Development of an active fly-by-wire flight control system

DEVELOPMENT OF AN ACTIVE FLY-BY-WIRE FLIGHT CONTROL SYSTEM Charles A . Anderson General Dynamics Corporation Convair Aerospace Division Fort Worth Operation S U M M A R Y This paper p r e s e n t s a summary of t h e YF-16 f l i g h t c o n t r o l system. The b a s i c f u n c t i o n s of t h e f l i g h t c o n t r o l system are discussed, as w e l l as t h e unique f e a t u r e s such as Relaxed S t a t i c

Longitudinal S t a b i l i t y (RSS) , Fly-By-Wire (FBW) , and S i d e - S t i c k

P i l o t ' s C o n t r o l l e r (SSC). I n a d d i t i o n , t h e b a s i c philosophy be- hind the s e l e c t i o n of t h e f l i g h t c o n t r o l system f u n c t i o n s and unique f e a t u r e s is discussed.

INTRODUCTION The YF-16 is t h e f i r s t aircraft developed i n which an Active F l i g h t Control System was incorporated from i t s i n c e p t i o n . In the p a s t , the design of a f l i g h t c o n t r o l system w a s undertaken after t h e b a s i c aircraft aerodynamic design w a s set and was used mainly t o improve handling q u a l i t i e s . This u s u a l l y involved l i t t l e more than augmenting p i t c h and l a t e r a l - d i r e c t i o n a l damping.

A s a i r c r a f t handling and performance requirements increased, s o d i d t h e complexity of t h e f l i g h t c o n t r o l system. The desire t o o b t a i n uniform aircraft response t o p i l o t commands r e s u l t s i n command augmentation systems being used i n t h e f l i g h t c o n t r o l system. S i n c e t h e s e systems r e q u i r e d l a r g e a u t h o r i t y s u r f a c e commands t o achieve t h e d e s i r e d response, t h e requirement f o r highly r e l i a b l e e l e c t r o n i c systems was generated and achieved.

The achievement of t h i s r e l i a b i l i t y has allowed the a p p l i c a t i o n Active Control System i n t h e YF-16.

of an SYMBOLS aerodynamic c e n t e r A.C.

normal a c c e l e r a t i o n A n d r a g c o e f f i c i e n t CD l i f t coef f f c i e n t CL l i f t of t h e wing body due t o a n g l e of a t t a c k L o L W B t o t a l l i f t of t h e wing-body-tail WBT l i f t of t h e t a i l due t o a n g l e of attack LaT l i f t of t h e t a i l due t o d e f l e c t i o n L8T left-hand LH M < 1 Mach less than one M > 1 Mach g r e a t e r than one MAC mean aerodynamic chord t o t a l p r e s s u r e PT s t a t i c p r e s s u r e PS RH right-hand RSS relaxed s t a t i c l o n g i t u d i n a l s t a b i l i t y S M s t a t i c margin t r a i l i n g edge T.E.

weight W a n g l e of attack s i d e s l i p a n g l e p i t c h rate h o r i z o n t a l t a i l d e f l e c t i o n DISCUSSION The design of f l i g h t c o n t r o l systems has evolved from p u r e l y mechanical t o active o v e r t h e p a s t two decades, as depicted i n Fig- u r e 1. The advent of high-performance a i r p l a n e s i n t h e mid-1950's t h a t were required t o o p e r a t e over larger performance envelopes n e c e s s i t a t e d t h e development of three-axis e l e c t r o n i c s t a b i l i t y augmentation systems. O r i g i n a l l y , t h e B-58 u t i l i z e d single-branch e l e c t r o n i c s i n i t s three-axis augmentation system. The following generation of a i r p l a n e s , e. g. , t h e F-111, employed t r i p l e - r e d u n d a n t e l e c t r o n i c s i n s t a b i l i t y and command augmentation system due t o t h e l a r g e r a u t h o r i t y requirements ' However, p i l o t mechanical c o n t r o l s were r e t a i n e d s o t h a t t h e aircraft could be flown s a f e l y i n the event of e l e c t r o n i c f a i l u r e s .

Limited F B W f u n c t i o n s w e r e incorporated i n t o c o n t r o l system such as t h e s p o i l e r s , t e r r a i n following r a d a r c a p a b i l i t y and l o w speed t r i m compensator on t h e F-111. I n a d d i t i o n , several spe- c i a l i z e d a i r p l a n e r e s e a r c h and test programs have used d u a l , t r i p l e and quadruple redundant e l e c t r o n i c s i n t h e i r c o n t r o l systems.

