On many jet and turboprop aircraft, roll spoilers are used to assist the ailerons in banking, and therefore turning the plane. Roll spoilers are flat panels mounted on the upper wing surfaces, which deploy up into the slipstream on the down wing only, disturbing lift and thereby aiding the down wing aileron in effecting the turn.

Roll spoilers are interconnected with the ailerons, so as to perform in harmony with them. In many aircraft roll spoiler deployment is also regulated as a function of airspeed. For example, on the Dash-8 two roll spoilers deploy on each wing with aileron usage below 140 knots, but only one operates per wing above that airspeed.

The reason roll spoilers are required on many high-speed aircraft, is that they must operate under such high speed ranges -- these planes must resolve the aerodynamics of high speed cruise with the slow flight and high lift required for safe takeoffs and landings.

Planes can fly fastest with small, thin, low-camber swept wings with high wing loading, to minimize drag. Safe take-offs and landings, on the other hand, require high-camber, high lift wings, with low wing loading (and therefore high drag). In essence, two different airplanes are required, one that can go fast, and one that can get everybody off the ground in less than ten miles of runway!

This challenge is resolved through extensive use of those big flaps and leading edge devices (LEDs) you see on airliners, which effectively convert the wing of a jet from one shape to the other. The problem is that in order to make wing size (and drag) small for optimum cruise speeds, those high lift devices (LED's and flaps) must extend across as much of the wing's span as possible, for adequate low speed effectiveness.

With those big flaps installed, there's only so much room left on the wing for ailerons. Small ailerons may be fine for high speed cruise, but they're often too small for adequate roll response at low airspeeds, like at takeoff and landing times. One solution to this problem is to put multiple ailerons on each wing, separately activated as a function of airspeed. The other solution is to install roll spoilers to help the ailerons. The Boeing 767, for example, carries roll spoilers AND two ailerons per wing (the outboard ones are locked above 240 kts).

On a few aircraft types with very small wings, such as the Mitsubishi MU-2, the flaps must be so big to achieve reasonable landing speeds, that there's no room left on the wings for ailerons at all! So on MU-2s all roll control is done by roll spoilers. Since spoilers effect roll by destroying lift, crosswind techniques for such aircraft must be modified under marginal take-off and landing situations.