Piper J3
Well Known Member
I normally fly round-robin cross-countries of less than 150 miles total so I fly at altitudes of maybe 3500MSL.
Two weeks ago, I flew a couple of hundred-mile legs and ventured up to 7500MSL to achieve a good economy but found that my fuel flow actually increased with altitude. Today I flew 330nm and again at 7500MSL. I copied Van’s POH Cruise Performance and took it with me for comparison. The speeds look to be spot-on but the fuel burn increases with altitude instead of decreases.
Here are two data points from today’s flight:
2500 MSL 115 KTS 5.0 GPH @ 5500 RPM - MAP=25.0 / EGT~1300F
7500 MSL 115 KTS 7.4 GPH @ 5500 RPM - MAP=23.0 / EGT~1300F
Today’s flight was flown in loose formation with a friend’s RV-12. He saw the same speeds but his fuel burn was similar to Van’s published Cruise Performance. Of note is that his EGT’s run about 100F higher than mine (1400F Vs. 1300F).
So, maybe I’m running a little richer mixture? And maybe my carbs are not leaning correctly at altitude?
Any thoughts appreciated. I thought I’d post here first, and then if no luck, I’ll ask in the Rotax forum…
Two weeks ago, I flew a couple of hundred-mile legs and ventured up to 7500MSL to achieve a good economy but found that my fuel flow actually increased with altitude. Today I flew 330nm and again at 7500MSL. I copied Van’s POH Cruise Performance and took it with me for comparison. The speeds look to be spot-on but the fuel burn increases with altitude instead of decreases.
Here are two data points from today’s flight:
2500 MSL 115 KTS 5.0 GPH @ 5500 RPM - MAP=25.0 / EGT~1300F
7500 MSL 115 KTS 7.4 GPH @ 5500 RPM - MAP=23.0 / EGT~1300F
Today’s flight was flown in loose formation with a friend’s RV-12. He saw the same speeds but his fuel burn was similar to Van’s published Cruise Performance. Of note is that his EGT’s run about 100F higher than mine (1400F Vs. 1300F).
So, maybe I’m running a little richer mixture? And maybe my carbs are not leaning correctly at altitude?
Any thoughts appreciated. I thought I’d post here first, and then if no luck, I’ll ask in the Rotax forum…