Glider Handling on Tow

[John Carlyle]

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

Occasionally I’ve received an aero tow in my LS-8 (dry) at 55 kt (minimum recommended aero tow speed is 54 kt). The plane is heavy and unresponsive at that speed; it’s an extremely stressful experience! But once off tow the same plane handles like a dream at speeds down into the upper 30 kt range.

Before anyone asks, yes, I do immediately ask the tow pilot for 10 kt more speed - right now! I’d just like to understand what the root aerodynamic cause of the poor handling might be.

-John, Q3

[Mike the Strike]

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

[Papa3[_2_]]

On Tuesday, November 5, 2013 1:10:33 PM UTC-5, John Carlyle wrote:
> A question for any aerodynamicists out there: 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

[HGXC[_2_]]

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 there: 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.

[Steve Leonard[_2_]]

On Tuesday, November 5, 2013 3:06:37 PM UTC-6, HGXC wrote:
I don't think there is anything wrong with renewing a few classic discussions.

Classic discussions. Hmm. Ginger, or Mary Ann?

;-)

[Craig Funston[_2_]]

On Tuesday, November 5, 2013 1:35:15 PM UTC-8, Steve Leonard wrote:
> On Tuesday, November 5, 2013 3:06:37 PM UTC-6, HGXC wrote:
>
> I don't think there is anything wrong with renewing a few classic discussions.
>
>
>
> Classic discussions. Hmm. Ginger, or Mary Ann?
>
>
>
> ;-)

Mary Ann, Definitely.

Craig

[John Carlyle]

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 there: 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

[son_of_flubber]

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

> I don't think there is anything wrong with renewing a few classic discussions.

The problem with searching for old discussions before posting a question is that newbies (like me) pick up a lot on RAS from questions that we never think to ask. So if people search for their question and don't repost the question, our ignorance remains unrequited.

And that ignorance is deep and wide. I did not even realize that the PW-5 was Spawn of Satan. Seriously, there seem to be an endless stream of "good to know" and even a few "really important to know" topics. You make us less dangerous to you by educating us.

I wish there were a book of "soaring lessons learned". Last Fall I learned the hard way that the "caster wheel" on a glider trailer tends to vibrate loose and drag on the pavement. Where am I suppose to have learned that? My pappy did not get his hands dirty very often and my education is deficient in many areas of practical knowledge.

Civilized online communities maintain FAQ lists. In lieu of that... keep the repeat questions coming.

[Steve Leonard[_2_]]

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

[[email protected]]

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

[Papa3[_2_]]

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 there: 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.

[Papa3[_2_]]

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.
>
>

[[email protected]]

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

[John Carlyle]

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

[John Carlyle]

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.

[Steve Leonard[_2_]]

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

[[email protected]]

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

[John Carlyle]

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

[GC[_2_]]

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

[Steve Leonard[_2_]]

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

[Bruce Hoult]

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 there: 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!!!)

[darrylr]

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 there: 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

[Bruce Hoult]

On Friday, November 8, 2013 7:29:51 PM UTC+13, darrylr wrote:
> 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 there: 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

At one stage I felt the PW5 towed both extremely nose high and with a lot of back stick to keep position.

This did improve once I realized that with the towplane climbing at well over 1000 FPM (the vario needle is simply always pegged, regardless of lift/sink) vs 600 FPM in the DG1000/Grob/Janus, flying with the towplane in the normal position on the horizon meant that I was flying waaay above his flight path. Dropping down to where the tailplane and wing/struts looked normal improved things a lot.

[John Carlyle]

Thanks again for your thoughts, Steve. Sorry for my delay in responding.

All of our slow tow incidents have occurred above 500 feet; none of us has ever had a problem near the ground. Our airport elevation is 700 feet with unobstructed horizons, and the temperatures on the days of the incidents were mostly in the 70s. The Pawnee pilots claimed that they had been maintaining 80 mph during the climb to that point; but when they went to 90 mph the handling problem for the remainder of the tow was gone.

