y s i s o f data used i n t h i s paper Th e a u t ho r w ould a l so lt k e to ex pr ess hts appre c i a ti on t o O r. J . Le u ng, of t h e NASA Am e s Rese a r c h C enter. f or hi s assis ta nce i n t h e r e processtng and a n a lys ts o q selec ted data, REFERENCES i 1. Ed e nbo ro ug h , 14. K . , "In ves ti g a t i on of T t l t R o tor V T OLA irc r af t R oto r - Py lon S tabi l i t y ," S e n Ae ros p a ce ) l Scie n ces M eet tn q , New York, N . Y ., Jan. Z 3-26, 1967, A I A A Paper No. 67-17 , January 196 7 .
Z . Ha ll, g. Ear l J r. , "Pro p-Rotor S t a b i l it y a t 141g hHover R ati os, " J . _ . 14elt r . op t a r S ot., vol. 2, n o. 2 , Ap rt l 1966o pp. 1 1 - 2 6.
3. k dv a nc _n t o f Propro tor Tec h nolo g y, T a sk l! - Wtnd Tunnel T est 14 esul t s, NA S A C ontr a ctor Repo r t CR - 11 41 6 3, S eptlmber 1 97 1 .
4 . Yen , j t ng, We b e r, G. E ., C _ llff e y , T. M . , "A S tudy of F old tn g Proprot or Oyn amt cs, " AFFOL - TR- _ 7 , feb r uary 1971.
5. Jo h n s on, g., "P r ed i cte d Dyn ami c C har a c teris t i cs o f t h e XV-1S T l l tt ng P r op r o t o r A t r cr|ft i n F l i g h t a n d i n t he 40- b y 80- F OOt g t nd Tun n el." NA S ATM X-73158. June 1976.
6. Johnso n . W ., "Th e I n flu en ce of P t tc h oLog Cou p l i n g o n the Pre dicted Ae r oeles tt c St ab tl | t y of t h e XV-1S Tll t tn g I _ o p r o t O r Ai r craft," ft4 sA TI4 X-73 Z 13 _ 7. C ole. 14 , A,, J r .. " On-L i n e Fai lu r e 0e r e c t ion I nd Damp i n g Ne e su r lm en t of Ae r os p ace Structures by Rando m Oecrm n t Signa t u r es," NAS A CR . Z 20S, _ rc h 1973.
8 . Otl g e r , J. / 4. en d He r r . R . C., "Results of St r uctu r al O y n _ ic T e s ti ng of t he XV. ' S T | tt Rotor R e s u rc h A t r cr l f t , " AH S Pa p er a l-$3, / aa y lg ql .
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