C - 1 4 1 , NASA F-8, and TWeaD programs.

These i n c l u d e t h e F-4 SFCS, Since only s i n g l e - f a i l u r e p r o t e c t i o n i s provided w i t h t r i p l e - redundant e l e c t r o n i c systems, an active c o n t r o l system must employ quadruple-r edundan t e l e c t r o n i c s t o provide the t w o - f a i l u r e pro tec - t i o n t h a t is r e q u i r e d . The development of a quadruple-redundant system has been a s t r a i g h t f o r w a r d and low-risk extension of the 10 years of highly s u c c e s s f u l t r i p l e - r e d u n d a n t e l e c t r o n i c a p p l i c a t i o n experience on t h e F-111 program and t h e quadruple-redundant experi- ence gained during the F-4 SFCS program.

T h e YF-16 Control System The f u n c t i o n s of the YF-16 f l i g h t c o n t r o l system are very s i m i l a r t o those of most o t h e r new high performance aircraft. The b a s i c f u n c t i o n s of t h e f l i g h t c o n t r o l system t h a t are common are

a i r d a t a scheduled g a i n s , s t a b i l i t y augmentation (dynamic) , i n t e r -

connects between r o l l and yaw a x i s and command augmentation. The unique f e a t u r e s and f u n c t i o n s of the f l i g h t c o n t r o l system are s t a t i c l o n g i t u d i n a l s t a b i l i t y augmentation (RSS) , minimum d i s p l a c e -

ment s i d e - s t i c k c o n t r o l l e r (SSC) , t o t a l Fly-By-Wire implementation

(FBW) and angle-of-attack and normal a c c e l e r a t i o n l i m i t i n g .

Why Relaxed S t a t i c S t a b i l i t y

For t h e primary design mission of t h e YF-16 - a i r s u p e r i o r i t y -

t h e importance of maneuverability and range r e s u l t s i n t h e RSS con- c e p t providing s u f f i c i e n t b e n e f i t s t o j u s t i f y i t s i n c o r p o r a t i o n , The b a s i c RSS concept can be stated i n a very simple way: Balance the a i r p l a n e f o r optimum performance 1.

2. Rely on t h e f l i g h t c o n t r o l system t o provide t h e d e s i r e d level of s t a t i c s t a b i l i t y as w e l l as dynamic c h a r a c t e r i s tics.

I l l u s t r a t i o n s of t h e d i f f e r e n c e s between a conventionally-balanced a i r p l a n e and an a i r p l a n e w i t h relaxed static s t a b i l i t y are given i n Figures 2 and 3 .

I n t h e subsonic f l i g h t regime (Figure 2) t h e conventionally- balanced a i r p l a n e i s shown t o have i t s wing-body l i f t a c t i n g f o r - ward of t h e c e n t e r of g r a v i t y and t h e t o t a l l i f t a c t i n g a f t of t h e c e n t e r of g r a v i t y , Since i n a s t a b l e system t h e moment produced by t h e wing-body l i f t as a f u n c t i o n of a n g l e of a t t a c k must be less than t h a t produced by t h e t a i l , t h e t a i l must be d e f l e c t e d i n a d i r e c t i o n t o reduce t h e t o t a l t a i l l i f t i n o r d e r t o trim t h e system. Therefore, t h e t o t a l trimmed l i f t a v a i l a b l e a t a given a n g l e of a t t a c k is reduced f o r a conventionally-balanced aircraft.

The RSS-balanced aircraft has both t h e wing-body and t h e t o t a l l i f t a c t i n g forward of t h e c e n t e r of g r a v i t y . I n t h i s case t h e moment produced by t h e wing-body l i f t as a f u n c t i o n of a n g l e of a t t a c k i s g r e a t e r than t h a t produced by t h e t a i l and t h e t a i l must be d e f l e c t e d i n a d i r e c t i o n t o i n c r e a s e t h e t o t a l t a i l l i f t i n o r d e r t o t r i m t h e system. Therefore, t h e t o t a l t r i m m e d l i f t avail- a b l e a t a given a n g l e of a t t a c k is increased f o r an RSS configuration a , In Figure 3 , t h e s a m e information i s shown f o r a supersonic f l i g h t c o n d i t i o n , I n t h i s case, b o t h t h e conventionally-balanced and RSS a i r p l a n e s have b o t h t h e wing-body and t o t a l l i f t a c t i n g a f t of t h e c e n t e r of g r a v i t y . Because t h e RSS a i r p l a n e has a f a r t h e r a f t c e n t e r of g r a v i t y than t h e conventionally-balanced a i r p l a n e , t h e down load on t h e t a i l r e q u i r e d t o t r i m t h e system is much smaller. Therefore, t h e RSS aircraft has a higher t o t a l l i f t a v a i l a b l e than a conventional balanced aircraft a t t h e same a n g l e

