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