Spoilers are small, hinged plates on
the top portion of wings.
Spoilers can be used to slow an aircraft, or to make an aircraft
descend, if they are deployed on both wings.
Spoilers can also be used to generate a
rolling motion for an aircraft, if
they are deployed on only one wing.
This slide shows what happens when the pilot only deflects the
spoiler on the right wing.
Spoilers Deployed on Both Wings
When the pilot activates the spoilers, the plates flip up into the
air stream. The flow over the wing is disturbed by the spoiler, the
drag of the wing is increased, and the lift is decreased. Spoilers
can be used to "dump" lift and make the airplane descend; or they can
be 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. The lower the lift, the better the brakes
work. The additional drag of the spoilers also slows the plane
down.
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 download your own copy of this simulator for use off line. The program
is provided as Spoil.zip. You must save this file on your hard drive
and "Extract" the necessary files from Spoil.zip. Click on "Spoilview.html"
to launch your browser and load the program.
Spoiler Deployed on Only One Wing
A single spoiler is used to bank the aircraft; to cause one wing tip to move
up and the other wing tip to move down.
The banking creates an unbalanced side force component of the
large wing lift force
which causes the aircraft's flight path to
curve.
(Airplanes turn because of banking, not because
of the force generated by the
rudder.
On the 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.
The lift force (F) is applied at the
center of pressure
of the segment of the wing containing the spoiler.
This location is some distance (L) from the aircraft
center of gravity which creates a
torque
T = F * L
about the center of gravity.
The net torque causes the aircraft to rotate
about its center of gravity. The
resulting motion will roll the aircraft to the right
(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 download your own copy of this simulator for use off line. The program
is provided as Spoilrol.zip. You must save this file on your hard drive
and "Extract" the necessary files from Spoilrol.zip. Click on "Rolsview.html"
to launch your browser and load the program.
[You can also test the roll 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!)