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.

@Steve, that's a great description, I'll go try it with the other direction. My natural slip tendency is to bank left because I'm right handed-- it's easier and more accurate for me to push left than pull right.

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.

That's an excellent reference. It certainly describes what I see, but it's not in our POH (http://www.franconiasoaring.org/pdf-...Rev%20 9.pdf). I wonder why not?

At 14:12 28 September 2020, Kenn Sebesta wrote:
@Steve, that's a great description, I'll go try it with the other
direction. My natural slip tendency is to bank left because I'm right
handed-- it's easier and more accurate for me to push left than
pull right.

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.

That's an excellent reference. It certainly describes what I see, but
it's
not in our POH
(http://www.franconiasoaring.org/pdf-
files/Flight%20Manual%20Grob%20103%20Astir%20FH%20Rev%2
09.pdf).
I wonder why not?


You have a Twin Astir (I) first generation. The above quote is
probably from a Twin II manual. Twin I's have a center hinged
rudder with no factory seals. IIRC, Twin II's have a side hinged
rudder with a tape seal on the hinge side.

The rudder on the Twin I can be made noticeably more effective by
adding seals with Z tape running just in front of the rudder gap IAW
LTB Lindner (Grob certificate holder) Service Letter SL-12.

http://www.ltb-lindner.com/service-letter.html

Putting seals and Z tape on a Twin Astir rudder costs little in time
or materials and yields much better rudder efficacy. I assume that
your glider does not yet have this mod, so get it done ASAP. You
won't be sorry, and you won't be complaining about a small and
ineffective rudder anymore.

RO

On Monday, September 28, 2020 at 11:30:06 AM UTC-4, Michael Opitz wrote:
At 14:12 28 September 2020, Kenn Sebesta wrote:
@Steve, that's a great description, I'll go try it with the other
direction. My natural slip tendency is to bank left because I'm right
handed-- it's easier and more accurate for me to push left than
pull right.

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.

That's an excellent reference. It certainly describes what I see, but
it's
not in our POH
(http://www.franconiasoaring.org/pdf-
files/Flight%20Manual%20Grob%20103%20Astir%20FH%20Rev%2
09.pdf).
I wonder why not?

You have a Twin Astir (I) first generation. The above quote is
probably from a Twin II manual. Twin I's have a center hinged
rudder with no factory seals. IIRC, Twin II's have a side hinged
rudder with a tape seal on the hinge side.

The rudder on the Twin I can be made noticeably more effective by
adding seals with Z tape running just in front of the rudder gap IAW
LTB Lindner (Grob certificate holder) Service Letter SL-12.

http://www.ltb-lindner.com/service-letter.html

Putting seals and Z tape on a Twin Astir rudder costs little in time
or materials and yields much better rudder efficacy. I assume that
your glider does not yet have this mod, so get it done ASAP. You
won't be sorry, and you won't be complaining about a small and
ineffective rudder anymore.

RO
Michael, how we can make seat in the back more comfortable ?
There is pure torture in the back seat of our ACA Twin Astir now .
Thx
Ryszard

Michael, how we can make seat in the back more comfortable ?
There is pure torture in the back seat of our ACA Twin Astir now .
Thx
Ryszard

This guy is asking the important questions.

Not all of the two seat Grobs were terrible. The one that had the worst back seat comfort was the Grob with the retractable landing gear. That hump for where the retracted landing gear is at the most inconvenient spot, right under your tailbone. They must have had a person with a really weird body shape be the person they designed it for.

I think somebody could come up with a contoured seat cushion that would form around the retracted wheel hump. The penalty, of course, would be if the instructor is a tall fellow. That extra cushion would make the head-bump on the canopy even more likely.

But if somebody could come up with a torture-reduction seat cushion for obtusely-shaped rear seat Grobs -- now THAT would be a cool winter project.

Thankfully my club has two ASK-21s.
Sorry, Ryszard.

-- Piet

At 04:35 30 September 2020, RW wrote:
On Monday, September 28, 2020 at 11:30:06 AM UTC-4, Michael
Opitz wrote:
At 14:12 28 September 2020, Kenn Sebesta wrote:
@Steve, that's a great description, I'll go try it with the other
direction. My natural slip tendency is to bank left because I'm
right
handed-- it's easier and more accurate for me to push left than
pull right.

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.

That's an excellent reference. It certainly describes what I see,
but
it's
not in our POH
(http://www.franconiasoaring.org/pdf-

files/Flight%20Manual%20Grob%20103%20Astir%20FH%20Rev%2
09.pdf).
I wonder why not?

You have a Twin Astir (I) first generation. The above quote is
probably from a Twin II manual. Twin I's have a center hinged
rudder with no factory seals. IIRC, Twin II's have a side hinged
rudder with a tape seal on the hinge side.

The rudder on the Twin I can be made noticeably more effective
by
adding seals with Z tape running just in front of the rudder gap
IAW
LTB Lindner (Grob certificate holder) Service Letter SL-12.

http://www.ltb-lindner.com/service-letter.html

Putting seals and Z tape on a Twin Astir rudder costs little in
time
or materials and yields much better rudder efficacy. I assume
that
your glider does not yet have this mod, so get it done ASAP. You
won't be sorry, and you won't be complaining about a small and
ineffective rudder anymore.

RO
Michael, how we can make seat in the back more comfortable ?
There is pure torture in the back seat of our ACA Twin Astir now .
Thx
Ryszard


Ryszard,

I am 6'2" tall, so I can't use too many cushions, but a thin yoga
mat underneath, and one of those memory foam lumbar back
support pillows works for me. You can also try an inflatable lumbar
pillow to try and get just the right amount of support for yourself. I
don't have any problems with 3-4 hour flights in the back seat.
We do have one club member (who is somewhat shorter than I)
who has made a custom foam cushion system for himself by carving
the foam with one of those electric kitchen carving knives.

Incidentally, the Twin I Trainer version with the sprung fixed gear
also has the same rear seat configuration. One has to go to a
Twin II with the 3 wheel configuration to get rid of the Twin I gear
well issue (due to C/G and main wheel locations).

I also find that some yoga mat material on the armrest keeps my
elbow from getting sore.

Hope this helps...

RO