When the handing problem first happened to me I did think that I’d somehow gotten low and hit the wake. It didn’t make sense, because the proper sight picture in my LS-8 on tow has the tow plane touching the top of the glare shield, so it’s super easy to see if you’re too low (or too high). I climbed 20-30 feet, but when it didn’t help I asked for more speed and all was well. I haven’t asked the other pilots if they thought they’d hit the tow plane wake, but I will.

It may be a while before I can see if going into low tow will relieve our slow tow handling problems. I can’t fly for another week and the weather is getting more problematic. But when I do the experiment, I’ll make sure that I’m climbing, as you advise.

I must say I’m very pleased to now have an aerodynamic understanding of why gliders on tow can have handling problems. Several more experienced pilots had told me that I’d be fine as long as the tow plane kept on flying. But they were flying G-103s, not high performance ships, and it’s possible that they’d never experienced what I was seeing.

-John, Q3

On Thursday, November 7, 2013 2:05:07 PM UTC-5, Steve Leonard wrote:
> 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

[JS]

Of course, there are no examples of Pawnee ASIs being incorrect!
Many of us have towed fully ballasted behind 235HP Pawnees at Ely, NV. Field elevation at Ely is six thousand five hundred feet MSL. Predictably, the main problems there seem to be ground roll length and climb angle.
Short of calibrating the ASI or adding a radio so you easily communicate without the awkward "speed up" signal, perhaps make a higher standard tow speed for anything that doesn't look antique?
Jim

[JS]

My mistake on Ely, field elevation only 6256'.
But density altitude at launch time is typically close to ten thousand.
Jim

[Eric]

>
>===========
>
>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).
>
The above is the correct explanation. I would like to add the
following;

Don't confuse downwash with wake turbulence. The wake is the
turbulence from the propellor slipstream deflected down by the wing.
There is downwash above and bellow the propellor slipstream.

The more higher downwash angle the towplane produces, the worse the
effect. So for a given tow speed...
If the tug has just refueled Worse.
If the tug is two up Worse
If the tug has a high wing loading and lots of high lift devices like
a Wilga Much worse.

The new light tugs like the Eurofox can tow an empty ASW27 comfortably
at 55kts because they are light, low wing loading and at 55kts are
well above their stall speed, all of which give a small downwash
angle. As a result they can do most of what a 200hp tug can do on
100hp. The initial ground accelleration is their only weak point.

[Papa3[_2_]]

I've towed with my LS8 at many sites in many different conditions behind many different towplanes, and I've never had the problem you describe at an IAS (on my side) of above 65 kts. If the Pawnees (PGC?) were truly doing 80mph (70 kts), I can't imagine how this would be described as a "slow tow". I've had tows that dropped down into the low 60s or even high 50s (kts) where it felt squirrely, but that's a full 10kts slower than what you are describing.

Just a data point.

P3



On Saturday, November 9, 2013 10:40:05 AM UTC-5, John Carlyle wrote:
> Thanks again for your thoughts, Steve. Sorry for my delay in responding.

> All of our slow tow incidents have occurred above 500 feet; none of us has ever had a problem near the ground. Our airport elevation is 700 feet with unobstructed horizons, and the temperatures on the days of the incidents were mostly in the 70s. The Pawnee pilots claimed that they had been maintaining 80 mph during the climb to that point; but when they went to 90 mph the handling problem for the remainder of the tow was gone.
>

[John Carlyle]

Erik, I think you and Jim Staniforth have identified the key issue. Five pilots in four glider types at one site experiencing low tow speed handling problems behind one tow plane point to the ASI in the Pawnee reading incorrectly.

I searched the RAS archives and found a thread from 1997 discussing Pawnee ASIs reading too high, ie, giving the glider a low speed tow. Several posts blame the Pawnee ASI static arrangement (it seems in some Pawnees the ASI static ports to the cockpit while in others the ASI static goes to fuselage static ports). It is claimed that the first is affected by ventilation use, while the second is influenced by propeller airflow. One post asked if anyone had tried using the static that was a part of his pitot probe, but got no answer.

So apparently Pawnee ASI errors are known and of a long standing nature. Is there any good fix?

