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Old September 29th 20, 07:48 PM posted to rec.aviation.soaring
Kenn Sebesta
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Posts: 32
Default Grob Twin Astir getting "stuck" in a slip

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

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.