_-

of a t t a c k .

Now what t h i s a l l means is improved maneuverability and range.

R e pres en t a t ive tr i m requ ir emen ts f o r t h e c onven t ion a 11 y - ba l a n c ed and RSS c o n f i g u r a t i o n s are shown i n F i g u r e 4 f o r b o t h subsonic and supersonic Mach numbers. The b e n e f i t s t h a t are obvious from t h i s i l l u s t r a t i o n are: (1) h i g h e r trimmable l i f t c o e f f i c i e n t , and (2) lower t r i m d e f l e c t i o n s w i t h a t t e n d a n t drag r e d u c t i o n and lower t a i l loads.

The trimmed d r a g p o l a r s shown i n F i g u r e 5 are i l l u s t r a t i v e of t h e t r i m drag reduction a t t r i b u t a b l e t o t h e RSS balance. The reduced t r i m drag r e s u l t s i n higher s u s t a i n e d load f a c t o r s and increased range. Note t h a t t h e b e n e f i t s are most pronounced a t t h e higher l i f t c o e f f i c i e n t s , which i s an extremely important region f o r t h e YF-16. A secondary b e n e f i t of t h e RSS balance is a somewhat reduced weight because of reduced t a i l loads.

Why -F l y -By -W i r e ’ The d e c i s i o n t o employ t h e CCV concept of relaxed s t a t i c sta- b i l i t y (RSS) f o r t h e YF-16 brought w i t h it t h e r e s p o n s i b i l i t y f o r providing a r e l i a b l e , f u l l - t i m e - o p e r a t i n g , t h r e e - a x i s s t a b i l i t y and command augmentation system. Since a r e l i a b l e s t a b i l i t y and command augmentation system i s r e q u i r e d , adequate e l e c t r o n i c re- dundancy i s necessary t o f u l f i l l t h i s requirement. Therefore, t h e d e c i s i o n t o be made i s whether p i l o t commands should be t r a n s - m i t t e d via mechanical components ( l i n k a g e , b e l l c r a n k s , etc .) o r electrical s i g n a l paths e If mechanical components are chosen, e l e c t r i c a l components are s t i l l involved t o implement t h e command augmentation system. It follows then t h a t t h e r e t e n t i o n of mechan- i c a l components f o r transmission of p i l o t s t i c k commands i s u n j u s t - i f i a b l e , s i n c e an u n s t a b l e a i r p l a n e cannot be c o n t r o l l e d i n f l i g h t without t h e b e n e f i t of a f u l l - t i m e - o p e r a t i n g s t a b i l i t y and command augmentation system. Therefore, fly-by-wire (FBW) is a n a t u r a l out- growth of a redundant e l e c t r o n i c c o n t r o l system required f o r an augmentation system i n a n u n s t a b l e ( i . e e , RSS) a i r p l a n e .

An active c o n t r o l system o f f e r s f o u r b e n e f i t s which t h e YF-16 a i r p l a n e enjoys: (1) p r e c i s i o n c o n t r o l and optimum response; (2) design f l e x i b i l i t y , o f f e r i n g growth c a p a b i l i t y and easy acceptance o f design changes; (3) improvements i n a m a i n t a i n a b i l i t y and s u w i v - a b i l i t y as a r e s u l t of s i m p l i f i e d equipment i n s t a l l a t i o n s ; and ( 4 ) improved a i r p l a n e performance, s i n c e t h e i n t r o d u c t i o n of CCV concepts i s compatible w i t h FBW.

