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Overbanking tendency

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Overbanking tendency

Old 29th Oct 2021, 13:38
  #41 (permalink)  
 
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Can you expand on the AOA being higher? I'm not seeing it. Are you talking about the roll-in, or the steady part of the turn?
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Old 29th Oct 2021, 14:38
  #42 (permalink)  
 
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Sounds like roll damping/subsidence, which is a dynamic effect. In a steady state turn AoA is the same across the wing but the airspeed differs from one end to the other.
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Old 29th Oct 2021, 15:32
  #43 (permalink)  
 
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In the very simplest terms, the wing foil itself has to be pitched up to the approaching air to generate lift. Separate the angle of flow into the horizontal and vertical components. The horizontal component is the wing's speed and makes drag, and vertical could be said to be the amount of air needed to deliver the lift. Now take the two wings of a turning aircraft. The same amount of air must be coming up vertically to each wing, but one is travelling faster. Recombine the two sets of vectors for each wing.

Last edited by boguing; 29th Oct 2021 at 19:48.
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Old 30th Oct 2021, 13:16
  #44 (permalink)  
 
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Boguing,

I see where you're coming from. For a propeller, there is the rotational component, (in-plane with the disk) that increases in magnitude at a further-out radius; and the forward component, which is the same at all stations. The resultant of those two is the relative wind, which changes angle due to the change of the rotational component's magnitude. (Notice that so far, I have not brought the chordline into the picture. All we're concerned with, is the relative wind and the components that go into it. Without referencing the chordline, we understand the change in relative wind. But this changing relative wind is why the twisting chordline changes along with it, to keep AOA the same.)

But the analogy from this, doesn't carry. For the (wings of) the plane in a turn, there is no motion component analogous to the forward component of the prop. The plane is moving forward, of course, but THAT IS the rotational component. The whole picture is turned up 90 degrees. If you simplify it down to look at a plane doing a zero-bank turn with rudder only, different stations along the wing are moving at different speeds, but the direction does not change. It is precisely horizontal. There is no motion 90 degrees to that, to add and come up with a resultant at an angle.

If you remove the zero-bank simplification and look at a normal banked turn, the path of the wings sweeps out a cone instead of a disk, but the relative wind at each station still comes from the horizon...with no component from above or from below. Upon rereading your post, this could have been a lot more succinctly put to say "there is no vertical component." Anyway. There is no relative wind change, therefore there is no AOA change. (Of course most planes have washout that varies the AOA, but this is mirrored left vs right.)
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Old 30th Oct 2021, 15:01
  #45 (permalink)  
 
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There can't be an angle (of attack) without a vertical component. There may not be one for the aircraft as a whole, but there is for its wing.

If this weren't the case how would you explain the tip stall of an incipient spin?
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Old 30th Oct 2021, 15:52
  #46 (permalink)  
 
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Or if youíre taxying around in a circle on a wingtip wheel, does the AoA vary along the wing? No? Take the ground away - does it change anything?

The best non-mathematical description of AoA that Iíve seen is that itís the difference between where the wing is pointing and where itís going. For an aircraft in a steady banked turn, this is the same all along the wing, otherwise it would be rolling...
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Old 31st Oct 2021, 03:31
  #47 (permalink)  
 
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boguing,

"There can't be an angle (of attack) without a vertical component."

This is not true. Look at at a simple plane, flying at a mid speed and mid AOA, straight and level. There is no vertical component, but there is an AOA.

"If this weren't the case how would you explain the tip stall of an incipient spin?"

In this case there is a vertical component. But there doesn't have to be.
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Old 31st Oct 2021, 11:19
  #48 (permalink)  
 
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I've asked MadFltScientist (and would like to ask Genghis the Engineer but his pms box is full...) to chip in on the AoA point I may foolishly be trying to make.

Last edited by boguing; 31st Oct 2021 at 18:07.
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Old 2nd Nov 2021, 23:30
  #49 (permalink)  
 
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Thatís is whatís called spiral mode of the aircraft. Aircraft can have stable or unstable spiral mode. If itís stable when you bank the aircraft and let it go it tends to recover itself from the bank over time. On the other hand, if itís unstable, then bank of the aircraft gradually increases and ends up with whatís known as graveyard spiral.

Itís very closely related with dihedral effect. If aircraft has high wings or delta wings or wings that has positive dihedral (upward wingtips) then itís tend to be more spirally stable. Meaning it will recover from the banks.

There is a tradeoff between spiral stability and dutch roll stability. The more spirally stable an aircraft is the more nasty dutch roll it has and vice versa. The spiral stability is not regarded as important as the other stability modes. And if there needs to be a tradeoff between spiral mode and dutch roll, the spiral stability is out of the window. And the reason for that is, even if itís unstable it grows very slowly and by the time it becomes dangerous pilot already corrects it.

I have seen quite some aircraft that are spirally unstable. But hardly see one that is unstable in dutch roll.
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Old 3rd Nov 2021, 01:20
  #50 (permalink)  
 
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In addition to the outboard wing traveling at a greater speed (thus generating more lift), the sideways component of the relative airflow in a sideslip contributes to create overbanking tendency.

The sideways component flows around the fuselage from inboard wing's root, decreasing the angle of attack of the inboard wing, flows under the fuselage and then flows in an upward direction at the root of the outboard wing, increasing its angle of attack.

Quite a bit of a mouthful to write without an illustration available.


On a high wing aircraft, the sideways component contributes to lateral stability.


These images are taken from OAA PPL collection ebook.

Last edited by Banana Joe; 3rd Nov 2021 at 01:45.
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