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THRUST VECTORING - THE COMBAT EDGE

Nasa's
F-18 HARV
  Written 1996                                                                                                                                       Nasa

BACKGROUND

Development of short-to-medium range all-aspect missiles has increased the need for a fighter aircraft to rapidly point its nose at an opponent to obtain a first-shot opportunity during air combat manœuvring.  Rapid nose pointing is of particular importance within visual range.  Even though long-range missiles are also continually improving, failures and rules of engagement will inevitably produce close-in encounters.  During close-in air combat, the need to acquire a position in the opponent's rear quarter has diminished with the continual improvement of short-to-medium range all-aspect missiles.  Due to the enhanced capability of these missiles, a great deal of research has gone into developing high-performance aircraft that can rapidly manœuvre at angles of attack (AOAs) greater than the maximum lift; at these high AOAs very high nose-turning rates can be generated.  The reason that such high turn rates can be generated is because at high AOA, and therefore low speed, the aircraft can be manœuvred round its velocity vector (or direction of travel).  This can be likened to a 'spinning top' rotating on its point.  The velocity vector can be thought of as the aircraft's 'point' round which it can spin.  This is in complete contrast to an aircraft turning using normal aerodynamic forces where the manœuvre is performed about a point in space away from the aircraft; thereby, giving a radius of turn or pitch change.  This can result in a lot of sky being flown through in order to change direction (either turning or pitching).  However, at high AOA the aircraft is simply turning or pitching about a point on the aircraft (the centre of gravity).   Simulation studies and flight test have shown that an operational capability and good controllability in this high-AOA flight regime provide tactical manœuvre advantages during close-in air combat with current missiles and guns.  The success in simulated air combat studies has led to research aircraft that have the capability to manœuvre at post-stall AOAs

The limiting parameter that has in the past prevented manœuvres at high AOAs is control power, or to put it another way, the ability of the aircraft to generate enough force, in the required direction, to move the aircraft in a controlled fashion.  In recent years, three high-AOA flight test aircraft (X31, the F18 High-Alpha Research Vehicle (HARV), and the F16 Multi-Axis Thrust Vectoring (MATV)) have been manœuvring successfully at post-stall AOAs which were not previously achievable. The X31 aircraft and HARV are designed to manœuvre at AOAs up to 70 degrees; the MATV aircraft has performed sustained manœuvres up to an AOA of 84 degrees with excursions beyond that.  Thrust vectoring controls provide these aircraft with the added control power required to manœuvre in the high-AOA flight regime which was not achievable with purely aerodynamic controls.

The UK and the USA are presently engaged in collaborative work to study the control law design of high-AOA aircraft.  This has in the past only be in the form of simulation.  However, as a natural extension of the collaborative efforts the UK were invited to flight test the F18 HARV.  This gave the first flight test opportunity to investigate the benefits of a thrust-vectoring-control system and post-stall manœuvring in combat aircraft.  This report is based on that flight test and three years of simulation, both on the Large Motion Simulator at the Defence Research Agency site at Bedford and on the Differential Manœuvring simulator at the NASA Langley Research Centre.

 

THE PROBLEM WITH HIGH-AOA MANŒUVRING  (back to top)

A major factor that limits the high-AOA manœuvring effectiveness of current fighter aircraft is the degradation of aerodynamic control effectiveness as AOA is increased.  With these aircraft pitch and yaw control power degrades rapidly as the AOA approaches maximum lift due to the conventional control surfaces becoming immersed in the low-energy stalled wake shed from the wings and fuselage.  High performance aircraft, like the F18, have to some extent overcome these problems in pitch and can already attain high AOAs, albeit with limited manœuvre capability and reduced controllability.  However, the more critical problem is the rudder capability; the level of yaw control required actually increases with AOA due to the increasing yaw rates required to co-ordinate the rolling manœuvre about the velocity vector.  The typical result is that the level of yaw control required exceeds the amount available, beginning at an AOA substantially lower than the AOA for maximum lift.  The resulting deficiency in yaw control results in an inherent roll-rate limiting of the aircraft and, therefore, reduced manœuvring effectiveness.  In the F18 in roll-rate is limited above 8 degrees AOA due to insufficient yaw control available to adequately co-ordinate the rolling manœuvre.  In the extreme there is a danger of departure with poor roll co-ordination due to roll-yaw coupling.