How The F l i g h t Control System B a s i c a l l y Works The YF-16 quadruple-redundant system employs f o u r inde- pendent s i g n a l branches, i . e . , each i n p u t s i g n a l source ( p i l o t , i n e r t i a l s e n s o r s , etc.) o r i g i n a t e s as f o u r s i g n a l s , designated Branches A , B, C , and D. This redundancy concept i s depicted f o r t h e p i t c h a x i s only i n Figure 6 . Each of t h e f o u r branches are processed independently i n t h e F l i g h t Control Computer. This com- p u t e r c o n t a i n s various f u n c t i o n s which modify i n p u t s i g n a l s from each of t h e t h r e e c o n t r o l axes, e.g. , c o n t r o l dynamics, s t r u c t u r a l f i l t e r s , gain-scheduling, s e l e c t o r s , power monitors, and various i n t e r c o n n e c t i n g e l e c t r o n i c c i r c u i t r y between t h e t h r e e c o n t r o l axes. Once t h e i n p u t s i g n a l s have been gain-adjusted, f i l t e r e d , are s e n t t o each of and a m p l i f i e d , t h e r e s u l t i n g o u t p u t signals t h e f i v e l a r g e - a u t h o r i t y , high-response, command servos. Each s e r v o , i n t u r n , drives i t s r e s p e c t i v e s u r f a c e power a c t u a t o r , as shown i n Figure 6. The b a s i c l o c a t i o n of t h e hardware components of t h e f l i g h t c o n t r o l system i s shown i n Figure 7.

I F l i g h t p a t h c o n t r o l is achieved through t h e a c t u a t i o n of an all-movable, d i f f e r e n t i a l h o r i z o n t a l t a i l f o r p i t c h and r o l l con- t r o l , wing-mounted f l a p e r o n s f o r r o l l c o n t r o l , and a conventional rudder f o r yaw c o n t r o l . Maneuver c a p a b i l i t y a t high angles of a t t a c k is enhanced by automatic p o s i t i o n i n g of t h e f u l l - s p a n lead- i n g edge f l a p .

Important Design Considerations The d e c i s i o n t o employ an active c o n t r o l system i n l i e u of a conventional c o n t r o l system r e q u i r e d t h e a d d r e s s i n g of several important design c o n s i d e r a t i o n s p e c u l i a r t o t h e s e systems. These i n c l u d e : e l e c t r o n i c c i r c u i t f a i l u r e monitoring, electrical power f a i l u r e s , engine f a i l u r e s command s e r v o s , s u r f a c e a c t u a t o r s , and branch s e p a r a t i o n . ,

When employing redundant e l e c t r o n i c systems , c o n s i d e r a t i o n

must be given t o t h e problem of proper signal s e l e c t i o n and f a i l - u r e monitoring. The F-111 a i r p l a n e u t i l i z e s t r i p l e - r e d u n d a n t e l e c t r o n i c s w i t h middle-value s i g n a l s e l e c t i o n . With more than

350,000 a i r c r a f t f l i g h t hours , t h e r e has

been only one known d u a l e l e c t r o n i c f a i l u r e experienced a (The p i l o t landed t h e a i r p l a n e without i n c i d e n t ) . With r e l i a n c e on demonstrated o p e r a t i o n a l ser- vice, t h e YF-16, quadruple-redundant system likewise u t i l i z e s middle-value s i g n a l s e l e c t i o n on t h e processed i n p u t commands _- (which r e s u l t from t h e f o u r s e p a r a t e e l e c t r o n i c branches) t h a t are ready f o r outputs t o t h e command servos. To i l l u s t r a t e , s i g n a l Branches, A, B, and C a r e compared. The m i d d l e value i s s e l e c t e d and then quadrupled so t h a t f o u r i d e n t i c a l s i g n a l s a r e a v a i l a b l e as output commands. I f , f o r example, signal Branch B v a r i e s a predetermined amount from t h e o t h e r t w o , then Branch D is sub- s t i t u t e d instantaneously f o r B. If one of these t h r e e subse- say A, then t h e minimum value s i g n a l of C o r D i s quently f a i l s , chosen. By u s i n g t h i s type of f a i l u r e monitoring and s i g n a l se- l e c t i o n , t h e c o n t r o l system i s protected a g a i n s t dual f a i l u r e s .

