Steve, again:
Rudder moves TE to th right, tail moves to the left because the lift vector points to the left.
Tail keeps moving to the left until it assumedly sees the wind from the left. Now the lift vector changes to the right. Maybe at this point, you won't feel something strange because the forward section of the fuselage will over-compensate the missing force to the left.
Until now, you can't separate the aerodynamics of the fin and the rudder into one generating a force to the left, and the other one to the right - it is one single (vertical) wing with a defined circulation, and it's the right side which has the low pressure condition.
Tail keeps moving to the left until the vertical wing stalls, which makes the pressure *increase* overall on the right side. But the pressure might also locally decrease at the location of the rudder (rudder deflections are large, so the flow might well re-accelerate from the hinge on) which would make the rudder stuck to the right.

All in all, tricky and interesting topic. I'l give it some special attention next time I'll slip (that would be on an ASK21 though - no sucked rudder, but large slipping angle).

Le mercredi 30 septembre 2020 Ã* 16:40:25 UTC+2, Steve Leonard a écritÂ*:
Steve Leonard
TW, think about this a bit more. Rudder moves TE right. Tail of aircraft moves left. Yaw string goes right. Airflow is now moving left to right across the plane, as well as front to back across the plane. So, airflow hits left side of vertical fin. Rudder pushes tail to the left, but the vertical fin is trying to push the tail back to the right. If the airflow is far enough off to the left, the right (downwind, lee side, "upper surface" or however you want to think of it) side will have its airflow separate near the leading edge of the vertical fin. The airflow over that entire side of the tail separates, and the resulting lowered pressure pulls the rudder hard to the stop. The rudder is still hard right, so the tail stays deflected to the left. The plane does not straighten out because instead of the vertical producing a side force at a long moment arm, it produces primarily a drag or aft force, with a moment arm of the sine of the yaw angle.

It is well known among aerodynamicists that surface hinged controls (as opposed to center hinged controls) have asymmetric authority. It is also known that the surface has more authority when deflect towards the hinge than away from the hinge. That is why I suspect that this may happen one direction and not the other with the Twin Astir

And note, I did NOT say it does not happen with center hinged surfaces. It happens in spades on my Nimbus 3. It transitioned from no force to hold rudder deflection to the opposite pedal pushing my foot back with great vigor. From my weight and how hard I pushed, I am guessing 50-60 lbs force was required to center the rudder. It was MUCH more than full pedal for rolling into a turn. I have spent some time assisting with rudder and brake pedal force measurement calibrations on GA aircraft, so I do have a feel for home much I can push with my feet and or my legs.

To see this flow behavior, tape some yarn to the right side of your vertical fin and rudder, attach a GoPro or similar to the right tip of your horizontal tail, and go do some slips. Both left and right. Report back.

PS: I have done this with my Nimbus 3.

And, Mike, Twin Astirs (Retract gear, at least) have a side hinged rudder.. Right side if memory serves me correctly.

Steve Leonard