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HANDLING QUALITIES REQUIREMENTS FOR CONTROL CONFIGURED VEHICLES R . J . Woodcock and F . L. George A i r F o r c e Flight Dynamics Laboratory SUMMARY The r a p i d emergence of fly-by-wire and control-configured v e h i c l e concepts challenges us t o account adequately f o r t h e i r p o t e n t i a l e f f e c t s on f l y i n g q u a l i t i e s . F a i l u r e mode p r o b a b i l i t i e s and consequences must be considered.
Adequate c o n t r o l l a b i l i t y must be provided f o r aerodynamically unstable air- c r a f t a t extreme f l i g h t conditions. Nonclassical o v e r a l l dynamics of highly augmented a i r c r a f t create t h e need f o r new approaches t o specifying design criteria. New c o n t r o l modes such as d i r e c t f o r c e r e q u i r e d e f i n i t i o n of bound- aries f o r usefulness as w e l l as d e s i r a b i l i t y . These considerations are being incorporated i n t h e continuing e f f o r t a t t h e AF.Flight Dynamics Laboratory t o review and revise t h e formal military f l y i n g q u a l i t i e s requirements. This paper w i l l review t h e r a t i o n a l e and present c u r r e n t r e s u l t s addressing t h e above considerations with regard t o M i l i t a r y S p e c i f i c a t i o n MIL-F-8785B, "Flying Q u a l i t i e s of P i l o t e d Airplanes".
INTRODUCTION Recently w e were asked t o clear f o r f l i g h t t e s t i n g an a i r p l a n e which, with- out added b a l l a s t , w a s p r e d i c t e d t o be somewhat u n s t a b l e i f t h e s t a b i l i t y aug- mentation system (SAS) should f a i l . Considering t h e expected degree of inac- curacy i n aerodynamic and r e l i a b i l i t y p r e d i c t i o n s , w e recommended p u t t i n g t h e c e n t e r of g r a v i t y somewhat forward of t h e SAS-off maneuver point--where s t i c k f o r c e and d e f l e c t i o n p e r g go t o zero. Contrary t o t h e MIL-F-8785B requirement, w e did not f e e l compelled t o i n s i s t on a c.g. l o c a t i o n t h a t would a s s u r e static speed s t a b i l i t y .
"What?", our Laboratory Deputy Director asked. "Here we've put so much of our resources i n t o developing control-configured v e h i c l e s t o t o l e r a t e relaxed s t a t i c s t a b i l i t y , and now you tell m e a l l t h a t refinement i s n ' t neces- sary--you say a p l a i n unaugmented a i r p l a n e can f l y t h a t way s a f e l y . Have w e wasted a l l t h a t t i m e and money?"
Well, t h e r e is more t o CCV than t h a t i n s e v e r a l dimensions, including t h e degree of allowable bare-airframe i n s t a b i l i t y .
But h e had made a v a l i d point one t h a t has bothered some of us a l l along. W e know through observation t h a t p i l o t s can c o n t r o l a moderately u n s t a b l e v e h i c l e i n t h e r i g h t circumstances.
Haven't h e l i c o p t e r s been f l y i n g f o r a long time--and unstable a i r p l a n e s too!
Quoting Amos Root's observations of t h e Wright b r o t h e r s ' experiments a t t h e Huffman P r a i r i e i n t h e summer of 1904 , "When I f i r s t s a w t h e apparatus it p e r s i s t e d i n going up and down l i k e t h e waves of t h e sea. Sometimes it would d i g its nose i n t o t h e d i r t , almost i n s p i t e of t h e engineer. A f t e r repeated experiments it w a s f i n a l l y cured of its f o o l i s h tricks, and w a s made t o go l i k e a steady old horse. This work, mind you, w a s a l l new. Nobody l i v i n g could give them any advice. It w a s l i k e exploring a new and unknown domain.
S h a l l I t e l l you how they cured it of bobbing up and down?
Simply by loading its nose o r f r o n t steering-apparatus with cast i r o n . I n my ignorance I thought t h e engine w a s n o t l a r g e enough; b u t when f i f t y pounds of i r o n w a s fastened t o its 'nose' (as I w i l l p e r s i s t i n c a l l i n g i t ) , it came down a t o l e r a b l y s t r a i g h t l i n e and c a r r i e d t h e burden with ease. There w a s a reason f o r t h i s t h a t I cannot explain here... Over one hundred f l i g h t s have been made during t h e p a s t summer. Some of them reached perhaps 50 o r 60 f e e t above t h e ground. On both t h e s e long t r i p s seventy pounds i n s t e a d of f i f t y of cast i r o n w a s c a r r i e d on t h e 'nose'."
O r read Maj. Gen. Benjamin D. Foulois' account2 of h i s experience a t F t . Sam Houston i n 1910 as t h e U . S . Army's a i r p l a n e p i l o t : "We wanted t o develop t h e a i r p l a n e i n t o a s t a b l e platform f o r a i r reconnaissance work. Old Number One w a s t h e last of t h e K i t t y Hawk models, and with its two e l e v a t o r s out i n f r o n t i t w a s about as s t a b l e as a bucking bronco. W e con- tinued experimenting t h e r e while t h e Wright b r o t h e r s made modifications back at Dayton, Ohio. When one of t h e e l e v a t o r s up f r o n t w a s moved around t o t h e back, s t a b i l i t y improved somewhat b u t not enough. I later found out t h a t by using j u s t one e l e v a t o r , t h e rear one, I had a platform t h a t worked very w e l l . I could l e t go of t h e levers and make notes and It got t o b e a n a i r p l a n e t h a t could b e used f o r sketches.
real m i l i t a r y reconnaissance."
