Winch Physics

Hi Bill--

When that sultry day comes, I hope Craig has had time to reinstall the
doors and to rig up the air conditioner!

Well, I don't think I am fiddling with power formulas. Initially I did
drive myself crazy by tracing the pulls back to the drum and working
through the moment arms, line speeds, revs and such like, but I finally
saw the light.

If you look at your copy of the spreadsheet you can see that I broke the
acceleration problem down into its two basic parts: A) acceleration of
the combined masses of the glider and the towline by whatever means,
including auto tow, using the classic acceleration of mass formulas and
B) acceleration of the drum. This second part proved to be the most
difficult for me as it involved the not-so-classic problem of rotational
acceleration. The textbook I have just mentioned the subject in passing.
English and metric units worked out to be the wierd slug-ft and kg-m^2.

Sorting out lb-force and lb-mass is never fun but it must be done! Then
everything kind of falls into place: lb-force times distance is work,
and work divided by the time it takes to do the work is power. I guess
that is a power formula fiddle after all! Anyway the real fiddle was
building the Excel spreadheet, but then it always is.

I believe we're in agreement on the altitude effects -- high launch
sites will want turbos, maybe attached to a diesel if that's the only
way they can be got. Andreas reports his turboed club winch has so much
power in reserve they can launch heavies with tailwinds. Now that is
RARE!

Bob

Bill Daniels wrote:

"Bob Johnson" wrote in message
...
Hi Bill:

You have flown our winch and must have felt me ease back on the throttle
to keep you from exceeding 55 kt. I'll readily concede to you and
Andreas that our winch at 6000 ft or hitched to a 2000 lb glider is not
going to perform as well as it does at Odessa's 3000 ft pulling the
Blanik.

Absolutely. Your winch has far more power than needed for the L-13 at
Odessa. Craig is one of the few winch builders in the USA that put enough
power in his winch. It'll be interesting to hear your comments about
launches on a 100+ degree day with no wind.

There's an issue that I don't have a good understanding of which is why you
and I keep fiddling with these power formulas. I think it's the density
altitude effects. DA has a very pronounced negative effect on power output
of a normally-aspirated engine combined with the need to accelerate the
glider to a higher speed. These two effects combine to really sap the
energy from a winch launch.

I find an unexpected power shortage to be quite alarming. I'd like the
power margin to be large so that under the worst imaginable conditions there
is still a considerable power reserve. Just because the power is there,
doesn't mean you have to use it.

Bill Daniels

Has anybody figured the effects of the
elasticity/stretch of the winch cable.
I seem to remember some references to plasma rope
instead of steel cable. This stretch probably
makes it easier to accelerate the drum to speed as the
line stretches. Now it the acceleration rates of the drum
and glider are non-linear at the start of launch, your calculations
of horsepower (should be torque) get really muddy fast.

Scott.

Hi Scott --

Plasma doesn't stretch, thank goodness. In fact stretchy winch lines in
general are bad news.

Bob

plasticguy wrote:

Has anybody figured the effects of the
elasticity/stretch of the winch cable.
I seem to remember some references to plasma rope
instead of steel cable. This stretch probably
makes it easier to accelerate the drum to speed as the
line stretches. Now it the acceleration rates of the drum
and glider are non-linear at the start of launch, your calculations
of horsepower (should be torque) get really muddy fast.

Scott.

Hey, Bob, go join the winch design group on Yahoo. We need your input.
http://groups.yahoo.com/group/winchdesign/

Bill Daniels

"Bob Johnson" wrote in message
...
Hi Scott --

Plasma doesn't stretch, thank goodness. In fact stretchy winch lines in
general are bad news.

Bob

plasticguy wrote:

Has anybody figured the effects of the
elasticity/stretch of the winch cable.
I seem to remember some references to plasma rope
instead of steel cable. This stretch probably
makes it easier to accelerate the drum to speed as the
line stretches. Now it the acceleration rates of the drum
and glider are non-linear at the start of launch, your calculations
of horsepower (should be torque) get really muddy fast.

Scott.

"Bob Johnson" wrote in message
...
Hi Scott --

Plasma doesn't stretch, thank goodness. In fact stretchy winch lines in
general are bad news.
Bob

Except when using polyprop rope! That bit of stretch helps get to flying
speed very quickly
and into the initial climb sooner, therefore higher launches.
Allows the winch engine to get to full revs,drums and drive gear at speed
before the glider
starts to move, (giant flywheel) equals less loading on your engine at the
critical moments
At $160 per length,life 800-1000 launches its cheap too.A man can pick up a
kilometre
of rope so that gives you an idea how much less weight the glider is
lifting.

gary


plasticguy wrote:

Has anybody figured the effects of the
elasticity/stretch of the winch cable.
I seem to remember some references to plasma rope
instead of steel cable. This stretch probably
makes it easier to accelerate the drum to speed as the
line stretches. Now it the acceleration rates of the drum
and glider are non-linear at the start of launch, your calculations
of horsepower (should be torque) get really muddy fast.

Scott.

We used this type of launch technique as early as 1980
in the F3B FAI class of model gliders.
We were limited by the size of the battery as well motor size.
Naturally in full size gliding one can not load up the glider as much.
I can see how advantage it must be to use this kind of launch
in full size winching.
Udo

"goneill" wrote in message
...

"Bob Johnson" wrote in message
...
Hi Scott --

Plasma doesn't stretch, thank goodness. In fact stretchy winch lines in
general are bad news.
Bob

Except when using polyprop rope! That bit of stretch helps get to flying
speed very quickly
and into the initial climb sooner, therefore higher launches.
Allows the winch engine to get to full revs,drums and drive gear at speed
before the glider
starts to move, (giant flywheel) equals less loading on your engine at the
critical moments
At $160 per length,life 800-1000 launches its cheap too.A man can pick up
a
kilometre
of rope so that gives you an idea how much less weight the glider is
lifting.

gary


plasticguy wrote:

Has anybody figured the effects of the
elasticity/stretch of the winch cable.
I seem to remember some references to plasma rope
instead of steel cable. This stretch probably
makes it easier to accelerate the drum to speed as the
line stretches. Now it the acceleration rates of the drum
and glider are non-linear at the start of launch, your calculations
of horsepower (should be torque) get really muddy fast.

Scott.