Glider Handling on Tow

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.

===========

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.

Having gone through all the posts on this thread, I'm amazed that no-one
has stated the obvious. 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 the 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

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. 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?

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. 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

On Tuesday, November 5, 2013 11:10:33 AM UTC-7, 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?



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