REVERSE-THRUST TECHNOLOGY FOR VARIABLE-PITCH
`
`FAN PROPULSION SYSTEMS
`
`David A. Sagerser, John W. Schaefer, and Donald A. Dietrich
`NASA Lewis Research Center
`
`During the past several years, a number of tests have been conducted to
`develop the technology necessary to meet the unique reverse-thrust performance
`requirements of a variable-pitch fan propulsion system. Areas that have been
`investigated include the losses and distortion associated with the air entering
`the fan and core compressor from the rear of the engine, the direction of fan
`blade pitch rotation for best reverse-thrust aeroacoustic performance, and
`engine response and operating characteristics during forward- to reverse-thrust
`transients. The test results of several scale fan models as well as a full-
`size variable-pitch fan engine are summarized. More specifically, these tests
`have shown the following: A flared exhaust nozzle makes a good reverse-thrust
`inlet, acceptable core inlet duct recovery and distortion levels in reverse
`flow were demonstrated, adequate thrust levels were achieved, forward- to ra-
`verse-thrust response time achieved was better than the goal, thrust and noise
`levels strongly favor reverse through feather pitch, and finally, flight-type
`inlets make the establishment of reverse flow more difficult.
`
`INTRODUCTION
`
`The short field lengths envisioned for short-haul aircraft operation have
`made reverse-thrust performance a critical part of the propulsion system's de-
`sign requirements. The conventional approach to providing reverse thrust in
`turbofan engines is to use target or cascade thrust reversers to redirect the
`engine exhaust flow in a forward direction. Considerabie study in recent years
`has been directed toward an alternate approach to reverse thrust - the variable-
`pitch fiin.
`
`Noise requirements for short-haul aircraft dictate that a low pressure
`ratio, high bypass ratio fan be used especially for an under-the-wing engine
`installation. For such requirements, engines designed with variable-pitch fans
`for reverse thrust have been shown (refs. 1 and 2) to be superior to those with
`fixed pitch fans and conventional reversers. The primary advantage is lower
`propulsion system weight. An added benefit is faster response times in forward
`thrust which are important for approach waveoff maneuvers. The faster forward
`thrust response times are a result of a variable-pitch fan's ability to provide
`approach thrust at high fan speeds (ref. 1).
`Because of these advanzages, a
`variable-pitch fan was incorporated in the under-the-wing engine of NASA'S
`Quiet Clean Short-Haul Experimental Engine (QCSEE) Program.
`
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`Obtaining reverse t h r u s t with a v a r i a b l e - p i t c h fan engine involves a new
`mode of engine operation.
`I n norm11 forward-thrust operation engine a i r e n t e r s
`t h e inl+:.t, passes through t h e e n g i t e , and is exhausted out t h e r e a r a s shown i n
`the. uy.p.r half of f i g u r e 1. I n r e v e r s e t h r u s t t h e f a n blade p i t c h i s changed
`s o tk..tt t h e f a n a f r flows i n t h e o p ~ o s i t e d i r e c t i o n . A i r must be drawn from
`t h c r ?.IC of t h e engine; t h e a i r is ro,quireJ t o t u r n 180° from its o r i g i n a l direc-
`.s shown i n t h e lower half of f i g u r e 1. P a r t of c h i s a i r must t u r n
`t'ion,
`near11 180° again t o supply t h e engino core. The r e s t of t h e a i r passes
`t.hrougil the fan and i s exhausted out t h e i n l e t . Requiring t h e a i r t o follow
`:hi s d i f f i c u l t path and operating t h e m g i n e during t h e forward t o r e v e r s e
`t h r u s t t r a n s i t i o n r a i s e s a number of design questions:
`(1') What nozzle shape is required ?o minimize t h e pressure l o s s e s and d i s -
`t o r t i o ~ i n reverse t h r u s t ?
`*Is.) W i
` pressure recovery and d i s t o r t i o n l e v e l s i n t o t h e c o r e compressor
`l
`l
`be sat:% s f a c tory?
`I n which d i r e c t i o n should t h e far\ blade p i t c h be changed f o r adequate
`(:')
`rl :verse - t h r u s t l e v e l s ?
`(41 Csn t h e forward t o reverse-thrust
`t r a n s i t i o n be accomplished i n t h e
`r e q u i r e i time without engine o p e r a t i o n a l problems?
`(5) What e f f e c t w i l l a fltght-type i n l e t have on r e v e r s e - t h r u s t operation?
