Tug/tow plane performance

There is a lot of good talk about takeoff distances. My hope is to
share some facts gained over 40 years of towing. I'm not going to
search back in my logs, but I know that I've towed with some pretty
weak ones and with as much as 600 hp. (Ag Cat) Since my business
involves towing gliders at high altitudes and at very high density
altitudes and over power lines, my comments may be of some value.
My favorite tug is the 235-250 Pawnee. So easy to fly...cheap to buy
(relative to a Super Cub) We tune our tugs in many ways to get the
best performance. We also tune our tow pilots as well. We do have a NO-
GO glider must release point. In addition, the glider pilot must
release if any thing is not right...on or before lift-off.
Because we do not have a survivable landing area for a pre-mature
termation of the tow,( landing straight ahead), we tow (composite
ships) at 65-70 knots before crossing the power lines, usually with
100-200 ft above at that point. We teach our students how to make a
steep turn of 180 degrees without losing or gaining altitude and
without getting anywhere near stall. It cannot be done by the
"average" pilot without a speed margain.

Tuning the Pawnees involves using a climb prop (fixed) twisted to
develope red-line RPM on take-off. (The tow planes do not fly cross
country.) We do reduce mixture to best RPM then back off a bit. The
best thing we have done is to adjust CG such that the elevator is not
plowing through the air to maintain the correct towing attitude. The
other significant thing we do is to insist that the glider pilot stay
directly,.. straight behind the tow plane. Never to fly higher than
the tug and only to fly low tow or box the wake for skill development
and proof of skill. Any other position creates drag. Drag reduces
power to climb. We typically climb at 600-700 fpm with a Grob 103 or a
DG500 loaded. At 8,000 msl that tends to drop to about 500-600 fpm.
Because we are in the desert, we have very high density altitudes.
As a research engineer at Lockheed, I was fortunate to work on Ground-
Effect vehicles. Ground effect is very useful if you know how to use
it. We teach "ground effect landings" to help students and others
appreaciate a very useful tool.
Take-off runway length has so many variables, I doubt that a chart
could be developed for all conditions. The atmosphere is always in
motion in all directions and is barely predictable. To develope such a
chart would require some risk to document minimums. Use the experience
of others and reasonable action.
The biggest risk for take-off is HURRY.
Lots of love to you all. Especially tow pilots and instructors. Fred