Hi Bill --

That's right, I'm just dealing with the initial problem of accelerating
the ship off the ground. The dynamics of the climb I'm leaving to others
smarter than me.

If your experience, of which I know you have in abundance, dictates
ADDING power when the ship begins its climb, then so be it. The big
block in the PSA winch has to be backed off from 3800 rpm to 3000 rpm in
the climb when launching our L-13, even in a low-wind situation as
prevailed last Saturday. I believe Craig is pulling back just right, as
he climbs at 55 kt.

As a crazy analogy, look at the Super Hornet. With engines spooled up
and in full afterburner, it takes a tremendous shove from the steam
catapault to get the ship in the air. Once in the air it immediately
climbs out like a banshee.

Thus my gut feeling that it takes more power to accelerate to flying
speed than it does to climb.

What am I missing here?

BJ

Bill Daniels wrote:

Bob, let me try a different tack and look at the instantaneous power demand
at the moment of highest wire speed.

Andreas will be airborne at 65 km/h but won't really begin his climb until
over 102 km/h. At 100 km/h he is still accelerating at 10 meters per second
per second with a line pull of 615 kg. At that instant, he is demanding 169
kW or 227 HP at the glider. Since, as you pointed out, the winch engine is
also accelerating the drum and cable as well as overcoming cable friction
with the runway the real power demand at the engine is much higher.

If the glider must be accelerated to a higher ground speed because of
density altitude, the power demand goes higher still. At a 10,000 foot
density altitude he will need to go 20% faster before beginning the climb
and require 20% more power.

It's difficult to forecast the worst case power demand so I've always
advocated a large power margin to insure the power demand can always be met.

Bill Daniels

"Bob Johnson" wrote in message
...
Hi Andreas and Bill --

You're right, a 2000-lb weak link and a turbo for high altitude launches
can make all the difference in the world.

I figure Andreas' DG 505 at 615 kg gross and using all of the strength
of a 8929 N (2000-lb) weak link should get airborne (65 km/h or 35 kt)
in about 1.5 sec and take 125 kW (165 hp) out of the engine in the
process.

That's ripping the nose ring out of the bull's nose!

Cheers and all the best,

BJ

Andreas Maurer wrote:

On Sat, 20 Mar 2004 10:47:47 -0600, Bob Johnson
wrote:

The Diesel, despite its other admitted advantages (or perhaps because
of
them) will take longer to "wind up". The diesel engine is more massive
than a petrol engine in the crankshaft and flywheel, and in all other
moving parts. I do not know the exact masses of the two types of prime
movers. Perhaps someone out there can contribute this information. The
dry weights of the two power plants might be sufficient information.

This longer "wind up" period results in fewer "G's" being applied to
the
sailplane being launched during the acceleration from rest to flying
speed and the subsequent start of the pull-up and climb.

I have no idea what Diesel winches you have seen so far, but it's very
easy to break the weak link during the initial acceleration with ours.
280 hp turbo Diesel and 3.500 ft of good-old fashioned steel cable.
Wind drivers in my club are instructed not to apply full power
immediately since the acceleration is so quick that it WILL break the
weak link.
The weak link I'm talking about is the 2.000 lbs (the strongest that
is available) onr for our DG-505.

Bye
Andreas