Airspeed control during ground launch?

"Andreas Maurer" wrote in message
...
On Mon, 2 Apr 2007 10:01:07 -0600, "Bill Daniels"
bildan@comcast-dot-net wrote:


Pilot
confidence in the system is strongly related to the consistency of the
launches. If pilots lose confidence in the winch launch system, they'll go
elsewhere.

... or choose to stick to the familiar aerotow...

Might this be one cause why winch launching is used that seldom in the
US?

Andeas, you are right. This is a big reason.



The advantages of automatic tension control are so large that there's
every
reason to adopt it.

Hmmm... as usual, I beg to differ.
Here in Germany there are hundreds of winches in use with power
ranging from 150 to 400 hp, extremely few of them equipped with
tension control (afaik only the few electric winches), yet the winch
launch works extremely well with very few safety issues.

Actually, if you think about it, the diesel winch with a Vioth Turbokuppling
is very close to a tension controlled winch. Diesels have a very flat
torque curve and torque increases linearly with throttle. The Voith
coupling just passes that torque to the drum at a 1:1 ratio. If the winch
driver makes a reasonable guess as to throttle setting based on the glider
to be launched and the wind, the result will be pretty good.

Even in Germany, however, it's likely that the average tension will be 50%
to 70% of the weak link breaking load. Increasing this to 70% to 90% will
significantly increase release heights. 80% - 90% would be even better but
that will likely be more precision than the average human winch driver can
manage. Automated tension control systems can shave it as close as you
like.


The major cause for the wide-spread use of winches in Germany is
simplicity: By using standard truck parts such as engine and rear axle
it was (and is) easily possible to build a reliable winch for moderate
costs. Virtually any winch in Germany is homebuilt.

I think adding complex parts (such as automatic gearboxes and tension
control) raises the barrier to build an own winch significantly
(complexity, reliability, engineering effort, cost) while the
efficiency gain is negligible.

I think in order to promote winch launching in the US it might help to
heep the design as simple as possible.


In the US for the last 50 years or so, 'simple' has meant a V8 engine and
automatic transmission from a wrecked car. The engine will not produce
anything like a flat torque curve and transmission will run wild shifting
whenever it pleases. Combine this with gliders without CG hooks and the
results are very inconsistent leading to absurd 'rules' like "raise nose to
increase speed".


A collateral problem related to adoption of lightweight 'plastic' winch
rope
is that the winch driver can no longer see the rope sag. With heavy steel
cable, winch drivers used cable sag as a gauge of tension. And, absent
airspeed telemetry, the winch driver has no information about the gliders
airspeed. This absence of feedback leads to inconsistent launches.

Frankly spoken, this is contrary to my own experiences on a vast
number of winches.
A properly designed 280 hp Diesel winch doesn't need any sort of
gearbox and tension control, yet can easlily and safely launch any
glider from Grunau Baby to ASH-25E to satisfying altitudes - even with
an inexperienced winch driver, and even with plastic rope.
Not even a radio communication between pilot and winch is necessary.


I suspect that the average German club member has a far better idea of what
a good launch feels like and how one would drive a winch to obtain that
launch than his US counterpart. In the US, a little intellegence built
into the winch will be a big help to a fledgling winch operation.

This is even more so when it's important to achieve the highest launch
possible to compete with aero tow. Automating cable tension control allows
higher tensions to be used with safety and thus higher launches.

Bill Daniels