Small turbofans have the same scale boundary problems Monty mentioned.
I
doubt seriously
that Williams will get the .5 pound of fuel per pound of thrust they
claim for their small turbofan engine. They refuse to give me test
numbers
even though I have asked for them for the past three years at OSH. The
engineers
recognize me coming and and exclaim: Oh no! Not Paul Lamar! He is
going to
ask
us what the actual fuel burn is again. Frankly they have scammed us
for
100 million
of our tax money.
Paul Lamar
A thrust specific of 0.5 at static is no big deal - I am sure that Perry
is
doing much better
than that. As all of the smart people have said (rightly) - static
thrust
is almost meaningless
unless you are flying at ultralight speeds - Hey! how about we put a
Tracy
redrive on
one of those little 142 lb 160 hp Solars on a KIS 2 place for a true
rotary
hot rod
for popping out to Fox field for a burger - we might have to tank up to
get
back -
but what a bitchen sound and smell - Vance
First; static thrust on a turbofan or pure jet is meaningful as thrust is
almost
constant with speed unlike a prop. Here is that very important chart
again.
Look at the upper left chart.
Second; a thrust specific of .5 for a small turbofan (Perry is using
rotary) is a big deal
as one is more common. The big 2700 pound thrust Williams is 0.5. Williams
claims the
little 750 thrust version is also .5 which is highly unlikely given the
scale effect.
This is why I think a 1000 HP turbo three rotor ducted fan would make a
good substitute
for a small turbofan on a business jet. Twice the range with half the fuel
burn.
Paul Lamar
The higher the bypass ratio of the turbo fan - the better the specific
consumption
versus thrust becomes, and the more it begins to look like a propeller. If
you
notice, those curves had no units up the left side because they were drawn
"free hand" with a pencil based on general trends rather than plotted data.
Three pounds of static thrust per shaft horsepower is relatively easy as
you approach prop type disc loadings, and driven by one of those thisty
little
Solars at 1.3, a static value of 0.5 would be easy. The question you should
ask the Williams folks to make them squirm, is what is the specific at
cruise. - Vance (an ex design engineer at GE AGTD during the dark ages
of turbofan design - ask an old timer what a blucket is)
I think the charts were based on constant HP in.
the reason the turbofan and pure jets are so bad at slow speeds is the thrust
efficiency is bad at low speeds due to poor disk loading as you mentioned. As speeds
and dynamic pressure increase they approaches ram jet operation. The trend is to larger
fans with gear boxes.
Paul Lamar
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