Charles Gibbs-Smith w r i t e s "So when t h e Wrights b u i l t t h e i r f i r s t g l i d e r i n 1900 it i d e a s which t h e b r o t h e r s w e r e t o u t i l i s e incorporated two throughout t h e i r e a r l y work--the i n t e n t i o n a l l y unstable aero- plane which could b e k e p t f l y i n g s a t i s f a c t o r i l y only by t h e p i l o t ' s s k i l l , and t h e warping of t h e wings f o r c o n t r o l i n r o l l . ' W e t h e r e f o r e resolved', wrote Wilbur, ' t o t r y a fundamentally d i f f e r e n t p r i n c i p l e . W e would arrange t h e machine s o t h a t it would not tend t o r i g h t i t s e l f . " ' This w a s t r u l y i n s t a b i l i t y , as w e have seen from t h e preceding accounts. It w a s e x a c t l y t h a t concept of i n s t a b i l i t y - - t o a manageable degree--that l e d Lilienthal, Chanute and t h e Wrights t o succeed where t h e "chauffeurs" of highly s t a b l e a i r p l a n e s could not achieve c o n t r o l l e d f l i g h t . But t h e e a r l y f l i e r s had a r a t h e r high accident rate which must b e a t t r i b u t e d i n p a r t t o t h e v e h i c l e ' s i n s t a b i l i t y . Our t o l e r a n c e today may b e less, even f o r emergencies, considering t h e higher speeds and poor weather t o which our f l y i n g now is sub j ect.
Even t h e Wrights soon recognized t h e need f o r improvement. I n a d d i t i o n t o b a l l a s t i n g f o r a forward c.g. and moving t h e canard s u r f a c e t o t h e t a i l i n order t o move t h e n e u t r a l s t a b i l i t y p o i n t a f t , they a l s o i n v e s t i g a t e d automa- t i c mean&. It is i n t e r e s t i n g t o n o t e t h a t t h e i r Patent No. 2913 f o r automa- t i c s t a b i l i z a t i o n preceded Gen. Poulois' rearranging t h e c o n t r o l s u r f a c e s with t h e Wrights' help. For p i t c h , "a pivoted vane a c t i n g under t h e i n f l u e n c e of wind pressure" sensed angle of a t t a c k t o c o n t r o l a supply of compressed air which actuated t h e e l e v a t o r . A pendulum w a s s p e c i f i e d "for lateral control".
Operation of t h e s e devices would n o t move t h e p i l o t ' s c o n t r o l levers. I n 1914 Orville Wright w a s awarded t h e C o l l i e r Trophy f o r h i s work on automatic s t a b i l i z a t i o n .
BACKGROUND OF CURRENT ACTIVITY I Why, then, have w e been less w i l l i n g i n r e c e n t t i m e s t o accept i n s t a b i l i t y , A number of reasons, each with some degree of v a l i d i t y , even f o r emergencies?
have l e d t o t h i s conservatism: U n t i l r e c e n t t i m e s t h e f a i l u r e rates of s t a b i l i t y augmenta- t i o n equipment gave t h e expectation of f r e q u e n t l y experiencing t h e basic-airframe c h a r a c t e r i s t i c s . Greater redundancy w a s not a t t r a c t i v e because of t h e increased c o s t and t h e maintenance burden t o keep it a l l operating.
L i t t l e i s y e t known about t h e cumulative e f f e c t s of several poor f l y i n g q u a l i t i e s together, except t h a t an a i r c r a f t t h a t i s s a f e with any one "unacceptable" q u a l i t y can become u n f l y a b l e with some combinations of t h e s e c h a r a c t e r i s t i c s . Further, a number of p l a u s i b l e s i n g l e and m u l t i p l e f a i l u r e s can degrade several handling q u a l i t i e s . Loss of j u s t t h e p i t c h axis of augnentation, f o r example, could degrade damping, frequency, maneuvering f o r c e g r a d i e n t s , f r i c t i o n and backlash. A p i l o t - induced-oscillation could not be stopped by clamping t h e c o n t r o l s t i c k i f d 6,/d n, is unstable.
Viable designs have g e n e r a l l y been possible. with b a s i c a l l y s t a b l e airframes--at least f o r conventional a i r p l a n e s .
L i t t l e experience h a s been obtained t o d e f i n e i n s t a b i l i t y boundaries s u i t a b l e f o r t h e speeds, t a s k s and weather t h a t are now commonly encountered i n operating a i r c r a f t .
From t h e d a t a c o l l e c t e d f o r MIL-F-8785B5 t h e t o l e r a b l e amount of t h e d a t a are i n s u f f i - i n s t a b i l i t y is a f u n c t i o n of t o t a l damping; c i e n t , however, t o draw a v a l i d requirement. "After studying t h e a v a i l a b l e d a t a , it is obvious t h a t many f a c t o r s i n f l u e n c e t h e amount of i n s t a b i l i t y which can b e handled. Because even a small i n s t a b i l i t y can b e q u i t e dangerous under some circum- s t a n c e s , it w a s decided t o r e q u i r e t h e a i r p l a n e t o b e statically s t a b l e even f o r Level 3."
