Er, did you actually read what I wrote Bill? I said
that UHMWPE cable is good stuff, and becomes doubly
advantageous for retrieve winching, where you are pulling
two cables into the air, and for longer runs. It also
doesn't try to untwist when new, unlike stranded steel
cable, or kink like piano wire. As Dan G has pointed
out it is also safer to handle.

The main downside is cost, plus the fact that on short
runs cable weight is not much of an issue and you don't
get that much of a height advantage by using synthetic
cable. We had a period at Lasham when one of our winches
had synthetic cable on one drum and steel cable on
the other, so we were able to do direct comparisons
on alternate launches in the same conditions. As a
percentage of the cable run in still evening air, we
averaged about 38% for steel cable and 41% for synthetic.
For a 4000ft run this works out at 1520 ft for steel
and 1640ft for synthetic, so there was an advantage
of just over 100ft in using synthetic. Off course the
longer the run the greater the advantage would become,
but then how many clubs have a winch run of more than
5000 feet?

Lasham didn't just throw UHMWPE cable onto an existing
winch! We had a number of modifications done in preparation,
to prevent drum crushing and to smooth the rollers,
as recommended by Skylaunch. Our Retrieve winch came
fitted with some very light, orange coloured, synthetic
string, but as I have already said that lasted no time
at all. The one synthetic main cable we tried actually
lasted longer than a steel cable, giving 2500 launches
as compared with about 1900 launches, but this was
not enough to offset the extra cost. I should point
out that we normally launch across two 50 metre wide,
extremely abrasive and rough WW2 concrete runways,
so we suffer high cable wear rates at the best of times.
There is a plan to dig these runways up (when there
is a local building project going on and we can get
a good price for the hard core) and replace them with
smooth narrow tarmac strips for winter operations.
We may then reconsider synthetic winch cables.

The retrieve winch pulls the cable back at about 50mph.
If you don't provide some pay-out braking, not only
will the main winch drum probably over-run when you
stop pulling, but the cable has enough momentum that
it won't stop when you cut the throttle on the retrieve
winch and you pull the rings into the rollers. This
has happened to me on one occasion as a retrieve winch
driver when the main winch driver didn't apply enough
braking. The Skylaunch retrieve winch is powerful enough
to pull back any sort of cable over any reasonable
distance.

I have carried a number of comparison tests in the
last year which I have reported on the Yahoo Winch
Launching Group website. OK I couldn't measure as many
parameters as I would have liked, but they were generally
carried out on relatively still evenings (I instruct
on an evening group) with the same winch and winch
drivers and a small group of pilots, so they should
be fairly indicative. Sometimes I just asked other
pilots and instructors how high they getting under
various circumstances. When we were using the retrieve
winch (we still often vehicle retrieve in the evening)
the most interesting comparison was between the last
few launches and the very final one when we disconnect
the retrieve cable and launch normally, so the cable
can be drawn fully into the winch so we can put it
away. That is how I derived the approximate height
loss attributable to the retrieve winch. At least I'm
not just drawing computer generated curves on a graph,
derived from questionable mathematical models, unlike
certain contributors to the group who have little or
no practical experience of winch launching!

Del Copeland

At 17:42 29 November 2007, Bill Daniels wrote:

'Dan G' wrote in message

.com...
On Nov 29, 2:57 pm, 'Bill Daniels' wrote:
We actually tested the power required to retrieve
Spectra and the starter
motor is way more powerful than needed. Not sure
what your problem is at
Lasham.

Derek will answer for himself, but I expect they retrieve
against the
pay-out brake on the winch, as per normal operation.
If you don't, the
drum will keep turning under its own momentum when
the retrieve stops
and you'll get cable everywhere. Not good.

Yes, they certainly pull against the heavy payout brake
- they have to,
they're using steel cable. You have to keep steel
under tension to prevent
tangles. Spectra/Dyneema doesn't need that tension.

Spectra/Dyneema doesn't ball up and tangle like steel
so you use much
lighter braking force. In fact, with tension control
used for braking, very
light reverse torque is more than enough to prevent
problems so very, very
little force is needed to pull out a Spectra cable
from a winch designed to
take advantage of Spectra/Dyneema.

To get all the benefits of Spectra/Dyneema, you have
to not only modify the
winch, you have to change operational techniques.
Just throwing it on an
old steel cable winch and using steel cable operating
techniques is
guaranteed to fail - as it did at Lasham.


The cost of Spectra is more like three times the cost
of steel not 5
times -
I checked the prices this morning. It takes a LOT
of power to pull steel
cable across an airfield - I can pull a mile of Spectra
with one finger.

We need verifiable data from retrieve winches. Your
tests at Lasham are
not
verifiable since you did no controlled experiments.

Having been to Lasham myself and talked to people
there (though not
yet witnessed the retrieve winch itself operating),
I'm perfectly
satisfied that what Derek says about the performance
of both UHMWPE
and retrieve winches is correct.


Derek says Dyneema doesn't work because it's too expensive,
wears out faster
than steel and doesn't provide any aditional height.
Evedence from many
other sites successfuly using Dyneema is to the contrary
on all points. The
difference is that successful sites did their homework
and made all the
neccessary changes.

Bill Daniels