Question about lift affecting airplane in a loop

  1. Say I have an airplane that is performing a loop at a constant speed. Why is it that at the top of the loop the normal force points inward and not upward due to the lift of the plane?

    That way, at the top of the loop, when the pilot is upside down, he feels lighter due to the lift of the plane and the seat which is attached to the plane moving upwards and the gravitational force pulling the pilot downwards.
  2. jcsd
  3. russ_watters

    Staff: Mentor

    If he's upside-down and still at a positive angle of attack, the lift vector points toward the ground!
  4. sophiecentaur

    sophiecentaur 13,923
    Science Advisor
    Gold Member

    You need to ask yourself which direction the force from the wing will be working when the plane is upside down with the controls in the same position as when it started the loop. Is it still 'lifting' the plane? How would it 'know' which way up it was?
  5. If the sum of all the forces acting on the plaine didn't point inwards (downwards) it wouldn't complete the loop. If it acted outwards (upwards) it would keep going up.

    Aside: It's not strictly necessary for the lift force to be acting downwards (towards the ground). If it's going to complete the loop all that matters is that the net force acting on the aircraft is towards the ground.
  6. A.T.

    A.T. 6,194
    Gold Member

    The plane is not moving upwards at the top of the loop.
  7. sophiecentaur

    sophiecentaur 13,923
    Science Advisor
    Gold Member

    It would depend upon the speed and radius of the loop but the 'lift' force would never be upwards unless the controls were changed. You would want a hint of centripetal force from the wings, though, to stop you drifting out of your seat when going over the top.
  8. sophiecentaur

    sophiecentaur 13,923
    Science Advisor
    Gold Member

    HAHA. If it were moving upwards, he wouldn't be at the top of the curve.
  9. Danger

    Danger 9,878
    Gold Member

    Some people don't realize that a lot (not sure of the percentage) of aerobatic birds have symmetrical wings which provide equal "lift" when upside-down. It takes the guesswork out of some pretty gnarly stunts.
  10. starting a loop with a strong tailwind, I'd expect to get a fair bit of upwards lift as I passed vertical the first time.
  11. AlephZero

    AlephZero 7,298
    Science Advisor
    Homework Helper

    The tailwind is irrelevant, unless the wind suddenly changes direction while you are flying the loop.

    Everything that happens to an aircraft is relative to the air, not to the ground - except during takeoff, landing, and crashing :smile:
  12. rcgldr

    rcgldr 7,577
    Homework Helper

    In real life, constant speed during a loop of any shape would be very difficult, requiring adjustment of thrust during upwards movement and requiring adjustment of some type of variable air brake during downwards movment.

    As mentioned before, at the top of the loop, the tangent to the path of the loop is horizontal, and the normal force would have to be downwards and perpendicullar to the loop because at the top of a loop, the loop is curving "downwards" by definition. If the net normal force (gravity + lift) was zero, the airplane would be flying inverted but in a straight line. If the net normal force was upwards, the airplane would still be climbing, so it wouldn't be at the "top" of the loop.
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