W e need t o reexamine t h e s e conservative requirements i n order t o provide more guidance on t h e circumstances and amounts i n which i n s t a b i l i t y i s s a f e . W e those present.
s o l i c i t t h e opinions of MIL-F-8785B AND CCV'S I n developing MIL-F-8785B w e gave much thought t o t h e conditions f o r allowing degraded f l y i n g q u a l i t i e s . W e wanted t o account as much as p o s s i b l e f o r real-world problems without overly complicating t h e requirements. Two causes of degradation w e r e considered. Flying q u a l i t i e s giving less perform- ance o r r e q u i r i n g more p i l o t a t t e n t i o n are allowed o u t s i d e t h e m i l i t a r y - s p e c i f i e d Operational F l i g h t Envelopes. This allows some c a p a b i l i t y f o r adapt- i n g t o changes i n mission without unduly p e n a l i z i n g a design f o r having a l a r g e r f l i g h t envelope than required. After r e l a t i v e l y infrequent f a i l u r e s (nominally once per hundred f l i g h t s ) t h i s s a m e l e v e l of degradation, Level 2, i s allowed i n t h e Operational F l i g h t Envelope, and f u r t h e r degradation is allowed o u t s i d e those boundaries. Only r a r e l y (once i n 10,000 f l i g h t s ) is degradation beyond Level 2 allowed i n t h e Operational F l i g h t Envelope. I n any case Level 3 is a r e l a t i v e l y s a f e f l o o r . Degradation beyond Level 3 r e q u i r e s s p e c i a l consideration on a case-by-case b a s i s , thus i n p r i n c i p l e giving t h e procuring a c t i v i t y t h e power of decision. The S p e c i a l F a i l u r e States which are s u b j e c t t o t h i s approval are of s e v e r a l categories. I n some cases other s p e c i f i c a t i o n s o r design p r a c t i c e s give acceptable assurance: t h e b a s i c air- c r a f t s t r u c t u r e is a common r e l i a b i l i t y standard. I n o t h e r cases judgment must b e used t o e s t a b l i s h a p o i n t of diminishing r e t u r n s : two, o r t h r e e o r four h y d r a u l i c systems are used t o power e s s e n t i a l f l i g h t c o n t r o l s , f o r example.
There a l s o w i l l b e cases i n which f a i l u r e is expected t o b e extremely remote i n p r o b a b i l i t y , b u t t h e c o s t of a change o r a d d i t i o n t o preclude t h e f a i l u r e o r l i m i t its e f f e c t is small enough t o warrant disapproval of a S p e c i a l F a i l u r e S t a t e . I n s t i l l o t h e r cases approval may b e granted i f s p e c i a l design o r test requirements are m e t .
Despite an occasional opinion t o t h e contrary, MIL-F-8785B does apply t o CCV's - as f a r as t h e s p e c i f i c a t i o n goes. Although t h e 8785B treatment of response t o atmospheric disturbances i s weak i n general, c l e a r l y t h e require- ments and t h e Level s t r u c t u r e apply t o conventional s t a b i l i t y and c o n t r o l augmentation. The S p e c i a l F a i l u r e S t a t e s provide a mechanism "to a s s u r e t h a t t h e f l i g h t s a f e t y , f l y i n g q u a l i t i e s and r e l i a b i l i t y a s p e c t s of dependence on s t a b i l i t y augmentation and o t h e r forms of system complication w i l l b e con- s i d e r e d f u l l y " . The l i m i t a t i o n s f o r CCV a p p l i c a t i o n are a l a c k of requirements on d i r e c t f o r c e c o n t r o l , and t h e expression of many requirements i n terms of classical modal parameters. I would l i k e t o evoke d i s c u s s i o n of t h e s e matters now, a t t h i s meeting.
Thrust/speed brake requirements were considered b u t omitted as beyond t h e scope of t h e s p e c i f i c a t i o n . W e are having second thoughts on t h a t now, and w i l l t r y t o arrange with t h e propulsion people f o r adequate coverage somehow between t h e two d i s c i p l i n e s . A l a c k of experience w i t h d i r e c t l i f t or s i d e - f o r c e c o n t r o l l e r s still precludes d e f i n i t i v e requirements f o r those c o n t r o l modes--despite t h e Japanese' s u c c e s s f u l use of a n automatic maneuvering f l a p i n air combat i n 1943; on t h e outstanding Kawanishi Shiden (George) fighter.6 THE FOR34 OF DYNAMIC RJ3QUIREMENTS / Reference t o short-period, dutch-roll, etc. modes is not as much a hindrance t o CCV a p p l i c a t i o n as one might f i r s t suspect. The i d e a , of course, is t o state t h e requirements i n a formwe are f a m i l i a r with, i n terms c o n s i s t e n t with t h e a i r c r a f t c h a r a c t e r i s t i c s t h a t form t h e d a t a base. Conventional sta- b i l i t y augmentation modifies t h e parameters but not t h e form of t h e response.
Recent f l i g h t c o n t r o l system designs, however, show a tendency t o introduce a d d i t i o n a l dynamic modes a t frequencies on t h e order of t h e a i r c r a f t response frequencies, giving rise t o o v e r a l l motions u n l i k e t h e conventional response.