`
`A n,mber of tests have been conducted ovzr t h e p a s t s e v e r a l years t o
`a n a e r t h e s e questions. The r e s u l t s nf some of t h e s e i n v e s t i g a t i o n s a r e dis-
`cussed i n t h i s r e p o r t t o provide an overview of r e v e r s e - t h r u s t technology f o r
`vbri 3b:l.e- p i t c h fan propulsion systems. To add perspective t o t h e test r e s u l t s ,
`t h e reverse-thrust requirements a r e discussed f i r s t .
`
`REVERSE-THRUST REQUIRkZAENTS
`
`Reverse-thrust r e g u l a t i o n s f o r short-haul a i r c r a f t have not been estab-
`1-ished. However, based on a number of a i r c r a f t systems s t u d i e s , reverse-chrust
`objectiuzis have been defined f o r QCSEE. They a r e compared t o t y p i c a l r e v e r s e
`t h r u s t c;.laracteristics f o r conventional ensines i n t a b l e I. The r e v e r s e - t h r u s t
`l e v e l £0,-r OCSEE, 35 percent of takeoff t h r u s t , is required f o r landing on i c y
`rlmways o r i n t h e ~\ve.lt of brake f a i l u r e ( a s describ2d i n r e f . 3). Although
`t h e QCSEE c b j e c t i v t .'alls on t h e low s i d e of t h e range f o r conventional a!.r-
`c r a f t , t h e resu'rtng a i r c r a f t d e c e l e r a t i o n i s comparable t o conventional a i r -
`c r a f t t~ecausc. *!CSEE is designed f o r an a i r c r a f t with a high thrust-weight r a t i o .
`
`t o reverse-response time o b j e c t i v e , o r t i m e t o reverse, f o r
`The 'orward-
`QCSEE !s considerably more s t r i n g e n t than f o r conventional a i r c r a f t because of
`t h e RhOrt f i e l d operation. However, t h e time t o reverse f o r conventional a i r -
` is longer mostly because of t h e time required t o i n c r e a s e t h e engiaz
`C ~ E '
`r. .leed from a near f'.ight
`i d l e condition a t t h e i n i t i a t i o n of r e v e r s e t h r u s t t o
`t h e design r e v e r c c - t h r u s t condition. Thus, some reverse t h r u s t J s being qener-
`ated durinfi m o ~ t of t h a t time.
`
`Opelacing an engine i n reverse t h r u s t a t low forward v e l o c i t i e s can r e e u l t
`i n cr'laust gas r e i n g e s t i o n , foreign object damagc ~ r o m t h e r e v e r s e j e t impinging
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`on t h e ground, and t h e impingement of hot exhaust g a s e s on a i r c r a f t s t r u c t u r e s .
`Because of t h i s , r e v e r s e - t h r u s t o p e r a t i o n i s u s u a l l y p r o h i b i t e d bclcw c e r t a i n
`forward v e l o c i t i e s . A comparison of t h e minimum forward-velocity limits
`( t a b l e I) shows t h a t t h e QCSEE o b j e c t i v e i s more s t r i n g e n t than conventional
`a i r c r a f t , a g a i n because o f t h e s h o r t f i e l d operation.
`
`The importance of low n o i s e i n a l l phases of short-haul o p e r a t i o n r e s u l t e d
`For 1C8 400 newtons of r e v e r s e
`i n a r e v e r s e - t h r u s t n o i s e o b j e c t i v e f o r QCSEE.
`t h r u s t a maximum no;'.se
`l e v e l of 100 PNdB on a 152.4-meter
`s i d e l i n e has been
`e s t a b l i s h e d .
`
`A I R INTAKE CHARACTERISTICS
`
`Exle L Performance
`
`To a s s i s t t h e flow of a i r i n t o t h e tear of t h e e n g i a e d u r i a g r e v e r s e t h r u s t ,
`t h e f a n nt)zzle can be opened t o fona a f l a r e d shape, c a l l e d a n "exlet," a s
`shown i n f i g u r e 1. A number of s c a l e e x l e t models were t e s t e d ( r e f s . 4 and 5)
`t o determine what geometry r e s u l t s i n t h e lowest t o t a l pressrlre l o s s and dis-
`t o r t i o n l e v e l . The e x l e t c o n f i g u r a t i o n s t e s t e d covered £!arc a n g l e s O from
`0'
`from 1.4 t o 2.8, and d u c t s with and with-
`t o 60°, c o n t r a c t i o n r a - i o s bhTIAS
`o u t simulated a c o u s t i c s p l i t t e r s .
