Frame of reference - forces in a turn (airplane)

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Discussion Overview

The discussion revolves around the depiction of forces acting on an airplane during a turn, comparing two different illustrations. Participants explore the implications of different reference frames—specifically inertial versus non-inertial—and how these affect the understanding of forces such as centripetal and centrifugal forces in the context of aerodynamics.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants argue that the first illustration is correct as it analyzes the forces from a stationary reference frame, suggesting that unbalanced forces indicate the airplane is accelerating towards the center of the turn.
  • Others propose that the second illustration shows how centripetal force increases during sharper turns, indicating that both images represent similar concepts but from different perspectives.
  • One participant emphasizes that the second image depicts a non-inertial reference frame where forces appear balanced, questioning the utility of this perspective for pilots since it suggests zero acceleration.
  • Another participant notes that while the airplane experiences forces towards and away from the axis of rotation, the interpretation of these forces can vary depending on the chosen reference frame.
  • There is mention of centrifugal force being considered fictional by some, which adds to the complexity of understanding forces in a rotating frame.

Areas of Agreement / Disagreement

Participants express differing views on the accuracy and utility of the two illustrations, indicating that multiple competing interpretations of the forces in a turn remain unresolved.

Contextual Notes

Participants highlight the dependence on reference frames and the implications of balanced versus unbalanced forces, but do not resolve the underlying assumptions or definitions that could clarify these points.

italia458
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Hi,

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

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

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!
 
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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.
 
italia458 said:
Hi,

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

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

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!

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:
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.

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.

Relative to the rotational reference frame, the forces the plane experiences are trying to push it away from the axis of rotation.

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

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