Safety of winch launch vrs. aero tow?

Bob Johnson wrote:

Robert --

I would like to learn how they do this.

The experiment I was talking about was done in another club,
so I have no direct information about it. However, when our winch
correctly delivers its power, we also get about 40% of cable length,
i.e. 400m with 1000m cable.

Our Blanik has perhaps the best
located CG hook on any sailplane I'm aware of, although some would say
the bridle is somewhat awkward to use.
On a typical tow, with the Blanik weighing about 1100 lb, the 300 hp
winch engine throttle is advanced to the forward stop in 3 seconds and
at this time the engine is already rapidly approaching redline 5000 RPM.
At this point the Blanik is already beginning its climb and the throttle
is eased.

There are some differences with what happens in my club: the reduction
ratio is not the same by us, engine RPM is 1800-2000. We never get
near redline, although the throttle is kept to the forward stop for
2 seaters (ASK21) until the glider seen from the winch crosses the
angle between the front window and the top window, i.e. cable angle
near 45 degrees. Our engine has only 200 hp.

This quick takeoff and climb performance is still not fast enough to
load the ship and its occupants to more than about 0.8 - 0.9 G
horizontal acceleration, which I think one would have do to attain any
more than the 1/3 line length releases we are attaining.

We limit our climb airspeed to 55-60 kt, which I believe is the POH
recommendation. Very little if any back stick is necessary, and the
Blanik attains the 55 kt climb pretty much on its own until the last
part of the 45 second tow, when some back stick is applied to counter
the downward (with respect to the ground) pull of the rope. Our
Spectra/Dyneema/Plasma plastic rope weighs less than its full 66 lb when
partially wound on the drum, so line weight (and its necessary
acceleration and ground contact friction) for us is practically
negligible.

If there is a better setup anywhere in the world, I would sure
appreciate a description.


Maybe a reduction ratio on the engine allowing full throttle without
crossing redline, keeping full throtte longer during the climb, climbing
at 60 kt rather than 55 kt. And probably a higher nose up attitude during
the first part of the climb, which would be allowed with more power and
speed during this phase. But maybe the difference is only due to the
better aerodynamic of the ASK21 compared to the Blanik. I don't remember what gliders
were used in the experiment I reported about, but probably modern
gliders, certainly not Blaniks (almost unknown in France).

Thanks for your reply,