In article ,
Bruce Hoult wrote:
In article 419b8d27$1@darkstar,
(Mark James Boyd) wrote:

I wonder if you could redo the numbers for the limiting (minimum)
case for launching a human. Lets say a 50kg launch. Then also
do the numbers for a 150kg launch. I'd love to see what this looks like for
10kg and 20kg of thrust.

Well, if you scale the weight, thrust and drag by the same amount then
all the speeds and times are the same.

So with half the thrust, half the drag, and half the weight,
the rest is the same? Hmmm...ok. If this is right then 25kg of
thrust will get the Sparrowhawk accelerated and aloft smartly.
20kg of thrust (45# of the AMT turbine) will do nearly as well.

I don't know how much is the right amount, but I was trying to guess for
a heavily loaded (full of water) single seater or light two seater on
grass on ground that isn't rock hard. Imagine putting 20 kg of weights
on a rope over a pulley, with the rope attached to a glider. Would it
move it? I don't think so. Would it keep it going if it was already
moving? Maybe, just.

I've considered the idea of using fishing line with a 50# rating attached
to a bicycle. We've used a 14-year old to pull an (empty) 2-33
this way (with stronger rope). I'd like to try this "bicycle launch"
with a very light glider (maybe a Russia) to see what happens. I'd
really love to see a glider break ground pulled by a guy on a bicycle!
I'm only just half joking here...

I'm interested in the minimum case because this is a natural starting
point. I've done these calculations and it seemed that 15kg of thrust
gave a ground run less than 600m, and a climb rate of more than 200fpm
(might have been 500fpm, but I don't recall).

Good God. I don't know where you fly, but most glider pilots don't have
that sort of takoff space available to them!

All of the places I've launched have at least 3000ft of takeoff
space available, mainly because this is a pretty minimum runway length
for aerotow of the heavier ships on warmer days.

I, for one, do *not* want to be stooging off the end of the runway and
overflying the houses at best L/D speed with 200 fpm of climb in still
air!

Depends on the price. If it costs me an additional $5,000 a year
for anything over 200fpm, and my runway is 6000 feet long,
I'd be happy at 200 feet crossing the end of the runway with consistent
200fpm climb, then a downwind turn. Is this safe? That seems clear.
Is it cost effective? Well, what's the price for more climb?
Everybody *wants* 1,000,000 fpm climb. Nobody *wants* to pay for it.
I choose 30 feet over the end of the runway and 200ft per NM specifically
because I don't know anyone who would accept less performance. So this
is a natural starting point for calculations. A very light glider with
a very low stall speed with moderate performance on a runway
that is of fairly common US length. What is the thrust needed?

It doesn't mean everyone will *want* this combination, just that
nobody wants anything less. Ergo it is the starting point.

But I think all the calculations and even the Alisport Silent
implementation on a 150%-200% scale point to this as a fully viable
solution with a lighter glider and one engine and still 500fpm climb
from an acceptable ground roll. I know a (creative) Russia motorglider
owner/A&P who is almost disgusted enough with his unreliable
engine that he's almost ready to try out an AMT450. I've seen that
twinkle in his eye and know he's an avid experimenter. Hmmm...
winter is upon us and he may need somethin' to tinker with, even
if it just ends up as a turbo
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Mark J. Boyd