More bad physics - turns

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AuxBatOn
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Re: More bad physics - turns

Post by AuxBatOn »

RedAndWhiteBaron wrote: Sat Oct 17, 2020 5:52 pm Forgive me if this is obvious - but every illustration I've seen in this thread fails to account for the force of lift (be that positive or negative) generated from the empennage. It would seem to me that any discussion of the forces involved in a turn cannot be considered to be a complete argument without that force factored in.

But then again, I know nothing.
Generally, all lift is consolidated in one force acting upon the center of gravity. It is only important to separate the forces when you investigate handling qualities using various stability derivatives.
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Re: More bad physics - turns

Post by tsgarp »

RedAndWhiteBaron wrote: Sat Oct 17, 2020 5:52 pm Forgive me if this is obvious - but every illustration I've seen in this thread fails to account for the force of lift (be that positive or negative) generated from the empennage. It would seem to me that any discussion of the forces involved in a turn cannot be considered to be a complete argument without that force factored in.

But then again, I know nothing.
Good point. I hadn’t thought of that. At first glance the main thing the tail does is contribute to the yaw through the centripetal component of its force vector.
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Re: More bad physics - turns

Post by photofly »

But what happens in an inverted turn? Or in the Southern Hemisphere?
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Re: More bad physics - turns

Post by RedAndWhiteBaron »

photofly wrote: Sat Oct 17, 2020 6:25 pm But what happens in an inverted turn? Or in the Southern Hemisphere?
In both cases, the frame of reference remains the same. We simply invert either gravity or the coriolis force.

To be fair, I have never intentionally flown inverted, or in the southern hemisphere (although the latter would be a rather entertaining story, were it unintentional)
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Last edited by RedAndWhiteBaron on Sat Oct 17, 2020 6:59 pm, edited 2 times in total.
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Re: More bad physics - turns

Post by RedAndWhiteBaron »

AuxBatOn wrote: Sat Oct 17, 2020 6:13 pm
RedAndWhiteBaron wrote: Sat Oct 17, 2020 5:52 pm Forgive me if this is obvious - but every illustration I've seen in this thread fails to account for the force of lift (be that positive or negative) generated from the empennage. It would seem to me that any discussion of the forces involved in a turn cannot be considered to be a complete argument without that force factored in.

But then again, I know nothing.
Generally, all lift is consolidated in one force acting upon the center of gravity. It is only important to separate the forces when you investigate handling qualities using various stability derivatives.
This would be an acceptable simplification to me in stable flight, but not in a turn. The lift generated by the tail (again, either positive or negative) in a turn is very much different from that generated in stable flight.
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Re: More bad physics - turns

Post by digits_ »

RedAndWhiteBaron wrote: Sat Oct 17, 2020 6:34 pm
AuxBatOn wrote: Sat Oct 17, 2020 6:13 pm
RedAndWhiteBaron wrote: Sat Oct 17, 2020 5:52 pm Forgive me if this is obvious - but every illustration I've seen in this thread fails to account for the force of lift (be that positive or negative) generated from the empennage. It would seem to me that any discussion of the forces involved in a turn cannot be considered to be a complete argument without that force factored in.

But then again, I know nothing.
Generally, all lift is consolidated in one force acting upon the center of gravity. It is only important to separate the forces when you investigate handling qualities using various stability derivatives.
This would be an acceptable simplification to me in stable flight, but not in a turn. The lift generated by the tail (again, either positive or negative) in a turn is very much different from that generated in stable flight.
Even if that were true, you still end up with a vertical and horizontal component in the lift. It doesn't matter what the exact value is for the issue that was discussed.

Why do you think the tail generates a different kind of lift in a turn?
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Re: More bad physics - turns

Post by RedAndWhiteBaron »

digits_ wrote: Sat Oct 17, 2020 6:37 pm Why do you think the tail generates a different kind of lift in a turn?
It doesn't generate a different kind of lift (if we can agree on what lift is - and from what I remember of an argument back in July, we can't...) . It generates a different magnitude of lift, and in a different direction. But not really.

In stable flight, one can simplify lift as the force produced by the wings and elevator that allows for stable flight. In a turn however, while the lift produced by the wings is dependent only on angle of attack, the lift produced by the tail is dependent both on AoA and the elevator position. Some portion of that rear lift will act towards turning the plane, and given its moment (or its distance from the centre of mass), a greater portion than that generated by the wings.

