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Answers
  1. 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

  2. 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

  3. 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.
  4. 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.
  5. 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.
  6. 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.
Please send any comments to:
Curator: Tom.Benson@grc.nasa.gov
Responsible Official: Kathy.Zona@grc.nasa.gov