-John, Q3


On Sunday, November 10, 2013 8:14:13 AM UTC-5, Papa3 wrote:
> I've towed with my LS8 at many sites in many different conditions behind many different towplanes, and I've never had the problem you describe at an IAS (on my side) of above 65 kts. If the Pawnees (PGC?) were truly doing 80mph (70 kts), I can't imagine how this would be described as a "slow tow".. I've had tows that dropped down into the low 60s or even high 50s (kts) where it felt squirrely, but that's a full 10kts slower than what you are describing.
>
> Just a data point.
>
> P3

[Andy Melville[_2_]]

At 13:14 10 November 2013, Papa3 wrote:
>I've towed with my LS8 at many sites in many different conditions behind
>ma=
>ny different towplanes, and I've never had the problem you describe at an
>I=
>AS (on my side) of above 65 kts. If the Pawnees (PGC?) were truly doing
>80=
>mph (70 kts), I can't imagine how this would be described as a "slow
tow".
>=
> I've had tows that dropped down into the low 60s or even high 50s (kts)
>w=
>here it felt squirrely, but that's a full 10kts slower than what you are
>de=
>scribing.=20
>
>Just a data point.=20
>
>P3
>
>
>
>On Saturday, November 9, 2013 10:40:05 AM UTC-5, John Carlyle wrote:
>> Thanks again for your thoughts, Steve. Sorry for my delay in
responding.=
>=20
>
>> All of our slow tow incidents have occurred above 500 feet; none of us
>ha=
>s ever had a problem near the ground. Our airport elevation is 700 feet
>wit=
>h unobstructed horizons, and the temperatures on the days of the
incidents
>=
>were mostly in the 70s. The Pawnee pilots claimed that they had been
>mainta=
>ining 80 mph during the climb to that point; but when they went to 90 mph
>t=
>he handling problem for the remainder of the tow was gone.=20
>>=20
>

[Andy Melville[_2_]]

At my club in the Uk we have a Pawnee and a Supercub for towing.
with 80 mph on the Pawnee the glider behind is gettting 60-65 knots...ok
for K21 's and the like but you get distressed LS8's and asg29 pilots
calling asking for more speed...especially if full of water.! This seems to
be caused by the pitot position with the Pawnee in a unatural climb
psositon with a glider on the back. Take the speed to 90mph and they are
happy as they get 70knots on the back!!
Now to the Super Cub( 180hp) With 60 mph on the dial the glider behind is
getting 60-65 Knots( same as Pawnee at indicated 80mph) Once again this I
believe is due to pitot and unnnatural climb angles. speed up to 65 mph and
the guys on the back are happy getting 65-70 knots
So we have 2 different tugs indicating 20 mph differences to give the same
speed to the glider!!!
If you know this , dont shout at the tug pilot but politely call on radio
for 'a few more knots please'
hope that helps
Regards from the Uk


At 18:44 11 November 2013, Andy Melville wrote:
>At 13:14 10 November 2013, Papa3 wrote:
>>I've towed with my LS8 at many sites in many different conditions behind
>>ma=
>>ny different towplanes, and I've never had the problem you describe at
an
>>I=
>>AS (on my side) of above 65 kts. If the Pawnees (PGC?) were truly doing
>>80=
>>mph (70 kts), I can't imagine how this would be described as a "slo
>tow".
>>=
>> I've had tows that dropped down into the low 60s or even high 50s
(kts)
>>w=
>>here it felt squirrely, but that's a full 10kts slower than what you are
>>de=
>>scribing.=20
>>
>>Just a data point.=20
>>
>>P3
>>
>>
>>
>>On Saturday, November 9, 2013 10:40:05 AM UTC-5, John Carlyle wrote:
>>> Thanks again for your thoughts, Steve. Sorry for my delay in
>responding.=
>>=20
>>
>>> All of our slow tow incidents have occurred above 500 feet; none of us
>>ha=
>>s ever had a problem near the ground. Our airport elevation is 700 feet
>>wit=
>>h unobstructed horizons, and the temperatures on the days of th
>incidents
>>=
>>were mostly in the 70s. The Pawnee pilots claimed that they had been
>>mainta=
>>ining 80 mph during the climb to that point; but when they went to 90
mph
>>t=
>>he handling problem for the remainder of the tow was gone.=20
>>>=20
>>
>
>

[Greg Delp]

