# Frame of reference - forces in a turn (airplane)

1. May 28, 2013

### italia458

Hi,

I've seen the forces in a turn depicted two different ways:

http://selair.selkirk.ca/Training/Aerodynamics/images/lf-turn.gif [Broken]

http://www.free-online-private-pilot-ground-school.com/images/forces_during_turn.gif

Which one is correct?

I think the first one would be since it's analyzing it from one reference frame and the second picture appears to analyze it from multiple reference frames.

Regarding the second picture: if the forces are all balanced (as depicted), the airplane wouldn't be accelerating. How can it turn then?

Cheers!

Last edited by a moderator: May 6, 2017
2. May 28, 2013

### SteamKing

Staff Emeritus
I think the second picture is merely showing how the centripetal force increases in a sharper turn, where the plane is banked more. IMO, both pictures show the same thing, only slightly different.

3. May 28, 2013

### TheFerruccio

The first picture shows unbalanced forces and it says "Force of centripetal acceleration" centripetal means "towards the center" if I recall. So, the plane is accelerating towards the center of the axis of its turning radius. The frame of reference, in this case, is stationary, from the ground, or a stationary observation tower, looking towards the airplane in that instant that it was facing away from the viewer.

The second picture is from a rotational reference frame. The forces are balanced, in this case, because it is a non inertial reference frame. The forces are balanced, but in relation to a reference frame that is already rotating. The airplane is "stationary" relative to the rotational reference frame, but is still rotating. A centrifugal force is mentioned, so, in this case, it means "from the center." Relative to the rotational reference frame, the forces the plane experiences are trying to push it away from the axis of rotation.

At least, that's how I interpret it.

Last edited by a moderator: May 6, 2017
4. May 28, 2013

### italia458

Is there any benefit for a pilot to see acceleration relative to a rotational reference frame... that is rotating at the same velocity as the airplane?! Meaning zero acceleration. To me it seems at least useless and just wrong, in some way.

How do you know which forces the airplane is "experiencing"? I thought all the forces drawn on that airplane were what it was experiencing. If that's correct, then it's experiencing a force away from the axis of rotation (as you stated) but it's also experiencing a force towards the axis of rotation (centripetal force) and they're equal and opposite.

I understand that if you decide to have your reference frame move in the exact path of your object then there isn't any acceleration. But what's the point of that? Every force would be always balanced then and there would never be acceleration.

5. May 28, 2013

### Delta Kilo

The point of the second picture is to show the resultant load or "g-force" experienced by the airframe.

6. May 30, 2013