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' OF FI CE NATIONA L D'I_ , 'TU D ES ET DE RE C HERCHES AEROSPATIAI,ES
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( . NASA_ . ,,,_ . _X_68,) u,, _) A 5UMMAPY OF WIND
TUNNEL _ESE . %ECi- ON TILT POTORS FPO_
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A SU MMARY OF W IND TUNNEL RESEARCIi ON TI L T-ROTORS
FROM IlOVERTO CRUISE FIJGIIT
R EVU E DE S I _ TU D E S E N SO U FFLE RI E S UR L ES R OT OR S B AS C UL AN TS,
D U VOL STATIO NNA IRE A U VOL D E C R O ISli _ RE
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REVUE DES ETUDES EN SOUFFLERIE SUR LES ROTORS B,a,SCULANTS, DU VOL STATIONNAIRE AU VOL DE CROISIERE D a n s le c adre d 'un a cc ord d e c oop 6r a ti o n N A S A / O N ER A, u n programme de re c herc h es exp _ rimentales a 6 t 6 c onduit sur une famil l e de roto r s bas c u l ants dest i n 6 s 6 des V T O L c o :_ vertib l e s 6 g r ande vttesse ; les essais ont 6 t 6 eff ec tues dans dive r ses ins t allat i ons " d ° e s sais a u x U.S. A . e t en Fran c e dep ui s le po in t fi x e jusq u '6 un nomb r e de Ma t h d e 0, 7 env i ron : - ban c point f i xe de I ' U.S. A i r F or c e 6 Wr i ght- F ie l d, - instal l ation d'essa i s de I ' U.S. Army 6 Ames, - g r ande soufflerie 40 × 80 p i eds de la NASA _ Ames, - souff l e r ie s o n i que de 8 m, $1 de I 'O.N. E . R .A. 6 Modane.
L 'ob i e c tif prin c ipal de ce tte re c her c he 6 ta i t d ' obtenir d e s r 6 sulta t s pr _:i s sur I 'i nf l ue nc e " : du vr ill age et de 1' 61 asti ci t 6 des pa l es sur les performan c es d ' un r otor bas c u l ant dons tou t _ i e domaine de vol.
C inq rotors rigides en dura l umin avec diverses lois de vr i l l age, et un rotor a _ ro 61 ast i que _ e , 1 fibres de ver r e (d i am _ tre : 4 m) o nt 6 t 6 essay 6 s, 6 1 ° 6c he i le 1 / 4 environ d'un c onvertib l e !_ t y p i que c al c ul 6 p o ur une vit e ss e de c roisi _ re sup 6 r i eure 6 300 nceuds.
,_ L es effo r ts g l ob a ux sur l e ro t or, les c on tr ain t es l oca l es e t l es d 6 forma t i o ns de pa l e o n t _ 6 t 6 compar 6s ave c les pr 6 visions th 6 or i ques d o n s un domaine 6 tendu d e nombres de Ma ch et i L i_ _ de R eynolds.
_ ' _ On d _. ;; i ci ce r ta i nes te c hniques nouve ll es d ' essais d 6 ve l o pp 6 es 6 I °o cc asion de c e pr o gramme de coop 6 rat i on et on donne u n bref r 6 sum 6 d e s prin ci paux r 6 su l tats obtenus dans _ l es deux I_ ay s .
I A S_I'4A B Y 3F "dX_D-TUN_;LL RESEARCH O;iTILT - E C TORS FROM HOVER TO CRUISE F L IGHT b y Ph. POISSON- Q UINTON, ONERA " a nd W.L. COOK. NASA _ " SUMMA_Y W i t h i n the fr am ework of a c oo p e rative s greement between NASA and ,) :; E R A, an ex p er i ment a l re s ea r c h progra m ha s b e e n c o nducted on a serie s of tilt r otors designed for a range of blades twist in the v arious ; in / tunnel f a ci l ities of the NASA, 0NERA and t h e ['SA A2 _RD L . _e faci l ities in c lude the ._ASA / Ames h0 x 80-f oo t '_ 4 i nd Tu n nel, 3 . JERA / ' _d_ne d meter s o nic wind tunn e l and the USA A _DL / Ames 7 - by 10-f oo t Win d Tunnel as well a s the Air Force static test facility at Wright-P a tters o n.
The m a in obje c t i ve of t h e e xperim e ntal p r og r a m w as t o obtain prec i se results a bo u t the in - fluence of bl a des twist a n d a eroel a sticity on tilt rotor perf o rm anc e, from hover to h i gh speed cruise , :_s=hnumbe r of abo u t 0.7.
