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The aerodynamics of a towplane in a kiting glider situation



 
 
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  #1  
Old May 17th 20, 11:48 PM posted to rec.aviation.soaring
Tom BravoMike
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Posts: 266
Default The aerodynamics of a towplane in a kiting glider situation

I know there is a thread going on about and around the latest tragic accident from 5-9-2020 and I preferred to start a new thread as I felt uncomfortable going into a dry technical discussion in the context. So the issue is:

I would like to know more about the aerodynamics of the towplane in all those accident situations (and I mean above certain altitude minimum): Statistically, do they crash being stalled or in a deep dive? What is the usual reaction to the tail being pulled up - does the towpilot try to overrule the pulling glider and he gets into a stall? He cannot dive until the rope breaks. Recovering from a stall he has to get out of a dive anyway. So it seems that pushing down into a dive instead of trying to pull up would result in a smaller loss of altitude (and cause the rope to break earlier), rather than in the sequence: struggle to overrule the pulling glider - stalling (loss of height) - pulling out of a dive (loss of height).

Just thinking loud, curious if any research has been done and if so, to what conclusions? Does anybody know?
  #2  
Old May 18th 20, 02:20 AM posted to rec.aviation.soaring
[email protected]
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Default The aerodynamics of a towplane in a kiting glider situation

The tow cable pulls on the tow plan with some force (F) and angle (theta). When the glider balloons, both the force and angle increase. This lifts the tail of the tow plane with a force Fsin(theta). When Fsin(theta) exceeds the control authority of the tow plane, the tow plane is forced into a dive.

Presumably, cable force has an upper bound of 80% to 200% of the glider gross weight (FAR 91.309).

It might be possible to design a tow plane release mechanism with something similar to a back release. I'm thinking of a torsion spring-loaded mechanism with some length (x) such that the tow cable tension exerts a torque xFsin(theta) on it.

  #3  
Old May 18th 20, 03:16 AM posted to rec.aviation.soaring
Shaun Wheeler
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Default The aerodynamics of a towplane in a kiting glider situation

On Sunday, May 17, 2020 at 5:48:22 PM UTC-5, Tom BravoMike wrote:
I know there is a thread going on about and around the latest tragic accident from 5-9-2020 and I preferred to start a new thread as I felt uncomfortable going into a dry technical discussion in the context. So the issue is:

I would like to know more about the aerodynamics of the towplane in all those accident situations (and I mean above certain altitude minimum): Statistically, do they crash being stalled or in a deep dive? What is the usual reaction to the tail being pulled up - does the towpilot try to overrule the pulling glider and he gets into a stall? He cannot dive until the rope breaks. Recovering from a stall he has to get out of a dive anyway. So it seems that pushing down into a dive instead of trying to pull up would result in a smaller loss of altitude (and cause the rope to break earlier), rather than in the sequence: struggle to overrule the pulling glider - stalling (loss of height) - pulling out of a dive (loss of height).

Just thinking loud, curious if any research has been done and if so, to what conclusions? Does anybody know?


Excellent topic.

Thank you.
  #4  
Old May 18th 20, 07:18 AM posted to rec.aviation.soaring
2G
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Default The aerodynamics of a towplane in a kiting glider situation

On Sunday, May 17, 2020 at 6:20:53 PM UTC-7, wrote:
The tow cable pulls on the tow plan with some force (F) and angle (theta).. When the glider balloons, both the force and angle increase. This lifts the tail of the tow plane with a force Fsin(theta). When Fsin(theta) exceeds the control authority of the tow plane, the tow plane is forced into a dive.

Presumably, cable force has an upper bound of 80% to 200% of the glider gross weight (FAR 91.309).

It might be possible to design a tow plane release mechanism with something similar to a back release. I'm thinking of a torsion spring-loaded mechanism with some length (x) such that the tow cable tension exerts a torque xFsin(theta) on it.