A'Harrah7, f o r one, h a s pointed out t h e d i f f i c u l t y i n a s s o c i a t i n g short-period requirements with a p a r t i c u l a r p a i r of poles on a r o o t locus. Nevertheless it i s o f t e n p o s s i b l e t o f i n d a n equivalent classical a i r c r a f t which matches t h e response of a more complicated dynamic system reasonably w e l l over a s u i t a b l e t i m e period o r frequency range. Then it should b e v a l i d t o compare those equivalent parameters with modal requirements. W e r e a l i z e t h e need f o r a more generally a p p l i c a b l e a l t e r n a t i v e and hope t o do b e t t e r , a t least with longitu- d i n a l requirements, i n our c u r r e n t r e v i s i o n e f f o r t .
A l t e r n a t i v e l o n g i t u d i n a l requirements are being i n v e s t i g a t e d which should b e more generally a p p l i c a b l e , but a t f i r s t t h e s e w i l l seem t o b e of less d i r e c t use t o t h e airframe designer. One p Q s s i b i l i t y i s Neal and Smith's closed-loop criteria which u t i l i z e p i l o t - v e h i c l e a n a l y s i s with a s p e c i f i e d p i l o t d e s c r i b i n g f u n c t i o n and parameter adjustment r u l e s . Other p o s s i b i l i t i e s , semi-empirical in o r i g i n , involve p r o p e r t i e s of t h e open-loop Bode phase angle v s frequency curve. I d e a l l y a requirement should apply t o a l l of: The complete a i r p l a n e a t t i t u d e response including a l l p e r t i n e n t modes (e.g., both phugoid and s h o r t period) The a i r p l a n e p l u s f l i g h t c o n t r o l system (i.e., including l a g s and t i m e delays) The various c o n t r o l element forms r e s u l t i n g from c u r r e n t f l i g h t c o n t r o l augmentation concepts The b a s i c inner a t t i t u d e response f e a t u r e s which are necessary r e g a r d l e s s of outer-loop c o n t r o l problems o r a u x i l i a r y c o n t r o l (e.g., d i r e c t l i f t ) Variations i n p i l o t c o n t r o l technique (e.g., closed- loop bandwidth) with c o n t r o l task o r f l i g h t phase.
(adapted from Ref, 9).
CURRENT ACTIVITY REGARDING LONGITUDINAL REQUIREMENTS W e are a l s o examining "envelope" criteria i n t h e t i m e and frequency domains--for example Malcom and Tobie's C*10 and t h e McDonnell A i r c r a f t re- finement.ll C* is a r a t i o n a l parameter t o i n v e s t i g a t e and t h e envelopes f a c i l i t a t e design. -Vhile t h e s p e c i f i c criteria which have been developed may work f o r t h e p a r t i c u l a r configurations i n v e s t i g a t e d , they s e e m t o lack v a l i d i t y i n general a p p l i c a t i o n . The r e f i n e d C* and E* criteria do not s e e m t o match t h e time-history r a t i n g s of Ref. 8 Vol. I1 much b e t t e r than t h e o r i g i - n a l C* c r i t e r i a do. However, as reference 12 p o i n t s o u t , it is not realistic t o expect any s i n g l e c r i t e r i o n t o encompass a l l p o t e n t i a l f a u l t s , e s p e c i a l l y f o r high-order o r multi-mode systems.
S,ince t h e p u b l i c a t i o n of MIL-F-8785B i n 1969, a number of research con- tracts have been sponsored by t h e AF F l i g h t Dynamics Lab both t o generate d a t a and t o develop new requirements t h a t encompass new technology. Among t h e proposed requirements c u r r e n t l y being reviewed a r e t h e Calspan proposed l o n g i t u d i n a l maneuvering c r i t e r i a i n reference 13. Longitudinal a t t i t u d e and normal a c c e l e r a t i o n c o n t r o l i s r e l a t e d t o frequency response c h a r a c t e r i s t i c s , considering d e s i r a b l e p i l o t compensation needs. An attractive f e a t u r e i s
elimination of t h e need t o i d e n t i f y s h o r t period frequency and damping - a
real advantage f o r highly augmented a i r p l a n e s . However, measurement of a s l o p e and phase a n g l e from t h e p i t c h frequency response amplitude v e r s u s phase angle p l o t is required. This does n e c e s s i t a t e knowledge of t h e a i r c r a f t / f l i g h t c o n t r o l system l o n g i t u d i n a l frequency response function. The p r a c t i c a - l i t y of i d e n t i f i c a t i o n with c u r r e n t l y a v a i l a b l e computer algorithms and f l i g h t test d a t a commonly recorded is being evaluated. Also being i n v e s t i g a t e d is t h e p r a c t i c a l i t y of generating an equivalent t r a n s f e r f u n c t i o n which would allow p r e s e n t a t i o n of requirements i n terms of "equivalent" parameters or, perhaps, required p i l o t compensation parameters. This concept of incorporating p i l o t workload and t r a n s f e r functions relates requirements more d i r e c t l y t o t h e designer; b u t t h e d i f f i c u l t y of a c c u r a t e l y f i t t i n g a n a r b i t r a r y frequency response curve with a s p e c i f i e d t r a n s f e r f u n c t i o n form is s i g n i f i c a n t .
