Royce Wise wrote:

Paul, care to answer this post from another newsgroup?
Royce

"S. Ramirez" wrote:

<snip>

The only place a turbine makes any sense is up high, real high.  A turbine
makes no sense at the altitudes  we fly in small experimental airplanes.

...

I'd rather cruise along with a Lycoming (or maybe a Subaru?) and be able

to

fly past a few fuel pumps.

dd

<unsnip>

Does anyone know why a turbine is more efficient at higher altitudes than at
lower altitudes?  Just curious.

Thanks.

Simon Ramirez, Aerocanard Builder
Oviedo, FL USA

The specific fuel consumption does not change. Pounds of fuel burned
per pounds of thrust generated. Aero drag is
lower at high altitudes. Thrust falls off but not as fast as aero
drag falls off so the airplane goes faster. Thrust does not fall off
as fast as it would but for the ram effect. This  increases pressure
in the inlet at the higher speeds. Sort of like how a ram jet works
to a minor extent.

Paul Lamar

Hey Paul I would also like to help answer Simons question. I fly turbine for a living. You are
absolutly correct with the prospect of air inlet pressure being greater at lower altitudes but the
serious difference is due to air compressability and the nature of a turbine engine. The Turbine
engine will also drop in fuel flow as pressure altitude increases because the density of air that
is flowing through the turbine is less. The turbine is capable of retaining higher thrust levels
because it is still able to compress the air efficiently.  As a matter of thrust, the same size
displacement on a turbine is able to far exceed its aero counterpart. The power generated is
capable of handling larger loads(ie. larger propellers, multi bladed propellers, or turbo fans to
operate more efficiently) and still have enough power to handle environmental conditioning needed
at altitude.Please keep in mind that an accessory section of a small turbine such as a
Pratt&Whitney PT-6 is turning 36,000RPM governed to generate electrical power for systems as well.
The dynamics of the propeller or fan are the restrictions as far as how high the airplane can fly.
Notice large transport aircraft use large bypass fans with very small turbine compressor sections.
That is because it needs to utilize as much  fan area to generate thrust at higher altitudes while
still trying to be efficient. When I fly overseas in the B-767/300 I usually weigh around 368,000
and flying at 33,000 to 35,000 feet. My fuel flow is 5,500 lbs/engine.
Simon I hope this helps and sorry for getting away from rotary topics.
Doug
RX-6  13BT
N900DM
Henderson,Nevada

 
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