Given the difficulty of finding suitable aerodynamic yaw controls, the designer of low aspect ratio tailless aircraft is usually forced into considering the use of yaw thrust vectoring (YTV) as a yaw control device. The primary advantage of YTV is that it retains its effectiveness in flight regimes where conventional aerodynamic yaw controls are ineffective, for example, at low airspeeds or high . YTV is also very effective in controlling the yawing moment due to an engine failure on a multi-engine aircraft having the engines mounted close to the centerline. The YTV can force the thrust vector of the remaining engine or engines to pass very close to or through the c.g. This eliminates, or at least minimizes, the yawing moment created by the failed engine.
There are two primary disadvantages of YTV. First, the engine power setting and the limited angular deflection of the thrust vector limits the total control power. Second, except for mechanical panels that are inserted directly into the exhaust jet, methods that have been developed and tested to change the direction of the engine’s exhaust typically have a lower bandwidth capability than conventional aerodynamic effectors.
Because aircraft are typically optimized to reduce the aerodynamic drag to a minimum, and the removal of conventional vertical tails reduces drag still further, the thrust required for cruise is also minimized. Combined with the limited angular deflection of the thrust vector, the yaw control power attainable with YTV is sometimes severely limited. In any case, the aircraft must remain under control at any point within the permissible flight envelope for any possible engine power setting. This includes the case of the total loss of engine thrust due to compressor stalls during high- maneuvers, fuel starvation due to fuel system malfunctions, etc. Control must be maintained for some period of time sufficient for engine restart or the clearance of malfunctions. Consequently, some aerodynamic yaw control device must be available to provide the necessary control power until the YTV can be restored.
Depending on how the thrust vectoring mechanism is implemented, the attainable bandwidth for YTV is typically limited to approximately 1.0–1.5 Hz for deflections in the range of plus or minus 50% of maximum deflection. This is entirely satisfactory for use as steady-state yaw trim devices or for slow, gentle maneuvers. However, it is not fast enough to provide the good lateral-directional handling qualities required for the rapid, large-amplitude maneuvers used by fighter aircraft. Once again, some high-bandwidth aerodynamic yaw control device is required to supplement the YTV.
For fighter aircraft with a high thrust-to-weight ratio, the use of thrust vectoring is beneficial for rapid, large-amplitude maneuvers at speeds up to about 250 kt calibrated airspeed. This corresponds to the upper left-hand corner of the Mach–altitude flight envelope. Fortunately, this is the part of the flight envelope where very high- maneuvering takes place and where the engine would typically be operating at maximum power settings. In the case of the F-22, the use of thrust vectoring is of most benefit in the pitch axis, where it is used to prevent pitch up during high- rolls. This unloads the horizontal tails and allows them to be used as both roll and yaw effectors to provide high rolling performance [2–4]. If the aircraft is designed to have neutral aerodynamic stability, the aerodynamic moments to be overcome during maneuvers are reduced to a minimum. Then thrust vectoring can be used for quasi-steady-state low-amplitude maneuvering throughout the flight envelope. For rapid, large- amplitude maneuvering at high airspeeds, aerodynamic control effectors must be used. Their effectiveness continues to increase as the square of the airspeed.
It is seen that YTV can be a useful yaw control for a tailless aircraft, but it can never be the primary or only yaw control device. Some type of aerodynamic control device is still always required, because control must be maintained even if all thrust is lost due to engine failure.
