At the front of all of the Wright brothers'
aircraft
one finds the elevators.
The elevators are a pair of movable wings which are controlled by the pilot.
This slide shows what happens when the pilot deflects the
elevators leading edges upward. How does changing the elevator angle
affect the aircraft?
As described on the inclination effects page,
changing the angle of attack of a wing or airfoil changes
the amount of lift generated by the foil. With greater upward
deflection of the leading edge, lift increases in the upward direction.
With a
downward deflection, lift increases in the downward direction.
The lift force of the elevator is applied some distance from the aircraft
center of gravity. This creates a
torque on the aircraft and the aircraft
rotates
about its center of gravity.
Pulling the leading edge upward will cause the entire aircraft
to
pitch
nose up. The pilot can use this ability to make the airplane
rise or dive.
Let's investigate how the elevator works by using a Java
simulator.
You can change the elevator setting by using the slider at the right.
You can also download your own copy of this applet by pushing the following button:
The program is downloaded in .zip format. You must save the file to disk and
then "Extract" the files. Click on
"Elv.html" to run the program off-line.
The placement of the elevators at the front of
aircraft is rather unique for the Wright flyer. Modern aircraft
typically have the elevator
at the rear, attached to the horizontal
stabilizer. The Wright's placed their elevator at the front to provide
protection to the pilot in the event of a crash. (The pilot of this aircraft
lies next to the engine on the lower wing.) But there is also a static performance
advantage when the elevator is placed forward. Lifting wings have a natural tendency
to flip tail over nose because of the way the pressure is
distributed.
To overcome this tendency, the horizontal stabilizer on modern aircraft are usually
set at a negative angle generating a negative lift force to keep
the tail down and the airplane trimmed.
This negative force must be overcome with
greater lift by the wings. But, with the elevator in the front, the normal angle
is set slightly positive to trim the aircraft and this positive lift is added
to the wing's lift to get the airplane off the ground. For a power-limited
aircraft like the Wright Flyer, this is a better arrangement than having the
elevator at the rear.
There is a disadvantage to this arrangement, however, which the Wright brothers
worked for several years to overcome. If the airplane is displaced in pitch,
by a gust of wind causing the nose to pitch up, the angle of attack on the elevator increases.
This increases the lift force of the elevator, which causes the nose to pitch
up even more. The aircraft can become quite unstable in pitch and hard to fly
depending on
the size of the elevator, the distance to the center of gravity, and the natural
moment (tail over nose) of the wing geometry. The Wright
1903
and
1904
flyers had serious pitch problems created by the forward elevator. It wasn't
until the re-design of
1905
that the brothers solved this problem.
The Wright flyers were
highly maneuverable but not very stable.
The brothers
had to constantly provide the stability for the aircraft by
working the flight controls to change the
elevator setting.
Like the Wright 1903,
modern fighters are also designed to be highly
maneuverable but not very stable. Fighters now use a computerized
stability augmentation system (SAS) to reduce the work load on the pilot.
You can view a short
movie
of "Orville and Wilbur Wright" explaining how the elevator
was used to control the pitch of their aircraft. The movie file can
be saved to your computer and viewed as a Podcast on your podcast player.
