SAFE Winch Launching

Don Johnstone wrote:
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...


That is the question that is puzzling me too. The tension sensed by the
winch will, and should be constantly changing during the launch, measuring
that would be, how can I put it, useless.
The tension at the glider release hook will be relatively constant but
measuring that introduces the problem of the addional weight of the device
that does it and a reliable transmission of data to the ground.

Which is why I suggested we think about metrics. If there were not big
changes in tension at the Winch end you would have a REAL problem. Go
check it - on our 2km run the glider has not moved before the winch is
1-1.5s into power up - until then there is a lot of tension going into
the cable at the winch and zero happening at the glider end.

We have had a ham fisted learner winch driver snap the cable neatly
(1880KN breaking strain cable) at the drum without even moving the chute
at the launch point.

So - The place it has to be reasonably gentle (smooth curve) is at the
glider hook.

Now we get into a whole new debate about the relative advantage of
UHMDPE (Spectra / Dyneema etc.) and steel cable.

Steel has much more elasticity and inertia to absorb those transient
loads. The spring steel wire acts like a spring storing energy and
smoothing loads. Inconveniently this means that when it breaks under
tension you get a backlash that can result in big overwinds and injuries.

The plastic rope is - counter-intutively given the name, far less
elastic and very light. Result is that winches so equipped may need to
have smoother delivery. And (I am not a good enough engineer here) it is
possible that the tension at the winch on these is similar to that
experienced by the aircraft hook. Would depend on hysteresis and all
sorts of other things that are probably best measured rather than
calculated / guessed.

Any one with facts?

"That pretty much means either finding a rebuildable Gehrlein,
importing an inexpensive used winch from Europe, or homebrewing a new
winch (based on the UK spec). I prefer the latter, as I think there is
still plenty of room for optimization, both in fabrication and
operation. While I respect those who are trying to push the envelope
with advance power systems and launch automation, a simple single drum
winch based on automotive components will be good enough for our local
group get started. "

I agree, however, I suggest a retrieve winch set up if you guys are
going for a single drum as otherwise, launch rate will be too slow.
Last year I flew from a 6 drum winch and it was aircraft carrier
style. From reading, I gather that with a single drum winch with a
retrieve winch set up can achieve a rate of 20 launches an hour. How
many tow planes would it take to achieve 20 launches/hour to 2,000
feet per launch? My guess is two tow plane. With only 1 tow plane that
many launches would take 2 hours - much more if they stray far from
the airport. The economics of winching are staggering compared to
aerotows - and above all - much more fun. This includes hunting for
that first lift at low altitude AGL. Even with multiple relights, it's
still cheaper and more fun.
Tom

Generally speaking, the only way to break a weak link or a cable at the
start of a winch launch is to open the throttle wide before all of the
slack has been taken out of the cable, so that you get a sudden snatch.

I have launched on both stranded steel and UHMWPE cable on a conventional
Tost winch with a changing automatic gearbox, and couldn't honestly feel
any real difference, except that you don't get any jolts from cleated
repairs going through the rollers with the synthetic cable. These
generally occured near the top of the launch, where we were most likely to
break cables with the Tosts, and are more down to the way we repair broken
steel cables, rather than the cables themselves.

I personally much prefer synthetic cables, because they give smoother and
higher launches, but the club management said no, due to the much higher
cost of the stuff!

Dyneema is definitely safer in that it has less tendency to spring back
after a break and no sharp ends to cut anybody who is unlucky or stupid
enough to stand in the way. My club has had a couple of minor injuries
that way.

Derek Copeland


At 06:55 15 July 2009, Bruce wrote:
Which is why I suggested we think about metrics. If there were not big
changes in tension at the Winch end you would have a REAL problem. Go
check it - on our 2km run the glider has not moved before the winch is
1-1.5s into power up - until then there is a lot of tension going into
the cable at the winch and zero happening at the glider end.

We have had a ham fisted learner winch driver snap the cable neatly
(1880KN breaking strain cable) at the drum without even moving the chute

at the launch point.

So - The place it has to be reasonably gentle (smooth curve) is at the
glider hook.

Now we get into a whole new debate about the relative advantage of
UHMDPE (Spectra / Dyneema etc.) and steel cable.

Steel has much more elasticity and inertia to absorb those transient
loads. The spring steel wire acts like a spring storing energy and
smoothing loads. Inconveniently this means that when it breaks under
tension you get a backlash that can result in big overwinds and
injuries.

The plastic rope is - counter-intutively given the name, far less
elastic and very light. Result is that winches so equipped may need to
have smoother delivery. And (I am not a good enough engineer here) it is

possible that the tension at the winch on these is similar to that
experienced by the aircraft hook. Would depend on hysteresis and all
sorts of other things that are probably best measured rather than
calculated / guessed.

Any one with facts?

