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Trimmed
Aircraft
Answers
- An airplane has
a wing lift of 30,000 lbs. which is centered 20 feet to the right of
the cg. The tail lift is centered 100 feet to the right of the cg. How
much force must the tail lift supply in order to keep the plane from
rotating? Is this force upward or downward?
Step 1: W
= 30,000 ibs. , dw = 20 ft. , dt = 100 ft.
Step
2: T
= -( W x dw ) / dt = -6,000 lbs. , downward
- Continuing with
Problem 1, suppose the lift force is located 20 feet to the left of
the cg. How much force must the tail lift supply in order to keep the
plane from rotating ? Is this force upward or downward?
(Hint:
Distances measured to the right of the cg are positive while distances
to the left are negative.)
Step 1: W
= 30,000 ibs. , dw = -20 ft. , dt = 100 ft.
Step
2:
T = -( W x dw ) / dt = +6,000 lbs. , upward
- Explain why the
tail lift force is different in Problems 1 and 2 while the wing lift
force is the same.The tail lift force creates
a torque that must cancel any torque the wing lift force creates. Torque
created by the wing lift force depends on the amount of wing lift force
and the center of pressure position with respect to the cg.
- Consider Problem
1 again. Suppose the fuel tanks become significantly lighter after a
long flight. This causes the cg to move toward the front of the plane
(to the left). Will a change in the position of the cg of 10 feet to
the left affect the previously trimmed (balanced) airplane? (Mathematically
support your answer.)
Step 1: W
= 30,000 ibs. , dw = 30 ft. , dt = 110 ft.
Step 2: T
= -( W x dw ) / dt = -8,181.82 lbs. , downward.
The change in the position of the cg will affect the trim of the airplane
because the new value of the tail lift required for balance is not the
same as the old value.
- If the movement
of the cg causes the airplane to become unbalanced, what do you think
the pilot will do to regain balance?
If the airplance becomes unbalanced, the
pilot may adjust the tail lift to its proper value by moving the control
surfaces (specifically the elevators) on the tail.
- Consider Problem
2 again. Suppose the same thing that happened in Problem 4 happens in
Problem 2. Will a change in the position of the cg of 10 feet to the
left affect the previously trimmed (balanced) airplane? (Mathematically
support your answer.)
Step 1: W
= 30,000 ibs. , dw = -10 ft. , dt = 110 ft.
Step
2: T
= -( W x dw ) / dt = +2,727.27 lbs. , upward.
The change in the position of the cg will affect the trim of the airplane
because the new value of the tail lift required for balance is not
the same as the old value.
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