The system i s f u l l y protected a g a i n s t power l o s s e s . Multiple e l e c t r i c a l power sources are provided by an engine gear box-driven generator, a standby hydraulically-driven generator, and from m u l t i p l e b a t t e r y power as a l a s t source. The standby generator, h y d r a u l i c a l l y driven by e i t h e r the engine o r emergency power u n i t (EPU), i s automatically a c t i v a t e d i n t h e event of improper genera- t o r voltage o r frequency. I f b o t h generators a r e l o s t , t h e The end b a t t e r i e s provide approximately 10 minutes of power.

r e s u l t i s t h a t t h e system receives uninterrupted regulated power w i t h automatic o r manual power switching c a p a b i l i t y . In a d d i t i o n t o t h e above normal e l e c t r i c a l p r o t e c t i o n , f u r t h e r p r o t e c t i o n relative t o engine f a i l u r e i s provided by t h e EPU which auto- m a t i c a l l y p r o t e c t s a g a i n s t l o w hydraulic system pressure.

Another c o n s i d e r a t i o n which i s a b s o l u t e l y e s s e n t i a l t o t h e Successful o p e r a t i o n of an a c t i v e c o n t r o l system i s t h e conver- s i o n of electrical command s i g n a l s t o mechanical s i g n a l s f o r com- manding each s u r f a c e power a c t u a t o r . Each c o n t r o l s u r f a c e i s powered by a tandem valve-on-ram power a c t u a t o r . I n conventional a i r p l a n e s , p i l o t s t i c k and p e d a l inputs a r e summed mechanically w i t h t r i m a c t u a t o r and damper (stability-augmentation) servo i n - puts t o command each power a c t u a t o r ' s valve through conventional linkage. I n t h e YF-16 a c t i v e c o n t r o l system, t h e inputs are summed e l e c t r i c a l l y and fed t o a command (secondary) servo which provides a mechanical input t o a power a c t u a t o r ' s valve through a very s h o r t linkage run, as indicated i n Figures 6 and 7.

Why Side S t i c k C o n t r o l l e r When t h e d e c i s i o n w a s made t o adopt t h e fly-by-wire f e a t u r e of t h e c o n t r o l system, t h e door w a s opened f o r simple implementa- t i o n of any one of a number of new p i l o t - c o n t r o l l e r concepts.

Should t h e c o n t r o l s t i c k be r e t a i n e d i n t h e conventional c e n t e r - l o c a t i o n o r would it be more effective on t h e s i d e ? Should it be a displacement s t i c k o r a f o r c e - s e n s i n g s t i c k ? With t h e s e ques- t i o n s i n mind, several s t u d i e s and r e s e a r c h programs were under- taken _ t o determine t h e b e s t s o l u t i o n f o r t h e YF-16.

A f t e r r e s e a r c h i n g SSC i n s t a l l a t i o n s t h a t had previously been tested cn such aircraft as t h e B-47, B-26, B-58, F-4, F-8, F-104, F-105, F-106, A-4, A-6, A - 7 , X-15 and o t h e r s , General Dynamics b u i l t a f l i g h t c o n t r o l s i m u l a t o r t o check o u t ideas and designs.

A number of c e n t e r - s t i c k and s i d e - s t i c k hand c o n t r o l l e r designs were evaluated i n a f l i g h t c o n t r o l s i m u l a t o r . Included i n t h e s e were f inger-type c o n t r o l l e r s palm c o n t r o l l e r s , conventional g r i p s

w i t h unconventional axes of r o t a t i o n , and force-sensing c o n t r o l l e r s

w i t h both low and high f e e l - f o r c e s . The s t u d i e s and e v a l u a t i o n s showed t h a t t h e f o r c e - s e n s i n g , s i d e - s t i c k c o n t r o l l e r w a s s u p e r i o r t o a l l of t h e o t h e r approaches, i n c l u d i n g displacement and f o r c e - s i d e s t i c k s .

s e n s i n g c e n t e r s t i c k s and displacement-type The most widely recognized advantages of t h e f o r c e - s e n s i n g s i d e - s t i c k c o n t r o l l e r are: (1) improved high g t r a c k i n g (based on r e s u l t s from t h e NASA Langley dual-mode s i m u l a t o r and t h e N A S A

bly-by-wire F-8 aircraft) , (2) improved access t o t h e instrument

panel and increased panel area, (3) ease of implementation of p i l o t i n p u t s i n t h e computer ( e l e c t r i c a l s i g n a l s p r o p o r t i o n a l t o s t i c k f o r c e ) , and (4) p i t c h and r o l l axes b e t t e r o r i e n t e d t o t h e p i l o t ' s a r m and shoulder muscles.