A r e c e n t experimental program14 studied t h e t a s k dependence of r e q u i r e - ments such as those described above. Using t h e AF v a r i a b l e s t a b i l i t y T-33, v a r i a t i o n s i n p i l o t r a t i n g w e r e shown f o r some high-order configurations as a f u n c t i o n of e v a l u a t i o n t a s k . The configurations most a f f e c t e d by t a s k v a r i a t i o n a l l exhibited r e l a t i v e l y high dominant n a t u r a l frequencies. I n evaluating t h e Calspan proposed requirements, Mayhew15 i l l u s t r a t e d t h e i n f l u e n c e of closed-loop bandwidth on N e a l and Smith's f l y i n g q u a l i t i e s parameters. H e h a s a l s o shown t h e r e l a t i o n s h i p between t h e proposed ,' requirements and t h e c u r r e n t f a m i l i a r s h o r t period criteria. While t h e Calspan proposal i n c l u d e s some provision f o r bandwidth v a r i a t i o n , f u r t h e r evaluation w i l l determine i f a d d i t i o n a l provision i s required.
A d i f f e r e n t approach t o f l y i n g q u a l i t i e s criteria, amenable t o u s e i n t h e design ph se, is based on t h e "paper p i l o t " concept f i r s t proposed by 1%
Anderson . Reference 1 2 developed a computerized method of handling quali-
ties a n a l y s i s based on t h i s i d e a which showed r e l a t i v e l y good c o r r e l a t i o n f o r conventional" a i r p l a n e s - b u t less successful f o r designs r e p r e s e n t a t i v e of CCV technology. However, t h i s r e s u l t is not conclusive because t h e empirical n a t u r e of t h e criteria involved r e q u i r e a good d a t a b a s e f o r v a l i d a t i o n . Such a b a s i s does n o t exist f o r CCV a i r p l a n e s . Hence, the general approach does warrant f u r t h e r study f o r f u t u r e a p p l i c a t i o n .
CURRENT ACTIVITY REGARDING LATERAL-DIRECTIONAL REQUIREMENTS The present l a t e r a l - d i r e c t i o n a l dynamic requirements are intended t o mini- mize undesirable yaw due t o r o l l , and dutch-roll e x c i t a t i o n . These goals may b e s a t i s f i e d by t h e b a s i c a i r p l a n e design o r by.incorporating augmentation (with proper a t t e n t i o n t o r e l i a b i l i t y ) . Consequently, t h e s e requirements are c o n s i s t e n t t o a high degree with CCV design approaches. S p e c i f i c a t i o n of response c h a r a c t e r i s t i c s such as posc/pav i s c o n s i s t e n t with t h e philosophy being explored f o r t h e l o n g i t u d i n a l requirements, though modal items are not.
Reference 9 has proposed a new requirement f o r heading c o n t r o l which is intended t o address t h e problem of adverse yaw more d i r e c t l y . The approach is t o e v a l u a t e t h e roll-yaw c o n t r o l coordination required i n a t u r n a g a i n s t a d e s i r a b l e standard f o r a coordinated turn. Obviously t h i s c r i t e r i o n could b e applied t o design of a CCV system as w e l l as evaluation of conventional air- planes. General a p p l i c a b i l i t y of t h i s c r i t e r i o n (or some v a r i a t i o n thereof) t o CCV designs i n c o r p o r a t i n g d i f f e r e n t c o n t r o l modes t o achieve heading c o n t r o l remains t o be i n v e s t i g a t e d , although r e f e r e n c e 9 i n d i c a t e s t h e c r i t e r i o n is i n s e n s i t i v e t o a i r p l a n e class o r type. Also, as with t h e proposed longitudi- n a l requirements, t h e p r a c t i c a l i t y of measuring o r i d e n t i f y i n g t h e response c h a r a c t e r i s t i c s needed remains t o b e e s t a b l i s h e d .
Direct s i d e f o r c e c o n t r o l is f r e q u e n t l y mentioned i n conjunction with CCV and as noted previously is an area where d e f i n i t i v e f l y i n g q u a l i t i e s d a t a are scarce. Before such d a t a can b e generated, a complete understanding of t h e way p i l o t s employ d i r e c t s i d e f o r c e i n various t a s k s ( F l i g h t Phases) muqt b e developed. For example, they may i n some cases employ s i d e f o r c e t o perform e i t h e r a f l a t t u r n o r s i d e s l i p i n tracking. Another a p p l i c a t i o n could b e t o t r i m o u t a crosswind e f f e c t . Also, t h e e f f e c t of i n t e r a c t i o n w i t h o t h e r c o n t r o l s and with o t h e r subsystems such as d i s p l a y s must b e explored. Recent e f f o r t s a t AFFDL have looked a t t h e weapon d e l i v e r y task17 and STOL landing18.
Additional work c u r r e n t l y underway w i l l hopefully b r i n g us t o t h e p o i n t of developing some new requirements.
I Display i n t e r a c t i o n and cockpit c o n t r o l l e r c h a r a c t e r i s t i c s i n general r e q u i r e f u r t h e r study b e f o r e d e f i n i t i v e requirements can b e developed t o encompass some a s p e c t s of CCV technology. I n some cases, it is simply a matter of generating data. For example, p i l o t r a t i n g and performance d a t a are necessary t o develop q u a n t i t a t i v e requirements on f o r c e levels and gradients (including n o n l i n e a r i t i e s ) f o r s i d e s t i c k s . Display i n t e r a c t i o n must be considered when evaluating f l y i n g q u a l i t i e s as a f u n c t i o n of t a s k and also as a function of c o n t r o l mode. For example, t h e evaluation of d i r e c t s i d e f o r c e c o n t r o l f o r weapon d e l i v e r y mentioned above considered only f i x e d gun- s i g h t s . To complete t h e e v a l u a t i o n it w i l l b e necessary t o consider t h e e f f e c t of a c t i v e gunsights on t h e p i l o t ' s use of d i r e c t s i d e foce.
