Ailerons are normally situated at the wing tip — Page 173, Lesson 233

Let me pick up right where we left off — we were talking about ailerons, and now we're looking at where they're placed and the structural price we pay for that placement. Ailerons are normally situated at the wing tip. Why? Because that's where they give the greatest moment for the force they produce. Remember, a moment is a turning effect — force times distance from the pivot point. The wing tip is the furthest point from the aircraft's centreline, so a given aileron force there produces the largest rolling moment. That's the whole point of an aileron — to roll the aircraft. But here's the trade-off. Because they're at the tip, they also cause the maximum twisting and bending loads on the wing. Think about it — the wing is a long cantilevered structure. A force applied at the very end of it creates the biggest bending moment at the wing root, and the biggest twisting moment along the span. So the very thing that makes the aileron effective — being at the tip — is also what loads the wing structure hardest. And this can lead to a serious problem: a loss of effectiveness, or even reversal of the aileron. Reversal is exactly what it sounds like — you deflect the aileron to roll one way, and the wing twists so much that the aircraft actually rolls the other way. That's a catastrophic failure of control, and it's driven by the structural distortion we just talked about. So how do we reduce these effects? There are two main approaches. First, we can mount the ailerons further inboard — closer to the wing root. That reduces the moment arm, so the same force produces less twisting and bending. But it also reduces the rolling moment, so we lose effectiveness. The second approach is to fit two sets of ailerons. One set at the wing tip, for use at low speeds — because at low speeds the aerodynamic forces involved are low, so the structural loads stay manageable. And one set inboard, for use at high speeds — because at high speeds the forces are greater and could cause greater structural distortion. So the inboard set, with its shorter moment arm, is the one that can handle the high-speed loads without twisting the wing dangerously. And here's the summary rule you need to remember: only the inboard ailerons are used when the flaps are retracted. When the flaps are out — at low speed, high lift configuration — the tip ailerons do the work. When the flaps are retracted — clean configuration, high speed — the inboard ailerons take over. So the key relationship to hold in your head: tip ailerons give maximum moment but maximum structural load; inboard ailerons give less moment but less distortion. And the aircraft switches between them based on flap position, which is really a proxy for speed.