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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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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.
  #2  
Old September 28th 20, 02:25 PM posted to rec.aviation.soaring
Steve Leonard[_2_]
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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, 02:34 PM posted to rec.aviation.soaring
[email protected]
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Default Grob Twin Astir getting "stuck" in a slip

Great explanation, Steve.

  #4  
Old September 28th 20, 04:37 PM posted to rec.aviation.soaring
Tango Whisky
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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

  #5  
Old September 28th 20, 05:03 PM posted to rec.aviation.soaring
Kenn Sebesta
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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.
  #6  
Old September 29th 20, 07:42 AM posted to rec.aviation.soaring
Tango Whisky
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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.

  #7  
Old September 29th 20, 07:48 PM posted to rec.aviation.soaring
Kenn Sebesta
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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.
  #8  
Old September 30th 20, 03:40 PM posted to rec.aviation.soaring
Steve Leonard[_2_]
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Default Grob Twin Astir getting "stuck" in a slip

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 √* 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

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


  #9  
Old September 30th 20, 05:15 PM posted to rec.aviation.soaring
Tango Whisky
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Default Grob Twin Astir getting "stuck" in a slip

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 √* 16:40:25 UTC+2, Steve Leonard a √©crit¬*:
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

  #10  
Old September 30th 20, 05:38 PM posted to rec.aviation.soaring
Michael Opitz
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Default Grob Twin Astir getting


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

 




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