Dave Siciliano (ADS)
(Top Gun)
09/20/08 01:50 PM
Re: Turbine P-Baron

Quote:

Dave,

That's a huge difference in FF. When I asked whether it was "both", I guess it's always a tradeoff between TAS and FF. For instance, at 10,000 ft, if FF was 440 PPH, I imagine that TAS would be in the 200-220 range. So, as one increases altitude, either TAS goes up, FF goes down, or both, depending on your power settings.

IIRC, the difference is much more severe for a turbofan engine vs a turboprop. I don't really understand the theory behind that, but is that correct?




There certainly are many folks out there that can explain this better than me, but I'll give it a general go and be happy for anyone else to chime in. The turbines are a bit different in the way they are run; unlike a prop, they seem to do better at recommended power settings; that is you don't run them too hot or too cold. So, you are correct in that if one ran 440PPH at 10,000 feet, TAS would be down, but one generally wouldn't run a low power setting like that a lot. The small amount of fuel savings is offset by the cycle cost on the plane.

While turbines could be more efficient lower, most are set up to get maximum efficiency higher. The compressors (which could be compared to the turbo on a recip) produce full power up to an altitude, then, power drops off. So, the old C-90 King air engines made full power up to about 14,000 on a standard day and power dropped off above that. One would still benefit from being higher as the air was thinner and TAS would still be high with lower fuel burn as in the expamle I provided. The -135s on the Blackhawk conversion plane, maintain full power to a higher altitude; so, the sweet spot is mid 20s as opposed to low 20s on earlier King Airs.

I yield to the next speaker that can explain turbine efficiency better <g>

Best,

Dave



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