On Tue, 14 Jul 2009 21:19:50 -0700 (PDT), tommytoyz
wrote:


The real
cost of a winch launch is something like $3-5 per launch including
maintenance, reserves and replacements. 30 launches a week x $10
profit = $1,200 extra a month. That's one very nice glider upgrade
every 5 years. This is one of the main reasons why European clubs have
such nice equipment.


Unfortunately this is not the reason...

There might be a few clubs in Europe that charge the prices you
mentioned, but all the clubs I know charge between 2 and 5 Euro,
sufficient only to cover the costs.

Cheers
Andreas

I think all (certainly almost all) UK clubs charge the equivalent of
USD10 or more. Winch launching makes a surplus, aerotowing tends to run
at a deficit, at the smaller clubs at least.

Andreas Maurer wrote:
On Tue, 14 Jul 2009 21:19:50 -0700 (PDT), tommytoyz
wrote:


The real
cost of a winch launch is something like $3-5 per launch including
maintenance, reserves and replacements. 30 launches a week x $10
profit = $1,200 extra a month. That's one very nice glider upgrade
every 5 years. This is one of the main reasons why European clubs have
such nice equipment.


Unfortunately this is not the reason...

There might be a few clubs in Europe that charge the prices you
mentioned, but all the clubs I know charge between 2 and 5 Euro,
sufficient only to cover the costs.

Cheers
Andreas

Andreas,
At my old club in Germany there were several prices depending if you
were a youth, club member in club aircraft, club member with your
private aircraft or non club member with your private aircraft. The
cheapest rates are as you mentioned. It also makes a big difference
that the German clubs have way more launches than 30 a week. It's more
like 150 a week or more. So the spread can be less.

Of course there are other criteria that the government supports these
clubs and some members to a small degree and they also charge by the
hour for their aircraft - though a small fee - but a fee none the
less. Their set up is just totally different, but my point is that
you can make good money as a club here in the USA with a winch where
you really can't with a tow plane. The tow plane is just very
expensive if operated by the club and if using an FBO, no money comes
back to the club.

All this changes, with a club owned winch and even charging the
astronomical fee (for a winch launch) of $15 per launch, it's a good
money maker which can be put to good use in a non profit 501 (c)(3)
club to buy aircraft, equipment, hangars, etc.....Even at $10 a launch
it's a good money maker. Perhaps the launch fees can be charged as in
the German clubs, depending on club status and aircraft. This would
encourage club membership as well as it's cheaper to launch as a club
member.

At 08:05 15 July 2009, tommytoyz wrote:
Even with multiple relights, it's
still cheaper and more fun.
Tom

Not if you finish up at the back of a queue of 20 gliders as the sea
breeze blows through and the launch point has to be moved to the other end
of the runway....

On Jul 15, 12:55*am, Bruce wrote:
Don Johnstone wrote:
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...

That is the question that is puzzling me too. The tension sensed by the
winch will, and should be constantly changing during the launch, measuring
that would be, how can I put it, useless.
The tension at the glider release hook will be relatively constant but
measuring that introduces the problem of the addional weight of the device
that does it and a reliable transmission of data to the ground.

Which is why I suggested we think about metrics. If there were not big
changes in tension at the Winch end you would have a REAL problem. Go
check it - on our 2km run the glider has not moved before the winch is
1-1.5s into power up - until then there is a lot of tension going into
the cable at the winch and zero happening at the glider end.

We have had a ham fisted learner winch driver snap the cable neatly
(1880KN breaking strain cable) at the drum without even moving the chute
at the launch point.

So - The place it has to be reasonably gentle (smooth curve) is at the
glider hook.

Now we get into a whole new debate about the relative advantage of
UHMDPE (Spectra / Dyneema etc.) and steel cable.

Steel has much more elasticity and inertia to absorb those transient
loads. The spring steel wire acts like a spring storing energy and
smoothing loads. Inconveniently this means that when it breaks under
tension you get a backlash that can result in big overwinds and injuries.

The plastic rope is - counter-intutively given the name, far less
elastic and very light. Result is that winches so equipped may need to
have smoother delivery. And (I am not a good enough engineer here) it is
possible that the tension at the winch on these is similar to that
experienced by the aircraft hook. Would depend on hysteresis and all
sorts of other things that are probably best measured rather than
calculated / guessed.

Any one with facts?

As you point out, the mfg, spec's call for about 1.5% elongation at
failure for UHMWPE (Spectra/Dyneema) That is much stiffer than
steel. And yes, any roughness or variability in winch power output
does get passed to the glider. That said, rope launches seem much
smoother than with steel cable.

A misconception seems to be that a tension controlled winch goes
instantly from 0 to 100% tension at the beginning of the takeoff
roll. In fact, tension ramps up smoothly and only becomes constant
during the main climb. That's why it's called a "tension controlled
winch" rather than a constant tension winch.

The steady tension make it far easier for the pilot to maintain good
airspeed control. There is much less effect from thermals and gusts.
It's a quieter, smoother easier launch. Because the "area under the
curve" is greater with constant tension in the main climb, more net
energy gets transferred to the glider than in the case of varying
tension so launches are higher with no increase in maximum climb
angle.

