 Ailerons:
The two ailerons, one at
the outer trailing edge of each wing, are movable surfaces that
control movement about the longitudinal axis. The movement is roll.
Lowering the aileron on one wing raises the aileron on the other.
The wing with the lowered aileron goes up because of its increased
lift, and the wing with the raised aileron goes down because of its
decreased lift. Thus, the effect of moving either aileron is aided
by the simultaneous and opposite movement of the aileron on the
other
wing.
Rods or cables connect the ailerons to each other and to the
control wheel (or stick) in the cockpit. When pressure is applied to
the right on the control wheel, the left aileron goes down and the
right aileron goes up, rolling the airplane to the right. This
happens because the down movement of the left aileron increases the
wing camber (curvature) and thus increases the angle of attack. The
right aileron moves upward and decreases the camber, resulting in a
decreased angle of attack. Thus, decreased lift on the right wing
and increased lift on the left wing cause a roll and bank to the
right.
Elevators:
The elevators control the movement of the airplane about its
lateral axis. This motion is pitch. The elevators form the rear part
of the horizontal tail assembly and are free to swing up and down.
They are hinged to a fixed surface--the horizontal stabilizer.
Together, the horizontal stabilizer and the elevators form a single
airfoil. A change in position of the elevators modifies the camber
of the airfoil, which increases or decreases lift.
Like the ailerons, the elevators are connected to the control
wheel (or stick) by control cables. When forward pressure is applied
on the wheel, the elevators move downward. This increases the lift
produced by the horizontal tail surfaces. The increased lift forces
the tail upward, causing the nose to drop. Conversely, when back
pressure is applied on the wheel, the elevators move upward,
decreasing the lift produced by the horizontal tail surfaces, or
maybe even producing a downward
force. The tail is forced
downward and the nose up.
The elevators control the angle of attack of the wings. When back
pressure is applied on the control wheel, the tail lowers and the
nose raises, increasing the angle of attack. Conversely, when
forward pressure is applied, the tail raises and the nose lowers,
decreasing the angle of attack.
Rudder:
The
rudder controls movement of the airplane about its vertical axis.
This motion is yaw. Like the other primary control surfaces, the
rudder is a movable surface hinged to a fixed surface which, in this
case, is the vertical stabilizer, or fin. Its action is very much
like that of the elevators, except that it swings in a different
plane--from side to side instead of up and down. Control cables
connect the rudder to the rudder pedals.
Trim Tabs:
A trim tab is a small, adjustable hinged surface on the trailing
edge of the aileron, rudder, or elevator control surfaces. Trim tabs
are labor saving devices that enable the pilot to release manual
pressure on the primary controls.
Some airplanes have trim tabs on all three control surfaces that
are adjustable from the cockpit; others have them only on the
elevator and rudder; and some have them only on the elevator. Some
trim tabs are the ground adjustable type only.
The tab is moved in the direction opposite that of the primary
control surface, to relieve pressure on the control wheel or rudder
control. For example, consider the situation in which we wish to
adjust the elevator trim for level flight. ("Level flight" is the
attitude of the airplane that will maintain a constant altitude.)
Assume that back pressure is required on the control wheel to
maintain level flight and that we wish to adjust the elevator trim
tab to relieve this pressure. Since we are holding back pressure,
the elevator will be in the "up" position. The trim tab must then be
adjusted downward so that the airflow striking the tab will
hold the elevators in the desired position. Conversely, if forward
pressure is being held, the elevators will be in the down position,
so the tab must be moved upward to relieve this pressure. In this
example, we are talking about the tab itself and not the cockpit
control.
Rudder and aileron trim tabs operate on the same principle as the
elevator trim tab to relieve pressure on the rudder pedals and side
ward pressure on the control wheel, respectively.
Hovercraft -
As you can see from the above description - all control surfaces
are in a free air stream , a Hovercraft with its limited speed has
only the duct exit as a sufficient airflow area where control
surface will make sense. Once you tilt the control surface in
excess of 25% off its centerline - you will reduce airflow and
thrust.
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