On Thu, 27 May 2004 08:59:42 -0700, "Michael Stringfellow"
wrote:

This would probably be easy to measure with a spring balance and a tow rope.

We'll do that too but getting a steady reading on rough ground will
likely be difficult.



The forces you need once airborne can readily be calculated - for instance,
an 800 pound glider with a 40:1 L/D has 20 pounds of drag. If you assume
this increases 50% with a deployed engine, this increases to 30 pounds of
drag. So a single engine with 40 to 45 pounds of thrust would probably be a
minimal sustainer. Two such engines would give you a climb rate in the
hundred feet a minute range.

Performance calculations for jet are pretty straightforward even
including the effect of thrust fall off with increasing airspeed. You
can calculate that from the mass flow of the engine and sea level
thrust.

Watching the jet-powered Silent on its recent visit to Arizona, the problem
appeared to be more the energy needed to accelerate and climb with the
glider's mass than to overcome friction. (The noise wasn't objectionable,
by the way. Certainly nothing like a pulse jet!)

The Silent has a max gross of 640 lbs according to their website. It
also is only a 31:1 glider. Jets have the interesting characteristic
that the power available increases with airspeed. Unlike propellor
aircraft the best rate of climb speed occurs at relatively high speed
which favors gliders with good performance at high speed.

With two 55lb thrust engines(which are going to be available soon) a
900lb glider will have a sea level climb rate of around 650 feet/min-
at 80 to 100 knots IAS. The takeoff run will be the big issue and the
rolling friction will play a large part in this. Preliminary estimates
are that it won't be all bad and will meet JAR22 in this regard. It
will also have single engine climb capability.

What altitude above sea level and what temperature when you saw the
Silent fly?

On the other hand, I know of a Nimbus 3DM that operates from a grass strip
and sometimes uses auto tow to get it airborne to reduce the take-off roll,
so friction on the ground is not negligible.

Having owned one of them and operated off a sealed strip at 2100 feet
on warm days I can say it has no problem. At 5000AMSL it was
considerably less good. At high altiude on grass I can see the
problem.


Mike