Grob Twin Astir getting "stuck" in a slip

[Kenn Sebesta]

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.

[Steve Leonard[_2_]]

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

[Matthew Scutter]

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.

[[email protected]]

Great explanation, Steve.

[Kenn Sebesta]

@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%20 9.pdf). I wonder why not?

[Michael Opitz]

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

[Tango Whisky]

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 écritÂ*:
> 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

[Kenn Sebesta]

On Monday, September 28, 2020 at 11:37:29 AM UTC-4, 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.

@TW, you might consider which side of the vertical stabilizer is seeing the relative wind. When the plane is yawed strongly to one side, let's say the right, and it is slipping in the other, i.e. to the left, then the left side of the vertical stab. is the windward side. At this point, the rudder deflection to the right side decreases the angle of attack, much like reflex flaps. So it is indeed geometrically possible to stall the rudder as @Steve described.

Turbulence caused by a control surface gap-- which @RO is absolutely right we have on this Twin Astir-- could easily trigger a flow separation condition across the rudder. And at this point, the pronounced relative wind from the slip combined with the stalling rudder/vertical stab assembly could easily cause the rudder to be forced to sustain full deflection. This windward pressure on the rudder would explain why I need to use force to center the rudder, exiting the slip. So I think it's safe to conclude it's not only possible, it's highly plausible.

[Tango Whisky]

Kenn,

if the fin sees the relative wind from the left, care to explain how it produces lift to the left?

Le lundi 28 septembre 2020 Ã* 18:03:30 UTC+2, Kenn Sebesta a écritÂ*:
> On Monday, September 28, 2020 at 11:37:29 AM UTC-4, 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..
> @TW, you might consider which side of the vertical stabilizer is seeing the relative wind. When the plane is yawed strongly to one side, let's say the right, and it is slipping in the other, i.e. to the left, then the left side of the vertical stab. is the windward side. At this point, the rudder deflection to the right side decreases the angle of attack, much like reflex flaps. So it is indeed geometrically possible to stall the rudder as @Steve described.
>
> Turbulence caused by a control surface gap-- which @RO is absolutely right we have on this Twin Astir-- could easily trigger a flow separation condition across the rudder. And at this point, the pronounced relative wind from the slip combined with the stalling rudder/vertical stab assembly could easily cause the rudder to be forced to sustain full deflection. This windward pressure on the rudder would explain why I need to use force to center the rudder, exiting the slip. So I think it's safe to conclude it's not only possible, it's highly plausible.

[Kenn Sebesta]

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

[RW[_2_]]

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

[Piet Barber]

> 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

[Michael Opitz]

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

[Nick Kennedy[_3_]]

On Wednesday, September 30, 2020 at 6:45:06 AM UTC-6, Michael Opitz wrote:
> 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 owned a retractable gear Grob Twin Astir and gave rides from the rear seat, and flew it on a bunch of long XC flights.
The factory shape of the rear seat is tough. With various cushions and astronaut foam and duct tape you can create a usable shape and then cover it all with a sheet to make it look OK.
The key for me was to get into a more upright seating position and moving forward to get closer to the stick. It takes quite a bit of padding behind your back to get you upright.
After getting a nice seat I had many 6 hour flights in that plane.
Another tip is to try and fly it as much as possible using the rudder, like a RC, your legs are stronger than your arms and the roll can get stick at higher speeds.
I liked that plane, alot, I was sad to see it go.
Nick
T

[Tango Whisky]

Kenn,

still puzzled about that. If you start deflecting the rudder to the right, the relativ wind for the fin is coming from the right, and the fin produces lift to the left.
If you end up this maneuvre with the fin stalled from relativ wind to the left, at some point the lift vector would have to change from the left to zero and than to the right before braking down upon stall. That should be something quizzy to feel - I'm doing banked slips on a regular basis with progressive slip angles (up to full controls deflections where I would guess the slipangle is something like 30 deg) and I have never experienced the "inversion".
Having said that, and imaging that the rudder is actually stalled, I strongly doubt that adverse yaw alone would stabilize the slip (plus, the stalled rudder still produces considerable drag which would pull the nose to the left). What does help stabilizing the slip is the additional drag of the forward fuselage section due to the slip angle - especially in the case of two-seaters.

Le mardi 29 septembre 2020 Ã* 20:48:16 UTC+2, Kenn Sebesta a écritÂ*:
> On Tuesday, September 29, 2020 at 2:42:27 AM UTC-4, Tango Whisky wrote:
> > Kenn,
> >
> > 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.

[Kenn Sebesta]

On Wednesday, September 30, 2020 at 9:12:14 AM UTC-4, Tango Whisky wrote:
> Kenn,
>
> still puzzled about that. If you start deflecting the rudder to the right, the relativ wind for the fin is coming from the right, and the fin produces lift to the left.
> If you end up this maneuvre with the fin stalled from relativ wind to the left, at some point the lift vector would have to change from the left to zero and than to the right before braking down upon stall. That should be something quizzy to feel - I'm doing banked slips on a regular basis with progressive slip angles (up to full controls deflections where I would guess the slipangle is something like 30 deg) and I have never experienced the "inversion".
> Having said that, and imaging that the rudder is actually stalled, I strongly doubt that adverse yaw alone would stabilize the slip (plus, the stalled rudder still produces considerable drag which would pull the nose to the left). What does help stabilizing the slip is the additional drag of the forward fuselage section due to the slip angle - especially in the case of two-seaters.

That's a great point about the forward fuselage section. It certainly is a lot draggier than the fine aft fuselage,. and must contribute some to the stabilized slip.

It might be that you can only get to this stabilized state by first applying full rudder and then bank. If you bank first and then apply rudder, I wouldn't expect the scenario I described above to be possible. In a progressive slip, once the rudder stalls you'd have a very abrupt step change. This would be a valuable test to perform in the air. Maybe if the next time I'm up in the Grob I can find some easy lift I can experiment with various control techniques.

BTW, it's possible that the vertical stabilizer isn't stalled but that the rudder is still experiencing a locking force to leeward. The airflow normal to the vert. stabilizer will want to deflect the rudder, and the flow parallel to it will want to realign the rudder. It could be that in a rudder lock situation the empennage is not stalled, but that something about the flow makes it so that the rudder deflection force overcomes the realignment force.

[Steve Leonard[_2_]]

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

[Tango Whisky]

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

[Michael Opitz]

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

All of ours are center hinged with the rudder pushrod actuator
connection on the right side. Some have had foam rubber
weatherstripping seals installed on both sides inside the tail fin.
We have just done the elevator pushrod TN inspection on 4
(with one left to go), so we have had to pull the rudder off
on all of them just recently.

RO

[Michael Opitz]

>Another tip is to try and fly it as much as possible using the rudder,
like
>a RC, your legs are stronger than your arms and the roll can get stick
at
>higher speeds.
>I liked that plane, alot, I was sad to see it go.
>Nick
>T
>

Nick,

Flying gliders, I tend to lead my turns with rudder and then
follow with aileron input anyway, but like you, I find this
technique to be especially effective in a Twin Astir. It is something
my father taught me when I was just starting out. He pretty much
stressed keeping aileron inputs to a minimum to avoid destroying
lift over a large part of the span, thus obtaining a better climb
result. Also keeping the stick quiet - no stirring like churning butter.
Maybe that's part of why I had fellow competitors at WGC's coming
up to me and asking how I was able to climb up through entire
gaggles on course.

RO