how it works: roll spoilers


On many jets and turboprops, roll spoilers are used to assist the ailerons in banking and thereby turning the plane. Roll spoilers are flat panels mounted on the upper wing surfaces, which deploy upward 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 spoilers deploy as a function of airspeed. For example, on the de Havilland Dash-8, two roll spoilers deploy on each wing with the aileron below 140 knots, but only one operates per wing above that airspeed.

The reason roll spoilers are often required on high-speed aircraft is because they operate across such great speed ranges — such planes must resolve high-speed aerodynamics with the slow flight required for safe takeoffs and landings. Planes fly fastest with small, thin wings and high wing loading. Safe take-offs and landings, on the other hand, require high-camber, high-lift wings, with low wing loading. 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 big flaps and leading edge devices (LEDs) like you see on jetliners, which effectively convert the wing from a high-speed shape to a low-speed, takeoff-and-landing shape. The problem is that in order to make wing size (and drag) small for optimum cruise speeds, the flaps must extend across as much of the wingspan as possible for adequate low-speed effectiveness. With all those flaps installed, there’s little 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 when taking off and landing. 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 utilizes roll spoilers AND two ailerons per wing — the outboard ones are locked above 240 kts).

On a few aircraft 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 for ailerons at all! So on MU-2s, all roll control is accomplished by spoilers. Since spoilers effect roll by destroying lift, crosswind techniques for such aircraft must be modified under marginal take-off and landing situations.

Incidentally, Transport Category Airplanes must be equipped with redundant or separated primary flight controls in order to overcome any control jams. So on planes that have them, the roll spoilers usually connect to one pilot’s flight controls, while the ailerons connect to the the other. The two control yokes are mechanically linked so ailerons and spoilers work together when turning either yoke. But if either the roll spoilers or the ailerons were to jam, a clutch connecting the two systems can be released or overcome, allowing one pilot to fly via the one that still works. Pretty cool, eh?!!

 To learn more about turbine aircraft and how they work, see The Turbine Pilot’s Flight Manual and included Aircraft Systems CD-ROM. ©2009 Gregory N. Brown

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