Glider Handling on Tow

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

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 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

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.

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

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

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

===========

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.

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.

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

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