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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).

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.