Fuel Efficiency at higher altitudes

[S3flyer]

Being a flatlander, my normal cruising altitudes center around 5500' and I burn 5gph (or slightly less) running at 5200 RPM from engine start to engine stop.

 

This past weekend I had the need to cruise for quite awhile up at 8500' (DA 10,250). I was turning about 5300 RPM for the 3.1hrs flight time (take-off to touch-down) and 3.4hrs of total engine time (from start to shutdown). I burned right at 14g so I either burned 4.5gph or 4.1gph, depending on how you want to measure.

 

I assume the 10-20% improvement was due to not generating 75% power?

[Ed Cesnalis]

Dave,

 

What was your throttle setting? You have to have the needle jet circuit in control to lean and save fuel. Theoretically at WOT the main jet will control and there will be no leaning. Some say this never fully happens at altitude. I mostly fly at 10,000' and burn over 5 but fly at WOT.

[S3flyer]

I'd guess WOT would have been a bit over 5400RPM and I was at 5300RPM.

[Ed Cesnalis]

I assume the 10-20% improvement was due to not generating 75% power?

 

If you are above 7,500' and have WOT and have your main jet in control then you would realize max fuel burn while not generating 75% power.

 

Since you were not at WOT and since its possible that your main jet gets control at altitude I would expect that the Bing carbs were leaning and saving the fuel.

[FlyingMonkey]

I believe I have read that most piston powered craft are most efficient in cruise at about 8000ft in a standard atmosphere, for best power production vs drag due to air density. That's kind of a high cruise for most flatlanders, but it might make sense to climb that high for a long cross country route.

[Jim Meade]

most

NATURALY ASPIRATED

piston powered craft are most efficient in cruise at about 8000ft in a standard atmosphere, for best power production vs drag due to air density. That's kind of a high cruise for most flatlanders, but it might make sense to climb that high for a long cross country route.

That assumes correct leaning, which one can't do on the Rotax with Bing carbs. As Dean Lockwood says, the Bings don't lean as well as a plain ord red knob on the dashboard.

 

In my experience, wind has as much or more to do as power or drag with regard to the most efficient altitude. That is, sometimes it's better to be inefficient but faster. It's best to go fast into the wind and one can afford to go slow with it. Often, I find myself going low westerly and higher easterly to accommodate previailing winds, but the 496 and the D120 are my friends when it comes to picking the best altitude while in flight.

[207WF]

I have experienced higher fuel flows at higher DAs for the same true airspeed, consistent with the carbs' "autolean" not leaning enough. For example, 4.6 gph at 6500 DA might become 5.1 gph at 12,000 DA. For me, the cruise altitude is mostly about the wind speed and direction. WF