Glider Handling on Tow

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.

On Tuesday, November 5, 2013 4:06:37 PM UTC-5, HGXC wrote:
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



I don't think there is anything wrong with renewing a few classic discussions. There are always new pilots like myself who can benefit but wasn't around back in the day. I have flown Hang Gliders for nearly 40 years and managed glider blogs and know there are from time to time repeated topics.I don't see the harm in reintroducing them.

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

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

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.

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 2:29:47 PM UTC-5, Steve Leonard wrote:
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

We fly low tow as standard practice and I don't see quite the degree of this effect that I see in "normal" high tow. This may be, in part because it is more obvious you are starting to be affected by the wake when you feel it nibbling on the vertical tail.
On the topic of speed, it is worth noting that short wings at high lift coefficients generate a more powerful tip vortex. Lower C/L at increased speed reduces the effect of the vortex.
Another voice heard from
UH

Thanks again, Steve, I appreciate your ideas. A few comments:

1) The people experiencing this problem at my club are flying a LS-8, a V2x, an ASW-27 and a Duo, all behind a Pawnee with tow ropes no shorter than 200 feet. Our G-103’s don’t seem to have the problem (longer wings? But then why the problem with the Duo?)

2) Our tow pilots are aware of the problem, and they’re working with us. We’re going to explicitly remind them of the potential for the problem each time they start towing us.

3) Next time I fly I’ll ask the Pawnee pilot (at altitude) to slow down and I’ll go into low tow to see if it helps. Since I fly an LS-8 which is reported particularly susceptible to the problem, if that works out for me it should help others (particularly at a site where the tow plane doesn’t have a radio). It won’t be rigorous, but a positive report might be of some use.

-John, Q3

On Wednesday, November 6, 2013 2:29:47 PM UTC-5, Steve Leonard wrote:
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 6/11/2013 05:32, Mike the Strike wrote:
The Discus 2 exhibits the same behavior. It's a problem mostly seen
in newer unflapped racing ships because of the angle of incidence of
the wing. This results in a nose-up attitude at low speeds and on
aerotow this results in a downward pull on the nose from the towrope.
This down-pull has to be counteracted by up elevator. With ballast,
I run out of up elevator around 60 knots or so, and I have had my
glider sink into low tow a couple of times with a slow tug and have
had to release a couple more. In free flight, the glider will stall
at its placarded speed and handles fine. Tows below 60 knots have
very poor aileron and elevator control.

There may be other things going on, but I suspect the relative angle
of the towrope to the fuselage on tow is responsible. Flapped ships
alter their pitch and hardly suffer from this effect.

Mike

Discus 2b


In that case, flying in low tow should reduce the problem.

Somebody care to experiment?

GC

On Wednesday, November 6, 2013 8:39:53 PM UTC-6, John Carlyle wrote:
Thanks again, Steve, I appreciate your ideas. A few comments: 1) The people experiencing this problem at my club are flying a LS-8, a V2x, an ASW-27 and a Duo, all behind a Pawnee with tow ropes no shorter than 200 feet. Our G-103’s don’t seem to have the problem (longer wings? But then why the problem with the Duo?) 2) Our tow pilots are aware of the problem, and they’re working with us. We’re going to explicitly remind them of the potential for the problem each time they start towing us. 3) Next time I fly I’ll ask the Pawnee pilot (at altitude) to slow down and I’ll go into low tow to see if it helps. Since I fly an LS-8 which is reported particularly susceptible to the problem, if that works out for me it should help others (particularly at a site where the tow plane doesn’t have a radio). It won’t be rigorous, but a positive report might be of some use. -John,

Just guessing here, but are most of these reported control problems starting right after takeoff, or at some point during the tow, when the towplane slows down a bit for one reason or another? I have seen lots of problems when the towplane takes off and is starting to climb before the glider can get off the ground. And when the glider does get off the ground, he starts off in the towplane's wake. Been there. Not a happy place to be.

For the LS-8, V2, 27, and Duo, if the issue is starting for right at liftoff, this could be the case. The Grob, with its lower wing loading, has more margin above stall, so maybe it doesn't get as bad. Just a turbulent ride.. With a lighter wing loading towplane that really doesn't like to be on the ground above 60 MPH, it is vital that the towpilot not start his climb with a higher wing loaded glider until he is sure the glider is airborne. This does not mean he should try to stay on the ground, just don't start climbing until you have more speed.

If it is happening once you are above, say, 500 feet, climbing normally and all was well before hand, it could be that the towplane slowed down, you got lower to keep him in sight, and dropped down into the wake. Back to that lower incidence thing and not being able to see the towplane at lower speeds, and maybe the longer rope or low tow position would help in resolving the handling issue.

As for 3, John, if you do that, be sure to still be climbing, as the pictures and sensations change for level flight versus climb. Good refresher work before the next Flight Review!

Steve