Bill Daniels wrote:
OK, I guess this is a top post thread.

It ain't horsepower, it's the PROP. To improve the 0 - 20 acceleration,
improve the propeller. Have you ever tried to figure out how much power
actually gets to the tow hook on the glider from a 260 HP Pawnee? Maybe 20
HP?

At low speeds, as at the beginning of the TO roll, the tug's prop is mostly
stalled and converting little of the engines HP to thrust. As it
accelerates, more of the prop disc becomes unstalled and the prop picks up
efficiency and therefore more acceleration. A headwind helps a lot here.

So, what to do?

1. A constant speed prop reduces the blade AOA and lets the engine rev to
its redline RPM. (Noisy)
2. Big, slow turning props make much more thrust at low speeds than small,
high RPM props. (Quieter but require PRSU gearbox)
3. Ducted fans work really well at low speeds but lose out to a standard
prop at 100 MPH +. (A LOT quieter)

The ducted fan looks very good in that it can turn at crankshaft RPM while
producing several times the thrust of an unshrouded prop at zero airspeed.

Great stuff, Bill. I hadn't thought before how a headwind
helps a fixed pitch prop on the ground. Interesting...

I'm still excited about turbines on self-launch gliders
(and small planes). I was disappointed to learn that
Avemco puts turbines, no matter how small, in
a different insurance category than regular ol' planes.
It turns out my insurance only covers borrowed aircraft
with one or less engines, six or less seats, and
non-turbine without floats.

So much for the twin turbine Sparrowhawk ;(
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Mark Boyd
Avenal, California, USA