`
`The r e s u l t s , along w i t h geometric d e f i n i t i o n s . e r e summarized i n f i g u r e 2
`f o r freestrean! v e l o c i t i e s Vm of 0 and 41.2 metern p e r second and a f a n duct
`Mach number Q of 0.4.. The r e s u l t s i n d i c a t e t h x t a f l a r e angle of 30° gave
`t h e h i g h e s t p r e s s u r e recovery. A t f l a r e a n g l e s o t h e r than 00, t h e d a t a f e l l i n
`a r e l a t i v e l y narrow band showing r e l a t i v e i n s e n s i t i v i t y t o c o n t r a c t i o n r a t i o
`and t h e presence of an a c o u s t i c s p l i t t e r . A f l a r e a n g l e of O0 r e p r e s e n t s a
`nozzle i n a forward t h r u s t p o s i t i o n and would n o t nonually be considered f o r
`I n
`r e v e r s e t h r u s t o p e r a t i o n except i n t h e eve:~t of a nozzle a c t u a t o r f a i l u r e .
`general, t h e e x l e t t e s t s showed t h a t t h e t o t a l p r e s s u r e recovery was high when
`t h e sharp t u r n t h e flow must make around t h e z x l e t l i p is considered. However,
`t e s t d a t a shown i n f i g u r e 2 a r e f o r smooth axisymmetric e x l e t s and c o n s t a u t f a n
`duct Mach number. Therefore, t h e e f f e c t s of d i f f e r e n c e s i n t h e s e c h a r a c t e r i s t i c s
`should a l s o b e considered.
`
`Exlet shapes with V notct.es which more a c c u r a ~ e l y r e p r e s e n t a v a r i a b l e
`a r c a nozzle were a l s o t e s t e d . These t e s t s showed t h a t f o r a c o n f i g u r a t i o n
`s i m i l a r t o t h e QCSEE nozzle t h e presence of notches would reduce recovery about
`0.5 percent ( r e f . 5 ) .
`
`The f a n duct Mach number has nn e f f e c t on recovery, but t o a l e s s e r e x t e n t
`than t h e free-stream v e l o c i t y ( r e f . 4 ) . For example, changing t h e d u c t Mach
`number from 0.4 t o 0.5 reduced recovery l e s s thqn 0.5 r e r c e n t .
`
`D i s t o r t i o n l e v e l s i n t h e f a n d u c t were a l s o measuzed. For t h e e x l e t geom-
`e t r i e s t e s t e d i n referelice 5 (except f o r t h e O0 f l a r e ) , t h e d i s t o r t i o n l e v e l s
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`.br
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`3
`
`were l e s s than 7 percept. Such l e v e l s were considered acceptable f o r a n engine
`like QCSEE.
`
`Core I n l e t Duct Performance
`
`Like t h e e x l e t , the core i n l e t duct o f f e r s a s i m i l a r sharp t u r n f o r t h e
`a i r t o negotiate. But i n terms of pressure l ~ s s , t h i s t u r n is more severe.
`The Mach number of t h e flow a t t h e beginning of t h e t u r n i s t h r e e o r four times
`t h a t f o r t h e e x l e t . Also, t h e f l o v must pass through t h e fan s t a t o r s and,
`depending on t h e core i n l e t design, t h e core i n l e t guide vanes. The l o s s e s i n
`the fan s t a t o r s a r e expected t o be low. However, t h e s e s t a t o r s impart a s w i r l
`t o t h e reverse flow which w i l l r e s u l t i n an unfavorable incidence angle on t h e
`core i n l e t guide vanes. This i n t u r n could r e s u l t i n more s i g n i f i c a n t losses.
`
`Core i n l e t recovery t e s t d a t a f o r two engine configurations a r e presented
`i n f i g u r e 3 from t e s t s described i n reference 6 and from an unpublished inves-
`t i g a t i o n by J. W. Schaefer of Cewis Research Center. The f i r s t engine con-
`f i g u r a t i o n shown ir f i g u r e 3 i s t h e f u l l - s i z e Q-fan T-55 engine and t h e second
`one shown i s a s c a l d moc;el (50.8-cm fan diameter) of t h e QCSEE engine. Both
`s e t s of d a t a show t h a t core--inlet t o t a l pressure recovery is a Cunction of f a n
`duct Mach cumber.
`
`The i m p o r t ~ n c e of core i n l e t recovery is shown by the c c r e l i m i t l i n e s on
`t h i s figure. These p o i n t s a r e operating conditions where f u r t h e r i n c r e a s e s in
`reverse t h r u s t l e v e l cannot be achieved without exceeding a core o p e r a t i ~ n a i
`t h e core operational l i m i t i s the compressor speed;
`l i m i t . For the Q-fan T-55,
`f o r the QCSEE engine, t h e calculated core l i m i t i s t h e t u r b i n e i n l e t temperd--
`t u r e .
`
`The s o l i d symbo1.s i n f i g u r e 3 show t h e point where the reqaired reverse-
`t h r u s t l e v e l i s obtained.
`In both cases the core recovery is adequate t o meet
`the required reverse-thrust l e v e l .
`
`A s can be seen from f i g u r e 3, both s e t s of d a t a a r e adequately represented
`by t h e same l o s s c o e f f i c i e n t l i n e of 1.5, even though the core i n l e t duct con-
`f i g u r a t i o n s a r e d i f f e r e n t . The Q-fan T-55 s p l i t t e r l i p i s more rounded than
`the sharp l i p of thf QCSEE model which would suggest higher l o s s e s f o r t h e
`the core i n l e t guide vanes of the Q-fan T-55 a r e located
`QCSEE model. Howe\:.r,
`i n t h e core i n l e t duct and a r e subject t o unfavorable incidence angles. The
`QCSEE core i n l e t guide vanes a r e e x t e r n a l t o t h e core duct which allows most of
`the core flow t o h y p ~ s s them i n reverse t h r u s t . Apparently, these configuration
`d i f f e r e n c e s have o f f s e t t i n g e f f e c t s which r e s u l t i n s i m i l a r l o s s characte.ris-
`t i c s .
`
`Distortion l e v e l s a t the compressor face were a l s o measured during t h e
`reverse-thrust t e s t s of the Q-fan T-55 and QCSEE models ( r e f s . 6 and 7). For
`t h e Q-fan T-55,
`the reverse-thrust d i s t o r t i o n l e v e l (conbined r a d i a l and c i r -
`cumferential) was about the same a s f o r the forward-thrust l e v e l . This unex-
`pected r e s u l t may be p a r t i a l l y a t t r i b u t e d t o t h e i n l e t guide vanes which a r e
`located i n the core i n l e t duct. This l o c a t i o n may help t o make t h e core flow
`more uniform. Results of QCSEE s c a l e model t e s t s indicated t h e r e v e r s e - t h r u s t
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`d i s t o r t i o n t o be h i g h e r than i n f o w a r d t h r u s t but a c c e p t a b l e f o r f u l l - s c a l e
`engine operation.
`
`FAN DESIGN AND OPERATION
`
`A b a s i c concern f o r t h e o p e r a t i o n of a v a r i a b l e - p i t c h f a n i s t h e d i r e c t i o n
`i n which t h e f a n b l a d e p i t c h should be changed t o develop r e v e r s e t h r u s t . The
`two p o s s i b l e ways a r e i l l u s t r a t e d i n f i g u r e 4.
`A c r o s s s e c t i o n of two f a n
`blades shown i n t h e i r normal forward-thrust p o s i t i o n is a t t h e t o p of t h i s
`f i g u r e . From t h i s p o s i t i o n , t h e b l a d e s can be turned through f l a t p i t c h , a
`c o n d i t i o n of zero l i f t , t o t h e r e v e r s e - t h r u s t p o s i t i o n a s shown on t h e l e f t of
`f i g u r e 4. Two t h i n g s should be noted f o r t h i s approach. F i r s t , a d j a c e n t b l a d e
`leading and t r a i l i n g edges must pass each o t h e r during t h e t r a n s i t i o n through
`f l a t p i t c h . T h i s r e q u i r e s t h a t t h e biade s o l i d i t y be l e s s than one a t a l l
`r a d i i . This can l i m i t f a n performance, e s p e c i a l l y a t t h e hub. Second, w h i l e
`t h e blade l e c d i n g edge remains t h e same r e l a t i v e t o t h e a i r f l o w , t h e b l ~ : e
`camber i s wrong f o r r e v e r s e - t h r u s t o p e r a t i o n .
`
`The a l t e r n a t e approach is t o t u r n t h e b l a d e s through f e a t h e r p i t c h , pass-
`i n g through a s t a l l condition. This is shown on t h e r i g h t s i d e of f i g u r e 4.