' Five _l,Lminium "rigid" rotors with v ar ious b l ade twists , a nd one fibergl a ss c omposite "d yn a - mi c aly s ca led" rotor were tested (sc a led 13 / 55th and 5 / 55th from a typical 55ft tilt rotor a ircraft design) ; glob a l for c es on t h e rotor, l o c al l o a ds and b l ade tor s io n al def l e c t i on me a s ur eme n ts were c om - p a red with theeretica l predi c tion s inside a l a r g e Re yn o l ds-Mac h enve l ope. Thi s p ap er des c ribe s s om e n ew test i ng tec h niq u e s deve l oped fo r t h is joint program and g i ves a br i ef s u m m a ry of t h e m ai n res u lts obt a ine d in t h e U.S. and French f a ci l ities.
_{OTAT I ON Cd Se c tio u dra 4 _, co effi c ient C I se c tion l i ft c oeffi c ient C T thr u st c oeffi c ient, _ / _n2D h Cp power c oeff ic ient, P / _n3D5 c b la d e c h ord D rotor di a m e t e r, feet DS s p inner d r ag C__ Z T _ 4 fig ur e of me r it, 0.798 P J adv a n c e ra t io , Vo / nD, w i t h n jt Voc°S_ n-- - SD ---- , w i th _ : rotor til t an gl e M M a ch n u mber Pb sp i nne r b a se p re ssur e Po fre e s tr e am pr ess ur e R rot or r ad i us r / R, loc al b lad e s t a t i on r a d i u s ra t i o R e Re ynolds num be r Sb sp in ner b ase area T G gro ss thr u s t T N ne t th ru s t , T o S V o f ree s t rea m v el ocity V t ve loc i ty at b lad e t i p a A u ista n t to t h e Sc i en tifi c D i re c t o r f or A er o n au ti cs Coord i na t i on, Of{ERA, 923 2 0 C NA TILLO N F RAt_ CE e, Ch i e f, A d v anced A ir c ra f t P ro g r am O ff i c# , NA S A, A m e s R e s ear ch Center , No ffe t t - Fi eld , 9 _ 03 5 C A LI F O RN I A tO-2 % blade section 5hi 2 _ess ro t or tilt smile rotor b la de p i tch a ngle _ blade twist a ngle, incremen t a l or tot&l rotor solidit y d e nsity b l ade effic i ency at f o rw ar d spe e d.
INTRODUCTION The ti l t-r o tor c on ce p t is o ne o f the m o st pr o misi n g c a ndid a tes f o r f u t u re VT O L A i rcraft be- / c a us e it provid e s a good c om p romise in d e s ig n re qu ir e ments for both hover a nd high spe ed c r u ise flight ; but two mai n pr obl e m s m us t b e c a ref ul ly st ud ied : - c om pr o m ise o f g e om e t r y l) r a e ro d y nam i c o pt imization o f ho v e r and c r u i s e effi c ie nc i e s , , - aeroelastic beh a v i o ur of th e r otor itse l f, a nd e l astic c oupl ing betw e e n r o tor s and eirfr a m e [ R _f. I] To answer a p a rt of these problems, the Researc h Progr a m su mm a rized in t h is pa p er was ini- ti a ted by NA S A (_4 ES Rese ar ch Center, Advanced Aircraft Progra m s Off i ce) and US A r my Aerona u tical Resear c h L a bo ra tory with a c o n + ract to the Bo eing C ompany, Ve r t o l Division ; the contr a ct objective w a s to e xp e ri- mental l y verif y p r edicted leve l s of hovering and cr u ise performa n ce [ 2] ; th i s pro g r a m was conducted as a joint NASA / ONERA effort u nder an Inte rn ation al C ooperative Ag r eem ent estab l ished in 1968 ; the a _reement pr ovi ded fo r t he use of F re nch an d U. S . f a c ili t i es a nd f or a n e x ch a nge o f th e re s ul ting data .
T he need f or a co m p r o mis e i n t h e d e si gn req ui re me n ts bet w een h over an d cru is e f l igh t o f a t il t -r o t o r Ai rcra f t i s i llustrated i n _ : i n hover flight (H) , t h e requ ireme nt f or e ffi c i en t per f ormance are high t h rust c oe ffi c i en t, hi g h ro t a- ti oual Jp eed ( R PM ) an d s m all value s of b lade t wist, wh er e a s the re qu ireme n ts f or ef fi c i en t cru is e per- f or m an c e (C ) ar e lo w t hr us t co e ff i ci en ts, r elat i vel y lower RP M, an d la rg e r value s o f b lade twi s t. T y T ical ro t or rad ial v ar i ati on s of loads and local M ac h nu mb er fo r hover ( H ) and cru i se (C) are al s o given in -i t i_--.._,w hic h i ll ustr a tes t h e g rea t d iffe r e nce i n loc al l if t coeffici en t a n d l ocal M ac h n u m be r fo r th e two c o nd itio ns of f li g h t.
