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Aviation History
1979
1979 - 4583.PDF
FLIGHT International, IS December 1979 2013 •W •*- MSB's unstable Starfighter IMAGINE a ball inside a bowl. Dis place the ball and it returns to its original position at the bottom of the bowl. Invert the bowl and balance the ball on top. Displace the ball and it rolls away. The first system is stable; the second is unstable. Aircraft are stable systems—usually. This is achieved by placing the centre of gravity ahead of the centre of lift, or aerodynamic centre. This creates a nose-down pitching moment which is trimmed out by download on the tailplane. Any increase in angle of attack generates more lift—and strengthens the nose-down moment. Left untrimmed, the aircraft will re sume its undisturbed attitude. A stable aircraft, therefore, resists efforts—<both external and internal— to change its position. There is a temptation to reduce stability to im prove aircraft response. Balance a book on your head and you find that hundreds of subliminal movements are required to keep it in its elevated position. Relax aircraft stability and pilot workload increases. A stability augmentation system (SAS) is re quired to sense and cancel out small disturbances. The extent to which stability can be relaxed is determined by the response of the SAS and by its authority, which is in turn dictated by the reliability. Ideally a fighter would be made longi tudinally unstable to increase pitch response and to convert a tail down load into a lift-enhancing upload. This requires a rapid-reacting flight-control system able to stabilise a naturally un stable aircraft. 3VIBB began a control-configured vehicle (CCV) demonstrator pro gramme in December 1974. The ob jectives were to develop an advanced CCV flight-control system, and to in vestigate demonstrator handling with up to 20 per cent negative static stability margin. Availability dictated modification of an F-404G Starfighter, although the aircraft has a high (around 20 per cent) positive static margin. To achieve a 20 per cent negative margin requires drastic measures. Heading Canard aileron actuator fairings on the fuselage and photographic reference markings distinguish MBB's fly-hy-wire Star fighter. Below Skewed airflow-direction sen sors and four redundant strapdown inertial platforms measure position and motion By GRAHAM WARWICK First, 600kg of aft ballast (lead shot) has been added under the jet- pipe. This moves the e.g. to its aft, tip-up limit. Secondly, an F-104 tail- plane has been mounted above the forward fuselage. This canard moves aircraft aerodynamic centre forward by roughly 35 per cent of the mean aerodynamic chord (area divided by span). Incidence of the canard is set at — 4°. This ensures that the wing and canard are lift free at the same angle of attack. Although the canard carries some 20 per cent of aircraft weight, the overall untrimmed lift coefficient is only slightly increased over the F-104G. Canard downwash creates a download on the wing, virtually negat ing the extra lift. Trimmed lift co efficient is increased considerably, however, because negative stability means a lifting tail. Maximum usable lift coefficient is increased 24 per cent. Conversely, drag is down 23 per cent at the F-104G maximum usable lift coefficient. The canard destabilises the CCV- F-104G up to 11° angle of attack. At this angle the canard stalls and the aircraft becomes positively stable. When the wing stalls at 16° angle of attack, both the canard and conven tional configurations pitch nose up. To prevent this, the F-104G has a stickpusher which triggers when angle of attack—allowing for pitching velo city—reaches 14°. A stickshaker pro vides artificial stall warning at 12°. On the CCV-F-404G the stickpusher actuator, located immediately ahead of the tailplane power jack, has been removed to make room for CCV series actuators. Pitch-up warning is still provided by the stickshaker. As a
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