So far as I know, there is no specific best point for
jet operation
(they stink everywhere, which is why they are almost extinct)
Now I know you know better than this Vance. Jets are the only thing
capable
of doing what they do. They do not stink everywhere. They stink at slow
speeds. They stink in our flight regime, historically. There is most
definitely a best point for a specific jet engine operation. They are
designed with an operating point in mind. The airframe and the jet must
be
matched closely to realize good efficiency. I will refer you to any
propulsion text to find the derivation of best cruise point Ve=2Va. As
far
as stinking, sit down and figure out that a modern turbofan has a TSFC
of
around .5 and can fly at 30K ft at M .8. Now figure out what Rare Bear
would
be doing at altitude and speed in terms of TSFC, not shaft SFC. Don't
like
that how about Turbo-shaft engines with compression ratios of over 20 to
1.
Specific fuel consumption numbers in the .45-.5 lb/hp/hr range. 2000 hp
in
something the size of a small block Chevy. The main problem is they do
not
scale down to our power levels very well, at least not with current
manufacturing techniques. Saying they stink in an unqualified manner is
a
bit harsh.
Short of some sort of gravity/electromagnetic alternative, I predict
that
jets will be with us for a LONGGGGGGGGG time.
As a post mortem on Perry's failed experiment I would say that you have
a
fan designed to operate in a duct, not in the open air. If you designed
a
fan to turn at 6000 rpm at 180 mph it would look a little different than
what you have. You also have an airframe that is not conducive to this
design. The Long EZ has a terrible aft body contraction problem. The air
going into the prop has been over the canard, canopy, fuse, gear,
engine,
wing....yuck. And the prop is not very far from the wing. Rule of thumb
is
at least one chord length behind the wing to let the air settle down.
Charlie Airesman moved the prop way back and had a nice faired
transition
into the hub. No offense Perry, your situation is not the same. Thanks
for
trying. Most people won't go out on a limb anymore. Remember it is the
EXPERIMENTAL Aircraft Association.
Monty
I think Vance was referring to pure jets Monty and not turbo fans
so you can't jump from one to the other in the same paragraph :)
I took the liberty of adding a bunch of pictures and illustrations to
prove your other points the vast majority of which I agree with.
Paul Lamar
Paul is correct - when I say jets, I mean straght turbojets - where all the
air
goes thru the combustion chamber, and they are indeed near extinct. I
certainly did not say turbines stink and you are right that big bucks, big
HP gas turbines are running really great specifics actually better than
Diesels on some points - and they do not scale worth beans to small
size and low cost. High bypass turbofans are often below 0.4 TSFC
(at static - they get poorer at speed) actually the TSFC static
for a Cessna 172 is probably about 0.15 also getting poorer with
speed. Turbofans are probably the best examples of the big mass,
lower velocity school. Those fans are running air slower than
flight speed to keep the blades well subsonic - raising the total
pressure of the downstream flow, and expanding up in velocity
in a contracting exit cone. Sound absorbing wall structure typically
fore and aft to be quieter. Those ducts are good for high power
fast, and the ducts we have been discussing are good for slow
(and safety on the blimps) and between it is rocky desert, where
a good prop is the best compromise - Vance
I have a engineer friend in the power plant business and he said the gas turbine
systems have bottoming cycle's wherein the waste heat is used to generate
steam for a steam turbine. Overall efficiency is about 50% while everything
else is down about 35% or lower. This is similar to the turbo compound idea.
Paul Lamar
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