D ESCRI P TIO N O F TILT RO TORS T h e _ met er (1 3 f oo t ) d i am et er ro to r mo del s ar e scal e d f ro m a 55 - fo o t d ia m ete r d e s ig n by Boel ng-V e r t ol (l ow di s c l o adin g Tilt R o tor Air cr aft, ace fi gurel) .
The 4 m d i a m e t er was c hos en in or d e r to pe r m it e x t en s i ve t eeth i n va rious f aci lities, i n - c lu d in g the 40 x 80 ft A mes T u n n e l , th e AFAP L- U S AFtes t st and , a _ d th e ON E RA - S I Modane s on ic 8-mete r dia m ete r t un ne l . Fig ures 2 and 3 give the g eneral ch ar a cte r isti c s o f t he r ot o rs.
T h e r oto r s hav e a 6 -p e rc e nt thic kne ss a t th e ro t or ti p , var yi n g t o 1 0- p er ce n t at a bo ut 30-perce nt o f t he ro t or rad ius . In b oard o f 30-percent , due to s tructu al re qu irem en t s o f t h e sm aller h m r ot or, it w a s n ec e ss ar y t o i nc re a s e the thi c kn e ss rap i dly t o a v alu e of 33-pe rce nt t h i ckne ss _t | 15- percent r ad i u s. This i n c rea s e i n t h i ckn ess had a n e ga t i v e e ffe ct o n h igh s peed c ruise pe r f or ma nc e ; ho w e v er , s u r v ey rak es meas ure m ent s w ere i ncluded i n the te s t s to de t er mi ne t h e per f o r ma n c e lo ss due to t he inc re a s ed t h i c kn e ss whi ch re s ul t ed i n a 2 t o 3-percen t reduct i on i n e ffi cienc y a t the M ach n um ber r an ge f ro m 0.5 t o 0. 68 . F ive s e mi -r _gi d ro t or de si gn s ,eac h ha v in g a d iff e re n t to t al v alue o f b lade twis t:2 6 . 6 ° , 29 ° , 36 ° , h0 .9 ° an d h_ o w e re t e st ed . T he detailed v_ r iati o _ of th i c k ne ss a t s e v er al r a d i al st a ti on s f or t h e ro t or s a n d the v ari ation o f b lade twis t f or b lad es D, E and F ( _, 3 6 an d 2 6,6 de gre e s o f t w is t re s - pectively ) a re sho wn i n _m_, 3.
) H O V E R PERFOR MAI_ CE Th e c al c ulate d v ar ia t i on of lo_al lift c oe ffi c ie n t a l o n g t h e bl ade r a d i us (figure ha) s h ow s t hat, wi t h the ; . _ o twist ed b l a d e s, a l arg e inner part o f t he rotor p e ne t ra t e s t he p re dicte d s t a ll b ou n da- r i e s, wh ere as t he lowe s t 2 6 .6 ° twist ed b lade s is far fr om t he separ ati on regim e ; th is expl ai n s t h e large "co m pu t ed d iff erence in h o v er e ffi c i ency ( fi gure o f me r i t) at t he de |i gn c ond iti on s (Vt i n - _ 30 m / | e c, 6 0 0 0 ft alt i tude) s ho w n on _ h b ; exper im en tal re sults ob ta / ned on t he s e t w o 13 ft °roto _are i n good : a _ ree me n t w i th t he pre d i c t ed (F . M.,-- -- 'C T) t rend : the l o ss i n ro t or hover per f o rmanc e is a b out 7 -percent a t _ o tw i_ t co mp ared t o 2 6.6 ° t w i s t.
The v ari ation o f th e h over fi gure o f m e rit m ea s ured f or the f ive rotors as a f u ncti o n o f the ir tw is t f_ o m the tw o te st faci l iti e s (A m e s _0 x 80 f t v i nd _ unnel an d U . S . Ai r F orce sta tic t e a t r i_) is gi - ven on_ f or the o_ t imum thru | t coe f ficient CT - 0.0 7_ : e xp e r im en ta l val ues are i n clo se _r ee- s e n t v l th th e p re d ic ted F . M , a nd decre a s e w hen th e b lad e twis t i ncre as e s .
