Glider Handling on Tow

Oh, and of course, Mary Ann.


On Wednesday, November 6, 2013 10:40:26 AM UTC-5, Papa3 wrote:
I didn't mean to be snarky - I promise. I forgot the smiley emoticon :-) I figured the parenthetical examples would be a clue that I meant this somewhat tongue-in-cheek.

On Wednesday, November 6, 2013 9:43:21 AM UTC-6, Papa3 wrote:
Oh, and of course, Mary Ann.





On Wednesday, November 6, 2013 10:40:26 AM UTC-5, Papa3 wrote:

I didn't mean to be snarky - I promise. I forgot the smiley emoticon :-) I figured the parenthetical examples would be a clue that I meant this somewhat tongue-in-cheek.





I wonder if doubling the length of the tow-rope would help (runway length permitting. That would get the glider further above the downwash. From my many winch tows back in Germany I can confirm that the glider comfortably flies at 55 knots at a steep enough angle (not to be confused with AoA) that you get to 1,400' in 35 sec.
Herb

No problem, Erik. I picked up on your clues. :-)

-John, Q3

On Wednesday, November 6, 2013 10:40:26 AM UTC-5, Papa3 wrote:
I didn't mean to be snarky - I promise. I forgot the smiley emoticon :-) I figured the parenthetical examples would be a clue that I meant this somewhat tongue-in-cheek.

BTW, this is one of the best discussions on R.A.S. where some good (and not so good) information has come out.

I did a search as Erik suggested. After reading a lot of posts, I found a very good explanation by Andreas Maurer, which I partially quote below. His explanation seems to account well for why the glider handling is so poor at slow tow plane speeds. But for me, several things are still unexplained:

1. Why does increasing the tow plane speed cause the poor glider handling to go away?

2. If the answer to (1) is that the down wash and wing tip vortices are now further below the glider, then why doesn't simply moving up higher above the wake of the slow tow plane remove the poor handling?

-John, Q3

===========

The following was written by Andreas Maurer, and posted on RAS 1/5/11:

The main factor for the seemingly odd flying characteristics behind
the tow plane is the downwash of the latter.

Let me explain:

The downwash has a significant angle (the air is deflected downwards
behind the tow plane's wing to up to four degrees!), but due to the
larger span of the glider it only affects the inner part of the
glider's wing.

Therefore, if the glider if lying laterally displaced, only one wing
is affected by the downwash of the tow plane - four degrees of AoA
difference between left and right wing need a lot of aileron to
correct.

Likewise, if the glider is flying straight behind the tow plane, the
downwash *decreases* the AoA of the affected inner part of the wing.
Getting the nose up by pulling back will restore the lift of the inner
part of the glider's wing, but now the outer parts of the wing have a
much higher AoA than they have in free flight.

Voila, meet the the conditions for poor aileron efficiency (high AoA!)
and tip stall.

The downwash is reduced by
- wingloading of the tow plane
- wing span of the tow plane

In other words: The more a tow plane looks like a motorglider (say, a
Dimona, or Katana Extreme), the less the flight characteristics of the
glider are affected.

Anyone who has ever been towed behind a motorglider or a microlight
will testify that problems like poor lateral control or running out
of elevator don't exist there, despite a far slower tow (55 kts
compared to a typical 70-75 kts behind a typical tow plane like
Reorqeur or Pawnee).

===========

On Tuesday, November 5, 2013 2:58:54 PM UTC-5, Papa3 wrote:
On Tuesday, November 5, 2013 1:10:33 PM UTC-5, John Carlyle wrote:

A question for any aerodynamicists out the why does low aero tow speed adversely affect the handling of a glider so drastically?

-John, Q3

Is it Winter already? This is one of those frequent threads (along with gelcoat maintenance, is the PW-5 the spawn of Satan, and the Downwind Turn) that come up every few years. As recently as two winters ago it was "Aerodynamics of Towing". If you search on some combination of "aerodynamics" and "tow" or "aerowtow" you'll see at least three primary generations of the thread. For instance:

https://groups.google.com/forum/#!searchin/rec.aviation.soaring/aerodynamics$20of$20towing/rec.aviation.soaring/C69yZmsaFe0/JqUTgv_G5HQJ

