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Old September 30th 20, 03:40 PM posted to rec.aviation.soaring
Steve Leonard[_2_]
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Default Grob Twin Astir getting "stuck" in a slip

On Monday, September 28, 2020 at 10:37:29 AM UTC-5, 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.
Le lundi 28 septembre 2020 Ã* 15:25:18 UTC+2, Steve Leonard a écrit :
Have you tried it both directions? The rudder is hinged on one side on the Twin Astir, so it likely behaves differently one way versus the other. It has to do with stalling the vertical fin. If you push right rudder, the tail of the plane moves to the left. The relative wind tries to stay parallel to the centerline of the rudder, so it is coming at the fin from the left. As you increase rudder deflection to the right, you are able to increase the AOA on the vertical fin enough to stall the right side of the vertical fin, which will pull the rudder full to the right. I am guessing that you can do this with a slip with rudder into the hinged side, and not the other way around. Why? The airflow can stay attached on the non hinge side due to the gentle radius at the fin to rudder transition, but not on the hinge side due to the abrupt change in contour at the fin/rudder. Since the flow stays attached, the rudder has a bit more authority in one direction than the other, so you can generate more sideslip and get into what is often called "rudder lock". This is where, as you described, the rudder stays completely deflected one direction, and you have to push, maybe very firmly, to get it to come back to center. But once centered again, it behaves normally.

It is not unique to the Twin Astir, but seems to be more common on planes with a side hinged rudder.

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