_ " -_- 3 fhe varl a tion of figure of merlt :'ira rotor olade tlp ': acn number (figure >_) _.nd thrust coefficlent (_) indicate a rapid red u ction in figure of merit at tip "dach above about 0.8, a nd a severe reuuctlon in figure of Tmrit occu r s at thrust coefficients a_ove about .09. for a given tlp Mach n u .nuer. The data an_ calc u lated values indicated that, at desi_n condltions( tip "_ach nu m ber of 0.67 a nd thrust coefficient of O.075} the figure of merit v a lues range from a ma_ x imum of about 0.79 for the low e r values of twist below 30 ° to less th a n 3.72 for v a lues of blaCe twist greater th a n hh degrees.
fo il l ustrate the sc a le effect, _ presents the v a ri a ti o n of figureof merit as a fu nc - tion of thrust coefficient for the 13-foot rotor a n d for the 5-foot rotor tested by the U.S. A r m_ (A A RL Static rig), with the s a me j6 ° twist law.
A _-percent loss in figure of merit with th sm a ller scale 5-foot di a meter rotor at the design C% 1 c a n be explained by a Re y nolds number effect a lre a dy evidenced on higher disc loading propellers (see._, profile drag versus He, and Hcf. I).
fhe 13-foot rotor falls on the flat part 9f the cu r ve (F.M., Re) above critic a l Reynolds n u _nber, whereas th e 5 -ai am eter r otor (mea n Reynolds n um ber of O.7 million) is on the steep part of the curve. From these curves it '_ ou ld u e expected that a _5-a± u _eter rctor "_ould have about 2 percent higher figure of merit tn a n the 13-foot dieu u eter roto r a nd also th a t the low disc loading p r op r otor having / abo u t 10 t o 12 pounds per sq u _re f oo t disc l oa d i n g w oul u h a ve from I._ to 2-pe rc en t lower figu r e o f merit than the m a ximu m perfor_ a n c e envelope v_lues for nigher disc lo a ding p r opellers.
Abo u t h o ver performa n c e measu r ements on scaled rotor m odels, a new m ethod has been recently developed in the O Jk R A S I " _od a ne wind-tu n nel, b a sed o': an extrapol a t i on to zero speed of the res u lts oh- ' t_*ined at ver y low speed inside the closed test sectlon (fi_ /re_ ). ' _ For the first time, it w a s possible to c he c k the validlty of this method by c omp a rison with "tr u e" static tests obtained w i t!, the s a me 13 ft ro t or o n the U.S.A.F. / Wright-Fie]d rig : it c a n be seen in figure 7 that the extrapolated results on power and t h rust c oeff ic ients obtained at ve r y low v a lues of the a dvance r e " _o J in S I _odane are in close a greement with tho s e measured on the special AF / A P L st • tic f a cility ; a lso shown, on the (CT, J) graph, is the theoreti c al t:1_nd, ass um ing th a t CD and F.'4.
a t low J v alu es are t he same as th o se o bt a ined o n a s ta t ic rig a t J = 0 (fr o m th e r e lL t i on s_ i p :
CT Vl
To use such low tess speed it is necessa r y : - to stop the wlnd-t u n nel fans, - to fit a low permeabili t y s : reen to thL rear of the first diffusor (to reduce the mass flow induced by k the rotor inside the retu r n el ; cult), - to meas ur e this ve r y l o w speed with special inst r ument a tion (double-ventu r i tube).
w _ na l l _, t he spe e d was r e clueed to 2-h m / s e e with the lowes t s creen po r osity, but attained 1 0- 1 5 m / s ee wit hou t any scr ee n a t t he fi r s t d iff_ or e x it.
FQ:_WA/_D SPnED TESTS. RIGID BLAD_LS Shown in f_g u re 8 is a shematic of the Oi_ERA / Modane 8-meter test faci l ity and the two t e st rigs used for these investigations wherein one rig is utilized for low speed tests for a l ar ge range of tilt a_gles, 0 to ;O-degrees, with a minim u m sized after body (this later rig w as also used for t_e pre- vious "quasi-static" tests).
Th is lar ge t u n nel [R_f. 3 3 h a s • c on tin u o us op er a tin g m od e wit h th r e e in ter c han _ e & ble t e st s e c t i ons , one o f the m t e i ng m a i nly u s ed _ or rotor and propeller, dr iv e n b y a g rou p o f _ a _ t urb i ne s ( 1000 H P ) the t un n el is p o w ered b y t w o co un ter-rota ti n _ f e rns dri v en b y hy dr au l i c t re phi ne s (2 x 55 000 HP) ; th e s t a - g nat io n pre ss ure is a tm o s ph e r ic Pe t " 0 , 9 b ar , at t he l o c al 3300 f t alt i tude) _ the c i r c uit e ool i n _ is o b ta i ned by ai r e xch an g e ; the te s t s peed c an r eac h M ach ;, b ut f or the pu r po s e o f the s e rot or te s t s t he M ath n u mb er w a s l_mi ted to • b out 0 . 7 .