On Tuesday, November 5, 2013 4:36:45 PM UTC-6, John Carlyle wrote:
I did a search as Erik suggested. After reading a lot of posts, I found a very good explanation by Andreas Maurer, which I partially quote below. His explanation seems to account well for why the glider handling is so poor at slow tow plane speeds. But for me, several things are still unexplained: 1. Why does increasing the tow plane speed cause the poor glider handling to go away? 2. If the answer to (1) is that the down wash and wing tip vortices are now further below the glider, then why doesn't simply moving up higher above the wake of the slow tow plane remove the poor handling? -John, Q3 =========== The following was written by Andreas Maurer, and posted on RAS 1/5/11: The main factor for the seemingly odd flying characteristics behind the tow plane is the downwash of the latter. Let me explain: The downwash has a significant angle (the air is deflected downwards behind the tow plane's wing to up to four degrees!), but due to the larger span of the glider it only affects the inner part of the glider's wing. Therefore, if the glider if lying laterally displaced, only one wing is affected by the downwash of the tow plane - four degrees of AoA difference between left and right wing need a lot of aileron to correct. Likewise, if the glider is flying straight behind the tow plane, the downwash *decreases* the AoA of the affected inner part of the wing. Getting the nose up by pulling back will restore the lift of the inner part of the glider's wing, but now the outer parts of the wing have a much higher AoA than they have in free flight. Voila, meet the the conditions for poor aileron efficiency (high AoA!) and tip stall. The downwash is reduced by - wingloading of the tow plane - wing span of the tow plane In other words: The more a tow plane looks like a motorglider (say, a Dimona, or Katana Extreme), the less the flight characteristics of the glider are affected. Anyone who has ever been towed behind a motorglider or a microlight will testify that problems like poor lateral control or running out of elevator don't exist there, despite a far slower tow (55 kts compared to a typical 70-75 kts behind a typical tow plane like Reorqeur or Pawnee).

Back to seriousness. Ever looked and the con-trials behind an airline in flight? It is very narrow right at the plane, and spreads out as the plane moves away. The wake and downwash field are moving down and spreading out. The con-trail spreads out as it is marking a limit of the wake. All the air above the actual wake and downwash field is also pulled down by the downwash field behind the wing. Some very large distance away, this downward pull of air will be negligible, but in the amount of altitude you can rise above the wake on tow without seriously upsetting your tug, the downwash will still be about the same. So, getting above the wake may solve some of the awful handling because you aren't being trounced around, but you will still be very nose high and feel very uneasy. Interesting videos show the wake actually gets narrower for some distance behind the wing, and then spreads back out. So, there probably is some effect of the wing of the sailplane extending beyond the wake of short wing towplanes.

Why does increasing the towplane speed help? Two things. First, many things aerodynamic happen with the square of the speed. 10% increase in speed gives a bit over 20% increase in stall margin. Second, when the towplanes wing doesn't have to work as hard to keep it in the air, the size and intensity of the wake and downwash field decrease. So does the downwash angle. So, by adding speed, the downwash angle is reduced, your required angle of attack is reduced, your margins and comfort levels all go up. And by the way, the wake will actually be higher where you are at the end of the same length tow rope, as it won't be angling down as much as it comes off the wing of the towplane.

As Andreas said, long wing towplanes don't create large angular downwash fields when flying at 55 knots. Pawnees and the like do.

Also worthy of note, I believe the downwash field increases in angle but decreases in velocity as a function of horizontal distance from the wing. So, the length of rope you chose to hang yourself with might also affect slow tow speed handling. Hint: I would try longer for better handling at the same tow speed. Gets you further horizontally and vertically from the wake..

Well, that was more than just two cents worth!

Steve

After carefully reading all the above I can only come to one conclusion: Mary Ann

Thanks for the clear explanation, Steve. Let me summarize: The root cause of poor handling on tow is the tow plane down wash producing either differential lift if you’re not centered on the tow plane, or tip stalling if you are. Having the tow plane go faster gets you further above the stall speed-wise, and reduces your AoA because the down wash moves up.

Since turbulence interaction is causing the problem, it implies that you could combat a slow tow plane pilot who refuses to speed up by flying below his down wash. But comments in a previous thread indicate that those who tried that maneuver still had handling problems. Any idea why?

-John, Q3

On Tuesday, November 5, 2013 6:28:54 PM UTC-5, Steve Leonard wrote:
Back to seriousness. Ever looked and the con-trials behind an airline in flight? It is very narrow right at the plane, and spreads out as the plane moves away. The wake and downwash field are moving down and spreading out. The con-trail spreads out as it is marking a limit of the wake. All the air above the actual wake and downwash field is also pulled down by the downwash field behind the wing. Some very large distance away, this downward pull of air will be negligible, but in the amount of altitude you can rise above the wake on tow without seriously upsetting your tug, the downwash will still be about the same. So, getting above the wake may solve some of the awful handling because you aren't being trounced around, but you will still be very nose high and feel very uneasy. Interesting videos show the wake actually gets narrower for some distance behind the wing, and then spreads back out. So, there probably is some effect of the wing of the sailplane extending beyond the wake of short wing towplanes.

