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.
But those two conditions are special cases and most winches will
probably never encounter the challenge they pose.
All I've tried to do is investigate the acceleration phase of the
launch.
BTW, the S-Hornet at 66,000 lb gross requires a weak link of 218,000 lb
minus the 44,000 lb thrust of the engines to attain 151 kt flying speed
in the 306 feet of flight deck it has to get away. 3.3 G's.
Bob
Bill Daniels wrote:
The craziness is in the horsepower formula. The Super Hornet's engines at
full thrust on the carrier deck produce no horsepower at all - just thrust.
Once it starts to move, the speed term in the HP formula kicks in and the HP
output soars. A bit of trivia, at about 325 knots, one pound of thrust
equals one HP which means that at 325 knots the Super Hornet's engines with
44,000 pounds of combined thrust are producing 44,000 HP vs. 0 HP one second
before the cat shot.
Using units I am more familiar with, 1 HP= 550 foot pound seconds. Or in
the case of a winch launch, cable tension in pounds times cable speed in
feet per second divided by 550. Because of the FPS term, the HP demand
peaks at the maximum cable speed just as the glider arcs up into the climb.
Unfortunately, without an energy storage system, the winch engine has to be
sized to meet peak demand even if that demand lasts only a second or two.
Bill Daniels
"Bob Johnson" wrote in message
...
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