TILT ROTOR TRANSITIO N RE G DdE On e o f t h e _on ls of t he co o per • t ire N A S A / 0 _ E RA pr og ra m w as t o com pa r e t he re sul t s ob t_dn ed on t he sam e r o to r (8_ - 1 2 , 5 m 2 ) i n si d e two ve ry d iffe rent t e s t se ct ion s iz es , at Am e s ( S T " _ 6 _ m 2 ) an d • t M oda n e ( S T - 5 0 m_), dur i ng t he tra n si t ion regim e 6f f l i gh t. The e ff ect o f t u mn el wall o n til t rot o r t est data,part i cular] 4 • t low sp eed and hig h tilt an _Les_ ha s b e en t he s ubject o f c o n si dera b le c ontr ov e r sy an d di sc u ssi on. The ;3-f oo t d i ame t er rotor w a s _ e s ted i n th e Am e s h( > ° by 8 0-f oo t w ind t u nne l thro u gh a r a n g e o f t ilt an _l e s f r om 0 t o 78 e an d • ran g e o f s pee ds . The t u nne l w a ll e ff e c t s o f t he _3 -To t or i n the _ 0- b_ 80 fo ot v i nd t u nne l ar e p resu m a bly ver y s ma ll a t dis c lo adin _ o f _ 0 p o un d s per s qu _ re f oot and a t t h i s v e ry low area rati o SR / S T _ 5_ . Th e s a _ e rotor 1 3- f oo t di am eter was tested un der the za m e tilt an _l e and v elo - ci ty c ond i t i o ns i n the 26-f oot or 8- m eter wi nd t un n el wh ere i n t he ar e a rati o S R / S T is v er y lar g e : _5_ ; z o ne i n v e s ti _m tor_ ha v e c la im ed t h at t hi s ra t io w ou l d b e too gre a t % o eve n con s ider te sti n gs. _ e _ ert h ele as, t he resu l t s o f th ese t e s t s i n t h e tw o w lnd -t u m nels ar e sho w n i n _ 9 : The t h ru s t c oe ffic ie n t a s _ n c- - tin- o f th e e ff e c ti v e a d v an c e rat i o J ' w h ic h is J c o s _ (_, _le cf t --' _l t ) i n dic ate s t han t h e,_ is K(_) d c orrelatio n b et v ee _ th e res ul t s o f t h e te s t s in the tw o vin d-t u m n els -rid th us the t u nne l w all e ff e c t s of the ; 3 -f oo t rotor te s t e d in the C3 E R A 8 _e t e r w ind t u nn el are o f l i tt le sig ni f ican ce f or r es e arch stud i e s o f th is type of V _ T O L p ro p ul s io n s ys te m, s t l ow d i s c l oad i n g. _ show _ tha t the prewio us h o v er t e sts ' 18-b in the hO' x 80' Ames t u nnel (made w ith o pen overhea d wi n d-tunnel d o o rs to minimize fr om recircu l ation) give a good extr a polation at J' = O.
CR U ISE TEST3 The cr u ise mode ( a xial flow) investigation was cond u cted at the S I Mod an e tunnel on a specia l axial support s ystem (Db - 0,6 m) eq u iped with a n internal 6 component balance ; be c a us e the axial for c es mea s ure d on the d y n a m o m ete r in cl u d e d t h e dra g of the sp i nner, i t was n e cess a r y to t a re t his dra_ to obt a i n e d the "net" thrust in v iew of c ompa r isons w i t h rotor perfo r ma nc e pre d i c tions ; FiG ure 10 expl a in s the two s ue- c es s i v e s me%hods u se d f or m e a s u r ing the s p i n n e r ta re d r a _ : The fi rs t on e i s b as e d on a spin ne r b as e pr e ssu re t e chnique : the s pinn e r form d r ag c o e fficient wa s d e t e rmined fro m "bl a d es -off" tot a l b al ance fo r c e and ba se p res s u r e me as _ T e m ents ; d u _i n E the r oto r te s t s them s el v es, th e to ta l spinner d rs _ Ds w a s obt ai ned by a dd / n g thi s fo r _ drag to the b ase pr ess ur e drag (m ea sur e d under roto r o pe rating c on dition s ) , and then the net th rus t was giv e n by T N - T G + DS, ; T G bein g t h e m ea su r e d gross th r us t .