Why does increasing the towplane speed help? Two things. First, many things aerodynamic happen with the square of the speed. 10% increase in speed gives a bit over 20% increase in stall margin. Second, when the towplanes wing doesn't have to work as hard to keep it in the air, the size and intensity of the wake and downwash field decrease. So does the downwash angle.. So, by adding speed, the downwash angle is reduced, your required angle of attack is reduced, your margins and comfort levels all go up. And by the way, the wake will actually be higher where you are at the end of the same length tow rope, as it won't be angling down as much as it comes off the wing of the towplane.

As Andreas said, long wing towplanes don't create large angular downwash fields when flying at 55 knots. Pawnees and the like do.

Also worthy of note, I believe the downwash field increases in angle but decreases in velocity as a function of horizontal distance from the wing. So, the length of rope you chose to hang yourself with might also affect slow tow speed handling. Hint: I would try longer for better handling at the same tow speed. Gets you further horizontally and vertically from the wake.

Well, that was more than just two cents worth!

Steve

On Wednesday, November 6, 2013 10:20:06 AM UTC-6, John Carlyle wrote:
Thanks for the clear explanation, Steve. Let me summarize: The root cause of poor handling on tow is the tow plane down wash producing either differential lift if you’re not centered on the tow plane, or tip stalling if you are. Having the tow plane go faster gets you further above the stall speed-wise, and reduces your AoA because the down wash moves up. Since turbulence interaction is causing the problem, it implies that you could combat a slow tow plane pilot who refuses to speed up by flying below his down wash. But comments in a previous thread indicate that those who tried that maneuver still had handling problems. Any idea why? -John, Q3

Yep, pretty much. And since the wake moves around, you are never centered in it.

Faster gets you lower AOA. Deck angle also goes down a bit more due to reduced downwash angle from the towplane wake. Below the wake, the air will be also getting pushed down, similar to how above the wake, it will be getting pulled down. So if you do get below the wake when being towed slow, you are still going to feel precariously nose high. And sometimes, pilot "gain" is heightened to the point where the pilot can drive what seems to be an instability (PIO). Us humans are good at being able to accidentally couple with a natural frequency and drive it. I know I have done it.

I do think it could be a worhtwhile program for someone to study. I do think that a longer rope will get you further away from downwash influences, if you are flying high tow. If you are flying low tow, the longer rope will likely mean that you have to go well below the altitude of the towplane to get below the wake. And you will still end up close to the wake and downwash, so a longer rope for low tow would likely provide no benefit.

It is also possible that the if glider being pulled too slowly by the short wing towplane can get up above the wake, the poor handling could be resolved and leave you with only a very nose high deck angle. Of course, it is very much counter to your thinking to be nose high, buffetting, and struggling with roll control to then pull further back on the stick while still hooked to the towplane.

This has mainly shown up (please correct me if I am wrong here) on newer Std Class planes with less wing incidence and less airfoil section camber in their wings. This improves high speed performance, but gives a more nose high attitude at low speed. And nose high at low speed on tow requires a lower tow position to be able to see the towplane and keep him safe while he is pulling you to altitude. So, you get closer to the wake, drop into it or start out in it when the towplane lifts off, and get into the controlability issues of low tow speed, particularly behind short winged tugs with more energetic wakes.

Steve

On Wednesday, November 6, 2013 11:36:45 AM UTC+13, John Carlyle wrote:
I did a search as Erik suggested. After reading a lot of posts, I found a very good explanation by Andreas Maurer, which I partially quote below. His explanation seems to account well for why the glider handling is so poor at slow tow plane speeds. But for me, several things are still unexplained:



1. Why does increasing the tow plane speed cause the poor glider handling to go away?



2. If the answer to (1) is that the down wash and wing tip vortices are now further below the glider, then why doesn't simply moving up higher above the wake of the slow tow plane remove the poor handling?



-John, Q3



===========



The following was written by Andreas Maurer, and posted on RAS 1/5/11:



The main factor for the seemingly odd flying characteristics behind

the tow plane is the downwash of the latter.



Let me explain:



The downwash has a significant angle (the air is deflected downwards

behind the tow plane's wing to up to four degrees!), but due to the

larger span of the glider it only affects the inner part of the

glider's wing.



Therefore, if the glider if lying laterally displaced, only one wing

is affected by the downwash of the tow plane - four degrees of AoA

difference between left and right wing need a lot of aileron to

correct.



Likewise, if the glider is flying straight behind the tow plane, the

downwash *decreases* the AoA of the affected inner part of the wing.

Getting the nose up by pulling back will restore the lift of the inner

part of the glider's wing, but now the outer parts of the wing have a

much higher AoA than they have in free flight.



