New 29 Palms Winchfest Video

"Andreas Maurer" wrote in message
...
On Mon, 17 Dec 2007 16:40:49 -0800 (PST), tommytoyz
wrote:


from the pilot's point of view, it should be a
simple matter.

I think that's the main point.

@Bill:
I really love the fact that you are trying to promote winch launching
in the US - but (please correct me if I'm wrong) from the comments
here in this newsgroup I get the impression that you US guys regard
the winch launch as something pretty hazardous that needs a lot of
technology to be safe.
Consequently, your approach to a US winch design looks a little
over-engineered to me, I have to admit. Complex and expensive to
build, hard to set up correctly. Possibly prone to mechanical
problems.

Us European guys seem to see things in a more relaxed way - we tend to
advocate training instead of technological devices to achieve safe
winch operations.

Considering the number of accident-free winch launches per year on my
side of the pond, don't you think that it might be useful to stick
more closely to the KISS-principle ("keep it simple, stupid"), forget
all that high-tech stuff and concentrate on correct training?

Nearly all the winches here are basically very simple.
Anything a winch needs is a strong diesel engine, two drums, 3.500 ft
of cable (be it steel or plastic). No rocket science involved.




Bye
Andreas

Andreas, I have great respect for the long experience you have on that side
of the pond. I have spent years researching just what you do over there.
That's where I got my ideas.

But, there are problems with the very basic KISS principle. If you ask the
winch driver to control the glider's airspeed, you have to give him a way to
to judge how well he does that. If the pilot has an airspeed indicator with
which he judges the quality of a launch but the winch driver has nothing but
"feel", that's unfair to the winch driver since the pilot has MUCH more
information than he does. It's interesting to see that a German company is
now selling an inexpensive airspeed telemetry unit to address this.

If you ask the winch driver to control tension instead of airspeed, he has
several things to help him do that - things like cable sag (if using steel)
throttle setting and the sound and feel of the winch. It's even simpler for
him if there is a tensiometer. It's still better if you can link the
instantaneous cable tension to a throttle control loop so the winch does it
automatically.

The critical point is that neither the pilot or winch driver can control
both airspeed AND tension at the same time. If you tell the pilot to just
"go along for the ride" and task the winch driver with controling both, you
are giving the winch driver an impossible task.

If the winch driver controls only tension, the pilot now has to step up and
control his own airspeed. Once I realized I could do this, launches assumed
a high level of smoothness and consistency. This REALLY works SWEET. The
math shows it works right up to the cable tension that would break the weak
link so winch engine potential power or torque has nothing to do with it.

However, this is a hard concept to teach someone who has been trained
otherwise. I've had people who were jumping up and down with indignation
since they "knew" that pulling harder would make the glider speed up. "Show
me", I said. They lost the bet - every time.

A good analogy is old cars with drum brakes. When they got wet, braking
action vanished until they dried out. There is an almost overpowering
sensation that the car actually speeds up when you step hard on the brakes
and nothing happens.

I'm sure this is what is confusing glider pilots on a winch. As the nose
rises during the rotation, the glider is still accelerating and, since the
airspeed doesn't drop when they pull, they assume pulling on the stick makes
the glider speed up - or at least that they have no control over airspeed.

Once the glider has stabilized in the climb, the old pitch/airspeed
relationship is restored and you can control airspeed. Pitch up to slow and
down to speed up. As the glider nears the top of the launch, the ability to
reduce airspeed is much reduced but releasing back pressure will definitely
make the glider speed up.

There are things that make this hard to see. For one thing, the airspeed
response is not instantaneous - it takes a little time for a pitch input to
result in an airspeed change. For another, if the glider is loaded with the
CG at or even beyond the forward limit, you will likely not have enough up
elevator authority to slow the glider. This is a W&B problem - not a winch
problem. It's is the "two 200+ pounders in a glider with a 380 pound
maximum cockpit load" problem.

So, is this approach unnecessarily complicated? I don't think so. All I am
saying is to give the winch driver the ability to control tension extremely
accurately no matter what. Any driver, any wind, any glider or pilot and
the tension is always exactly right. What's not to like about that?

All you ask of the pilot is to control airspeed with pitch exactly like it
is done in all other flight situations. In other words, "Just fly the
glider". What's complicated about that?

An AOA indicator helps the pilot because his wings are loaded to the
equivalent of 3.5 G's so the "loaded stall airspeed" is much higher than in
1G flight. Glider's have stalled while on the wire with disastrous results.
A safe AOA indication is an absolute guarantee that you won't stall - no ASI
indication gives that level of assurance.


Bill Daniels