Aircraft Pitch Control

 OPERATION AND EFFECT OF

PITCH CONTROL

ELEVATORS

The elevators are the control surfaces, which govern the movement of the aircraft around the lateral axis (pitch). They are normally attached to hinges on the rear spar of the horizontal stabilizer. When the control column or stick in the airplane is pulled back, the elevators are raised. The force of the relative wind on the elevator surfaces tends to press the tail down, thus causing the nose to pitch up and the angle of attack of the wings to increase. The reverse action takes place when the control column or stick is pushed forward. 

During flight of an airplane the operation of the elevators is quite critical, especially at low speeds. When power is off and the airplane is gliding, the position of the elevators will determine whether the airplane dives, glides at the correct angle, or stalls. Remember that an airplane will not necessarily climb when the control is pulled back. It is the power developed by the engine that determines the rate of climb of an airplane rather than the position of the elevators. If the elevators are held in a fixed position, the throttle alone can be used to make the airplane climb, dive, or maintain level flight.

The position of the elevator is important, however, to establish the most efficient rate of climb and a good gliding angle when power is off. It is also most essential for proper control when "breaking the glide" and holding the airplane in landing position.

STABILATORS

A special type of elevator that combines the functions of the elevator and the horizontal stabilizer is called a stabilator. A stabilator is an all-moving tail that works by changing the angle of attack of the stabilator and thereby changing the amount of downward lift that is generated by the tail. When this type of control airfoil is installed on an airplane, there is no fixed horizontal stabilizer. The stabilator is an airfoil that responds to the normal elevator control and serves as an elevator as well as a stabilizer.

VARIABLE INCIDENCE STABILISERS

In sweptback wings when fuel is consumed by the engines the aircraft C.G. shifts greatly within the limits. Hence there is a need to trim the aircraft by use of the elevators to prevent the aircraft pitching about. By deflecting elevators trim drag is increased and elevator authority is reduced. Instead of deflecting elevators if the horizontal stabilizer itself could be moved to trim the aircraft we can reduce trim drag and retain full authority of the elevators. Variable incidence stabilizers (Trimmable horizontal stabilizers) do just that.

Incorporating a conventional elevator control system, the variable incidence horizontal stabiliser is often used for pitch trim. Normally a powerful electric motor is used to vary its angle of attack when trim switches on the flight deck are operated.