Tension through the ground roll and rotation phase follows essentially
the same profile as with an excellent conventional winch launch.
There are limits set to preclude too rapid acceleration and the
tension cannot reach the weak link or rope breaking levels. The
automation make one launch pretty much like the next with little
variation due to the winch operator.

By the way, tension control is nothing new. I recall an article in
Sailplane and Gliding from the 1960's on the subject. Soaring
Magazine had an article on an auto tow system using a tensiometer made
from a hydraulic master cylinder and a pressure gauge from about the
same time. Many of today's hang glider winches and auto tow systems
use tension control.

At 23:52 15 July 2009, bildan wrote:

A misconception seems to be that a tension controlled winch goes
instantly from 0 to 100% tension at the beginning of the takeoff
roll. In fact, tension ramps up smoothly and only becomes constant
during the main climb. That's why it's called a "tension controlled
winch" rather than a constant tension winch.

The steady tension make it far easier for the pilot to maintain good
airspeed control. There is much less effect from thermals and gusts.
It's a quieter, smoother easier launch. Because the "area under the
curve" is greater with constant tension in the main climb, more net
energy gets transferred to the glider than in the case of varying
tension so launches are higher with no increase in maximum climb
angle.

How can you possibly know that without measuring the "tension" at the
release hook. You certainly cannot measure the increased tension caused by
a thermal at the winch as the inertia of the steel cable is the main cause
of the increase in tension experienced at the release hook. This
"tension spike" cannot be measured at the winch. From a point at which
the cable is at 45 degrees to the ground the "tension" imparted to the
glider, through the cable release, is increasingly provided by the weight
of the cable, and therefore measuring tension at the winch has no meaning
whatsoever, in fact the tension there should be almost 0.

Tension through the ground roll and rotation phase follows essentially
the same profile as with an excellent conventional winch launch.
There are limits set to preclude too rapid acceleration and the
tension cannot reach the weak link or rope breaking levels. The
automation make one launch pretty much like the next with little
variation due to the winch operator.

You may get good results by serendipidy but you certainly won't get them
through control of tension. What you are measuring at the winch bears
absolutely no direct relationship to what is experienced at the glider
end, and that is what counts.
With a good qualified winch driver you get consistent launches, sounds to
me you are worrying far too much about not being able to train your winch
drivers properly, and given some of the theories expounded that is perhaps
understandable.
What you are trying to achieve in a complicated way we solve by using
robust equipment and trained winch drivers who do understand what they are
doing.

By the way, tension control is nothing new. I recall an article in
Sailplane and Gliding from the 1960's on the subject. Soaring
Magazine had an article on an auto tow system using a tensiometer made
from a hydraulic master cylinder and a pressure gauge from about the
same time. Many of today's hang glider winches and auto tow systems
use tension control.

On Jul 15, 7:00*pm, Don Johnstone wrote:
At 23:52 15 July 2009, bildan wrote:

A misconception seems to be that a tension controlled winch goes
instantly *from 0 to 100% tension at the beginning of the takeoff
roll. *In fact, tension ramps up smoothly and only becomes constant
during the main climb. *That's why it's called a "tension controlled
winch" rather than a constant tension winch.

The steady tension make it far easier for the pilot to maintain good
airspeed control. *There is much less effect from thermals and gusts.
It's a quieter, smoother easier launch. *Because the "area under the
curve" is greater with constant tension in the main climb, more net
energy gets transferred to the glider than in the case of varying
tension so launches are higher with no increase in maximum climb
angle.

How can you possibly know that without measuring the "tension" at the
release hook. You certainly cannot measure the increased tension caused by
a thermal at the winch as the inertia of the steel cable is the main cause
of the increase in tension experienced at the release hook. *This
"tension spike" cannot be measured at the winch. From a point at which
the cable is at 45 degrees to the ground the "tension" imparted to the
glider, through the cable release, is increasingly provided by the weight
of the cable, and therefore measuring tension at the winch has no meaning
whatsoever, in fact the tension there should be almost 0.

First, we're not using steel cable, we're using UHMWPE (Specra/
Dyneema). Lightweight "plastic" rope only weighs about 1 pound per
hundred feet and has very little inertial at all. Tension measured at
the winch is within a percent or so of that at the glider and has
essentially no effect on tension measurement.


Tension through the ground roll and rotation phase follows essentially
the same profile as with an excellent conventional winch launch.
There are limits set to preclude too rapid acceleration and the
tension cannot reach the weak link or rope breaking levels. *The
automation make one launch pretty much like the next with little
variation due to the winch operator.

You may get good results by serendipidy but you certainly won't get them
through control of tension.

Gee, I must be dreaming But we seem to be getting exactly what I'm
telling you we're getting - with tension control. This isn't theory,
we're actually doing it. There must be some sort of magic effect no
one knows about. Or maybe the laws of physics are different over here.