LIMITING FACTORS I n concluding, then, w e reiterate t h a t i n many r e s p e c t s t h e c u r r e n t f l y i n g q u a l i t i e s requirements are compatible with CCV technology. I n some areas, new requirements o r expansion of old ones is needed. I n t h e s e areas, where new requirements are being formulated, w e are c e r t a i n l y considering CCV and where necessary attempting t o gather new d a t a . The following b a s i c f l y i n g q u a l i t i e s considerations, however, might b e termed as l i m i t a t i o n s on t h e general a p p l i c a t i o n of CCV technology.
Haw much s t a t i c i n s t a b i l i t y can b e t o l e r a t e d s a f e l y ? An a b s o l u t e bound is apparent from "critical task'' studies", which show t h a t divergence of a simple system is c o n t r o l l a b l e i f its t i m e t o double amplitude is w i t h i n cer- t a i n bounds, depending upon p i l o t workload. Boeing SST simulationsz0 found a c r i t e r i o n of T 2 6 sec t o set t h e s a f e a f t c.g. l i m i t . The c r i t i c a l t a s k has also been used as a s i d e t a s k i n p i l o t - v e h i c l e s t u d i e s , t h e magnitude of t h e c o n t r o l l a b l e u n s t a b l e t i m e constant being a measure of p i l o t workloadz1. The amount of divergence, then, which can b e handled s a f e l y i s seen t o depend upon t h e amount of a t t e n t i o n a p i l o t can devote t o c o n t r o l l i n g it. That i n t u r n i s a f u n c t i o n of t h e task's i n h e r e n t d i f f i c u l t y (e.g., landing a p p r o a c h 2 c r u i s e ) and t h e level of o t h e r f l y i n g q u a l i t i e s (e.g., concurrent a i l u r e s of
d
command augmentation o r i n another a x i s of s t a b i l i t y augmentation) .
Another necessary l i m i t on s t a t i c i n s t a b i l i t y is t h e amount of c o n t r o l Proposed c r i t e r i a have ranged from l i t t l e more than remaining f o r recovery.
s t a t i c balance22 t o MIL-F-83300's 23 h a l f t h e nominal c o n t r o l moment ( f o r forward f l i g h t ) and s p e c i f i e d a t t i t u d e changes i n 1 second ( f o r hover). S e n s i t i v i t y Any requirement i s bound t o be somewhat a r b i t r a r y t o g u s t s is a consideration.
Here too w e s o l i c i t opinions and data.
because experience i s limited.
Control s u r f a c e rate must also b e adequate, even i n emergency conditions.
A 1972 General Dynamics s t u d y shows convincing t i m e h i s t o r i e s of t h e wild maneuvers t h a t can r e s u l t from i n s u f f i c i e n t s u r f a c e rate f o r s t a b i l i t y aug- mentation. I n a n i n t e r n a l study, Watson, Bennett and Kouri s y s t e m a t i c a l l y v a r i e d t h e parameters of "a s m a l l CCV f i g h t e r a i r p l a n e design", seeking generalized design c r i t e r i a . That a t least is a start toward a s p e c i f i c a t i o n requirement.
7 42 From considerations leading t o t h e c u r r e n t requirements5, w e have t h e following d i s c u s s i o n relative t o i n s t a b i l i t y and F a i l u r e States of t h e air- plane.
The Level 3 requirements g e n e r a l l y apply i n t h e worst p o s s i b l e F a i l u r e Statest, Except f o r approved S p e c i a l F a i l u r e S t a t e s , then, MIL-F-8785's static s t a b i l i t y requirement does not permit basic-airframe speed i n s t a b i l i t y ( e l e v a t o r s u r f a c e f i x e d ) . Cases w i l l arise, however, i n which t h e procuring a c t i v i t y i s asked t o consider allowing basic-airframe i n s t a b i l i t y as a S p e c i a l F a i l u r e State. Even i f t h e r e l i a b i l i t y o f s t a b i l i t y augmentation should b e judged s u f f i c i e n t l y high, o r i f t h e degree of i n s t a b i l i t y seems acceptable i n i t s e l f , a number of a s p e c t s of combined airframe-flight c o n t r o l system behavior i n normal operation need t o b e examined b e f o r e accepting appreciable i n s t a b i l i t y i n a S p e c i a l F a i l u r e S t a t e .
Obviously, extremes of e i t h e r s t a b i l i t y o r i n s t a b i l i t y r e q u i r e more c o n t r o l t o balance t h e a i r p l a n e throughout an angle-of-attack range. I n t h e s t a b l e case, a t t h e c o n t r o l l i m i t t h e a i r p l a n e a t least has a r e s t o r i n g tendency. But when a n a i r p l a n e h a s an unstable v a r i a t i o n of elevator-surface p o s i t i o n w i t h airspeed, t h e s u r f a c e p o s i t i o n required t o maintain off-trim airspeeds is i n a d i r e c t i o n which reduces t h e c o n t r o l a v a i l a b l e t o i n i t i a t e recovery t o t h e t r i m speed. I f t h e unstable g r a d i e n t is l a r g e enough, t h e p i l o t could f l y f a r enough off t h e t r i m speed t h a t t h e r e would b e no e l e v a t o r c o n t r o l a v a i l a b l e f o r recovery. the s t o p s , t h e With t h e e l e v a t o r a g a i n s t a i r s p e e d would continue t o diverge and t h e p i l o t would b e powerless t o prevent it from doing so. Examples of t h i s behavior can b e found i n Mach tuck f o r subsonic a i r p l a n e s and during wave-offs f o r some propeller-driven a i r p l a n e s .
