OT WWII Memoirs (was SAFE Winch Launching ...)

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.

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.

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.

At Feshiebridge, Scotland, AIUI they developed telemetry for
indicating glider airspeed to the winch driver, and it was said to
work very well. IIRC, it did not send an airspeed number in knots, but
rather an indication of too slow, a bit slow, OK, a bit fast, or too
fast (or something like that) which is what the winch driver really
needs to know, they believed. The unit in the glider was calibrated
for the glider type.

It needs somebody with first hand experience of it to tell more, and I
don’t know if it has been kept going.

Chris N.

Skylaunch sell a telemetry system called 'Launch Assistant' which relays
the actual airspeed of the glider to an LCD display mounted somewhere in
the view of the driver. It costs a few hundred dollars in total. See:

http://www.skylaunchuk.com/

Derek Copeland


At 16:49 27 July 2009, Chris Nicholas wrote:

At Feshiebridge, Scotland, AIUI they developed telemetry for
indicating glider airspeed to the winch driver, and it was said to
work very well. IIRC, it did not send an airspeed number in knots, but
rather an indication of too slow, a bit slow, OK, a bit fast, or too
fast (or something like that) which is what the winch driver really
needs to know, they believed. The unit in the glider was calibrated
for the glider type.

It needs somebody with first hand experience of it to tell more, and I
don=92t know if it has been kept going.

Chris N.

We have the Skylaunch telemetry installed in one of our ships as a
trial, and have found it to be a very useful tool for the winch
driver. The winch cab display "lights up" at about the time of
rotation and a quick scan of the glider airspeed is very useful in
assessing the airspeed trend during the climb. It is particularly
useful as the glider begins the round out as an airspeed cue during
the reduction in line tension. The low cost makes this a very nice
innovation and we plan to install the units in our other club gliders.

Bob


On Jul 27, 1:45�pm, Derek Copeland wrote:
Skylaunch sell a telemetry system called 'Launch Assistant' which relays
the actual airspeed of the glider to an LCD display mounted somewhere in
the view of the driver. It costs a few hundred dollars in total. See:

http://www.skylaunchuk.com/

Derek Copeland

At 16:49 27 July 2009, Chris Nicholas wrote:





At Feshiebridge, Scotland, AIUI they developed telemetry for
indicating glider airspeed to the winch driver, and it was said to
work very well. IIRC, it did not send an airspeed number in knots, but
rather an indication of too slow, a bit slow, OK, a bit fast, or too
fast (or something like that) which is what the winch driver really
needs to know, they believed. The unit in the glider was calibrated
for the glider type.

It needs somebody with first hand experience of it to tell more, and I
don=92t know if it has been kept going.

Chris N.- Hide quoted text -

- Show quoted text -

I suspect fitting each and every glider with a telemetry transmitter
would be prohibitively expensive and resisted by most. A member of my
club did this as an experimental demonstration and I was able to drive
the winch to the appropriate airspeed. Getting accurate airspeed from
a rope mounted device for the whole fleet could be problematic.
Tension telemetry would be much easier to construct and ruggedize and
limiting tension by glider type (and loading) is doable, probably more
easily with hydraulically or electrically driven drums than with the
reciprocating and rotating weight of IC engines in a direct drive
(including automatic transmissions and drive trains) where airspeed
output would be more easily used. Tension control should allow for
wind gradients and shears but the control algorithms need to smooth
the changes to avoid surges and hunting. Manually driven winches
should not be discounted, but I have some ideas about standardization
of controls and operating methods.

Frank Whiteley


On Jul 27, 9:30*am, Don Johnstone wrote:
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.

Don Johnstone wrote:
...
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.


We have recently tested the Launch Assistent that is sold by Skylaunch,
http://www.skylaunchuk.com/index.htm
and have found that is can be (as the name indicates) an *assistant* to
the winch driver to help him/her judge the speed of the plane. It helps
escpecially unexperienced winch drivers, or in adverse conditions like
shearing winds within the launch.

After 20 years of experience as a winch driver and winch instructor
(Tost winch) I still believe a well instructed and experienced winch
driver (and all winch drivers should have a certain minimum of launches
per year) is able to judge and control the launch as good (or even
better) than any automated (tension-controlled) system, that in the end
also relies on the correct behavior of the pilot instead of the winch
driver.

--
Peter Scholz
ASW 24 JEB

At 17:23 27 July 2009, Peter Scholz wrote:
Don Johnstone wrote:
...
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.


We have recently tested the Launch Assistent that is sold by Skylaunch,
http://www.skylaunchuk.com/index.htm
and have found that is can be (as the name indicates) an *assistant* to
the winch driver to help him/her judge the speed of the plane. It helps
escpecially unexperienced winch drivers, or in adverse conditions like
shearing winds within the launch.

After 20 years of experience as a winch driver and winch instructor
(Tost winch) I still believe a well instructed and experienced winch
driver (and all winch drivers should have a certain minimum of launches
per year) is able to judge and control the launch as good (or even
better) than any automated (tension-controlled) system, that in the end
also relies on the correct behavior of the pilot instead of the winch
driver.

--
Peter Scholz
ASW 24 JEB


Well, there you have it, problem solved by superior and relevant British
engineering and at reasonable cost.
Tension controlled winches are obsolete already as we have jumped a
stage.
Any move to automatic winch driving would be a big big mistake, a good,
well trained winch driver is the best way of ensuring safety.
Automatic takeoff, automatic landing, automatic crash.

Peter Scholz wrote:
Don Johnstone wrote:
...
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.


We have recently tested the Launch Assistent that is sold by Skylaunch,
http://www.skylaunchuk.com/index.htm
and have found that is can be (as the name indicates) an *assistant* to
the winch driver to help him/her judge the speed of the plane. It helps
escpecially unexperienced winch drivers, or in adverse conditions like
shearing winds within the launch.

After 20 years of experience as a winch driver and winch instructor
(Tost winch) I still believe a well instructed and experienced winch
driver (and all winch drivers should have a certain minimum of launches
per year) is able to judge and control the launch as good (or even
better) than any automated (tension-controlled) system, that in the end
also relies on the correct behavior of the pilot instead of the winch
driver.

--
Peter Scholz
ASW 24 JEB

There has been some interesting scientific research beeing done on this
topic by the Akaflieg group of the University of Karlsruhe (Germany)
some years ago. It would go to far describing it in detail, but one
diagram where they measured a winch launch is quite interesting:

http://www.akaflieg.uni-karlsruhe.de...%20Schlepp.pdf

The parameters measured we
Height (blue)
Airspeed (magenta)
Torque at winch drum axle (dark blue)
elevator position (green)
Acceleration in x-direction (brown)
Acceleration in z-direction (yellow)

It shows that what appears to be a relative smooth winch launch, looking
at airspeed and height, has a quite significant variation in torque,
elevator position and acceleration.

The findings of this sientific work in one simple sentence: A simple
"constant tension" cannot cope with all the parameters that influence a
winch launch and that are dependent on eacht other.

So the (also economically) best approach would be to aid the winch
driver doing his job right instead of inventing a highly sophisticated
fuzzy logic system that may take into account all parameters to act as
well as a winch driver.

The goal is not to have winch launches that are all perfect and one as
optimal as the other, but the goal has to be to have safe winch launches
with an acceptable performance. And safety lies within a quite large
corridor.

--
Peter Scholz
ASW 24 JEB