At 22:34 14 July 2009, Gary Emerson wrote:
Just exacly where are you making these tensions measurements????

Are you making them at the weak link??

Are you making these measurements at the winch end of things?

I'd really like to know...


I'm going to throw in my two cents here. I've been trying to get hard
data and information from first hand experiences for several years, now,
there just isn't much real stuff publicly available. There is, however,
a growing body of theory (which is good), speculation (not so good), and a
lot of bold statements that I have been, so far, unable to verify.

First, as far as I find out, the only tension/torque controlled winches
currently in day to day use use either electric or hydraulic drive
systems. For an electric winch, the torque applied to the drum can be
controlled based on drive current and drum speed. In the hydraulic case,
torque control is achieved by adjusting hydraulic pressure. In both
cases, this allows applying a specific amount of torque to the drum, which
approximates the desired amount of tension on the rope. In neither case is
the actual rope tension being measured. Drum inertia, rope oscillations,
etc., will also affect the rope tension. Apparently, though, at least
some of these winches provide tension controlled launches on a practical
basis.

There is one winch with an automated controls system under development in
the US, which uses a diesel engine and automatic transmission, with a
running line tensiometer (RLT) at the winch end. I have not been able to
obtain any information on whether the tension control program is actually
in use, and whether it is resulting in any real improvement over more
conventional winch launches.

One group in the UK has flown with a recording load cell to collect data
during a dispute with the manufacturer of their (conventional) winch. The
load cell was apparently placed at the weak link end of the rope. I've
seen about 10 distinct graphs produced by this device, and they look
terrible (large excursions in tension). However, despite loud
proclamations that "hundreds" of similar flights have been recorded on
several winches, I've never been able to get hold of any further data. I
also have been unable to obtain information on the device itself, how it
was calibrated, the circumstances under which the data was recorded, and
whether there were electrical, mechanical, driver, and/or pilot related
issues that could affect the data.

It does look rather straightforward to put a recording or wireless load
cell at the glider end of the rope, or use an RLT or even measure the load
directly from the winch axle (which I'll have provision for in the winch
I'm designing). So, I expect we'll be seeing more data in the future.

That's all I know at the moment, if there is anyone lurking out there
with solid data, I'd love to hear about it...

Marc