We tow with a Pawnee at our club as well. We were also having issues with the tow speeds showing approximately 5-10 knots (10-15 mph) slower on the glider's ASI than on the Pawnee's. This spread worsened when the Pawnee's side window was open. ( we had a window latch that was loose and occasionally would open during takeoff or tow ) There was a very apparent change in the pitch attitude required to maintain the proper indicated but wrong actual airspeed and the glider pilots would request a faster tow speed. I found the static line broken inside the fuselage, hidden between the fuselage fabric and steel tubing next to a bracket near the throttle. This leak obviously induced large errors into the static system especially with the window open. I imagine this could be an issue with many of the Pawnees around. The hard static line is very close to vibrating fabric and can also be damaged by ingress/egress from the cockpit if the fabric is pressed into the line. The line eventually work hardened and broke in two.

[Dave Nadler]

On Tuesday, November 12, 2013 6:50:00 AM UTC-5, Greg Delp wrote:
> I found the static line broken inside the fuselage,
> hidden between the fuselage fabric and steel tubing next
> to a bracket near the throttle. This leak obviously
> induced large errors into the static system
> especially with the window open.
>
> I imagine this could be an issue with many of the
> Pawnees around.

Right, the aluminum static line is prone to cracking.
You can also see the ASI jump by opening and closing
the air vent when there's a static leak.

And of course some Pawnees have ASI on cockpit static...

Even without leaks the ASI will seriously over-indicate.

Unfortunately, lots of Pawnee pilots are not trained
about the issues in this thread.

Be careful out there,
Best Regards, Dave

[John Carlyle]

Andy, you say for your gliders to indicate 60-65 kt (70-75 mph) on tow that your Pawnee must indicate 80 mph (70 kt) and that your Super Cub must indicate 60 mph (52 kt)? I figured that there might be some ASI error during climb, but that’s truly remarkable!

Greg, thanks for mentioning your Pawnee’s broken static line problem. If this is common, as Dave says, it may be the source of our problem (which just started this year).

-John, Q3

[Scott Calvert[_2_]]

At 11:50 12 November 2013, Greg Delp wrote:
>We tow with a Pawnee at our club as well. We were also having
issues with
>t=
>he tow speeds showing approximately 5-10 knots (10-15 mph)
slower on the
>gl=
>ider's ASI than on the Pawnee's. This spread worsened when
the Pawnee's
>sid=
>e window was open. ( we had a window latch that was loose
and occasionally
>=
>would open during takeoff or tow ) There was a very apparent
change in the
>=
>pitch attitude required to maintain the proper indicated but
wrong actual
>a=
>irspeed and the glider pilots would request a faster tow speed.
I found
>th=
>e static line broken inside the fuselage, hidden between the
fuselage
>fabri=
>c and steel tubing next to a bracket near the throttle. This leak
>obviousl=
>y induced large errors into the static system especially with the
window
>op=
>en. I imagine this could be an issue with many of the Pawnees
around. The
>=
>hard static line is very close to vibrating fabric and can also be
damaged
>=
>by ingress/egress from the cockpit if the fabric is pressed into
the line.
>=
>The line eventually work hardened and broke in two.
>

[Scott Calvert[_2_]]

At 11:50 12 November 2013, Greg Delp wrote:
>We tow with a Pawnee at our club as well. We were also having
issues with
>t=
>he tow speeds showing approximately 5-10 knots (10-15 mph)
slower on the
>gl=
>ider's ASI than on the Pawnee's. This spread worsened when
the Pawnee's
>sid=
>e window was open. ( we had a window latch that was loose
and occasionally
>=
>would open during takeoff or tow ) There was a very apparent
change in the
>=
>pitch attitude required to maintain the proper indicated but
wrong actual
>a=
>irspeed and the glider pilots would request a faster tow speed.
I found
>th=
>e static line broken inside the fuselage, hidden between the
fuselage
>fabri=
>c and steel tubing next to a bracket near the throttle. This leak
>obviousl=
>y induced large errors into the static system especially with the
window
>op=
>en. I imagine this could be an issue with many of the Pawnees
around. The
>=
>hard static line is very close to vibrating fabric and can also be
damaged
>=
>by ingress/egress from the cockpit if the fabric is pressed into
the line.
>=
>The line eventually work hardened and broke in two.
>
Our Pawnee shows an increase in climb rate if the window is
open.

