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Control

 

Control represents the balancing act of the aircraft. In level flight we have weight acting through the centre of gravity, lift through the centre of pressure, and some more lift generated at the tail of the aircraft. Thrust is provided by gravity and drag generated by the wing and the fuselage. The fuselage ties all of these forces together, distributing the weight such that the centre of gravity is generally fixed and located ahead of the centre of pressure of the wing. The centre of pressure shifts as we move the elevator up and down. The elevator is located at the extreme aft end of the fuselage and represents an up or down balancing force to the weight (centre of gravity) and the lift (centre of pressure). When we move the elevator, more or less lift is produced at the tail of the aircraft. This changes the wing’s angle of attack and in the process alters the wing’s potential lift, drag, and speed.

Rolling movements are produced about the longitudinal axis by the movement of flaps on the trailing edge of the wings that work in opposition to one another. When an aileron on one wing is deflected downwards, that wing effectively meets the wind at a greater angle and thus should produce more lift and more drag. The opposite aileron is deflected upwards, so that wing produces less lift and less drag. The result is one wing moves up and the other down. If we return the ailerons to their centralized positions, the lift on both wings will be more or less equal and the aircraft stops rolling. Steeper turns will have a shorter turn radius and consequently faster turn rate if airspeed is held constant.

We can move the aircraft about the normal or vertical axis, by generating more lift in one direction or another parallel to the lateral axis, by movements of the rudder. Since the rudder is located at the end of a long moment arm (fuselage) relative to the centre of gravity, it can have a powerful effect at rotating the aircraft about its centre of gravity.

Since the controls are all joined to the same aircraft via the wings, fuselage, and tail we might expect that they interact somewhat. While the rudder primarily affects yaw it eventually causes the plane to turn if applied alone. When the aircraft is yawed, the forward moving wing in the air will create more lift mainly because of its increased angle of attack relative to the wing that moves back. This will eventually cause the aircraft to bank. When the airplane is banked using the ailerons, one wing generates more lift than the other. Whenever lift is created a certain amount of induced drag is also present. This aileron drag that is produced causes the aircraft to yaw away from the direction of the turn. Since such a slipping turn is not desirable this adverse yaw can be corrected by proper use of the rudder.