This slide shows what happens when the pilot deflects a spoiler on one wing. A spoiler is a small, hinged plate on the top portion of a wing. Spoilers are used to disrupt or "spoil" the flow of air over a section of a wing.
When the pilot activates the spoiler, the plate flips up into the air stream. On this figure, the airplane's right wing spoiler is deployed, while the left wing spoiler is stored flat against the wing surface (as viewed from the rear of the airplane). The flow over the right wing will be disturbed by the spoiler, the drag of this wing will be increased, and the lift will decrease relative to the left wing. Because the forces are not equal, there is a net twist about the center of gravity in the direction of the larger force. The resulting motion will roll the aircraft to the rightt (clockwise) as viewed from the rear. If the pilot reverses the spoiler deflections (right spoiler flat and left spoiler up) the aircraft will roll in the opposite direction. We have chosen to name the left wing and right wing based on a view from the back of the aircraft towards the nose, because that is the direction in which the pilot is looking.
Let's investigate how the spoilers work by using a Java simulator.
You can deploy the spoilers by using the slider at the bottom.
[You can also test this effect yourself using a wooden glider. Just put some control tabs on the top of both wings. Bend one tab up and leave the other flat, and you will see the airplane roll when it is flown. The roll will be in the direction of the tab that is pulled up. The tabs can be yellow stick-ums or tape attached to the wings near the center of the chord (a straight line connecting the leading and trailing edges of an airfoil).]
When you travel on an airliner, watch the wings during turns. The pilot rolls the aircraft in the direction of the turn. You will probably be surprised at how little deflection is necessary to bank (roll) a large airliner. But be warned that there is a possible source of confusion on some airliners. We have been talking about rolling the aircraft by using a spoiler near the center of the wing chord to decrease the lift of one wing. On most airliners, the aircraft is rolled by using ailerons to increase the lift on one wing and decrease the lift on the other wing. This produces an unbalanced force, which causes the roll. You can tell whether an airliner is using spoilers or ailerons by noticing where the moving part is located. At the trailing edge, it's an aileron; between the leading and trailing edges, it's a spoiler. (Now you can dazzle the person sitting next to you on the plane!)
Spoilers can also be deflected in pairs; spoilers can be deployed on both wings at the same time. Deflecting spoilers in pairs will decrease lift and increase drag on both wings and is used to slow the airplane down as it prepares to land. When the airplane lands on the runway, the pilot usually brings up the spoilers to kill the lift, keep the plane on the ground, and make the brakes work more efficiently. (The friction force between the tires and the runway depends on the "normal" force, which is the weight minus the lift.)
Last Updated Wed, Jul 30 11:10:34 AM EDT 2008
by Ruth Petersen, 9/9/99