A more sa tisf a cto ry me t hod wa s later us ed where a s pec ial b alan ce inside the spinner gi v es d i rec t ly its drag wit h t h e bla d es -on and ro t ati ng ; a com pa r i s o _ betw e en t hese two met h o ds se e m s t o in di- c a te t h at this l a ter metho d , tak i n g a c co u nt of the p res e n ce of t h e bl ades in f l ow, gives a litt l e l arg er ' spi nne r drag, i .e . a hi g h er cru i se e ffi c i ency ( be tw e e n I% and 2 %).
Thi s sp inner t a re dra g w as f ound to b e a ve ry signifi c an t p art o f t he th r ust m e as u r em en t s a s s h o wn in _ : th e 6 ross thru s t T G meas u r ed h a d ne ga t ive v alues at M ath n u mb e rs a bo ve 0 .6, where t h e s p i nn e r d r a g D$ be c a me g re at e r than t he blad e t_ u s t TN . All t h e thr u st data sh ow n here ha ve the t o tal s p inn e r dr ag r e m oved and he n: e the per fo r m an c e charac ter is t ics ar e for t h e b lades al o ne and do n ot i n c lude t he spi nne r s k i n f r icti on or [_r o fi le drag w h ich wo uld ex is t on the rotor i n flig ht , w herea s the b ase drag w ou ld b e p ar t o f t he tota l ai r c r _ £ t dr ag . Also s ho w n i n figur e 11 ar e ty pic al a d vanc e rat i o , J , and thrus t coe ffi c i ent v a l ue s f or a t il t ro t) r a i rcra f t f or a ran g e o f M ach n u mbe rs f r om 0 .3 to a b out 0 . 67 w h ich are us ed i n the f oll owing a na l y sis ( V ti p - 1 8 0 m / s e e , Z = I 0. 00 0 i t ) . For al l t he s e re s u l t s f r om $1 M o da n e t unnel , the con v ent i onal G la uert w a l l c orrect i on s ha ve b een a ppl i ed (rat i o S R/ S T - 0, 2 5), but lo w l eve l s o f th r us t lo a d i ng r e s u l t i n sm all m a g n i tude of the s e c orrect i on s At a c ru is e Mach n u mb er o f 0. 4 55 , a b ou t 2 9 0 knots , t he ra dial s e c t i on l if t cn e ffici ent s an d c r uis e e ffi c i ency f or b lade s D and F, 44 .0 and 26 .6 ° o f t wis t ar e s ho w n i n figure 12 : al*_ ho _h b lade F a t 26.6 ° is t h e b e s t at h ov er , i t s s ec tion l o s di r _ is p o o r and sh ow s a p pr o x ima tel y on e h alf of t he bl ad e ( inb o ar d of a bo ut 5 5-p e r c e nt rad i al sta t ion ) wi th a n e g at iv e l ift w h i c h re sults i n a 1 5 -p erce nt l owe r v a - lue of c r u is e e ffi ciency t h an for b la d e D a t 44 ° whe r e p osi t i v e s ecti o n l if t c o e ff ic i en ts e x ist exc e pt o ve r t h e o utbo a rd 1 0 - pe r cent of t h e ra di us.
Th e data i n figure 13 ar e t h e t hr _ t c o e ff ic i e n t and e ffi c i en c y ( o ) f or a r an ge u f advance ra- t i os at t w o blade ang l e s at each o f three M ath n umb er , M - 0. 4 5 5, 0 .5 4 and 0.68. T h e des ig n thru s t coe f- f ic i ent _L r ia t i on wi th J an d the re s u l t i n g e ffi ciency a t each Mach n umb er is al s o i nd ica ted. B ec a use o f t he lo w d is c lo a d i n g o f t he ro t or the thr us t at m ax im u m e ffi c ie ncy o f the rotor is not use abl e and o f no va l ue f_ the nor m a l c ru is e op e r a t i o n o f t i lt rotor a ir cr af t ha v ing raa lon a b le level s o f drag . As s ho wn i n t h e figur e the m ax imum v alums of e ff ic i ency oc c ur a t thr un t c oe ff i c i e e ts and h ence p ow er coe ffici en ts t h at are 2 t o 4 ti m e s h igh er th an requ i red f or t h e a ir cra f1_ with the h igh er v al ue s oc curr i n g at t he l o w er M ac h n umb e r o f O . 455 . ' Ut i l i_ t l on o f the maxim u m va l ue s o f e ff ic i ency can on l y be obtained e c ono mi - ca lly b y util i z i n g D tor s or propellers ha vi n _ sig n ific ant l y h ig he r value s o f d is c l o a din g o f the order o f 2 50 t o 3 5 0 da _ / m 2 (5 0 to 7 0 p ou n d s per s quare f oot) i n s te a d o f abo ut 5 0 da N / m 2 ( 1 0 l b / s q, ft ) cho s en here.