States, then, over t h e e n t i r e p e r m i s s i b l e range of For Airplane Normal speed and a l t i t u d e , s a f e t y comparable t o t h a t of a s t a b l e b a s i c airframe would r e q u i r e p i l o t - c o n t r o l and control-surface a u t h o r i t y t o balance t h e a i r p l a n e a t p o s i t i v e and negative u l t i m a t e load f a c t o r s , with some margin of c o n t r o l power remaining, wherever t h e b a s i c airframe is unstable.
( I n f l i g h t test, of course, l i m i t load f a c t o r would not i n t e n t i o n a l l y b e exceeded.) For a given configuration, t h e e l e v a t o r s u r f a c e and c o n t r o l p o s i t i o n s f o r balance d e t e r - mine t h e amount of c o n t r o l a u t h o r i t y l e f t f o r s t a b i l i z a t i o n and control. The relative a u t h o r i t y and i n t e r a c t i o n s of command, augmentation and t r i m c o n t r o l s are important considerations. Authority and rate s a t u r a t i o n may be p a r t i c u - l a r l y important f o r dual-purpose c o n t r o l s such as elevons. With aerodynamic i n s t a b i l i t y and higher-order f l i g h t c o n t r o l system dynamics, l i m i t cycles a l s o become of i n c r e a s i n g concern.
I n both Normal and F a i l u r e States, t h e augmentation must maintain appropri- ate l e v e l s of s t a b i l i t y i n responses t o both c o n t r o l and disturbance inputs.
For a b a s i c a l l y u n s t a b l e airframe, t h e s i z e s of t h e s e i n p u t s should be s t a t e d s p e c i f i c a l l y , r a t h e r than t a k i n g a primarily q u a l i t a t i v e approach. Some margin above s t r u c t u r a l design g u s t s and turbulence might b e s u i t a b l e . The r e q u i r e d augmentation a u t h o r i t y may exceed t h e p i l o t ' s c o n t r o l a u t h o r i t y .
Hard-over f a i l u r e s should b e made impossible i n t h e f l i g h t c o n t r o l system; engine-failure t r a n s i e n t s conceivably could b e critical. Large c o n t r o l i n p u t s of various forms and phasing should b e considered. The response t o disturbances during commanded maneuvers must b e considered. The e f f e c t of f l i g h t a t o f f - t r i m conditions on a l l t h e s e f a c t o r s must b e examined.
P a r t i c u l a r a t t e n t i o n is needed f o r t h e stall and s p i n recovery requirements.
Increased dependence on c o n t r o l systems and a r t i f i c i a l s t a b i l i t y makes surviva- b i l i t y a f t e r damage o r f a i l u r e a n important consideration f o r h i g h - a n g l e s f - a t t a c k f l i g h t .
S t a l l l i m i t e r s and d e p a r t u r e preventers are already developed as f i x e s f o r c u r r e n t f i g h t e r air lanes--the F-111 S t a l l I n h i b i t o r System24 and t h e A-7 d e p a r t u r e preventer 35 f o r example. Manufacturers whose a i r c r a f t do not need such devices expound on t h e air combat advantage a t t a i n a b l e a t extreme angles of a t t a c k : r a p i d d e c e l e r a t i o n , f o r example, t o change p o s i t i o n s with an enemy a t t a c k i n g from t h e rear. Certainly aerodynamic design f o r s t a l l / p o s t - s t a l l s t a b i l i t y remains a n important consideration f o r CCV design, i n order t o The A i r Force F l i g h t T e s t avoid completely u n c o n t r o l l a b l e s i t u a t i o n s .
Center's S t a l l / P o s t - S t a l l / S p i n F l i g h t T e s t Demonstration Requirements f o r Airplanes, MIL-S-8369IAy r i g h t f u l l y stresses t h e need t o demonstrate extreme r e s i s t a n c e t o loss of control. The required t e s t i n g s u b j e c t s a l l a i r c r a f t t o a degree of "gross" abuse beyond normal maneuvers. Highly maneuverable air- c r a f t are t o b e even more completely wrung o u t . Thus limiters, while cer- t a i n l y u s e f u l , can supplement b u t not replace aerodynamic design a t high angle of a t t a c k .
I n determining t h e adequacy of s t a l l limiters, c o n t r o l a u t h o r i t y and rate, one must choose t h e s i z e of disturbance t o b e allowed f o r . Turbulence l e v e l i s important; both MIL-F-8785B and t h e proposed MIL-F-9490D f l i g h t c o n t r o l system s p e c i f i c a t i o n g i v e models and i n t e n s i t i e s f o r turbulence up t o thunder- storm t n t e n s i t i e s . Single disturbances are l i k e l y t o b e critical. These include gusts, wind s h e a r , wakes of b u i l d i n g s , etc. near t h e runway and jet wakes. The B r i t i s h revisers of AvP 970 f l y i n g q u a l i t i e s requirements are considering, i n a d d i t i o n t o Gaussian turbulence, p a i r s of ramp g u s t s t o evoke t h e worst response. Glyn Jones' development of t h i s approach is proceeding. 26 REFERENCES 1. Root, A.I.: "Gleanings i n B e e Culture", January 1905 (Reproduced i n C.H. Gibbs-Smith: The Aeroplane, H e r Majesty's S t a t i o n e r y Office, London, 1960. ) 2. Foulois, B.D, as t o l d t o Harold R. Craven: "The Day I Taught Myself t o Fly"; Airman, Vol. X I V , No. 3, March 1970 (Reprinted from Sept, 1965) 3. Gibbs-Smith, C.H.: Aviation, An H i s t o r i c a l Survey from its Origins t o t h e End of World War 11; H e r Majesty's S t a t i o n e r y O f f i c e , London, 1970.
