Flight Controls — Page 180, Lesson 234

Let's pick up with the flaperon, because it's a clever answer to a real design problem. The flaps and the ailerons both live on the trailing edge of the wing, and they're both competing for the same limited space. For good take-off and landing performance, you want the flaps as large as possible. For a good rate of roll, you want the ailerons as large as possible. But you can't have both at maximum size on the same trailing edge. One solution is the flaperon. The idea is to droop the ailerons symmetrically — that is, both ailerons deflect down together — to augment the flap area. So they act like extra flap when you need extra lift for take-off and landing. Then, from that drooped position, they move differentially to give lateral control. Differential means they move in opposite senses — one up, one down — to roll the aircraft. So the same surface does double duty: drooped together for flap, then split differentially for roll. There's also another system where the trailing edge moveable surfaces perform the operation of both flaps and ailerons in a more integrated way. But the flaperon is the classic example of combining the two functions on one surface. Now let's move to the spoiler, which is a completely different device. The spoiler consists of part of the upper surface of the wing which can be raised. It's illustrated in Figure 8.2. When you raise it, it disturbs the airflow over the wing and reduces the lift. That's the core principle — it spoils the lift, hence the name. To function as a lateral control, the spoiler is raised on the wing which is required to move downwards, and it stays retracted on the other wing. Think about that carefully. If you raise the spoiler on the right wing, you reduce lift on the right wing, so the right wing drops. That's how it rolls the aircraft. Now here's a critical difference from the aileron. The aileron can increase lift on one wing while decreasing it on the other. The spoiler cannot give an increase of lift — it can only reduce it. So a roll manoeuvre controlled by spoilers will always give a net loss of lift. You're always sacrificing some lift to roll. But the spoiler has several advantages compared to the aileron, and I want to walk through each one. First, there is no adverse yaw. With an aileron, the down-going aileron increases drag and tends to yaw the nose away from the turn. With a spoiler, the raised spoiler increases drag, and so the yaw is in the same direction as the roll. That's a proverse yaw — it helps the turn rather than fighting it. Second, wing twisting is reduced. The loss of lift from a spoiler is distributed across the chord rather than being concentrated at the trailing edge. The chord is the distance from the leading edge to the trailing edge of the wing. An aileron applies its force at the trailing edge, which twists the wing. A spoiler spreads that load out, so there's less twisting. Third, at transonic speed its effectiveness is not reduced by shock induced separation. At high speeds, shock waves can cause the airflow to separate from the wing, and that reduces aileron effectiveness. The spoiler doesn't suffer that problem. Fourth, it cannot develop flutter. Flutter is a dangerous oscillation of the control surface. The spoiler is immune to it. And fifth, spoilers do not occupy the trailing edge, which can then be utilized for flaps. That's a big deal — it frees up the trailing edge for the flaps, which is exactly the space problem we started with. Now, in practice, how are spoilers used? On a few aircraft, lateral control is entirely by spoilers. But in the majority of applications, the spoilers work in conjunction with the ailerons. Here's why. Ailerons alone may be inadequate to achieve the required rate of roll at low speeds, when the dynamic pressure is low. Dynamic pressure is the pressure exerted by the moving air — at low speed it's low, so the ailerons don't generate as much force. And at high speeds, ailerons may cause excessive wing twist, and begin to lose effectiveness if there is shock induced separation. So spoilers can be used to augment the rate of roll, but they may not be required to operate over the whole speed range. On some aircraft, the spoilers are only required at low speed, and this can be achieved by making them inoperative when the flaps are retracted. So when the flaps are up, the spoilers don't work as lateral controls. When the flaps are down, they do. Here's the mechanical detail. Movement of the cockpit control for lateral control is transmitted to a mixer unit. That mixer unit causes the spoiler to move up when the aileron moves up, but to remain retracted when the aileron moves down. So the spoiler only ever goes up — it never goes down below the wing surface. It augments the up-going aileron's effect but stays out of the way on the down-going side. Finally, let's talk about speed brakes. These are devices to increase the drag of an aircraft when it is required to decelerate quickly or to descend rapidly. There are two main situations. Rapid deceleration is required if turbulence is encountered at high speed, to reduce the speed to the Rough Air Speed as quickly as possible. Rough Air Speed is the maximum speed at which you can safely fly through turbulent air — you want to get down to it fast. And a high rate of descent may be required to conform to Air Traffic Control instructions, and particularly if an emergency descent is required. So the speed brake is your tool for shedding speed or altitude quickly. That figure shows the leading edge of the up-going aileron protruding below the lower surface of the wing — it's a nice visual of how the aileron geometry works in practice.