During towplane upset fatal accident in Ephrata (1999) the rope did break, but the attitude of the towplane was such that a recovery was impossible. What I envision (which will probably draw the boo-birds) is a state estimation computer that monitors pitch rate of change, along with other flight parameters, to predict an upset in progress. If an upset isn't detected early there is no hope of recovery by any means.

Tom
  #5  
Old May 18th 20, 08:40 AM posted to rec.aviation.soaring
Marton KSz
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Default The aerodynamics of a towplane in a kiting glider situation

I found this diagram last week:

https://photos.app.goo.gl/nLpWwLHFmgwE5dEM9

Here's my interpretation:

The system of the connected towplane + glider has a center of gravity, somewhere along the towrope, closer the towplane.

When the kiting begins, the system of the two connected masses start rotating around this CG (just like groundlooping a tailwheel aircraft).

However, since the glider is lighter, it rotates faster, which makes the impression that the towplane slingshots it.

Also, the glider has wings, and faster airspeed on the wings means more lift - the glider wants to go even higher.

All the energy for the extra lift + speed has to come from somewhe the supply is the kinetic energy of the towplane. As the kiting aggravates, the towplane drastically slows down. First it runs out of elevator control, then stalls.
  #6  
Old May 18th 20, 11:11 AM posted to rec.aviation.soaring
Sci Fi
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Default The aerodynamics of a towplane in a kiting glider situation

At 06:18 18 May 2020, 2G wrote:
On Sunday, May 17, 2020 at 6:20:53 PM UTC-7,
w=
rote:
Presumably, cable force has an upper bound of 80% to 200% of the glider

g=
ross weight (FAR 91.309).
=20


If that 80-200% is correct it could mean any weak link, for a 350kg glider
could be from 280 kg to 700kg breaking strain. Perhaps the lower figure
should be used for aero-tows, as it is sooo much safer. However I don't
know if weak links of less than 500kg are made.

  #7  
Old May 18th 20, 01:26 PM posted to rec.aviation.soaring
Jonathon May[_2_]
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Posts: 88
Default The aerodynamics of a towplane in a kiting glider situation

At 10:11 18 May 2020, Sci Fi wrote:
At 06:18 18 May 2020, 2G wrote:
On Sunday, May 17, 2020 at 6:20:53 PM UTC-7,


w=
rote:
Presumably, cable force has an upper bound of 80% to

200% of the glider
g=
ross weight (FAR 91.309).
=20


If that 80-200% is correct it could mean any weak link, for a

350kg glide
could be from 280 kg to 700kg breaking strain. Perhaps the

lower figur
should be used for aero-tows, as it is sooo much safer.

However I don'
know if weak links of less than 500kg are made.


I have seen a 4 ton winch moved on a 2 seater launch(1000Kg)
links you can stop a tug and strip it of all its airspeed quite easily.
Tost make a series of links down to 80kg for hang-gliders ,but I
do not think that is the answer.

The only way I can see it working is an ,and+and+and system.

If the elevator is hard up+the angle is above a set figure +the
load is above set figure the rope is released.

You would need a load sensor ,a light beam over the rope to limit
the deflection and a micro switch on the elevator.If all 3 occurred
together the rope is released.You would probably need a
magnetic coupling.
And you would still get nuisance releases.


  #8  
Old May 18th 20, 02:19 PM posted to rec.aviation.soaring
[email protected]
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Posts: 88
Default The aerodynamics of a towplane in a kiting glider situation

On Monday, May 18, 2020 at 8:30:04 AM UTC-4, Jonathon May wrote:
At 10:11 18 May 2020, Sci Fi wrote:
At 06:18 18 May 2020, 2G wrote:
On Sunday, May 17, 2020 at 6:20:53 PM UTC-7,


w=
rote:
Presumably, cable force has an upper bound of 80% to

200% of the glider
g=
ross weight (FAR 91.309).
=20


If that 80-200% is correct it could mean any weak link, for a

350kg glide
could be from 280 kg to 700kg breaking strain. Perhaps the

lower figur
should be used for aero-tows, as it is sooo much safer.

