At the rear of the
fuselage of
most aircraft one finds a vertical stabilizer
and a rudder. The stabilizer is a fixed wing section
whose job is to provide
stability for the aircraft, to keep it flying straight. The
vertical stabilizer prevents side-to-side, or
yawing,
motion of the aircraft nose.
The rudder is the small moving section at the rear of the
stabilizer that is attached to the fixed sections by hinges.
Because the rudder moves, it varies the amount of force
generated by the tail surface and is used to generate and control the
yawing motion of the aircraft.
This slide shows what happens when the pilot deflects the
rudder, a hinged section at the rear of the vertical
stabilizer.
The rudder is used to control the position of the nose of the aircraft.
Interestingly, it is NOT used to turn the aircraft in flight. Aircraft
turns
are caused by banking the aircraft to one side using either
ailerons or
spoilers.
The banking creates an unbalanced side force component of the
large wing lift force
which causes the aircraft's flight path to curve.
The rudder input insures that the aircraft is properly aligned to the
curved flight path during the maneuver. Otherwise, the aircraft would
encounter additional drag or even a possible adverse yaw condition
in which, due to increased drag from the control surfaces, the nose
would move farther off the flight path.
The rudder works by changing the effective shape of the airfoil
of the vertical stabilizer.
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 increased
deflection, the lift will increase in the opposite direction. The
rudder and vertical stabilizer are mounted so that they will produce
forces from side to side, not up and down.
The side force (F) is applied through the
center of pressure
of the vertical stabilizer which is
some distance (L) from the aircraft
center of gravity. This creates a
torque
T = F * L
on the aircraft and the aircraft
rotates
about its center of gravity.
With greater rudder
deflection to the left as viewed from the back of the aircraft, the
force increases to the right.
If the pilot reverses the rudder deflection to the right,
the aircraft will yaw in the opposite direction. We have chosen to
base the deflections 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 rudder works by using a Java
simulator.
You can change the rudder setting 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 Yaw.zip. You must save this file on your hard drive
and "Extract" the necessary files from Yaw.zip. Click on "Yawview.html"
to launch your browser and load the program.
[You can also test the yaw effect yourself using a paper airplane.
Just cut a control tab in the rear of the body. Bend the tab right to
make the tail go right and the nose go left, and the airplane will
turn to the left when it is flown. The same thing will work on a
simple wooden glider. The tab can be a yellow stick-um or tape
attached to the vertical stabilizer.]
On all aircraft, the vertical
stabilizer and rudder create a
symmetric airfoil.
This combination produces no side force when the rudder is aligned with the
stabilizer and allows either left or right forces, depending on the
deflection of the rudder. Some fighter planes have two vertical
stabilizers and rudders because of the need to control the plane with
multiple, very powerful engines.
You can view a short
movie
of "Orville and Wilbur Wright" explaining how the rudder
was used to control the yaw of their aircraft. The movie file can
be saved to your computer and viewed as a Podcast on your podcast player.
