Flight Controls — Page 180, Lesson 236

Let's pick up with the speed brake and its different types. I want to walk you through the ideal behaviour first, because that sets the standard for everything else. Ideally, a speed brake should produce an increase in drag with no loss of lift and no change in pitching moment. That's the perfect outcome — you want to slow down, not disturb the wing's lift or make the nose pitch up or down. The fuselage-mounted speed brake is the type best suited to meet those requirements. Because it sits on the fuselage rather than on the wing, it doesn't interfere with the lift being generated, and it doesn't create an asymmetric pitching moment. That's the cleanest solution. However, there's a practical consideration. The wing-mounted spoiler already gives an increase in drag, so it's convenient to use the spoilers as speed brakes in addition to their lateral control function. Lateral control means roll control — the spoilers help you bank the aircraft. So you get double duty from the same surface. To operate them as speed brakes, they're controlled by a separate lever in the cockpit, and they move symmetrically — both spoilers go up together by the same amount. Now, there's an important limitation here. Speed brakes are normally cleared for operation up to VMO. VMO is the maximum operating speed — the fastest speed the aircraft is certified to fly at. But even within that clearance, at high speeds the speed brake may "blow back" from the fully extended position. Blow back means the aerodynamic force pushes the surface back down from its full extension, so it doesn't stay fully deployed. And here's the key point: the spoilers will still function as a roll control whilst being used as speed brakes. They do this by moving differentially from the selected brake position. So if you've set both spoilers up symmetrically as a brake, and you want to roll, one spoiler moves further up while the other moves back down from that brake position — that differential movement gives you the roll control. Now let's look at the effect of speed brakes on the drag curve. The drag resulting from the operation of speed brakes is profile drag. Profile drag is the drag caused by the shape of the surface and the friction of air moving over it — it's not induced drag from lift generation. So operating the speed brakes doesn't just increase total drag; it also decreases Velocity Minimum Drag, which we abbreviate as Vmd. Vmd is the speed at which total drag is at its minimum. By adding profile drag, you shift that minimum drag point to a lower speed. This is advantageous at low speeds, because the speed stability will be better than with the aircraft in the clean configuration. Clean configuration means no flaps, no slats, no gear extended — just the basic airframe. So at low speeds, having the speed brakes out gives you better speed stability, meaning the aircraft is more resistant to speed changes. Now let's move to ground spoilers, which are also called lift dumpers. During the landing run — that's the roll along the runway after touchdown — the decelerating force is given by two things: the aerodynamic drag and the drag of the wheel brakes. The wheel brake drag depends on the weight on the wheels. But here's the problem: that weight on the wheels will be reduced by any lift that the wing is producing. If the wing is still generating lift, it's holding some of the aircraft's weight off the wheels, so the brakes have less weight to grip against and produce less braking force. The wing lift can be reduced by operating the wing spoilers. So by deploying the spoilers, you dump the lift, which increases the weight on the wheels, which increases the brake drag. Both the brake drag and the aerodynamic drag are therefore increased, and the landing run is reduced — the aircraft stops in a shorter distance. On many aircraft types, additional spoilers are provided for use on the ground. These are the ground spoilers, or lift dumpers. They're made inoperative in flight by a switch on the undercarriage leg, which is operated by the extension of the leg after take-off. So when the landing gear extends after take-off, that switch is triggered, and the ground spoilers are deactivated — they can't be used in the air. They're only available once the gear is down on the ground. Now let's move to directional control. Control in yaw is obtained by the rudder. Yaw is the rotation of the nose left or right around the vertical axis. The rudder is required to do several specific jobs. First, it maintains directional control with asymmetric power — that's when one engine is producing more thrust than the other, which would otherwise yaw the aircraft. Second, it corrects for crosswinds on take-off and landing — a side wind pushing the nose off the runway heading. Third, it corrects for adverse yaw — that's the tendency of the nose to yaw in the opposite direction to a roll, caused by the difference in drag between the up-going and down-going ailerons. Fourth, it recovers from a spin — a spin is an aggravated stall with autorotation, and the rudder is the primary recovery control. And fifth, it corrects for changes in propeller torque on single-engine aircraft — the torque reaction from the propeller tends to roll and yaw the aircraft, and the rudder counters that. That's the full picture of speed brakes, ground spoilers, and the rudder's role in directional control.