Theory of Flight

 AEROPLANE AERODYNAMICS AND FLIGHT CONTROLS

An airplane is equipped with certain fixed and movable surfaces, or airfoils, which provide for stability and control during flight. Each of these airfoils is designed to perform a specific function in the flight of the airplane.

FIXED AIRFOILS

The fixed airfoils are the wings, the horizontal stabilizer, and the vertical stabilizer (fin). The function of the wings has been previously discussed. The tail section of the airplane, including the stabilizers, elevators, and rudder, is commonly called the empennage.

MOVABLE CONTROL SURFACES

Movable flight control surfaces are divided into two categories;

1. Primary flight controls

2. Secondary flight controls

The primary control surfaces of an airplane include the ailerons, elevators, and rudder. The secondary control surfaces include flaps; trim tabs, spoilers, and slats. The principles of the operation of flaps and spoilers are discussed later in this module. The primary control surfaces are used to "steer" the airplane in flight to make it go where the pilot wishes it to go and to cause it to execute certain maneuvers. The secondary control surfaces are used to change the lift and drag characteristics of the aircraft or to assist the primary controls. Large turbine aircraft, gliders, and some other types of aircraft are equipped with lift-control devices called spoilers, which may be used as or assist the primary roll control.

CONTROL OF AN AIRPLANE

Whether an airplane is stable or unstable, it is necessary for the pilot to be able to control it, so that he can maneuver it into any desired position.

 Longitudinal control is provided by the elevators, i.e. airfoils hinged behind the tail plane.

 Lateral control is provided by the ailerons, i.e. airfoils hinged at the rear of the airfoils near each wing tip.

 Directional control is provided by the rudder, i.e. a vertical airfoil hinged to trailing edge of vertical stabilizer.

The system of control is the same in each case, i.e. if the control surface is moved it will, in effect, alter the angle of attack and the camber of the complete surface to which it is hinged, and therefore change the force upon it . The control surfaces are connected to controls in the pilot’s cockpit. The elevators and ailerons are both moved by a single column on a universal joint (or by a wheel or handle-bars)and the rudder by two rudder pedals.

The control is instinctive, i.e. a forward movement of the control column depresses the elevators, increases the lift on the tail, and so causes the nose of the airplane to drop. If the control column is moved to the left, the right aileron will depress and right wing will rise at the same time the left aileron will raise and left wing will lower causing airplane to bank left.