Control: Axes, Surfaces, Trim and Control Effectiveness
Axes and surfaces
An aircraft has three axes through the centre of gravity: longitudinal (roll), lateral (pitch) and normal or vertical (yaw). Ailerons control roll about the longitudinal axis, elevators control pitch about the lateral axis, and the rudder controls yaw about the normal axis. Candidates must resist the common error of pairing the rudder with the lateral axis or the elevator with the normal axis. Secondary effects matter: ailerons produce adverse yaw, the rudder produces roll, and elevator deflection changes angle of attack and hence speed.
Trim and effectiveness
Trim tabs allow the pilot to relieve steady control forces; they do not change the aircraft's stability, only the equilibrium stick force. Control effectiveness depends on deflection angle, dynamic pressure, surface area and moment arm. As speed rises, a given deflection produces more moment, which is why V speeds for manoeuvre and flap extension exist. At high speed, reversal and aeroelastic effects can reduce effectiveness.
Traps and worked example
Traps: assuming trim changes the aircraft's speed directly (it changes the speed at which the aircraft settles for a given power); assuming a control surface is effective in proportion to deflection alone; and ignoring the effect of slipstream and prop wash on elevator and rudder at high power and low speed.
Rule of thumb: control force for a given deflection varies with the square of speed.
Worked example: a 10° elevator deflection at 80 kt produces a certain pitching moment. At 160 kt the same deflection produces four times the moment, so only about 5° is needed for the same pitch change, and structural loads are four times greater.