Decoupled Controls
Airplane stability augmentation must be rethought when designers choose to add direct normal and side force control surfaces. For example, with direct lift control through a fast-acting wing flap, pitch attitude can be controlled independently of the airplane’s flight path, and vice versa. The utility of such decoupled controls for tracking, defensive maneuvers, and for landing approaches is reviewed by David J. Moorhouse (1993).
20.4 Integrated Thrust Modulation and Vectoring
An airplane’s propulsion system can be integrated into a stability augmentation system that uses aerodynamic control surfaces. The total system would operate while the airplane remains under the control of the human pilot, qualifying as a stability-augmentation system rather than as an automatic flight control system.
For comparison, the previous coverage of propulsion systems in this book included:
Chapter 4 the effects of conventional, or fixed-configuration, propeller-, jet-, and rocket-propulsion systems on stability and control;
Chapter 10, Sec. 8 thrust vector control to augment aerodynamic surfaces in supermaneuvering;
Chapter 11, Secs. 14 and 15 propulsion effects on modes of motion and at hypersonic speeds;
Chapter 12, Sec. 1 carrier approach power compensation systems, for constant angle of attack approaches;
Chapter 20, Sec. 11 Propulsion-controlled aircraft, designed to be able to return for landing after complete failure of normal (aerodynamically implemented) control systems.
Depending on the number of engines under control, thrust modulation and vectoring systems can supply yawing, pitching, and rolling moments, as well as modulated direct forces along all three axes. Thus, thrust modulation and vectoring integrated into a stability – augmentation system can augment or replace the aerodynamic yawing, pitching, and rolling moments provided by aerodynamic surfaces. The situation is similar to aircraft like the Space Shuttle Orbiter, which carries both aerodynamic and thruster controls. However, in the context of stability augmentation, thrust modulation and vectoring would be used normally at the low airspeeds of approach and landing, rather than in space.
While in principle thrust modulation and vectoring can take the place of aerodynamic control surfaces at the low airspeed where the aerodynamic surfaces are least effective, it is reasonable to ask whether thrust stability-augmentation systems could satisfy flying qualities requirements. In a simulation program at DERA, Bedford (Steer, 2000), integrated thrust vector control was evaluated at low airspeeds on the baseline European Supersonic Commercial Transport (ESCT) design. The nozzles of all four wing-mounted jet engines were given both independent pitch and yaw deflections, providing yawing, pitching and rolling moments. Nozzle deflections were modeled as first-order lags. Conventional pitch rate, pitch attitude, velocity vector roll rate and sideslip command control structures were programmed.
Pitch control by thrust vectoring at approach airspeeds was as good as aerodynamic or elevon control, for a reason peculiar to the very low wing-aspect-ratio ESCT configuration. That is, the airplane has high induced drag at approach angles of attack, requiring large levels of thrust to maintain the glide path, thus making available large pitching moments with thrust deflection. Low airspeed roll and sideslip thrust vector control were positive and suitably damped but did not satisfy MIL-STD-1797A criteria.