[Soarin Again[_2_]]

Granted this is best with no wind conditions. But for those who find that
their tow planes continually seem to give slow tow speeds.
The next relatively calm morning put a handheld gps displaying ground speed
in the tow plane where the tow pilot can easily see it while towing. If
his indicated airspeed is notably different than what he sees for gps
ground speed, you have good grounds for a discussion. However if your
indicated airspeed was notably different then his gps ground speed, the
discussion may be go differently.

Another interesting tidbit would be for a glider while on tow at say 60 to
65 kts glider indicated airspeed. Release from tow and maintain that same
deck angle for say 13 seconds to see what indicated air speed the glider
finally stabilizes to. But remember it is critical that you maintain the
same pitch attitude relative to the horizon that you had while on tow.


At 11:50 12 November 2013, Greg Delp wrote:
>We tow with a Pawnee at our club as well. We were also having issues with
>t=
>he tow speeds showing approximately 5-10 knots (10-15 mph) slower on the
>gl=
>ider's ASI than on the Pawnee's. This spread worsened when the Pawnee's
>sid=
>e window was open. ( we had a window latch that was loose and
occasionally
>=
>would open during takeoff or tow ) There was a very apparent change in
the
>=
>pitch attitude required to maintain the proper indicated but wrong actual
>a=
>irspeed and the glider pilots would request a faster tow speed. I found
>th=
>e static line broken inside the fuselage, hidden between the fuselage
>fabri=
>c and steel tubing next to a bracket near the throttle. This leak
>obviousl=
>y induced large errors into the static system especially with the window
>op=
>en. I imagine this could be an issue with many of the Pawnees around.
The
>=
>hard static line is very close to vibrating fabric and can also be
damaged
>=
>by ingress/egress from the cockpit if the fabric is pressed into the
line.
>=
>The line eventually work hardened and broke in two.
>

[Geoff Brown[_2_]]

Having gone through all the posts on this thread, I'm amazed that no-one
has stated the obvious. Re: the difference in handling between flying at 40
knots on tow and flying at 40 knots off tow.

In the first case you are trying to get the glider to climb so you have a
high angle of attack so you can follow the tow plane. The wings are
pulling more than 1g, you just can't fell it in the cockpit. In the second
you are descending, and you might even be pulling less than 1g.

Or did someone mention it and I missed it?

At 18:10 05 November 2013, John Carlyle wrote:
>A question for any aerodynamicists out there: why does low aero tow
speed
>=
>adversely affect the handling of a glider so drastically?
>
>Occasionally I=92ve received an aero tow in my LS-8 (dry) at 55 kt
>(minimum=
> recommended aero tow speed is 54 kt). The plane is heavy and
unresponsive
>=
>at that speed; it=92s an extremely stressful experience! But once off tow
>t=
>he same plane handles like a dream at speeds down into the upper 30 kt
>rang=
>e.=20
>
>Before anyone asks, yes, I do immediately ask the tow pilot for 10 kt
more
>=
>speed - right now! I=92d just like to understand what the root
aerodynamic
>=
>cause of the poor handling might be.=20
>
>-John, Q3
>

[Tony[_5_]]

On Tuesday, November 12, 2013 9:47:33 AM UTC-6, Geoff Brown wrote:
> Having gone through all the posts on this thread, I'm amazed that no-one
>
> has stated the obvious. Re: the difference in handling between flying at 40
>
> knots on tow and flying at 40 knots off tow.
>
>
>
> In the first case you are trying to get the glider to climb so you have a
>
> high angle of attack so you can follow the tow plane. The wings are
>
> pulling more than 1g, you just can't fell it in the cockpit. In the second
>
> you are descending, and you might even be pulling less than 1g.
>
>
>
> Or did someone mention it and I missed it?

I don't think anyone has mentioned that. Could you expand a little on why you feel this is true?

