Fuel Flow vs. Altitude

flymo

flymo

Well-Known Member
Looking through the performance manual for both a non-turbo Mooney and a Cardinal and I can't seem to wrap my head around the reason that the fuel flow increases with an increase in altitude at the exact same power setting.

Power setting of 22" of MP and 2400 RPMs gives a fuel flow of:
2500 ft: 9.5 gph
5000 ft: 9.8 gph
7500 ft: 10.1 gph

Why is that?
 
Just guessing, but ambient pressure at 7,500 ft is 22.6". So to get a manifold pressure of 22" the throttle would need to be wide open. At 2,500 ft, not so much.

The air mass flow must work out to be greater at higher altitudes for those engine settings.
 
Less exhaust back pressure at the higher altitude means that for the same MP and RPM, you're making more power, necessitating more fuel flow.

Or something like that.
 
22" at 2500 the throttle is closer to closed than open. At 7500 if it's not wide open, it's close.
 
Colder OAT up higher = denser air entering cylinders at same power setting = more hp = more fuel burned.

Yes I know the air is less dense up there but the engine is only getting 22” of MP. So it does not know what altitude it is at but if you cool the air entering the engine it will produce more power and burn more fuel.
 
flymo said:
Power setting of 22" of MP and 2400 RPMs gives a fuel flow of:
2500 ft: 9.5 gph
5000 ft: 9.8 gph
7500 ft: 10.1 gph

Why is that?
Click to expand...

2500 ft, 22"/2400 = about 65% power
5000 ft, 22"/2400 = about 75% power
7500 ft, 22"/2400 = about 85% power

As you go higher, you will open the throttle more to maintain a given MAP. This allows the engine to breath more air, and also consumes more fuel.
 
flymo said:
Looking through the performance manual for both a non-turbo Mooney and a Cardinal and I can't seem to wrap my head around the reason that the fuel flow increases with an increase in altitude at the exact same power setting.

Power setting of 22" of MP and 2400 RPMs gives a fuel flow of:
2500 ft: 9.5 gph
5000 ft: 9.8 gph
7500 ft: 10.1 gph

Why is that?
Click to expand...

What is the true airspeed listed for those given power settings/fuel flow/altitudes?
 
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