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Spoiler Aileron Applications
A very early application of plug ailerons was to the Northrop P-61 Black Widow, which went into production in 1943. The P-61 application illustrates the compromises that are needed at times when adapting a device tested in a wind tunnel to an actual airplane. The plug aileron is obviously intended to work only in the up position. However, it turned out not to be possible to have the P-61 plug ailerons come to a dead stop within the wing when retracting them from the up position. The only practical way to gear the P-61 plug ailerons to the cable control system attached to the wheel was by extreme differential. Full up-plug aileron extension on one side results in a slight amount of down-plug aileron angle on the other side. The down-plug aileron actually projects slightly from the bottom surface of the wing. Down-plug aileron angles are shielded from the airstream by a fairing that looks like a bump running spanwise.
Plug-type spoiler ailerons are subject to nonlinearities in the first part of their travel out of the wing. Negative pressures on the wing’s upper surface tend to suck the plugs out, causing control overbalance. Centering springs may be needed. There can be a small range of reversed aileron effectiveness if the flow remains attached to the wing’s upper surface behind the spoiler for small spoiler projections. Nonlinearities at small deflections in the P-61 plug ailerons were swamped out (as an afterthought) by small flap-type ailerons, called guide ailerons, at the wing tips.
Early flight and wind-tunnel tests of spoilers for lateral control disclosed an important design consideration, related to their chordwise location on the wing. Spoilers located about midchord are quite effective in a static sense but have noticeable lags. That is, for a forward-located spoiler, there is no lift or rolling moment change immediately after an abrupt up-spoiler deflection. Since airfoil circulation and lift are fixed by the Kutta trailing – edge condition, the lag is probably related to the time required for the flow perturbation at the forward-located spoiler to reach the wing trailing edge. Spoilers at aft locations, where flap-type ailerons are found, have no lag problems (Choi, Chang, and Ok, 2001).
Another spoiler characteristic was found in early tests that would have great significance when aileron reversal became a problem. Spoiler deflections produce far less wing section pitching moment for a given lift change than ordinary flap-type ailerons. The local section pitching moment produced by ailerons twists the wing in a direction to oppose the lift due
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Outboard Aileron
Figure 5.11 Open slot-lip spoilers on the Boeing 707. Note the exposed upper surface of the first element of the flaps. The open spoilers destroy the slot that ordinarily directs the flow over the flap upper surface, reducing flap effectiveness. The reduced lift improves lateral control power when the spoilers are used asymmetrically or the airplane’s braking power when deployed symmetrically on when the ground. (From Cook, TheRoad to the 707, 1991) |
to the aileron. This is why spoilers are so common as lateral controls on high-aspect ratio wing airplanes, as discussed in Chapter 19, “The Elastic Airplane.”
Slot-lip spoiler ailerons are made by hinging the wing structure that forms the upper rear part of the slot on slotted landing flaps. Since a rear wing spar normally is found just ahead of the landing flaps, hinging slot-lip spoilers and installing hydraulic servos to operate them is straightforward. There is a gratifying amplification of slot-lip spoiler effectiveness when landing flaps are lowered. The landing flap slot is opened up when the slot-lip spoiler is deflected up, reducing the flap’s effectiveness on that side only and increasing rolling moment (Figure 5.11).
