# Question about lift affecting airplane in a loop

1. Apr 28, 2013

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. Apr 28, 2013

### Staff: Mentor

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

3. Apr 28, 2013

### sophiecentaur

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?

4. Apr 29, 2013

### CWatters

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.

5. Apr 29, 2013

### A.T.

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

6. Apr 29, 2013

### sophiecentaur

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.

7. Apr 29, 2013

### sophiecentaur

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

8. Apr 30, 2013

### Danger

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.

9. Apr 30, 2013

### mikeph

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.

10. Apr 30, 2013

### AlephZero

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

11. Apr 30, 2013

### rcgldr

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.