However I don'
know if weak links of less than 500kg are made.


I have seen a 4 ton winch moved on a 2 seater launch(1000Kg)
links you can stop a tug and strip it of all its airspeed quite easily.
Tost make a series of links down to 80kg for hang-gliders ,but I
do not think that is the answer.

The only way I can see it working is an ,and+and+and system.

If the elevator is hard up+the angle is above a set figure +the
load is above set figure the rope is released.

You would need a load sensor ,a light beam over the rope to limit
the deflection and a micro switch on the elevator.If all 3 occurred
together the rope is released.You would probably need a
magnetic coupling.
And you would still get nuisance releases.


Commendable that such thought is being put in but it's design, creation, approval and implementation at the end of the day. There is a point below which nothing is going to save the life of a tow pilot, human nature being what it is. I would have thought the potential for such an upset being caused by a flight instructor would be near zero... The old duffers YES, (and they abound) 15 year old students, YES but not instructors. The USAFA and Front Royal incidents are proof positive that there is a disconnect when it comes to recognizing the need to release when you kite above the tow plane, reaction time is everything. If the glider doesn't release it is necessary that the tow pilot has an unfettered system, at least not a system that has been proven in the past to fail. That would be the TOST system or at least if a Schweizer hook is used it be inverted and a release handle with adequate mechanical advantage and accessibility. Again this would not have helped the most recent accident victim but the next time a glider kites at a recoverable altitude let's hope the tow pilot has an out.

Walt Connelly
Former Tow Pilot
Now Happy Helicopter Pilot
  #9  
Old May 18th 20, 02:25 PM posted to rec.aviation.soaring
[email protected]
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Posts: 281
Default The aerodynamics of a towplane in a kiting glider situation

On Monday, May 18, 2020 at 2:40:26 AM UTC-5, Marton KSz wrote:
I found this diagram last week:

https://photos.app.goo.gl/nLpWwLHFmgwE5dEM9

Here's my interpretation:


Great picture for discussion, but it is lacking a critical part of the aerodynamics at the glider.

Consider recovering from a slack rope situation with nose a little high and a bit above the tow but still in sight. If you do it with a jerk, there is a sudden lot of force in the tow rope pulling the bottom of the glider forward and down. Given that the CG hook is below the center of lift, this puts a pitch up torque on the glider. Even with the pilot putting stick full forward, some gliders can go quickly into a nose high attitude with the elevator stalled.

At this point, the only glider control inputs left are to lean forward to move the CG or pull the yellow handle.

An ounce of prevention is worth a pound of cure.

Stay lowish and don't take up slack nose high. Understand this sequence and no matter what happens in the cockpit, fly to avoid it. If all else fails, be ready to use the yellow handle.



  #10  
Old May 18th 20, 04:04 PM posted to rec.aviation.soaring
Tom BravoMike
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Posts: 266
Default The aerodynamics of a towplane in a kiting glider situation

On Monday, May 18, 2020 at 2:40:26 AM UTC-5, Marton KSz wrote:
I found this diagram last week:

https://photos.app.goo.gl/nLpWwLHFmgwE5dEM9

Here's my interpretation:

The system of the connected towplane + glider has a center of gravity, somewhere along the towrope, closer the towplane.

When the kiting begins, the system of the two connected masses start rotating around this CG (just like groundlooping a tailwheel aircraft).

However, since the glider is lighter, it rotates faster, which makes the impression that the towplane slingshots it.

Also, the glider has wings, and faster airspeed on the wings means more lift - the glider wants to go even higher.

All the energy for the extra lift + speed has to come from somewhe the supply is the kinetic energy of the towplane. As the kiting aggravates, the towplane drastically slows down. First it runs out of elevator control, then stalls.


That's quite an interesting picture, helping to understand the process and the discussion about the (initially) small angles.
The 800 ft minimum for the towplane to recover from a dive seems a lot. Could speak for using eg. touring motorgliders as tugs in hope they could recover faster.
 




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