Th e da ta s h o wn i n fig ur e 14 s u _ arize th e m e a sur e d cr uis e effici en c y fo r blades D , E , an d F o ve r th e Mach n u mb er ran ge te s ted 0 . 3 t o 0 .7 2 an d the var ia t ion of cr uis e e ffici ency w ith the f i v e dif - f . f eren tly t wist ed bla de s f or 0. 455 an d 0. 6 06 M a t h n um ber : - High Mac h n um ber tests (0 . 5< M <0 .72 ) , i. e . abo ve t he rotor de sig n ca l cula tio n ( M • O .4 55 , s ee R e f. 2 ), w er e r un t o i n v e stigate c_ pr essi b i l it y e ffec t s on c ru ise m ode t ilt-r o t or per f o rm ance ; fiRur e 1 4a s ho ws a l ar g e decre as e of th e cr ui se e ff iciency a bove M : 0 . 6 due p L rt ly to the l _ rge pro fi le thicknes s ( 3 3 _) a t the bla de r oo t (requ i red f or st ru ct m' al c ond i tion s on the s e s caled m odel s ) .
- T he bla de tw is t e ff ect ,fiEure lhb) is v e ry im portant , the b e s t eff i c ien c y being obta i ned with the l ar - g e s t bl ade t w i s t t e s ted ( i . e . an opp os ite tre nd th an s ho w n i n ho v er) ; the c a l cula ted va l ues give n here f or M _ 0 .455 [R e f . _] take s i nto ac c o, m t the m eas u red v elo c ity pro fi l e a t the dis c pl an e (a s m all f l ow ac - ) celer a tion wu detected aro un d t he '_i n imum b ody " i n S I M odane tun a el) ; t he pred ic ted v a lue s are h i gher th an tho s e m ea s ured , m a i n ly a t the l _w e st blade t wis t _ f or the " E" blade (3 6 ° _ wist ) th e experi m ental cru is e e ff ic i enc y i s 0. 71 _ai n s t O. 74 5 pred i ct e d.
To conclude ab out the need f or a c _ apr mmise between the pe r f or m anc es o bta ined in hover a nd i n cru is e m ode s, f i _ ure 1_ g ive s the m e axur ed an d pr e d i ct e d w _ lue s o f how _ r fig ure of m erit a s a f unc tion o f t he de si gn cr uise e ffi ciency at M • 0 .h_ 5 : it a pp e ar s that a bl m de t wist of about 36 ° ( " E " rotor) see m s t he best c o mp r oml se f or th i s typ ic al project (F .M . • 0 . 7 7_, _ c r • 0 . 71).
( e ) Th e s y m bol s on th ese g r aphs are di re c t ly r e prod _ ed f ro m th e a ut _t i c plott i n g o f a comp uter pro g ra m, w h i ch t ak e s a c c o un t o f a ll the c orr ec t i on s an d tar es .
i FS_. L_A_D SP_LJ '.'_ST '_ 3_ /d_POELAS TICALY SCALSD hbTO._ To study the effect of aeroel a sticity on roto r p e rformance, a specia¢ 13 foot dia m e te r fiber- gl a ss mod e l h a d b e en d e signed and bu i lt [R e f. 2] with th e o p t i mum twi s t shown pr evi o usly (ty p e "b'", 36 ° twist) ; the main puroose of the tests in tile cruise mode was to study the effect of Macl.number and lo a - ding on "live" twist deformation, and the effect of aeroelasticity on rotor / blade st a bility. Shown in f_ K, re 16 is ._ sketch of the a eroe3 a stics blades and the photog ra phic torsional blade deformation tech- nique utilized to measure the variation of "live twist" during tests of the rotor. In this method, an ultra nigh speed photographic flash unit was used to obtain stop action photogr a phs of the b a ck face of one of the r ot a ting b la de s wnich wa_ p ai nted w i th triangular t a rgets at regula r spanwise i ntervals. _e angu - l ar difference between s p a ir t a rgets and the blade root reference targets i s re l ated to the b lad e ra d i al twist di s t r ibuti o n . A c omparison of t his me as ured di s trib uti on under forward speed c ond i t io n s i s ma de w i th th a t of the stati c twist d i s t ri b u tion with the b lad es nonro ta ting wh ich gi v e s the instantaneou s ae- roelastic to r si on al def l ection f o r v ar i ou s r adial st a tions. A c o m p ari s on f o r a typica l test point is a l so shown in figure 16 w h ich indi ca tes abo u t a 2 degree torsional def l ection ne a r the r otor tip. An i nterest i ng a spect of the me a surements was th a t the so c al _d "rig i d b l ades" (b u ilt of alum_ . nium) h ad no- ticao l e bl a de deformation as shown in figure 17 and th a t there w a s a consistant one degree gre a te r twist • deform a tion with the aeroel_tic blades th_ with the "rLgid blades" ocer the entire M a ch numb e r a nd ad- vance ratio, J, range tested a s s hown in the figure.
