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at the aft end of the aircraft provides the counterbalancing
force to provide a level attitude in normal
flight. Normally, the pilot will "trim" the elevators,
by use of the trim tab control in the cockpit, to
cause the elevators to provide the correct elevator
balance force to relieve the pilot from constant
elevator control.
You can readily see that loading of the aircraft, which affects the CG, is a critical consideration in
properly balancing the aircraft and it's controllability.
If the pilot pulls back on the control wheel, an "up-elevator" condition results. This forces the tail
downward, causing the aircraft to assume a "nose up" attitude. Likewise, a forward movement of the
control wheel by the pilot causes a "down elevator" state. This causes the tail to rise, forcing the aircraft
into a "nose low" attitude. By use of the elevator trim control (a small wheel or crank in the cockpit), the
pilot can cause the aircraft to remain in a nose-up, level, or nose down attitude.
As can be seen in the diagram above, when the CG is forward, a greater downward force is required by
the elevators to produce a level attitude. Likewise, when the CG is aft, the elevators must produce less
downward force to maintain level flight. NOTE: If the CG gets behind the Center of lift (the fulcrum) the
aircraft becomes unstable because the CG is aft of the fulcrum. IT MAY BE POSSIBLE TO EXCEED
THE TRIM CAPABILITY OF THE ELEVATORS SUCH THAT THE AIRCRAFT ALWAYS WANTS
TO NOSE UP, AND BE UNSTABLE. Therefore the pilot must pay attention to proper loading of the
aircraft. This will be discussed in greater detain under the subject of Weight and Balance.
Effects Of Attitude Change
Aerodynamics
http://www.uncletom2000.com/gs/aerodyn.htm (2 of 12) [1/23/2003 11:18:49 AM]
When the wing is in a given attitude with respect to the Relative Wind (R W) as shown in the diagram
below, the wing produces a Vertical Lift Force (LIFT) which is perpendicular to the Relative Wind..
There is also a DRAG component operating parallel
to the Relative Wind in opposition to the forward
motion of the wing. Drag is created as a natural part
of producing lift. These two forces intersect at a
point called the CL (center of lift}, or is also called
the CP (center of pressure]. The LIFT and DRAG
force vectors can be resolved into a single force
vector called the RESULTANT force.
Envision if the Angle of Attack is increased. The
Vertical Lift decreases in value, and the horizontal
force of Drag increases. Therefore, when a pilot
wants to slow the aircraft, the nose of the aircraft must be slowly raised into a greater "nose up" attitude,
causing drag to increase, thus slowing the aircraft. This increase of angle of attack has limits, however.
The wing design of most small aircraft, the wing has a "Critical Angle Of Attack" (somewhere around
18° to 20°) at which point the wing ceases to create sufficient lift to fly, and the wing STALLS. The air
flowing over the wing becomes so disturbed that adequate lift to sustain flight ceases, and the aircraft
pitches "nose down". This is a STALL.
The primary way to recover from a stall is to push the nose further downward, thus decreasing the Angle
Of Attack so that the wing flies again.
Also, envision in the diagram, when the pilot pushes the nose down by use of forward elevator, the Angle
of Atack decreases, thus decreasing the drag. Therefore, when power is held constant, the angle of attack
(nose high, level, or nose low) provides "Airspeed Control".
Assume for example, an aircraft has been cruising at 120 knots. When the aircraft enters the landing
pattern of an airport, the pilot may want to reduce speed to 90 knots. The pilot must reduce power to
prevent an altitude increase, and concurrently raise the nose of the aircraft so that the drag is increased
sufficiently to slow the aircraft to 90. Later, when on the final approach for landing, the pilot may wish to
slow even further, say to 70 knots. Power can be further reduced and the nose raised further, to again
increase drag. In addition, the pilot may add 10,20 or 30 degrees of flaps to add an additional drag and
lift.
The important point is that ATTITUDE is the primary control of airspeed; not THROTTLE! However, if
level flight is to be maintained, appropriate changes in power must be made whenever the pitch attitude
is made to prevent gaining or loosing altitude.
The Turn
Aerodynamics
http://www.uncletom2000.com/gs/aerodyn.htm (3 of 12) [1/23/2003 11:18:49 AM]
In order to turn the aircraft, it must be placed into a BANKED state, where one wing is high, the other
　
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