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Old September 28th 20, 05:03 PM posted to rec.aviation.soaring
Kenn Sebesta
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Posts: 32
Default Grob Twin Astir getting "stuck" in a slip

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.