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Grob Twin Astir getting "stuck" in a slip



 
 
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  #1  
Old September 28th 20, 01:35 PM posted to rec.aviation.soaring
Kenn Sebesta
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Posts: 22
Default Grob Twin Astir getting "stuck" in a slip

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.
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  #2  
Old September 28th 20, 02:25 PM posted to rec.aviation.soaring
Steve Leonard[_2_]
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Posts: 1,047
Default Grob Twin Astir getting "stuck" in a slip

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
  #3  
Old September 28th 20, 02:32 PM posted to rec.aviation.soaring
Matthew Scutter
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Posts: 14
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.
  #4  
Old September 28th 20, 02:34 PM posted to rec.aviation.soaring
[email protected]
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Posts: 768
Default Grob Twin Astir getting "stuck" in a slip

Great explanation, Steve.

  #5  
Old September 28th 20, 03:12 PM posted to rec.aviation.soaring
Kenn Sebesta
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Posts: 22
Default Grob Twin Astir getting "stuck" in a slip

@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?
  #6  
Old September 28th 20, 04:18 PM posted to rec.aviation.soaring
Michael Opitz
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Posts: 315
Default Grob Twin Astir getting

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

  #7  
Old September 28th 20, 04:37 PM posted to rec.aviation.soaring
Tango Whisky
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Posts: 384
Default Grob Twin Astir getting "stuck" in a slip

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 √* 15:25:18 UTC+2, Steve Leonard a √©crit¬*:
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

  #8  
Old September 28th 20, 05:03 PM posted to rec.aviation.soaring
Kenn Sebesta
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Posts: 22
Default Grob Twin Astir getting "stuck" in a slip

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.
  #9  
Old September 29th 20, 07:42 AM posted to rec.aviation.soaring
Tango Whisky
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Posts: 384
Default Grob Twin Astir getting "stuck" in a slip

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 √* 18:03:30 UTC+2, Kenn Sebesta a √©crit¬*:
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.

  #10  
Old September 29th 20, 07:48 PM posted to rec.aviation.soaring
Kenn Sebesta
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Posts: 22
Default Grob Twin Astir getting "stuck" in a slip

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.
 




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