`I n thi.s case, b l a d e camber is c o r r e c t i n t h e r e v e r s e p o s i t i o n , but t h e l e a d i n g
`and t r a i l i n g edges a r e reversed,, During t h e t r a n s i t i o n t h e flow over t h e
`blades s e p a r a t e s o r stalls. The flow then r e a t t a c h e s i n r e v e r s e t h r u s t and
`moves i n t h e o p p o s i t e d i r e c t i o n r e l a t i v e t o t h e blade. With t h i s approach t h e
`blade s o l i d i t y may exceed one, although t h e bl.ade t w i s t and camber w i l l s t i l l
`l i m i t t h e hub s o l i d i t y t o some e x t e n t .
`
`I
`I
`)
`
`!
`
`Thrust
`
`To determine which approach is b e s t , both s t e a d y - s t a t e r e v e r s e t h r u s t per-
`formance and t r a t i s i e n t o p e r a t i n g c h a r a c t e r i s t i c s must be considered. A com-
`parison of s t a t i c r e v e r s e - t h r u s t l e v e l s a t nominal r e v e r s e - t h r u s t blade a n g l e s
`18 shown i n f i g u r e 5. The d a t a a r e from tests of t h e Q-fan T-55 and QCSEE
`s c a l e model (unpublished Lewis d a t a and r e f . 6 ) . Both t e s t s were conducted
`3 4 t h s i m i l a r f l i g h t - t y p e i n l e t s .
`I n a l l c a s e s t h e r e v e r s e - t h r u s t d a t a a r e pre-
`sented r e l a t i v e t o t h e design takeoff t h r u s t l e v e l . The design takeoff condi-
`t i o n , however, was never achieved i n t e s t s of t h e Q-fan T-55 due t o a c o r e
`horsepower l i m i t a t i o n . The f a n was designed f o r a higher horsepower model of
`t h e T-55 than what was t e s t 5 d . This horsepower l i m i t e d t o scme e x t e n t t h e
`maximum r e v e r s e t h r u s t a t t a i n e d . Reverse-thrust l e v e l s f o r t h e Q-fan T-55 a r e
`d i r e c t f o r c e measurements while r e v e r s e - t h r u s t l e v e l s f o r t h e QCSEE model a r e
`c a l c u l a t e d from measured p r e s s u r e s and temperatures.
`
`As shown i n t h i s f i g u r e , r e v e r s e - t h r u s t
`l e v e l s exceeding t h e 35-percent
`goal can be achieved through f e a t h e r p i t c h b e f o r e reaching t h e c o r e l i m i t i n g
`conditions. By comparison, r e b e r s e - t h r u s t l e v e l s through f l a t p i t c h a r e l e s s
`than h a l f of those through f e a t h e r p i t c h and, even a r t h e f a n lin;its, a r e con-
`s i d e r a b l y l e s s than t h e goal.
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`Noise
`
`Noise i s a n o t h e r f a c t o r t o c o n s i d e r when d e c i d i n g which way t o change f a n
`b l a d e p i t c h f o r r e v e r s e t h r u s t . Unsuppressed r e v e r s e - t h r u s t n o i s e level. d a t a
`(unpublished Lewis d a t a ) f o r t h e Q-fan T-55 a r e compared i n f i g u r e 6. The
`n o i s e d a t a show t h a t r e v e r s e through f l a t p i t c h i s a c o n s i d e r a b l y n o i s i e r way
`tl a c h i e v e r e v e r s e t h r u s t .
`
`T r a n s i e n t Performance
`
`The d a t a t h a t have been d i s c u s s e d s o f a r have a l l b e ~ n a t s t e a d y - s t a t e
`c o n d i t i o n s . C r i t i c a l t o which way b l a d e p i t c h should be changed and t o t h e
`whole i s s u e of a c h i e v i n g r e v e r s e t h r u s t w i t h a v a r i a b l e - p i t c h f a n engine i s t h e
`performance of t h e engine d u r i n g t h e forward- t o r e v e r s e - t h r u s t
`t r a n s i t i o n .
`T e s t s t o determine t r a n s i e n t performance have been conducted w i t h t h e Q-fan
`T-55 both a t Hamilton Standard and NASA Lewis t e s t f a c i l i t i e s (unpublished
`Lewis d a t a and r e f . 8). A photograph of t h i s engine a t NASA Lewis i s shcwn i n
`f i g u r e 7. The r e s u l t s of t h e s e tests a r e d i s c u s s e d by comparing a r e p r e s e n t a -
`t i v e example of a through f l a t p i t c h a d :'-rough f e a t h e r p i t c h t r a n s i e n t .
