Flight Controls — Page 173, Lesson 231

Let’s start with mass balance, because it’s the first thing in this excerpt and it’s a classic exam favourite. Mass balance is a weight attached to the control surface forward of the hinge. Most control surfaces are mass balanced. The purpose is to prevent control surface flutter. Flutter is an oscillation of the control surface which can occur due to the bending and twisting of the structure under load. If the centre of gravity of the control surface is behind the hinge, its inertia causes it to oscillate about its hinge when the structure distorts. In certain circumstances the oscillations can be divergent, and cause failure of the structure. So the fix is to add weight to the control surface in front of the hinge line. That brings the centre of gravity of the control forward to a position normally close to, or slightly in front of, the hinge — but always to the point required by the designers. This reduces the inertia moments about the hinge and prevents flutter developing. Now, longitudinal control. Control in pitch is usually obtained by elevator or by a moving tailplane. The controls must be adequate to balance the aircraft throughout its speed range, at all permitted CG positions and configurations, and to give an adequate rate of pitch for manoeuvres. Lateral control is by the ailerons, producing a rolling moment by increasing the lift on one wing and decreasing it on the other. Now the key problem: adverse aileron yaw. The increased lift on the up-going wing gives an increase in induced drag, whereas the reduced lift on the down-going wing gives a decrease in induced drag. The difference in drag on the two wings produces a yawing moment opposite to the rolling moment — a roll to the left produces a yawing moment to the right. That’s adverse yaw. Three main methods reduce it. Differential ailerons: the linkage causes the up-going aileron to move through a larger angle than the down-going aileron. That increases drag on the up aileron and reduces it on the down aileron, reducing the drag difference. Frise ailerons: the Frise aileron has an asymmetric leading edge. The leading edge of the up-going aileron protrudes below the lower surface of the wing, causing high drag; the leading edge of the down-going aileron remains shrouded and causes less drag. Aileron-rudder coupling: the aileron and rudder systems are interconnected, so when the ailerons are deflected the rudder automatically moves to counter the adverse yaw. Also, if roll spoilers are used to augment the roll rate from the ailerons, they reduce adverse yaw, because the down-going wing gets an increase in drag due to the raised spoiler. Finally, inboard ailerons. Ailerons are normally at the wing tip to give the greatest moment for the force produced. But that also means they cause the maximum… and that’s where the excerpt cuts off.