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Old September 28th 20, 02:32 PM posted to rec.aviation.soaring
Matthew Scutter
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Default Grob Twin Astir getting "stuck" in a slip

On Monday, September 28, 2020 at 2:35:39 PM UTC+2, Kenn Sebesta wrote:
I was flying my club's Twin Astir and noticed that if I put it into the deepest slip I can, by first pushing the rudder all the way to the floor and then compensating for yaw by using opposite bank, the plane doesn't come out of the slip very willingly. I have to actually put opposite rudder to get the desired timeliness of response.

On anything powered I've ever flown, as well as for my one flight in a 2-33, the planes snap out of the slip on their own. By removing rudder pressure, the plane reduces slip accordingly. I've certainly never had to *push* on the opposite rudder to resume normal flight.

Of course, those planes have super boxy and wide fuselages, whereas the Grob has a much finer shape. The Grob also has a smallish rudder and vertical stabilizer compared to, say, a Cessna.

Lastly, the Grob has a T-tail, which could lead to some weird airflow issues, but typically I associate T-tails with attitude control issues, not yaw.

Anyone seen this kind of behavior? If so, is this normal for all fine fuselages, or is this unique to the Twin Astir?

P.S. This doesn't happen in shallow slips, there seems to be a knee in the flight behavior.


It happens in lots of gliders, particularly older generation trainers, at least learning in Australia it was part of sideslip approach training. Puchatek's are great for it.

It's also in the flight manual (see page 27)

The side-slip is quite controllable and, if needed,
this manoeuvre can be used for steeper approaches.
It is effective by using a 15 degrees angle of sideslip and should be finished of a safe hight (98 km/h;
54 kts; 61 mph). Rudder effect reversal have not
been observed.
17 th march 1982
FLIGHT MANUAL GROB G 103 27
The temporary control force to overcome the
force reversal or rudder lock is calculated
approximately 5 to 6 daN (rudder pressure).
The aileron does not change its force direction, rather it returns independently from the
full deflected position.
Rudder lock can be relieved without pilot input on the rudder. After moving the aileron
into neutral position, the Sailplane rolls out
of the Slip into wing level position. Thereafter the rudder frees itself from the full
deflected position and the force reversal is
relieved. Using this method to end the Slip
the Sailplane does not adopt unusual flight
attitudes and deviates only slightly from its original flight course.
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