I suppose I'm arguing that lift produced by the wings can be argued to be static, and not dependent on the state of the flight controls (notwithstanding flaps etc...), whereas lift produced by the tail is very much dependent on the state of flight controls. I apologize, I seem to be having some difficulty placing my argument into words.

So in a way, yes, it is a different kind of lift. I find these illustrations to be incomplete.
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Re: More bad physics - turns

Post by AuxBatOn »

RedAndWhiteBaron wrote: Sat Oct 17, 2020 6:34 pm
AuxBatOn wrote: Sat Oct 17, 2020 6:13 pm
RedAndWhiteBaron wrote: Sat Oct 17, 2020 5:52 pm Forgive me if this is obvious - but every illustration I've seen in this thread fails to account for the force of lift (be that positive or negative) generated from the empennage. It would seem to me that any discussion of the forces involved in a turn cannot be considered to be a complete argument without that force factored in.

But then again, I know nothing.
Generally, all lift is consolidated in one force acting upon the center of gravity. It is only important to separate the forces when you investigate handling qualities using various stability derivatives.
This would be an acceptable simplification to me in stable flight, but not in a turn. The lift generated by the tail (again, either positive or negative) in a turn is very much different from that generated in stable flight.
What about the lift component generated by the nose? By the struts? By the vertical tail?
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Re: More bad physics - turns

Post by RedAndWhiteBaron »

AuxBatOn wrote: Sun Oct 18, 2020 3:55 am What about the lift component generated by the nose? By the struts? By the vertical tail?
None of these are appreciably affected by the state of flight controls - that is, for a given attitude and airspeed, they will produce a known quantity of lift, whereas lift generated by the tail is very much dependent on the elevator position.
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Re: More bad physics - turns

Post by digits_ »

RedAndWhiteBaron wrote: Sat Oct 17, 2020 6:49 pm
digits_ wrote: Sat Oct 17, 2020 6:37 pm Why do you think the tail generates a different kind of lift in a turn?
It doesn't generate a different kind of lift (if we can agree on what lift is - and from what I remember of an argument back in July, we can't...) . It generates a different magnitude of lift, and in a different direction. But not really.

In stable flight, one can simplify lift as the force produced by the wings and elevator that allows for stable flight. In a turn however, while the lift produced by the wings is dependent only on angle of attack, the lift produced by the tail is dependent both on AoA and the elevator position. Some portion of that rear lift will act towards turning the plane, and given its moment (or its distance from the centre of mass), a greater portion than that generated by the wings.

I suppose I'm arguing that lift produced by the wings can be argued to be static, and not dependent on the state of the flight controls (notwithstanding flaps etc...), whereas lift produced by the tail is very much dependent on the state of flight controls. I apologize, I seem to be having some difficulty placing my argument into words.

So in a way, yes, it is a different kind of lift. I find these illustrations to be incomplete.
I"m not entirely sure I understand the point you are trying to make. The small change in lift generated by the elevators on the tail, causes a big change in lift on the wings, by changing it's angle of attack. That's the primary function of the elevator, right?

Are you trying to say that the, proportionally, the tail generates a different amount of lift in a turn vs in straight flight? If in straight and level flight, the wings generate 95% of the lift, and the tail 5%, then during a during it might be 96% and 4%? (just random numbers to clarify the example)

Regardless if that's true or not, the whole airplane is being kept in the air by a certain amount of lift generated by the whole airplane. The sum of all those different lift components is added up and represented by the single L vector. This representation is valid, as long as the lift is symmetrical on the left side vs the right side of the airplane.

If, during an established turn, there would be a significant difference of lift on the left vs right wing, then yes, the drawings would be wrong and inaccurate.

