Bill Freeman wrote:

Tracy beat me to it.  I was going to say that this was really
neat, but the ugly truth is that small turbines have EXTREMELY
bad fuel specifics.   I was going to guess 3X the fuel flow of
an equivalent piston engine.  Let's see -----

  36 kg x 2.2 = about 80 lbs thrust

at 150 mph ===> (220 ft/sec)  x 80 lbs = 17,600 ft-lb/sec

divided by 550 ft-lb/sec-hp   = about 32 hp thrust.

Assume a 82% efficient prop and you have total 39 hp at the
engines.  With a 0.42 BSFC you should burn 16.4 lb/hr or
2.73 gal/hr.   If the fuel specifics were ONLY 3X as bad,
this would be 8.2 gal/hr.

>From Tracy's number of 18 gal/hr you are 6.23 times the
fuel flow.    YIKES!  Perhaps the thrust is 36 kg PER ENGINE,
in which case the 3X is about right.  Either way pretty
catastrophically bad.  Bet they cost an arm and a leg to
buy, too.

When I was at Union Carbide in their machinery technology
group at the tech center (about 100 yrs ago :-) ), we were looking
at installing small turbines to recover some of the energy in
some process flows (like recovering ~50,000 HP from one
ethylene stream).  We brought in DeLaval and did some
prelim engineering work.  The main thing I remember these
many years later is that the efficiency of turbines REALLY
STINKS as they get smaller.   I believe one of the main
problems is that the tip effects don't scale, so short blades
are extremely inefficient.  I forget the details but we gave up
on the power recovery turbines, the there was some REAL
money being expended 24/7, but it still wouldn't pay.
There were other problems besides efficiency, like polymerization
IN the turbine, but we won't go into that.

There are some improvements that have been made in small
turbines in the last 10-15 yrs and I think that the Williams
tiny turbine MIGHT have a chance, but it is far from easy.
The key to the Williams will not be will it work, but will the
fuel specifics and first cost "work" (economically).

Does anybody know the fuel flows on some of the small
Williams certified turbines like the FX44 (IIRC)?  How good
are the real A/C small turbofans that are in production ?

Still pretty neat, tho.   ;-)

Bill

I asked that question at the William's booth. In fact the engineer recognized me when
I walked up. He said "Oh no! Not Paul Lamar!". He then said you are going to ask me
the 
specific fuel consumption aren't you. The answer is the same as it was last year.
"I can't tell you". So much for our 100 million dollars which is now nearly gone.
The price of the engine is now up to $100,000 which is out of my price range.
Further funding is being sought which will raise the final price even more.

What did we get for our 100 million dollars? NOTHING!

Some small turbo prop engines are slightly below .65 BSFC.

Paul Lamar
 
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