Sender: <marv@lancaironline.net> (Marvin Kaye)
To: lml
Date: Wed, 31 Jul 2002 11:07:48 -0400
Message-ID: <redirect-1660740@logan.com>
From: "Gary Casey" <glcasey@adelphia.net>
Subject: Turbine power
MIME-Version: 1.0
Content-Type: text/plain;
	charset="Windows-1252"
Content-Transfer-Encoding: 7bit
Importance: Normal

Lots of good opinions - but we need to keep a sense of realism here, too:
1.  Throttling would work well if you had a way to suck out the exhaust at
the same pressure.  Piston engines can be throttle with only a moderate fuel
economy penalty because the air flow is relatively low - way less than half
the air flow through a turbine.
2.  The ATP engine uses an existing core that has been around for many years
and as far as I can tell runs a turbine inlet temperature of about 1700 to
1800F.  To think that they are going to break new ground in fuel consumption
numbers is unrealistic.  I can't see how the fuel injection system could
improve fuel consumption over the original continuous flow system.
3.  As you scale turbines down in size they inherently get less efficient,
so you can't expect a 300 hp engine to get the brake specific of a 3,000 hp
engine.  Brake specific is a function of turbine and compressor efficiency
(these inherently go down with size) and turbine inlet temperature and I
don't see any breakthrough's here.
4.  I would expect a full-load brake specific of about 0.7 with the engine.
It could go a touch lower, but could be quite a bit higher.  And the best it
could do would be at the max rated turbine inlet temperature.
5.  Yes, some turbine engines are "flat-rated" because the power is limited
by torque or some other factor (like the aircraft itself) than temperature.
Is the ATP engine flat-rated?  Don't know, but I kind of doubt it.
6.  Generally the power rating of a turbo-prop is in shaft horsepower.
There is significant power available in the way of exhaust thrust, so this
is an good way to get the effect brake specific down.  ATP says that the
200-hp engine can have an exhaust thrust of 100 pounds.  Sounds a little
high to me, but with a good exhaust design one can get a few more horsepower
out of the engine.  I think 50 pounds would be more realistic.
7.  Yes, you could burn furnace oil, but the brake specific or power output
might not be as good as the fuel system may need to be recalibrated.
8.  Yes, you can certainly use the extra useful load to carry more fuel -
problem is you need to buy it every time.
9.  As was mentioned, the turbine engine is essentially "naturally
aspirated" in that the power goes down (at a fixed temperature) with
altitude just like a non-turbocharged piston engine.  The plane is most
efficient at it's service ceiling so, just like airliners, the optimum
flight path would be to always be continuously climbing until time to go
back to idle thrust and descend.
10  Compared to a piston engine run LOP I would bet that the consumption
would be 50% higher - and that would be on a relatively long leg with
minimum ground operation.

No silver bullet, the ATP turbine engine is still very, very tempting.
"Whishhhh....."

Gary Casey
ES project