FLIGHT AT EXTREMELY LOW SPEEDS

S tall speed is defined as the lowest airspeed at which 1 'G' level flight can be achieved. However it is also possible to fly the airplane at speeds below the defined stall speed. This regime is outside the certified flight envelope. There are several important factors that a pilot must know when the airplane is at extremely low speeds.

The aerodynamic lift that the wings and tail generates is dependent on both the angle of attack and the velocity over the surfaces. Angle of attack alone determines whether the surface is stalled. At very low airspeeds, even far below the strictly defined stall speeds, an un-stalled surface (one at a low angle of attack) will produce lift. However, the magnitude of this lift force will probably be very small. For a surface in this condition, the lift generated will not be enough to support the weight of the airplane. In the case of the lift generated by the tail, at very low airspeeds, it may not be great enough to trim the airplane that is to keep it from pitching.

With small aerodynamic forces acting on the airplane, and gravity still pulling towards the earth, the trajectory will be largely ballistic. It may be difficult to command a change in attitude until gravity produces enough airspeed to generate sufficient lift (this is possible only at angles of attack below the stall angle). For this reason, if airspeed is decreasing rapidly it is very important to reduce angle of attack and use whatever aerodynamic forces are available to orient the airplane so that a recovery may be made when sufficient forces are available.

When thrust is considered, the situation becomes slightly more complicated. With engines offset from the centre of gravity, thrust produces both forces and moments. As the airspeed decreases, engine thrust demand generally increases. With engines below the centre of gravity, there will be a nose up moment generated by engine thrust. At high power settings, this may contribute to even higher nose up attitudes and even lower airspeeds. Pilots should be aware that as aerodynamic control effectiveness diminishes, with lower airspeeds, the forces and moments available from thrust become more evident. Until the aerodynamic control surfaces become effective, the trajectory will depend largely on inertia and thrust effects.