4 . Howard, R.W.: "Automatic F l i g h t Controls i n Fixed Wing Aircraft--The F i r s t 100 Years"; The Aeronautical J o u r n a l V. 77 No. 755, November 1973.
7 44 5. Chalk, C.R., e t al: "Background Information and U s e r Guide f o r MIL-F- 8785B(ASG), ' M i l i t a r y Specification--Flying Q u a l i t i e s of P i l o t e d Air- planes'"; AFFDL TR 69-72, W - P AFB, August, 1969.
7. A'Harrah, R.C.: "Flight Simulation--A S i g n i f i c a n t Aid i n A i r c r a f t Design", AGARD Conference Proceedings N r . 119 on S t a b i l i t y and Control; Braun- schweig, 10-13 A p r i l 1972.
8. Neal, T.P. and Smith, R.E.: "An I n - f l i g h t I n v e s t i g a t i o n t o Develop Control System Design Criteria f o r Fighter Airplanes", AFFDL TR 70-74 Vof. I & 11, W - P AFB, December 1970.
9. Ashkenas, I.L.: "Recommended Revisions t o Selected Portions of MIL- F-8785B(ASG) and Background Data"; AFFDL TR 73-76, W - P AFB, August 1973.
10. Malcom, L.G. and Tobie, H.N.: "New Short Period Handling Quality C r i t e r i o n f o r Fighter A i r c r a f t " ; Boeing Co:Document D6-17841 T/N, November 1965.
11. K i s s l i n g e r , R.L. and Wendl, M.J.: "Survivable F l i g h t Control System Interim Report No. 1, Studies, Analysis and Approach, Supplement f o r Control Criteria Studies"; AFFDL TR 71-20, Supplement 1, W - P AFB, May 1971.
12. B r u l l e , R.V. and Anderson, D.C.: "Design Methods f o r Specifying Handling Q u a l i t i e s f o r Control Configured Vehicles"; AFFDL TR 73-142 Vol I & 11, W - P AFB, November 1973.
13. Chalk, C.R., et al: "Revisions t o MIL-F-8785B(ASG) Proposed by Cornel1 Aeronautical Laboratory Under Contract F33615-71-C-1254"; AFFDL TR 72-41, W - P AFB, A p r i l 1973.
14. Boothe, E . M . , et al: "A Two Phase I n v e s t i g a t i o n of Longitudinal Flying Q u a l i t i e s f o r Fighters"; AFFDL TR 74-9, W - P AFB, i n publication.
D.R.: "A Proposed Approach t o Revise t h e Short Period Require- 15. Mayhew, ments of MIL-F-8785B"; WP N r . 1, AFFDL, W - P AFB, May 1974.
16. Anderson, R.O.: "A New Approach t o t h e S p e c i f i c a t i o n and Evaluation of Flying Qualities"; AFFDL TR 69-120, W - P AFB, June 1970.
17. H a l l , G.W. and Weingarten, N.C.: "An In-Flight I n v e s t i g a t i o n of t h e Influence of Flying Q u a l i t i e s on P r e c i s i o n Weapon Delivery", AFFDL TR 72- 120, W - P AFB, J u l y 1973.
18. Boothe, E . M . e t al: "Direct Side Force Control (DSFC) f o r STOL Crosswind Landings"; AFFDL TR 73-2, W - P AFB, February 1973.
7 45 19. Jex, H.R., et al: "A 'Critical' Tracking Task f o r Man-Machine Research Related t o t h e Operator's Effective Delay Time"; NASA CR 616, Ames Research Center, November 1966.
20. Tomlinson, L.R.: "Control System Design Considerations f o r a Longitudi- n a l l y Unstable Supersonic Transport", Journal of A i r c r a f t , Vol 10 N r . 10, October 1973.
21. McDonnell, J . D . : "Pilot Rating Techniques f o r t h e Estimation and Evalua- t i o n of Handling Qualities"; AFFDL TR 68-76, W - P AFB, December 1968.
22. Kisslinger, R.L. and Lorenzetti, Maj. R.C.: "The Fly-by-Wire System Approach t o A i r c r a f t Flying Qualities"; presented t o NAECON, Dayton, OH 15-17 May 1972.
23. "Military Specification--Flying Qualities of Piloted V/STOL Aircraft", MIL-F-83300, 31 December 1970.
24. Lee, R.E., Jr., et al: "Evaluation of t h e F-111 S t a l l I n h i b i t o r System/ Landing Configuration Warning SystedAdverse Yaw Compensation Modifi- cations", FTC TR 73-27, AFFTC, July 1973.
25. Chen, R.T.N., e t al: "Development and Evaluation of an Automatic Departure Prevention System and S t a l l Inhibitor f o r Fighter Aircraft"; AFFDL TR 73-29, W-P AFB, April1973.
26. Jones, J . G . : " S t a t i s t i c a l Discrete Gust Theory f o r Aircraft Loads.
A Progress Report"; RAE TR 73167, Farnborough, November 1973.