`
`Considering f i r s t r e v e r s e through f l a t p i t c h t r a n s i e n t s , time h i s t ~ r i e s
`For f a n b l a d e a n g i e , t h r u s t , f a n speed, and f a n b l a d e s t r e s s a r e shown i n f i g -
`u r e 8 f o r a r e p r e s e n t a t i v e t r a n s i t i o ~ ~
`from a l a t d i n g approach t o a r e v e r s e -
`I n t h i s f i g u r e t h e t r a n s i e n t i s i n i t i a t e d a t time e q u a l s
`t h r u s t c o n d i t i o n ,
`zero. The b l a d e p i t c h was changed a t a r a t e of about 100° per second s t a r t i n g
`from t h e design a n g l e i n forward t h r u s t and moving t o t h e r e v e r s e a n g l e , 80° i n
`t h e f l a t p i t c h d i r e c t i o n . The t h r o t t l e wa: held c o n s t a n t i n t h i s t r a n s i e n t .
`T h r u s t , presented a s a p e r c e n t of measured t a k e o f f t h r u s t , responds t o t h e
`b l a d e a n g l e change and f a l l s o f f smoothly. The f i n a l r e v e r s e - t h r u s t
`i e v e l i s
`reached i n somewhat l e s s than 1 second. The f a n speed d u r i n g t h i s time a c c e l -
`e r a t e s q u i c k l y a s t h e load i n t h e f a n b l a d e s i s reduced. A s t h e b l a d e l o a d i n g
`i n c r e a s e s a g a i n i n r e v e r s e t h r u s t , ths f s n speed p:.aks and t h e n converges on
`t h e f i n a l r e v e r s e - t h r u s t value. Fan b l a d e v i b r a t o r y s t r e s s e s g z a d u a l l y b u i l d
`up d u r i v ~ t h e t r a n s i e n t and reach a l e v e l s l i g h t l y over t w i c e t h a t i n forward
`t h r u s t . This l e v e l i s w e l l w i t h i n t h e limits of normal b l a d e design.
`
`The primary o p e r a t i o n a l problem cncountered i n t h e r e v e r s e through f l a t
`p i t c h t r a n s i e n t s is t h a t t h e f a n tends t o overspeed. There a r e two ways t o
`h e l p reduce t h i s e f f e c t . F i r s t , t h e t r a m i e n t can be i n i t i a t e d a t a reduced
`fan speed t o a l l o w more overspeed margin :s was done i n t h e example of f i g u r e '
`This could reduce t h e e n g i n e ' s forward-thrust
`r e s p ~ n s e time f o r waveoff manue-
`v e r s . Second, s t a r t i n g from a h i g h e r i n i t i a l f a n speed, t h e f u e l flow can
`i n i t i a l l y be c u t back i n an attempt t o reduce t h e a v a i l a b l e e l g i n e power w h i l e
`t h e fan blades p a s s through f l a t p i t c h . This r e q u i r e s c a r e f u l c o n t r o l of t h e
`f a n b l a d e p i t c h and engine t h r o t t l e d u r i n g t h e t r a n s i e n t t o reach t h e r e v e r s e
`b l a d e p o s i t i o n w i t h t h e f a n speed a t t h e d e s i r e d l e v e l .
`
`A somewhat d i f f e r e n t sequence of e v e n t s o c c u r s d u r i n g a r e v e r s e through
`f e a t h e r p i t c h t r a n s i e n t which i s shown i n f i g u r e 9. The t r a n s i e n t was i n i t i -
`a t e d from t h e same approach t h r u s t l e v e l a s t h e t r a n s i e n t i n f i g u r e 8 but a t
`
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`a higher f a n speed f o r b e t t e t waveoff response c a p a b i l i t y . The f a n b l a d e
`p i t c h was changed, i n t h e f e a t h e r p i t c h d i r e c t i o n , a t about 130° per second.