Could you make a drawing of what you think it should look like, taking into account the effects on the tail you are trying to describe?
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Re: More bad physics - turns

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https://www.businessinsider.com/israeli ... ing-2016-4

F15 flying with wing missing will get you thinking :)
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Re: More bad physics - turns

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RedAndWhiteBaron wrote: Sun Oct 18, 2020 10:19 am
AuxBatOn wrote: Sun Oct 18, 2020 3:55 am What about the lift component generated by the nose? By the struts? By the vertical tail?
None of these are appreciably affected by the state of flight controls - that is, for a given attitude and airspeed, they will produce a known quantity of lift, whereas lift generated by the tail is very much dependent on the elevator position.
I can tell you that lift from the body (especially the nose area) affects greatly the position of your elevator.
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Re: More bad physics - turns

Post by RedAndWhiteBaron »

digits_ wrote: Sun Oct 18, 2020 10:36 am I"m not entirely sure I understand the point you are trying to make. The small change in lift generated by the elevators on the tail, causes a big change in lift on the wings, by changing it's angle of attack. That's the primary function of the elevator, right?

Are you trying to say that the, proportionally, the tail generates a different amount of lift in a turn vs in straight flight? If in straight and level flight, the wings generate 95% of the lift, and the tail 5%, then during a during it might be 96% and 4%? (just random numbers to clarify the example)

Regardless if that's true or not, the whole airplane is being kept in the air by a certain amount of lift generated by the whole airplane. The sum of all those different lift components is added up and represented by the single L vector. This representation is valid, as long as the lift is symmetrical on the left side vs the right side of the airplane.

If, during an established turn, there would be a significant difference of lift on the left vs right wing, then yes, the drawings would be wrong and inaccurate.

Could you make a drawing of what you think it should look like, taking into account the effects on the tail you are trying to describe?
I'm not sure if I could make a drawing - my computer drawing skills are... mediocre, at best.

I'll try to rephrase it. In level flight, we can simplify lift as a single vector because our frame of reference is unchanging. The sum of all lift is directly above and opposite to the sum of all weight.

In a turn, the horizontal stabilizer will generate less lift (or at least, I believe this to be a safe assumption), when expressed as a proportion of total lift. This generates a turning moment around the lateral axis. When not level, this turning moment becomes one of the forces that act to turn the plane to its new heading. The aircraft turning therefore depends partially on the different magnitude of lift generated by the tail, and without factoring this turning moment in, the plane would slip.

Or, to put it another way - the diagrams in the first post of this thread could be said to fail to account for the fact that the centre of lift changes (moves forward, I think) in a turn compared to level flight, and that a large part of the reason why is the difference in lift generated by the tail.
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Re: More bad physics - turns

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RedAndWhiteBaron wrote: Sun Oct 18, 2020 1:31 pm I'll try to rephrase it. In level flight, we can simplify lift as a single vector because our frame of reference is unchanging. The sum of all lift is directly above and opposite to the sum of all weight.
We can always simplify lift into a single vector. When we talk about lift we're normally talking about net lift. Net lift is a single vector.
RedAndWhiteBaron wrote: Sun Oct 18, 2020 1:31 pm In a turn, the horizontal stabilizer will generate less lift (or at least, I believe this to be a safe assumption), when expressed as a proportion of total lift. This generates a turning moment around the lateral axis. When not level, this turning moment becomes one of the forces that act to turn the plane to its new heading. The aircraft turning therefore depends partially on the different magnitude of lift generated by the tail, and without factoring this turning moment in, the plane would slip.
I think you're getting confused between the transition into (or out of) the turn, versus a steady-state turn. Entering a turn, the aircraft has to transition from flying a straight line while maintianing orientation to flying a curve while rotating. This transition requires unbalanced moments. But once the rotation has started, it will continue without the requirement for unbalanced moments. So in a steady turn, there is no net moment acting on the aircraft -- rotation has already been established.
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Re: More bad physics - turns

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Conflicting Traffic wrote: Sun Oct 18, 2020 1:52 pm
RedAndWhiteBaron wrote: Sun Oct 18, 2020 1:31 pm I'll try to rephrase it. In level flight, we can simplify lift as a single vector because our frame of reference is unchanging. The sum of all lift is directly above and opposite to the sum of all weight.
We can always simplify lift into a single vector. When we talk about lift we're normally talking about net lift. Net lift is a single vector.
RedAndWhiteBaron wrote: Sun Oct 18, 2020 1:31 pm In a turn, the horizontal stabilizer will generate less lift (or at least, I believe this to be a safe assumption), when expressed as a proportion of total lift. This generates a turning moment around the lateral axis. When not level, this turning moment becomes one of the forces that act to turn the plane to its new heading. The aircraft turning therefore depends partially on the different magnitude of lift generated by the tail, and without factoring this turning moment in, the plane would slip.
I think you're getting confused between the transition into (or out of) the turn, versus a steady-state turn. Entering a turn, the aircraft has to transition from flying a straight line while maintianing orientation to flying a curve while rotating. This transition requires unbalanced moments. But once the rotation has started, it will continue without the requirement for unbalanced moments. So in a steady turn, there is no net moment acting on the aircraft -- rotation has already been established.
I'm not so sure. If we assume a stable aircraft (one that will return to straight and level flight in the absence of control inputs), there will be some force acting on the aircraft trying to return it to stable flight, and it is the control surfaces that counter this - the elevator among them.

