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



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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).



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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!!!)