Spoiler Ailerons Reduce Wing Twisting in Rolls

Spoiler ailerons as a fix for wing aeroelastic twisting in rolls apparently had their first trial on a Boeing B-47. Spoiler ailerons have lower section pitching moments for a given lift change than flap-type ailerons, which means lower wing twisting moments. The proposal to use spoilers on the B-47 came from Guy Townsend, who had experience with spoilers on a Martin airplane. From an unpublished Boeing document by Cook:

In order to test them quickly, we first tried a pop-up scheme, where the several segments would fully extend in sequence as the pilot’s wheel was rotated. Electric solenoid valves at each hydraulic cylinder were programmed to open in sequence. However, this was too jerky, and proportional control was found to be required. This was later done on the B-52 mechanically.

The next step in complication was on the -80 [prototype Boeing 707], when it was decided to use the spoiler not only for roll control, but also for drag brakes in the air, and on the ground to unload the wing for better braking. This required a “mixer box.” While this system has proved reliable on 707 and subsequent models, the programming of spoilers electrically saves space and weight, and probably would provide roll control with safety by using the redundancy provided by the multiple segmented spoilers. The mechanically controlled aileron still provides a good backup for emergency [Figure 19.3].

Upper surface spoilers for lateral control, sometimes augmented by flap-type ailerons for low-speed control, are a standard feature on modern high-aspect-ratio swept-wingjets. They can be seen on a great variety of airplanes, such as the Douglas A3D-1; the Lockheed L-1011 and C-5A; the Convair 880M; the McDonnell-Douglas DC-8, DC-9, DC-10, and MD-11; the Airbus A310 and A320; and the Boeing B-52, 727, 737, 747, 757, 767, and 777 (Figure 19.4). When installed just ahead of slotted wing flaps, spoilers become slot – width control devices when the flaps are down, providing an additional bonus of powerful low-airspeed lateral control.

Aileron reversal is still a potential problem even in this modern age of supersonic air­planes and digital computers, on airplanes with straight as well as swept wings. This is indicated by the chart comparing various aileron designs for Boeing’s 2707 SST proposal. Spoiler ailerons would be needed to avoid major losses in aileron control power due to wing twist at high airspeeds, even for the 2707’s low-aspect-ratio wing (Figure 19.5).