New 29 Palms Winchfest Video

On Tue, 18 Dec 2007 10:14:05 -0700, "Bill Daniels"
bildan@comcast-dot-net wrote:


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.

Yup.
Here in Germany we call this training.

A new winch driver needs 100 winch launches on 10 different days
(read: different weather situations) to acquire his winch driver
license.
Any winch driver student is able to drive the winch safely after a
dozen launches using his feeling, an RPM meter and a look at the cable
sag.
Driving a winch is pretty simple if you ask me.


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.

Nice to have... but we use a much simpler way: We have an RPM meter.

Winch driver accelerates to a certain RPM (depending on glider type).
This RPM is modified according to the wind condition. Once the glider
goes through 45 degrees "winch driver head up", RPM is slowly
decreased.
Works like a charm 99.99 percent of the time.


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.

I think you are wrong here. The fact alone that nearly all winches in
Europe operate the way I described above means that this task is easy
to handle, don't you agree?


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.

Who cares?
I can assure you that with the KISS principle we reach the same
altitudes as you do.


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.

LOL... here comes the urban legend "pilot controls airspeed" again...


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.

I don't think so.
Being able to control the airspeed safely by pulling on the stick has
been an urban legend for ages. We sometimes talk about it, have a good
laugh, and return to the standard technique.


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.

I can ssure you - even a very tail-heavy DG-505 (the glider that can
break *any* weak-link) is unable to slow a winch with halfways
sufficient power down, although its elevator authority is sufficient
to stall it in any phase of the winch launch.
Fly one if you get the chance - it's interesting to see the loss of
aileron authority when you approach the stall during the winch
launch...


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?

The complexity.
The fact that extremely few winch operators feel the need to build a
winch according to yxour ideas points out that there might be other
priorities.


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?

It's not how it's done.
Taking the example of the 505 - if I tried to reduce airspeed by
pulling at more than 115 kph, I'd break the strongest weak link *any
time*.

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.

I disagree completely.

Take a closer look at causes for winch accidents:

Most accidents happen due to ground loops (just look at the video that
started this thread if you want to see some close cases) and badly
executed rope-break procedures.

There's hardly any case (I know of none) where the glider stalled at
an altitude of more than 200 ft. Nearly all stall related accidents
happen due to a too-sudden transition into full climb, followed by an
immediate highspeed stall and cartwheel or spin.
It's extremely simple to avoid this kind of accident: Keep the stick
in the recommended position for their first 150 ft of the winch launch
before pulling back smooooothly to enter the full climb angle.

Usually such a stall occurs extremely quickly - it's nearly impossible
to recover before impact.

These pilots were not able to hold the stick halfways in the correct
position, nor were they able to detect the very sudden transition into
a steep nose-up attitude. Nor, obvously, were they able to perform the
very simple corrective measu Push the stick forward immediately.

I strongly doubt that these pilots had been able to watch, interpret
and act according to an AoA meter.




Bye
Andreas