As I see it the "constant tension" theory relies on being able to measure
the tension being exerted on the glider release at the winch drum. Quite
how this might be achieved is very puzzling and has no relevance to the
information required to give a safe and effective launch.
If you were to say that measuring the tension at the glider release and
using telemetry to pass this information to the winch then that might
indeed work, however if you were going to the trouble of doing that you
might as well send useful information, like the airspeed of the glider, so
the winch driver could maintain a constant speed.
Cable tension during a winch launch has sod all to do with anything except
as an indicator to the winch driver of possible over or underspeed. It is
the speed which is of relevance and importance.

At 13:15 27 July 2009, Del C wrote:
The 'constant tension' theory of winch launching was dreamed up by
someone in the US who has no practical experience of winch launching
whatsoever!

So far nobody has managed to built a true tension winch (which would
measure actual cable tension), so we don't know if the theory would
work
or not. The concept seems to have become a bit of a Holy Grail in the
US,
which is probably inhibiting the design and building of more
conventional
winches that would work just fine.

On the Yahoo Winch Design site I have suggested carrying out some
autotowing experiments, where it would fairly easy to mount an in-line
load cell to find out if CT would work, but this suggestion was rejected
by the above person and his followers as not being relevant due to the
mass and inertia of the towcar. Such an experiment would work in calm
conditions.

There are a number of constraints in real life winch launching.

1) The minimum airspeed has to be at least 1.3 x the normal stalling
speed
(Vs), to avoid the risk of stalling or spinning at the increased wing
loading due to the cable pull. At the high levels of pull suggested this
might increase to 1.4 Vs.

2) The optimum climbing airspeed for best gain of height seems from
practical experience to be in the range 1.5-1.6Vs.

3) Most gliders have a fairly low maximum winch launching speed (Vw),
which is set for structural reasons. There should also be a weak link
(fuse) included in the cable line which will break before the glider
does.


4) Many gliders, particularly older ones such as the K13, only have a
very
limited speed range in which they will climb safely and well without
exceeding Vw. The stalling speed of a K13 can increase to over 50knots
near the top of the launch, its optimum climb speed is about 56knots and
its Vw is 58knots. Some more modern types such as the K21 are a bit more
speed tolerant.

5) You have to fly the glider in such a manner that you can always
recover from a cable break or winch power failure, and not risk a stall
or
flick spin. This entails a fairly shallow initial climb followed by a
controlled rotation rate of not more than 10 degrees per second. I
believe
the Germans once managed to kill 12 pilots in one year (1995) by
carrying
out what are known as 'kavalier starts' where the glider climbs very
steeply straight off the ground to maximise height. We have also had a
few
such accidents in the UK, always on very powerful winches so rapid
acceleration doesn't make them safe.

The theory behind constant tension is you provide a pull or tension that
is close to the breaking strain of the weak link. Thus you maximise the
pull and the height gain in accordance with the Goulthorpe formula:
h = P/W/(1+P/W) x l
where h = height, P= Pull, W = glider weight and l = notional cable run
from the point of rotation.
Thus for a Pull equal to the weight of the glider you would expect to
get
a height of 50% of the effective cable length.

However, the above equation is idealised and assumes zero cable weight
and
zero drag, and is based on 100% transfer of energy.

For many years I launched on very powerful manually driven Tost winches.
Many of the launches were way over Vw until you signalled too fast, but
it
was quite rare to break a weak link in the early part of the climb. I
therefore suspect that the constant tension as a large fraction of the
weak link strength idea would just vastly overspeed the launches. In
order
to contain the speed according to the theory, you would have to climb at
an
achieved climb angle of about 60 degrees. Most gliders run out of up
elevator well below this angle. Such an angle would also represent more
than a 'kavalier start' as described above!

The other idea in the 'constant tension' theory is that the glider
pilot
would control the speed by pulling back harder to slow the launch down
and
easing forward to speed up. However I worry that a pilot trying to
control
the speed at the same time as the winch is trying to sense and control
the
tension would just lead to an oscillating or hunting situation. As a
winch
driver myself, I always try to avoid 'chasing the glider pilot' as
this
generally makes things worse. If I have to make a speed adjustment I
just
move the throttle to a slightly different setting and then hold it still
again. The technique for controlling the airspeed from the glider end
does
work on a Skylaunch winch where you are giving a constant power setting
and
also works on constant torque Supacat (diesel + fluid flywheel))
winches.
With either type of winch you have to start backing off the throttle
setting near the top of the launch to avoid overspeeding the glider.

We don't know if constant tension would give a constant and appropriate
airspeed, or whether it would need to be varied for different stages of
the launch to achieve this.

Derek Copeland



At 19:45 26 July 2009, Don Johnstone wrote:
As interesting as it is the discussion about who did what in the last
war
has about as much relevance to gliding and safe winching as a tesion
controlled winch.

The differences are too numerous to mention except that a Spitfire,
Hurricane and Mustang all worked and did a useful job, unlike the
mythical
tension controlled winch.