The fly-by-wire a s p e c t of t h e f l i g h t c o n t r o l system i s p a r - t i c u l a r l y compatible w i t h a force-sensing c o n t r o l l e r . Advantages of t h i s combination i n c l u d e : (1) no linkage dynamics o r f r i c t i o n f e l t a t t h e c o n t r o l l e r , (2) no linkage balancing problems, (3) enhanced sys t e m s u r v i v a b i l i t y , ( 4 ) g r e a t e r freedom i n airframe design ( i n c l u d i n g ease of change), and (5) p o t e n t i a l f o r weight and c o s t r e d u c t i o n .

The p i l o t ' s c o n t r o l l e r shown i n Figure 8 i s a f o r c e - s e n s i n g (minimum d e f l e c t i o n ) , s i d e s t i c k , mounted on and extending above t h e right-hand console. The l o c a t i o n w a s developed t o ensure easy access f o r t h e 5 t h through 95th p e r c e n t i l e p i l o t . An a d j u s t a b l e a r m support i s provided t o enhance p i l o t c o n t r o l . The a r m support adjustments are vertical, f o r e and a f t , and tilt. The f o r c e - s e n s i n g element, which c o n t a i n s quadrex transducers i n both t h e p i t c h and r o l l axes is i d e n t i c a l t o t h e s t i c k - s e n s i n g u n i t em- ployed i n t h e A-7 aircraft, except f o r t h e l e v e l of redundancy s i n c e t h e r e is a l s o mechanical linkage. The s e n s i n g element has been adapted t o an F-111 g r i p .

The p i l o t introduces p i t c h and r o l l commands by applying a p p r o p r i a t e forces t o t h e s t i c k . The f o r c e s imparted t o t h e s t i c k by t h e p i l o t cause electrical signals t o be produced by t h e t r a n s - ducers located i n t h e lower p o r t i o n of t h e s t i c k ; t h e s e s i g n a l s are i n p u t t o t h e f l i g h t c o n t r o l computer. The t r i m button on t h e t o p of the stick g r i p allows the convenient and conventional i n p u t of p i t c h and r o l l t r i m commands. Other s t i c k g r i p switches are provided t o c o n t r o l elements of t h e armament system, nose-wheel s t e e r i n g , and aerial r e f u e l i n g .

Why Angle-of -Attack and Normal Acceleration Limiting Since by d e f i n i t i o n an a i r s u p e r i o r i t y aircraft i s highly maneuverable over i t s e n t i r e o p e r t i n g envelope, t h e r e are areas i n which it i s easy t o o b t a i n l a r g e values of a n g l e - o f - a t t a c k o r normal a c c e l e r a t i o n . There are several ways t h a t t h e p i l o t can be protected a g a i n s t such occurrences r a t h e r than r e q u i r i n g him t o spend h i s t i m e looking a t c o c k p i t instruments. One of t h e s e ways i s t o b u i l d i n t h e r e q u i r e d p r o t e c t i o n during a i r c r a f t d e s i g n by p u t t i n g on l a r g e enough aerodynamic s u r f a c e s ( i . e . , b i g v e r t i - c a l t a i l ) and enough s t r u c t u r a l weight t o a s s u r e that t h e p i l o t cannot s p i n o r break t h e a i r c r a f t , no matter what he does w i t h the s t i c k . As you might surmise, t h i s approach would s e v e r e l y p e n a l i z e t h e aircraft's b a s i c performance from a weight and d r a g s t a n d p o i n t .

Another method t o p r o t e c t t h e p i l o t is t o b u i l d i n enough aerodynamic r e s i s t a n c e t o s t a l l throughout t h e u s a b l e angle-of- a t t a c k range and enough s t r u c t u r a l weight t o o b t a i n t h e r e q u i r e d a 1.5 s a f e t y f a c t o r and depend on t h e p i l o t t o keep t h e "g" plus a i r c r a f t w i t h i n l i m i t s . The t h i r d method i s t o u s e t h e f l i g h t c o n t r o l system t o l i m i t angle-of - a t t a c k and normal a c c e l e r a t i o n which r e s u l t s i n t h e l i g h t e s t , b e s t performing a i r c r a f t , b u t a very complex c o n t r o l system.