`T h i s change i n i t i a l l y i n c r e a s e s t h e aerodynamic l o a d s on t h e blades. A s t h i s
`happens, t h e f a n speed is lo!rer;?d a s t h e f a n r o t a t i o n a l energy i s converted
`i n t o a t h r u s t i n c r e a s e , The t h r o t t l e i n t h i s c a s e was immediately r e s e t t o t h e
`f i n a l r e v e r s e - t h r u s t l e v e l . As t h e b l a d e p i t c h c o n t i n u e s t o change, t h e f a n
`e v e n t u a l l y s t a l l s and t h e t h r u s t f a l l s suddenly t o zero. This unloads t h e
`blades t o some degree and causes t h e f a n speed t o i n c r e a s e . S h o r t l y a f t c r t h e
`b l a d e s r e a c h t h e i r xeverse p o s i t i o n , t h e flow r e a t t a c h e s and r e v e r s e t h r u s t is
`l e v e l i s reached about 1 second a f t e r the
`obtained. The f i n a l r e v e r s e - t h r u s t
`t r a n s i e n t was i n i t i a t e d .
`
`During t h e t r a n s i e n t , t h e f a n b l a d e s t r e s s e s b u i l d up and peak a s t h e
`b l a d e s t a l l s . A second peak, g e n e r a l l y somewhat h i g h e r than ,be f i r s t , o c c u r s
`as flow r e a t t a c h e s t o t h e b l a d e s i n t h e r e v e r s e d i r e c t i o n . These s t r e s s peaks,
`w h i l e high r e l a t i v e t o forward-thrust l e v e l s , d i d n o t l i m i t t h e t r a n s i e n t t e s t s
`of t h e Q-fan T-55.
`Even thougt t h e s e r e s u l t s a r e e.ncouraging, f u r t h e r t e s t s of
`h i g h e r p r e s s u r e r a t i o v a r i a b l e p i t c h f a n s , such a s QCSEE, a r e needed b e f o r e
`more g e n s r a l conclusions can be drawn-
`
`INLET BACKPRESSURE
`
`?: 1
`
`. .
`
`T e s t s of tho Q-fan T-55, a s w e l l a s t h e QCSEE s c a l e model, showed t h a t a
`. f l i g h t - t y p e i n l e t can produce a backpressure on t h e f a n which t e n d s t o prevent
`t h e establishment of r e v e r s e flow. This can occur when t h e f a n i s s t a r t e d from
`r e s t with t h e b l a d e s i n i z i a l l y i n a r e v e r s e p o s i t i o n o r , more i m p o r t a n t l y ,
`d u r i n g a f o r ~ a r d t o r e v e r s e t r a n s i e n t through f e a t h e r p i t c h . T h i s e f f e c t can
`be explained by n o t i n g t h a t when t h e fan is s t a l l e d , flow i n t h e d u c t i s p r i -
`marily t a n g e n t i a l and tends t o r o t a t e with t h e fan. When t h e f a n is u n s t a l l e d
`and producing r e v e r s e t h r u s t , t h e flow i s n e a r l y a x i a l . Photographs of t u f t s
`i~ t h e f a n i n l e t i n f i g u r e 10 show t h e s t a l l e d and u n s t a l l e d flow f i e l d s .
`
`I n o r d e r f o r t h e s w i r l i n g flow i n t h e s t a l l e d c o n d i t i o n t o be exhausted
`o u t t h e s m a l l e r diameter t h r o a t of t h e i n l e t , t h e flow v e l o c i t y must i n c r e a s e
`t o ccnserve a n g u l a r momentum.
`Since t h e s t a t i c p r e s s u r e a t t h e f r o n t of t h e
`i n l e t is ambient, a h i g h e r t h a n ambient p r e s s u r e a t t h e f a n f a c e is implled.
`The f a n must, t h e r e f o r e , avercomc t h i s backpressure t o c l e a r s t a l l . The magni-
`tude of t h e backpressure w i l l depend on t h e i n l e t geometry.
`
`Test d a t a showi.ng t h i s e f f e c t a r e presented i n f i g u r e 11 f o r t h e Q-fan
`T-55 a t a r e v e r s e through f e a t h e r b l a d e a n g l e . Wall s t a t i c p r e s s u r e s divided
`by ambient p r e s s u r e a r e compared f o r a bellmouth and a f l i g h t - t y p e i n l e t both
`i n s t a l l e d and u n s t a l l ~ d c o ~ d i t l o n s f o r t h e same f a n speed. Of primary i n t e r e s t
`i s t h e u t a t i c p r e s s u r e a t t h e f a n f a c e . A s can be seen from f i g u r e 11, a higher
`p r e s s u r e does e x i s t L L L ~ I t h e f l i g h t - t y p e i n l e t i n a s t a l l e d c o n d i t i o n . The
`n e a r i y i d e n t i c a l s t a t i c p r e s s u r e s f o r t h e two c o n f i g u r a t i o n s i n t h e u n s t a l l e d
`c o n d i t i o n demonstrated t h a t t h e i n l e t backpressure e f f e c t is due t o more than
`j u s t t h e 0r.e-dimensional d i f f e r e n c e i n t h r o a t a r e a s .
