Roll Control by the Use of Spoilers

A spoiler is a device on the upper surface of an airfoil that, when extended into the flow, causes the flow to separate, resulting in a loss of lift. The effect is illustrated in Figure 8.32, which also illustrates several types of spoilers.

Roll control can be accomplished by the use of spoilers installed on the outboard extent of each wing in place of conventional ailerons. If a roll to the right is desired, the spoiler on that side is raised, reducing the lift on the right wing and causing a positive rolling moment. The drag will also be increased on the right wing, producing a favorable yawing moment.

Spoilers for roll control are currently used on several military airplanes and most jet transports to augment the primary aileron controls. They are also used on at least one general aviation airplane, the twin-turboprop Mitsubishi MU-2, as the sole means of roll control. In this case, the use of spoilers only for roll control permits the use of full-span Fowler flaps with a higher wing loading. According to Reference 8.10, “As the requirements increase for higher wing loading to improve ride and performance, better handling quali-

ties, and improved high lift devices, spoilers will probably be applied more extensively to light aircraft in the future.”

Figure 8.33 (taken from Ref. 8.8), presents some rolling moment data typical of the retractable spoiler aileron of the type pictured in Figure 8.32. Note that for aileron projections in excess of 2% of the chord, the variation of Ci with the projection is quite linear. However, for projections less than this amount, the graphs are highly nonlinear. In fact, little or no rolling moment is produced until the projection exceeds % of the chord. This “dead” region would not produce a very satisfactory feel to the pilot.

The configurations of Figure 8.32b and 8.32e are intended to remove the dead region at low spoiler deflections. As soon as the gap is opened, its influence is felt in producing separated flow. The configuration of Figure 8.32e is the type used on the MU-2.

In the past, spoilers have not been used as the sole primary source of roll control because of their nonlinear characteristics. At high angles of attack their effectiveness can be reduced or even reversed. This nonlinear behavior is also reflected in the control forces that result from nonlinear hinge moments. In addition, a lag can occur in reestablishing the attached flow pattern when the spoiler is retracted. Objections to spoiler ailerons have also been based on the fact that the rolling moment is produced by the loss of lift on one side of the wing. This results in an accompanying loss in altitude.

It appears that by careful design, these objections can be overcome. Also, actual flight experience and simulation studies indicate a negligible

difference in the roll dynamics between conventional and spoiler-type ailerons. Thus the loss in altitude associated with spoiler control does not seem to be a problem.