In article ,
Bruce Hoult wrote:

A low stall speed isn't what you want. By far the most important thing
is a high Vne. The best way to climb in a jet is to accelerate in level
flight (in ground effect if you can) to Vne and then climb at Vne.

This may be the best way in terms of efficiency and power, but
I'm not sure it is as practical as the alternative.

I'm thinking of a situation where acceleration to a speed where
climbing out of ground effect is possible. If we assume a
glider weighing 400 lbs total, it takes a certain amount of thrust
to accelerate it to above stall speed, and then Vx and Vy. It then takes
a lot more thrust to get it to Vne.

Will a turbine produce 4 times as much thrust at 120 knots compared to
50 knots? I don't know. I do know that if the stall speed is pushed
way up, you need a lot more runway or a lot bigger engine.

So look at the Sparrowhawk polar, and assume 400 lbs. What is the
minimum thrust turbine engine that can launch this thing from a
2000ft long runway with 30 feet of clearance over the far end
and thereafter at least 200 fpm climb per NM (no wind)?

Assuming the same turbine is used, I'm guessing moving the polar to
the right (with a new wing) doesn't help, even if the weight remains
the same. The issue is acceleration to flying speed, and this is
helped by lowering the flying speed (Vy or Vx) and lowering the weight.

So any of you math/aerodynamics guys out there got a guess?
I guessed 45 pounds of continuous thrust would do it, but
this was a SWAG, and I have no idea what a turbine rated at 45lbs
really puts out at 0 airspeed. I also didn't account for any drag
during the acceleration, and used interpolation for climb.

How much more thrust is needed to do the same thing, but with 500 fpm
climb per NM after launch?

I really don't know, but I'd love to see a java program where
you put in the weight numbers, polar data, and thrust at different
speeds, and get the results...

Other than a lightweight, inefficient turbine, I don't see
any other "revolutionary" powerplants for gliders which might
bring the cost down and create more widespread interest.

There are also rockets. Lighter, simpler, less fuel efficient, and
(probably) cheaper than jets.

It looks like $20 for 4lbs of thrust for 8 seconds. Each launch looks like
at least hundreds of dollars (worth of commercial rockets sold
by Public Missiles, Ltd and the like).

Perhaps these can be constructed as reusable and experimental,
for much less cost, but I'm just not familiar with this.

If you can give us some estimates on costs and thrust and burn time,
that would be great Perhaps the largest barrier to this
is unfamiliarity and not knowing how such a burn is controlled.
How does one perform an aborted takeoff?

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Mark J. Boyd