Voila, meet the the conditions for poor aileron efficiency (high AoA!)

and tip stall.



The downwash is reduced by

- wingloading of the tow plane

- wing span of the tow plane



In other words: The more a tow plane looks like a motorglider (say, a

Dimona, or Katana Extreme), the less the flight characteristics of the

glider are affected.



Anyone who has ever been towed behind a motorglider or a microlight

will testify that problems like poor lateral control or running out

of elevator don't exist there, despite a far slower tow (55 kts

compared to a typical 70-75 kts behind a typical tow plane like

Reorqeur or Pawnee).



===========



On Tuesday, November 5, 2013 2:58:54 PM UTC-5, Papa3 wrote:

On Tuesday, November 5, 2013 1:10:33 PM UTC-5, John Carlyle wrote:



A question for any aerodynamicists out the why does low aero tow speed adversely affect the handling of a glider so drastically?



-John, Q3



Is it Winter already? This is one of those frequent threads (along with gelcoat maintenance, is the PW-5 the spawn of Satan, and the Downwind Turn) that come up every few years. As recently as two winters ago it was "Aerodynamics of Towing". If you search on some combination of "aerodynamics" and "tow" or "aerowtow" you'll see at least three primary generations of the thread. For instance:



https://groups.google.com/forum/#!searchin/rec.aviation.soaring/aerodynamics$20of$20towing/rec.aviation.soaring/C69yZmsaFe0/JqUTgv_G5HQJ

That would explain why the effect is much more pronounced in a PW5 than in a 20m wingspan twin.

(don't cross the threads!!!)

On Thursday, November 7, 2013 7:13:57 PM UTC-8, Bruce Hoult wrote:
On Wednesday, November 6, 2013 11:36:45 AM UTC+13, John Carlyle wrote:

I did a search as Erik suggested. After reading a lot of posts, I found a very good explanation by Andreas Maurer, which I partially quote below. His explanation seems to account well for why the glider handling is so poor at slow tow plane speeds. But for me, several things are still unexplained:







1. Why does increasing the tow plane speed cause the poor glider handling to go away?







2. If the answer to (1) is that the down wash and wing tip vortices are now further below the glider, then why doesn't simply moving up higher above the wake of the slow tow plane remove the poor handling?







-John, Q3







===========







The following was written by Andreas Maurer, and posted on RAS 1/5/11:







The main factor for the seemingly odd flying characteristics behind



the tow plane is the downwash of the latter.







Let me explain:







The downwash has a significant angle (the air is deflected downwards



behind the tow plane's wing to up to four degrees!), but due to the



larger span of the glider it only affects the inner part of the



glider's wing.







Therefore, if the glider if lying laterally displaced, only one wing



is affected by the downwash of the tow plane - four degrees of AoA



difference between left and right wing need a lot of aileron to



correct.







Likewise, if the glider is flying straight behind the tow plane, the



downwash *decreases* the AoA of the affected inner part of the wing.



Getting the nose up by pulling back will restore the lift of the inner



part of the glider's wing, but now the outer parts of the wing have a



much higher AoA than they have in free flight.







Voila, meet the the conditions for poor aileron efficiency (high AoA!)



and tip stall.







The downwash is reduced by



- wingloading of the tow plane



- wing span of the tow plane







In other words: The more a tow plane looks like a motorglider (say, a



Dimona, or Katana Extreme), the less the flight characteristics of the



glider are affected.







Anyone who has ever been towed behind a motorglider or a microlight



will testify that problems like poor lateral control or running out



of elevator don't exist there, despite a far slower tow (55 kts



compared to a typical 70-75 kts behind a typical tow plane like



Reorqeur or Pawnee).







===========







On Tuesday, November 5, 2013 2:58:54 PM UTC-5, Papa3 wrote:



On Tuesday, November 5, 2013 1:10:33 PM UTC-5, John Carlyle wrote:







A question for any aerodynamicists out the why does low aero tow speed adversely affect the handling of a glider so drastically?







-John, Q3







Is it Winter already? This is one of those frequent threads (along with gelcoat maintenance, is the PW-5 the spawn of Satan, and the Downwind Turn) that come up every few years. As recently as two winters ago it was "Aerodynamics of Towing". If you search on some combination of "aerodynamics" and "tow" or "aerowtow" you'll see at least three primary generations of the thread. For instance:







https://groups.google.com/forum/#!searchin/rec.aviation.soaring/aerodynamics$20of$20towing/rec.aviation.soaring/C69yZmsaFe0/JqUTgv_G5HQJ



That would explain why the effect is much more pronounced in a PW5 than in a 20m wingspan twin.



(don't cross the threads!!!)

How does a PW5 handle on tow?

Who cares.

:-O