On 11 Jan 2004 22:15:26 -0700, (Mark James Boyd)
wrote:

In article ,
Mike Borgelt wrote:
On 11 Jan 2004 14:39:20 -0700, (Mark James Boyd)
wrote:

In article ,
Mike Borgelt wrote:

I've come to much the same conclusion as Mike. I'd use a single
more powerful turbine (maybe the 1500) instead of 2, but
the numbers seem to work for even fairly short fields.

The heat on the tail scares me though. Hmmm...how do we
get rid of the glider tail?


The guys with the Silent don't seem to have a problem with the heat on
the tail - the two AMT 450 turbines seem to be mounted parallel to the
centerline. If this still worries you a V tail as on HP gliders or the
Salto is the easy answer.

I've never seen a turbine airplane design that allows the hot exhaust
to reach a control surface of the aircraft. The fact that the Silent
flew a few times in this configuration is not convincing to me.
If the owner would put it on a stand in a hangar and
run it for an hour with his face right in front of the rudder/stab,
I'd change my tune. I don't have any hard facts or figures,
but my intuition sets off some warning flags here...


A few of us were brainstorming this this afternoon and we think that
mounting the turbines in the inside of the swing out doors solves this
completely. One engine ends up over each wing root. These things are
installed in R/C models so the heat issues are obviously manageable.
You can easily then cant then out a little if you want. Problem
solved(and a good argument for two engines)


I first thought of using the AMT1500 but when you do the numbers two
AMT450s (and soon the XP versions with about 10% more thrust) are
quite adequate for a 400kg glider.

The larger engine isn't for more thrust than two engines, but
just for the lower complexity of using one engine. Two engines are
best used in aircraft with high wing loading that carry
passengers through turbulence. Two engines in a light-wing
loaded aircraft is just unneccesary, IMHO. Engine failure is
a non-issue due to the glide ratio, and the reliability of turbines.
The added weight, wiring, two starters, fuel lines, etc. seem
silly if a single turbine can be used instead.

Given you will have only an electric fuel pump you are going to want
two anyway even for one engine. You already have two fuel tanks. You
might want two batteries as well to be sure of getting a start when
about to land out. Each of two smaller engines is lighter and simpler
to swing out than one larger one.
This looks one one of those issues where the "obvious" solution isn't
so obvious on reflection. The cost of the engines seems to scale
roughly with thrust so it is dollars per Newton you pay for.
The smaller engines also have thousands of hours operating history
which is worth a lot. And I *love* the idea of engine out capability
plus with two you really aren't going to fail to get at least one
running to avoid an outlanding.

Great for motorising motorless gliders as the weight in the fuselage
is minimal. Convert part of the water tanks/bags for jet
fuel.60Kg(75liters) will give you one hour.

Figuring out how to manage fuel from two tanks is a minor
complexity, and being able to dump fuel should ensure
one doesn't fly "chinese style" (won weeng lo).
It does seem using the fuel as ballast is an excellent feature,
but I'd want to really think hard about fire dangers.
Perhaps use less flammable fuel? I guess there is quite a
variety of fuel choices available...

I figured on one tank in each wing anyway and jet fuel is much less
flammable than gasoline anyway.

Now look at a Sparrowhawk
One AMT 450 will self launch this adequately.
Two smaller engines may still be optimum for slightly increased thrust
and engine out capability.

More power than adequate = better. One can always throttle
back for fuel savings.

Climb to 1000 feet or so and shut one down, then go find a thermal.


I suspect the designers used two
engines instead of one because the 1500 may not be readily tested/
available rather than due to the need for redundancy.

Yes and the redundancy is really nice to have. If the glider was not
capable of climbing on one I'd agree with you that one engine is
desirable but what is the point of designing around an engine that
isn't readily available with lots of operating history? The packaging
of two is also easier.


I've flown some twins and they have their uses; a powered glider
isn't a good match for two turbine powerplants
(just overkill/expense)...

Think about it some more and look at the prices. I didn't think jet
gliders were at all viable until I ran the numbers.

Hope the Windward Performance guys have a plan to increase production
because if this works they might be swamped by customers.

The Sparrowhawk may be ideal for this application, but other
light gliders also have comparable potential. And I personally
would want to see a competitor which could taxi well.
A self-launch glider which has trouble taxiing is less
interesting to me personally than something more flexible.
Besides, the noise may get one banned from the gliderport and forced
to use a gasp towered airport... ;P

I've seen and heard the R/C model jets fly. They aren't that noisy at
all. Two smaller engines over the wingroots actually shields the
people on the ground from much of the noise.

Taxiing is still going to be problematical but then very few existing
self launchers taxi well(as opposed to Stemme's, Katanas etc and even
they would have trouble at our airport.

I've got some time in the Jet Caproni about 20 years ago and I wasn't
that impressed. When I last flew in it 10 years ago the owner had
figured it out and it was good. We are talking the same sort of thrust
weight for a 400Kg glider(most 15/18m gliders) with two AMT 450's.


As for complexity of two jet engines compare with a horrible, heavy,
noisy vibrating two stroke with reduction drive and propeller, complex
and heavy extend /retract mechanism, heat issues if the engine is
buried in the fuselage and complex operating procedures with very
limited climb speed range. Look at the prices being charged for these
contraptions.


Mike Borgelt