[[email protected]]

On Tuesday, November 12, 2013 9:47:33 AM UTC-6, Geoff Brown wrote:
> Having gone through all the posts on this thread, I'm amazed that no-one
> has stated the obvious. Re: the difference in handling between flying at 40
> knots on tow and flying at 40 knots off tow.
>
>
>
> In the first case you are trying to get the glider to climb so you have a
> high angle of attack so you can follow the tow plane. The wings are
> pulling more than 1g, you just can't fell it in the cockpit. In the second
> you are descending, and you might even be pulling less than 1g.
>
> Or did someone mention it and I missed it?
>
Alas, this is not true. On tow, you are pulling exactly 1 g and flying at exactly the same angle of attack. If anything, in fact, the slight upward pull of the towrope means a slightly lower lift from the wings. Imagine if it were pulling you straight up -- zero angle of attack, zero lift on the wings.

You are flying in an upward trajectory. The wings have a slightly higher angle relative to the horizon, but not relative to the oncoming air.

The discomfort may be because the nose is pointed higher. But since the glider is going up, this higher nose angle does not mean higher angle of attack.

Similarly, an earlier poster suggested releasing from tow and maintaining pitch attitude relative to horizon. This will not work, as now the air is coming at you in a descending direction. In fact, a major fault of many students after a rope break is NOT promptly lowering the pitch attitude so they can maintain the SAME angle of attack and airspeed.

I think we're down to the downwash effect.

The one time you need temporarily more angle of attack is if the towplane climbs suddenly. I think this is why it's particularly annoying to be close to the ground and the towplane takes off, then climbs abruptly while losing speed while the glider is still basically on the ground.

John Cochrane

[John Carlyle]

Thanks for this idea. Three out of the four gliders have the ability to show ground speed, as well as an independent IAS from their SN-10s to cross-check their ASIs. Getting a ground speed value from the tow plane to compare to the tow plane ASI as well as the glider ground speed during flight is also doable. Not sure when we can do this, but I intend to try it!

-John, Q3

On Tuesday, November 12, 2013 10:45:33 AM UTC-5, soarin wrote:
> [snip}
> The next relatively calm morning put a handheld gps displaying ground speed
> in the tow plane where the tow pilot can easily see it while towing. If
> his indicated airspeed is notably different than what he sees for gps
> ground speed, you have good grounds for a discussion. However if your
> indicated airspeed was notably different then his gps ground speed, the
> discussion may be go differently.
> [snip]

[John Carlyle]

Mike,
You said that poor handling during low speed towing is mainly felt by newer standard class racing ships. This is probably correct if we accept Andreas’ explanation that downwash interaction (requiring higher AOA which leads to tip stalling) is the root cause of the poor handling problem, as flaps would allow a lower AOA than with a standard ship. However, a look at section 4 of several pilot handbooks showed that the 15/18 m ships have higher minimum tow speeds:

Ship, dry min tow, wet min tow
D2, 54 kt, 65 kt
LS-8, 54 kt, 65 kt
Duo, 54 kt solo, 65 kt dual
V2, 65 kt, 70 kt
ASW-27, 65 kt, 70 kt

Do you know what the 15/18 m pilots experience when being towed below 65 kt?

-John, Q3

On Tuesday, November 5, 2013 1:32:35 PM UTC-5, 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

[Mike the Strike]

On Tuesday, November 5, 2013 11:10:33 AM UTC-7, John Carlyle wrote:
> A question for any aerodynamicists out there: why does low aero tow speed adversely affect the handling of a glider so drastically?
>
>
>
> Occasionally I’ve received an aero tow in my LS-8 (dry) at 55 kt (minimum recommended aero tow speed is 54 kt). The plane is heavy and unresponsive at that speed; it’s an extremely stressful experience! But once off tow the same plane handles like a dream at speeds down into the upper 30 kt range.
>
>
>
> Before anyone asks, yes, I do immediately ask the tow pilot for 10 kt more speed - right now! I’d just like to understand what the root aerodynamic cause of the poor handling might be.
>
>
>
> -John, Q3

Not sure how different the experience of the flapped ships is, but the biggest complainers behind our Pawnees seem to be the D2 and LS8. From a wingtip video, I eyeballed the towrope angle and thought that its vector was below the fuselage center line. however, the down-wash theory is also certainly plausible. The dynamics of aerotow are certainly not simple and the problem could be a combination of several effects.

Mike