So far we were focusing on the wing and its sectional profiles, considering the geometrical parameters of the aerofoil (wing) and the parameters of the flow to which it is exposed. The pressure loading, lift and drag associated with aerofoil were discussed for both two-dimensional (infinite) and three-dimensional wings. In this chapter, let us consider a complete flying machine and study some of the basic flights associated with it. A flying machine and its control surfaces are schematically shown in Figure 10.1
When the control surfaces are in their neutral positions the aircraft, like the aerofoil, has a median plane of symmetry, and when properly located the center of gravity G lies in this plane.
The fixed fin of the aircraft is in the plane of symmetry, as shown in Figure 10.1. For simplicity, let us assume the aircraft to be in straight level (horizontal) flight, with all the control surfaces in their neutral positions. The lift is generated by the wings (port and starboard wings) and tail (the lift associated with the body of the aircraft is ignored). The thrust produced by the engine overcomes the drag, that is the thrust is assumed to be horizontal.
In Figure 10.1, the lateral axis Gy is perpendicular to the plane of symmetry and positive to starboard (that is to right). The symmetry will not be disturbed if the elevators are deflected. Raising the elevators will decrease the lift on the tail, and will cause a pitching moment, positive when the nose tends to be lifted. Moving the rudder to starboard will cause a yawing moment, tending to deflect the nose to the starboard, the positive sense.
The ailerons move in opposite senses, one up, one down, by a single motion by the control column. If we depress the port aileron and therefore simultaneously raise the starboard one, the lift on the port wing will increase and that on the starboard wing will decrease so that the rolling moment will be caused tending to dip the starboard wing, and this sense will be positive. This movement also causes a yawing moment, for the drag on the two wings will likewise be altered. To minimize this the ailerons are generally geared to move differentially so that one moves through a greater angle than the other.
Motion of ailerons or rudder will disturb the symmetry of the aircraft. A single-engine aircraft also has a dynamical asymmetry (tendency to tilt).