And if we assume a higher elevator position in a level turn (which I again think to be a safe assumption), the centre of lift must move forward, whereas the centre of gravity remains the same. This will cause the aircraft to rotate around the lateral axis. If the elevator position is the same in a turn as it is in stable flight, you would be correct. But it isn't.
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Re: More bad physics - turns

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RedAndWhiteBaron wrote: Sun Oct 18, 2020 2:03 pm If we assume a stable aircraft (one that will return to straight and level flight in the absence of control inputs) ...
This is not what "stable" means in the context of flight dynamics. A stable aircraft will return to it's trimmed angle of attack and trimmed slip angle (which ideally is zero). That's it. Stability doesn't require anything wrt straight and level.
RedAndWhiteBaron wrote: Sun Oct 18, 2020 2:03 pm And if we assume a higher elevator position in a level turn (which I again think to be a safe assumption), the centre of lift must move forward, whereas the centre of gravity remains the same. This will cause the aircraft to rotate around the lateral axis.
If we're talking about a steady turn, the center of lift is unchanged from S&L. There will be more download on the tail (or less lift), but there will also be more lift produced by the wings, and that lift is forward of the CG. So the net moment about the CG will be zero (and unchanged from S&L).

This isn't true when we alk about the transition into the turn, which requires angular acceleration.
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Re: More bad physics - turns

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Conflicting Traffic wrote: Sun Oct 18, 2020 3:26 pm If we're talking about a steady turn, the center of lift is unchanged from S&L. There will be more download on the tail (or less lift), but there will also be more lift produced by the wings, and that lift is forward of the CG. So the net moment about the CG will be zero (and unchanged from S&L).
This doesn't sound right.
The total lift the wing generates increases significantly, to keep the vertical component roughly the same. That would move the center of lift more forward, no? Not sure why the position of COG is relevant to determine the location of the center of lift?
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Re: More bad physics - turns

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RedAndWhiteBaron wrote: Sun Oct 18, 2020 2:03 pm
And if we assume a higher elevator position in a level turn (which I again think to be a safe assumption), the centre of lift must move forward, whereas the centre of gravity remains the same. This will cause the aircraft to rotate around the lateral axis.
Yes, but that would create more lift. Eventually you'll reach an equilibrium with a certain amount of lift, speed, bank angle, and then you get the situation as depicted in the original pictures.
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Re: More bad physics - turns

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digits_ wrote: Sun Oct 18, 2020 3:58 pm The total lift the wing generates increases significantly, to keep the vertical component roughly the same. That would move the center of lift more forward, no? Not sure why the position of COG is relevant to determine the location of the center of lift?
The center of lift is the point about which the lift doesn't produce a moment. For the wing only, assuming the wing has positive camber (usually the case), this point moves forward at higher angles of attack. But for the whole aircraft, we can use the elevator to control it's location (without moving the elevator, it moves aft with an increase in AOA). One of the requirements for an aircraft to be stable and controlable is that we have to be able to get a moment of zero about the CG (i.e. - we need the whole-aircraft center of lift to be at the CG). This is exactly what happens in a steady turn (and also in S&L, steady climbs, etc.) -- zero moment about the CG and zero angular accleration.
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Re: More bad physics - turns

Post by AuxBatOn »

The total lift stays the same. The total aerodynamic force changes.

By definition, lift directly opposes weight. Weight doesn’t change and you are not climbing therefore lift remains the same. The horizontal components of the aerodynamic force make up the drag and also contribute to the change in velocity vector to make the aircraft pitch into the turn.
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