The v a r ia tion of thr u st coefficient and e fficiency for Mach numb e rs of 0.h55, 0.5_ and 0.6 0 6 for s range of advance r a t i os for the _roelast i c b l ades i s c o m p ared in fig ure 18 to th e rigid blade d ata J shown previous l y. Altho u gh th e re is a significant increase in t he shs_ ne ss of the sl op e of thrust c o ef- fic i ent v a ri a tio n w i th ad v anc e ra ti o , th e act u al c rui se eff;ciency f o r th e cr uise Mach n umbers is n earl y eq u al to th e cr u ise eff i ciency with the rigi d b l ad e s, ag ai n in d icat i ng a very smal l twist defo r mation e_uivalent to the I deg_e _asure. The steepness of the thr_t coefficient c_e with velocity f or the _roeiutic rot o r relative to the rigi d rotor me_v be of great conce_ at the higher :_ch n_be r s d ue to the potenti a l high sensitivity of thr_t t o s_ed _ d blade angle. This sensitivity ma_ it difficult to control th r _t d_ing the win d t_nel tests _rticul_ly near values of low or ne_ti_ thrust where ins- tabilities d id occ_ with the a er_lastic r oto r , and p _vented testing a _ve a : dachn_ber of 0.606 (the "rigid" r ot or was tested to a _ch n_r of 0.72 with no indic at ion of instability).
A su_ c ur _ is ah own in _ which com p a r e_ the _as ur ed c_ise efficiency f o r the 36 d egre e twi s ted b la de rot or for the v a ri ou s r ig id an d _eroela_tic b] e_le rot or s tes t ed si n ce 1 96 8 wi th the c a l culated cr uis e eff i c ie ncy for t he sLme r o to r. AB cam be se e n above a Mac h n t_mber of abo u t 0 ._5 t he te s t da t a sh ows a si g nificant l ov e r c r uise e fficiency than calculat ed . A t 0.72 Msch n umbe r th e measured valu e s are of th e o rd e r of 0. _ 5 wh e rea_ calcula te d is ne Lr 0 .6_. Th e c o rre l ation of the vari o u s t es t data is g oo d in as m_ch a s the a c cura cy o f the t e st data i s .+O.01 in efficiency.
I n __ is sh o wn a trace o f a specific d i vergence of roto r lo ads at a Mach n umbe r of 0 .6 3 w i th the ae r oe l amt i c b l ades that p revented any furthe r tests a b o ve a Mach number of 0. 8 06. As can be see n , a rap i d an d d i ver g en t i nc re a s e i n t hru s t (o n t he mai n balan ce ) a nd i n l o cal bla de-e l e me n t t _ r si on a n d ch ord - wis e str e ss (meas_ w e d at 23% and 2 _ R, se e fig u r e 21) w e re me a s u r ed ; a bl ade fl u t ter o c curred pr i or t o t he quic k stop i n itia t e d a8 thes e meas ure m ent s were b e ing m o nit ored . Fu rth er analyses on s tr ess d a ta mea - sured on this el a stic ro t or , ta k e n at I0 de_ ree a n _le o f attan k _t l o w M a c h n umbe r (f _ rm 21), i n _ n e f - f ort to b e tter _ nd e r s t _ nd t he sta b i l it y prob l e m th a t v a n e n c o u nte r ed dur i n g t he t e s t s . But i t m u s t be r e- memb ered th at t h e , _ ch numb er o f 0 . 63 wh e re th e i n st a bilit y or f l u t ter occurred is si_ n lf i cant_ ab o v e t h e antici p at e d d e si gn cruis e M a c h n u mbe r o f ab ou t O ._6 f o r p et e nti a l tilt r o t o r ty p e a i r c ra ft b eing con s i - der e d .
REFERENCE Ll ] G I LL_RE K .B . - S urvey o f t ilt rotor te c hno l o gy de v e l op m en t Agar d Me e ting on A d vanc ed R o t orcr aft I(AEA / Lan_Iey, sep t. 1971 - A_ / r. _ .P.
P ro cee d i n g.
[2] B O _ I N G - V_ R TO L C y" Inv es t igation of t h e perfo rman ce of low d isc- load i n g ti l t ro t or s i n ho v er i n g and cr uis e f lights (NASA c ont r a c t I iA_-_0 2 _) - _L ' iG / Ver t o l D i v is i on re po rt D160 - 10013 - - I and 2 , March 1971 .
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