Grob Twin Astir getting "stuck" in a slip

Kenn,

if the fin sees the relative wind from the left, care to explain how it produces lift to the left?

Le lundi 28 septembre 2020 Ã* 18:03:30 UTC+2, Kenn Sebesta a écritÂ*:
On Monday, September 28, 2020 at 11:37:29 AM UTC-4, Tango Whisky wrote:
Nice explanation, but it doesn't work.
First of all, other ships with centrally hinged rudder also lock the rudder in a full slip (Janus comes to my mind).
Secondly, if you apply and hold full right rudder, the vertical's lift vector points to the left. If it wouldn't, the nose wouldn't stay on the right side. So the relative wind is coming from the right side of the fin, not the left side.
Stall always occurs on the lift vector side, never on the opposite side..
@TW, you might consider which side of the vertical stabilizer is seeing the relative wind. When the plane is yawed strongly to one side, let's say the right, and it is slipping in the other, i.e. to the left, then the left side of the vertical stab. is the windward side. At this point, the rudder deflection to the right side decreases the angle of attack, much like reflex flaps. So it is indeed geometrically possible to stall the rudder as @Steve described.

Turbulence caused by a control surface gap-- which @RO is absolutely right we have on this Twin Astir-- could easily trigger a flow separation condition across the rudder. And at this point, the pronounced relative wind from the slip combined with the stalling rudder/vertical stab assembly could easily cause the rudder to be forced to sustain full deflection. This windward pressure on the rudder would explain why I need to use force to center the rudder, exiting the slip. So I think it's safe to conclude it's not only possible, it's highly plausible.

On Tuesday, September 29, 2020 at 2:42:27 AM UTC-4, Tango Whisky wrote:
Kenn,

if the fin sees the relative wind from the left, care to explain how it produces lift to the left?

In the scenario we're describing the rudder ceases to develop lift, as it is stalled quite deeply. So in short, the explanation is that it doesn't produce lift to the left.

The purpose of dihedral is to couple bank and turn, so with a left bank we would expect a left turn to develop after a few seconds of uncoordinated flight. Since this left turn doesn't happen, it means we must have some kind of right yaw. When experiencing thee deeply stalled rudder, I suspect the balancing yaw moment is driven by the adverse yaw from the ailerons.

In short, we might think of rudder lock ia what happens when, for whatever reason, a slip's beta angle of attack causes the rudder to stall, resulting in the rudder being pushed to the leeward side. During the slip heading is maintained by adverse yaw. The banked slip will not end on its own without opposite rudder force.

Of course, this is first-principles speculation and so we can't know anything of sure without better references, either empirical or simulation results.

Kenn,

still puzzled about that. If you start deflecting the rudder to the right, the relativ wind for the fin is coming from the right, and the fin produces lift to the left.
If you end up this maneuvre with the fin stalled from relativ wind to the left, at some point the lift vector would have to change from the left to zero and than to the right before braking down upon stall. That should be something quizzy to feel - I'm doing banked slips on a regular basis with progressive slip angles (up to full controls deflections where I would guess the slipangle is something like 30 deg) and I have never experienced the "inversion".
Having said that, and imaging that the rudder is actually stalled, I strongly doubt that adverse yaw alone would stabilize the slip (plus, the stalled rudder still produces considerable drag which would pull the nose to the left). What does help stabilizing the slip is the additional drag of the forward fuselage section due to the slip angle - especially in the case of two-seaters.

Le mardi 29 septembre 2020 Ã* 20:48:16 UTC+2, Kenn Sebesta a écritÂ*:
On Tuesday, September 29, 2020 at 2:42:27 AM UTC-4, Tango Whisky wrote:
Kenn,

if the fin sees the relative wind from the left, care to explain how it produces lift to the left?
In the scenario we're describing the rudder ceases to develop lift, as it is stalled quite deeply. So in short, the explanation is that it doesn't produce lift to the left.

The purpose of dihedral is to couple bank and turn, so with a left bank we would expect a left turn to develop after a few seconds of uncoordinated flight. Since this left turn doesn't happen, it means we must have some kind of right yaw. When experiencing thee deeply stalled rudder, I suspect the balancing yaw moment is driven by the adverse yaw from the ailerons.

In short, we might think of rudder lock ia what happens when, for whatever reason, a slip's beta angle of attack causes the rudder to stall, resulting in the rudder being pushed to the leeward side. During the slip heading is maintained by adverse yaw. The banked slip will not end on its own without opposite rudder force.

Of course, this is first-principles speculation and so we can't know anything of sure without better references, either empirical or simulation results.

On Wednesday, September 30, 2020 at 9:12:14 AM UTC-4, Tango Whisky wrote:
Kenn,

still puzzled about that. If you start deflecting the rudder to the right, the relativ wind for the fin is coming from the right, and the fin produces lift to the left.
If you end up this maneuvre with the fin stalled from relativ wind to the left, at some point the lift vector would have to change from the left to zero and than to the right before braking down upon stall. That should be something quizzy to feel - I'm doing banked slips on a regular basis with progressive slip angles (up to full controls deflections where I would guess the slipangle is something like 30 deg) and I have never experienced the "inversion".
Having said that, and imaging that the rudder is actually stalled, I strongly doubt that adverse yaw alone would stabilize the slip (plus, the stalled rudder still produces considerable drag which would pull the nose to the left). What does help stabilizing the slip is the additional drag of the forward fuselage section due to the slip angle - especially in the case of two-seaters.

That's a great point about the forward fuselage section. It certainly is a lot draggier than the fine aft fuselage,. and must contribute some to the stabilized slip.

It might be that you can only get to this stabilized state by first applying full rudder and then bank. If you bank first and then apply rudder, I wouldn't expect the scenario I described above to be possible. In a progressive slip, once the rudder stalls you'd have a very abrupt step change. This would be a valuable test to perform in the air. Maybe if the next time I'm up in the Grob I can find some easy lift I can experiment with various control techniques.

BTW, it's possible that the vertical stabilizer isn't stalled but that the rudder is still experiencing a locking force to leeward. The airflow normal to the vert. stabilizer will want to deflect the rudder, and the flow parallel to it will want to realign the rudder. It could be that in a rudder lock situation the empennage is not stalled, but that something about the flow makes it so that the rudder deflection force overcomes the realignment force.