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`A technique t o overcome t h e e f f e c t of i n l e t backpressure and promote quick
`establishment of r e v e r s e flow was demonstrated during r e v e r s e through f e a t h e r
`t r a n s i e n t tests of t h e Q-fan T-55 (unpublished Lewis d a t a ) . With t h i s tech-
`nique, t h e fan blades a r e moved beyond the f i n a l r e v e r s e p o s i t i o n , held t h e r e
`f o r a shor:t period of time, and then returned. This temporarily reduces t h e
`angle of a t t a c k on t h e blades which allows r e v e r s e flow t o be e s t a b l i s h e d .
`This technique was shown t o be e f f e c t i v e without i n c r e a s i n g response time.
`
`CONCLUDING REMARKS
`
`The t e a t s conducted t o develop reverse-thrust technology f o r variable-
`p i t c h f a n engines have done much t o demonscrate t h e v i a b i l i t y of t h i s approach
`f o r powered-lift propulsion systems. More s p e c i f i c a l l y , these tests have shown
`the following:
`1. A f l a r e d erhaust nozzle is an acceptable reverse t h r u s t i n l e t .
`2. Acceptable core i n l e t duct recovery and d i s t o r t i o n l e v e l s i n r e v e r s e
`flow have been demonstrated.
`3. Adequate reverse-thrust l e v e l s can be achieved.
`4. Forward- t o reverse-thrust response times b e t t e r than t h e goal have been
`demonstrated without any s i g n i f i c a n t o p e r a t i o n a l problems.
`5. Thrust and noise l e v e l s strongly favor reverse thraugh f e a t h e r pitch.
`6. Flight-type
`i n l e t s make t h e establishment of r e v e r s e flow more d i f f i -
`c u l t , but moving the f a n blades beyond t h e i r normal r e v e r s e t h r u s t p o s i t i o n f o r
`a s h o r t period of time was e f f e c t i v e i n overcoming i n l e t backpressure.
`
`Areas where v a r i a b l e - p i t c h fan technology f o r r e v e r s e t h r u s t needs t o be
`expanded include t h e following :
`1. Effect of forward v e l o c i t y on t h e establishmerlt of r e v e r s e flow
`e s p e c i a l l y with f l i g h t - t y p e i n l e t s
`2. Fan blade s t r e e s l e v e l s f o r higher pressure f a n s during r e v e r s e through
`f e a t h e r t r a n s i e n t s
`3. A i r c r a f t i n s t a l l a t i o n e f f e c t s - flap-exlet I n t e r a c t i o n and r e v e r s e - j e t
`ground impingement
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`1. Neitzel, R. ; Lee, R. ; and Chamay, A. J. : QCSEE Task 2: Engine Installation
`Preliminary Design. (General Electric Co. ; NAS 3-16726. ) NASA CR-134738,
`1973. (FEDD dtstribution.)
`2. Helms, H. E. : Quiet Clean STOL Experimental Engine Study Program, Task 1 -
`
`Parametric Propulsion Systems Studies. (Detroit Diesel Allison; NAS 3-
`161 27.) NASA CR-135015, 1976. (FEDD distribution.)
`
`3. Howard, D. F.; et al.:
`Quiet Clean Short-Haul Experimental Engine, Prelimi-
`nary Under the Wing Flight Propulsion System Analysis Report. (General
`Electric Co.; NAS 3-15021.) NASA CR-134868, 1976. (FEI)D distribution.)
`
`4. Dietrich, Donald A,; Keith, Theo G.; and Kelm, Gary G.:
`Aerodynamic Per-
`formance of Flared Fan Nozzles Used as Inlets. NASA TM X-3367, 1976.
`
`5. Vier, W. F.:
`Quiet, Clean Short-Haul Experimental Engine (QCSEE) Test Re-
`sults from a 14 cm Inlet for a Variable Pitch Fan Thrust Reverser. (Gen-
`eral Electric Co.; NAS 3-18021.) NASA CR-134867, 1975. (FEDD distribution.)
`
`Quiet Clean Short-Haul Experimental Engine, Aero-
`6. Ciffin