On 12 Jan 2004 09:41:18 -0700, (Mark James Boyd)
wrote:

Mike Borgelt wrote:

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.

Compared to a two-stroke, four jet engines is an improvement.
But again, IMHO, one jet engine of the same power is better than
more engines in light wing-loaded aircraft. The complexity of
pilot management, and the extra workload to manage the feeding and
maintenance is the downside.

Even the jet Caproni was fly by wire for engine management. These
engines already come with automatic electronic controls. Not a heck of
a lot of complexity.


And redundancy is, I believe,
notional. I'd rather run out of gas and then switch to a full
tank than run out of gas on one engine and then, a few seconds
later, run out of gas on the other side.


I'm not worried about running out of gas, just Murphy's Law.

I hope we can agree as gentlemen to disagree on this one...
I'm strongly in favor of a single turbine engine for this
application.

They simply are not available yet. Two smaller turbines are and have
some advantages and I beleive aren't likely to cost significantly more
than one large one.

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...

You can easily then cant then out a little if you want. Problem
solved(and a good argument for two engines)

I just don't know how large the heat cones are out of these engines, so
I can't really agree or disagree...I don't think I can solve this
one from an armchair...

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.

Out of gas is out of gas, period. Turbines get more reliable as they
get larger,

Maybe when you are talking about GE90 vs JT8D although I wouldn't bet
on it without doing some research. Down in the sizes we are talking
about do we have any real numbers?


and are lots more reliable than anything with a prop.
The reliability card simply has negligible meaning in this context.
And again, the cost isn't the acquisition or fuel costs, it's
continuing cost...

We aren't talking certified airplane engines here for either the
AMT450 or the AMT1500/1700. Redundancy may indeed be very nice to
have.

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

The fuel is slightly less flammable but the heat danger is much
greater than a pure glider (of course). My point is just that
if one has a choice, maybe use the least flammable fuel? You
can still fill up with Jet A if needed...
And I'm also emphasizing that I think the fire risk is really
something to pay attention to and minimize by design...

I'm not sure thay make the fuel used in the SR71 anymore and it likely
wouldn't run in these engines.
The fuel and heat issues have been solved in the Jet Caproni with a
buried engine 25 years ago. There is a group in Australia converting
some non jet Caproni airframes to jet power. Same idea as Caproni but
allegedly a better and more refined installation and better
performance. I've seen it fly.

The little engines are used in R/C models. Obviously the heat issues
are solvable.

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?
Completely true. If we MUST use two because of
marketing/availability/testing reasons
then fine. Two in the hand is better than none in the bush. But
accepting a sub-optimal design instead of making some extra phone
calls means somebody else is gonna compete with you later, at
a better price offering reduced maintenance/complexity...

Until he has a customer have an engine failure right after takeoff.

The packaging
of two is also easier.
Boy I gotta strongly disagree with that.
Installing, testing, wiring, instrumenting, fueling,
operating, shutting down, diagnosing in flight, etc.
for two engines is wholly different than one.
There's a reason 727s have three crewmembers instead of one,
and it isn't because of the complexity of the passengers
or so the Captain can take a nap...

The 727 was designed in the early 60's. Even 747s with 4 engines don't
carry FE's anymore.

We are talking something that weighs 5 pounds. Installing and mounting
it isn't that difficult. Check out the model jet websites.

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.

My comment about noise meaning you may get banned from the gliderport
was tongue in cheek. Here's the that should have been there...

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.

Mike Borgelt

Yes, I'd very much like to see taxi capability. A short wingspan and
light weight like a Sparrowhawk is excellent for this turbine.
The extra stuff to make it taxi well would sell it to the
biggest market, "power pilots," with the best success.

You would need to check the fuel consumption at reduced power. You
might get a nasty surprise. The taxi to the strip might have fuel
consumption comparable to driving an M1A2 the same distance.

I can easily live without the taxi ability. I'm looking forward to the
Ventus Ca17.6TJ(that's TwinJet)

Mike Borgelt