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

In article ,
Bruce Hoult wrote:
In article 419af6e1$1@darkstar,
(Mark James Boyd) wrote:

Thrust is cheap. The amount of fuel used is to a first approximation
independent of the thrust of the engine (in fact to a certain point more
powerful engines result in less fuel used). But an engine that will
give you only 200 fpm of climb will take *forever* to get you to any
reasonable flying speed.

http://groups.google.com/groups?selm=an_595515430

I looked at your previous calculations, thanks for the reference to
the earlier post.

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.


I also looked at some of the other numbers in the post,
and they seemed a little off.
20kg of drag until liftoff speed seems a bit of an overestimate.
The Sparrowhawk would seem to have at most 5kg of drag
while accelerating to best L/D, assuming no
wheel friction

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.


Here were some of the calculations, which you did and I found enlightening:

thrust 50kg 100kg
Ground run 204m 76m
Dist at low level 945m 420m
climb angle 8deg 17deg
powered time 104s 45s
climb rate? 1000 fpm?

I'd be intereted to see what thrust is needed if the weight is reduced to
150kg, and the ground run is about 600m, using a Sparrowhawk polar.
Then I'd like to see how this changes if the ground run is allowed
to be 1200m. By ground run I'm assuming we mean accelerating
to something between Vs and Vy. I'd love to see what the climb angle
and climb rate then become.

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!

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!

--
Bruce | 41.1670S | \ spoken | -+-
Hoult | 174.8263E | /\ here. | ----------O----------