View Full Version : piston fuel consumption?

7th Feb 2004, 21:04
i heard from a mate of mine that any piston engined a/c's average fuel consumption in gph can be adduced by taking ten percent of the max.horsepower,and dividing by two.this is certainly true for the c172 series 1 acft,giving a gph of 8.anyone else heard of this formula for average fuel consumtion & is it reliable?:confused:

7th Feb 2004, 21:18
It's a rule of thumb. Fuel consumption varies by an enormous amount depending on mixture setting, % power set, proportion of time in cruise vs take offs / hour etc etc.

7th Feb 2004, 21:31
thanks! its actually quite scary because the flight manuals are all based on new acft operating at optimum conditions-and the figures given there are often far off the mark.going by the book,for example,a 2.2hr flight at density alt of 10000' with one t/o and 1 landing gives something like a 5.8 gph consumption as an average,whereas in reality in the conditions i operate under,you're looking more at an actual measured consumption of about 7.8gph!

7th Feb 2004, 23:55
Fuel consumption obviously depends on power setting but since you're turning chemical energy in the Avgas into useful work, you'd expect the energy you get out of a gallon to be predictable.

I'd estimate that you can get 14 horsepower hours per US gallon from a properly leaned engine. So for example a 200 hp IO-360 running at 70% power should cost you 10 USG/hr.

7th Feb 2004, 23:59
thanks! thats much closer to the actual than the book says!

8th Feb 2004, 09:10
John Deakins says:

for aspirated engines (8,5:1 compression ratio) - 14,9HP/GPH

for fire breathing turbos (7,5:1 compression ratio) - 13,7HP/GPH

Observation: those numbers are for lean of peak operation only

THE mandatory link for everyone who wants to lear A LOT about recips: http://www.avweb.com/news/columns/182146-1.html

Thank´s John. You made much much less ignorant.

8th Feb 2004, 11:16
Genarally speaking, for turbo and non-turbocharged piston engines, figure...one half pound of fuel per HP per hour.

Again, generally speaking, the only exception to this was with the CurtisWright R3350 DA/EA series turbocompound piston engines which had three Power Recovery Turbines (exhaust driven) fitted, which provided extra power directly to the crankshaft via fluid couplings. With these engines, the fuel consumption was (approximately) .36 pounds of fuel per HP per hour.
However these engines, in order to be operated as designed absolutely required 115/145 avgas, generally not available today.

The longest range piston engine airliner was fitted with these very advanced engines, the Lockheed 1649A Constellation.
23 hours endurance....provided you did not run out of oil.:ooh:
A superb aircraft.:ok:

8th Feb 2004, 23:34
Isn't there that line about them being the best 3 engined aircraft ever invented? :p

8th Feb 2004, 23:59

Well, I dunno. Flew this particular model for about 200 hours, and all four kept a turnin'....:ok: