This page is intended for college, high school, or middle school students.
For younger students, a simpler explanation of the information on this page is
available on the
Aircraft Roll Motion
In flight, any aircraft will rotate about its
center of gravity,
a point which is the average location of the mass of the aircraft.
We can define a three dimensional coordinate system through the
center of gravity with each axis of this coordinate system perpendicular
to the other two axes.
We can then define the
of the aircraft by the amount of
rotation of the parts of the aircraft along these principal axes.
The roll axis lies
along the aircraft centerline. A
is an up and down movement of the wings of the aircraft as shown in the
The rolling motion is being caused by the deflection of the
ailerons of this aircraft. The aileron is
a hinged section at the rear of each wing.
The ailerons work in opposition; when the right aileron goes
up, the left aileron goes down.
As described on the shape effects slide,
changing the angle of deflection at the rear of an airfoil will
change the amount of lift generated by the foil. With greater
downward deflection, the lift will increase in the upward direction;
with greater upward deflection, the lift will decrease in the upward
direction. Since the ailerons work in pairs, the lift on one wing increases
as the lift on the opposite wing decreases. Because the
forces are not equal, there is a net twist, or
about the center of
gravity and the aircraft rotates about the roll axis. The pilot can
use this ability to
the aircraft which causes the airplane to turn.
On this page we have demonstrated an aircraft roll induced by movement
of the ailerons, but there are other ways to produce a rolling motion
on an aircraft.
The Wright brothers used a method called
Their wings were wired together in such a way that the outer panels
of each wing could be twisted relative to the inner panel. The twisting
changed the local
angle of attack of sections of the wing which
changed the lift being generated by that section. Unequal forces on the
wings caused the aircraft to roll.
Many modern airliners use a
spoiler to roll the aircraft.
A spoiler is a plate that is raised
between the leading and trailing edges of the wing.
The spoiler effectively changes the
shape of the airfoil, disrupts the flow over the wing, and causes a
section of the wing to decrease its lift. This produces an unbalanced
force with the other wing, which causes the roll. Airliners use
spoilers because spoilers can react more quickly than ailerons and
require less force to activate, but they always decrease the total
amount of lift for the aircraft. It's an interesting trade! 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!)
You can view a short
of "Orville and Wilbur Wright" explaining how wing warping
was used to roll their aircraft. The movie file can
be saved to your computer and viewed as a Podcast on your podcast player.
Here is a still slide of the animation:
Aircraft Roll Motion:
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