For t h e YF-16 w e chose t o u s e a combination of methods two and t h r e e which r e s u l t e d i n an a i r c r a f t w i t h e x c e l l e n t performance c h a r a c t e r i s t i c s w i t h a minimum of complication i n t h e f l i g h t con- t r o l system. Using t h e above approach, i . e . , minimum s i z e s u r - faqes and s t r u c t u r a l weight combined w i t h angle-of - a t t a c k and nor- m a l a c c e l e r a t i o n l i m i t i n g , has r e s u l t e d i n a h i g h performance f i g h t e r type aircraft which t h e p i l o t may t r u l y maneuver w i t h "Complete Abandon. 'I . I YF-16 F l i g h t T e s t S t a t u s Thirty-one f l i g h t s have been made by YF-16 N o . 1 between 2 February and 13 A p r i l 1974 a c c r u i n g 33:45 t o t a l f l i g h t t i m e w i t h 1:39 being supersonic. S i x p i l o t s (2 c o n t r a c t o r , 2 AFFTC and 2 TAC) have flown t o d a t e w i t h USAF p i l o t s making t h e i r f i r s t f l i g h t s on f l i g h t Nos. 4, 1 2 , 16 and 28.

P i l o t acceptance of t h e advanced technology i t e m s , such as s i d e s t i c k c o n t r o l w i t h f o r c e i n p u t s , fly-by-wire f l i g h t c o n t r o l s w i t h relaxed l o n g i t u d i n a l aerodynamic s t a b i l i t y and maneuvering are Leading edge f l a p s , has been e n t h u s i a s t i c . Typical comments I 1 performance and a g i l i t y e x c e p t i o n a l , e a s i l y and p r e c i s e l y con- t r o l l a b l e , impressive r o l l response w i t h almost immediate s t o p a t release of s t i c k , comfortable and enjoyable t o f l y immediately, no d i f f i c u l t y experienced i n a d a p t i n g t o t h e s i d e s t i c k c o n t r o l l e r . " Confidence i n t h e redundant active c o n t r o l system had been s o f i r m l y e s t a b l i s h e d during s i m u l a t i o n , ground tests and checkouts, t h a t a l l f l i g h t s ( i n c l u d i n g takeoff and landing) have been made i n a s t a t i c a l l y u n s t a b l e c o n f i g u r a t i o n w i t h t h e normal c . g . f o r a11 f l i g h t s t o d a t e being 36%% MAC ( a i r c r a f t aerodynami- c a l l y u n s t a b l e i n p i t c h a t subsonic and t r a n s o n i c c o n d i t i o n s ) .

Some of the s i g n i f i c a n t i t e m s demonstrated t o d a t e include: \ 1. Level f l i g h t a c c e l e r a t i o n t o Mach numbers i n excess of 1.6 2. Wind-up t u r n s t o 7+ g ' s a t subsonic and supersonic speeds F l i g h t t o a n g l e s of a t t a c k of 2 2 O a t low sub- 3 .

s o n i c speeds and 18' a t high subsonic speeds, and 9' s i d e s l i p .

Conclusions and Remarks Although t h e YF-16 f l i g h t c o n t r o l system J e p r e s e n t s another i n a long l i n e of advanced c o n t r o l system concepts, i t s implemen- t a t i o n has been accomplished u s i n g c u r r e n t state of t h e a r t tech- niques and hardware. The r e l i a b i l i t y of t h e hardware t o d a t e has 66 H-904 been exceptional as w e l l as the p i l o t ' s acceptance of the system.

The f l y i n g q u a l i t i e s and performance o f the f l i g h t control system have been outstanding and w e feel have provided the A i r Force with / a n outstanding air superiority f i g h t e r prototype.

ELECTRON I CS I 8, SERVOS Figure 1 FLIGHT CONTROL SYSTEM EVOLUTION

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T.E. UP CONVENT I ONAL T.E. DOWN I 1T.E. DOWN CL CL Figure 4 REDUCED TRIM REQUIREMENTS M>1 c =15% )* CL CONVENTIONAL -CONVENTIONAL: CD Figure 5 MANEUVERABILITY IMPROVEMENT Figure 6 P I T C H AXIS REDUNDANCY CONCEPT FLAPERON COMMAND PILOT'S CONTROLLER (SIDE STICK) AL TAIL SERVOS RUDDER PEDAL ASSY Figure 7 FLY-BY-WIRE F L I G H T CONTROLS Figure 8 CREW STATION ARRANGEMENT