Fuel Consumption

[pmccarthy]

The figure below shows the fuel consumption of my Rotax 912ULS over 200 hours of operation. Consumption rises a bit for longer flights as less time is spent idling, warming up and taxying. This is Hobbs time and the fuel useage is measured with a well-calibrated dipstick. The main variables affecting consumption would be power setting (I use 65% or 75% depending on the reason for the flight) and density altitude.

 

I wonder how the consumption can vary so much. I also wonder what is a reasonable planning consumption. I use 20 l/h but it seems conservative as the average is 15 l/h. And sometimes it might not be enough. I should add that all the fuel was added in 20 litre jerrycans which correlated with the dipstick readings.

 

 

 

[crashley]

Great graph. I haven't worked out my 912's consumption to that degree but it is a lot higher than yours, about 20 to 22 lph

 

I cruise between 5200 and 5400rpm what is your rpm range

 

 

[alf jessup]

Great graph. I haven't worked out my 912's consumption to that degree but it is a lot higher than yours, about 20 to 22 lphI cruise between 5200 and 5400rpm what is your rpm range

Crashley,

 

My Tecnam Sierra I use between 18lph & 19lph at 5000rpm giving me an indicated airspeed of between 100 & 105 kts depending on the density of the air, I usually cruise at 4800rpm (95kts) which is between 15lph & 17lph going by my FS450 fuel flow meter, this corresponds pretty consistently to the amount of fuel I put back in over the hours flown.

 

I used BP 95 octane only.

 

Cheers

 

Alf

 

 

[crashley]

thanks alf I will have a look at the figures from my last trip and do a comparison My skyview fuel flow reads a little high normally and a bit higher again if I do a lot of taxiing or idling as it measures fuel to the engine but not the return fuel

 

 

[Downunder]

Great graph!!!!

 

I think some of the major discrepancies come from the amount of oxygen in the ambient air.

 

We generally fly by a "speed" setting. Lower ambient oxygen levels (such as in summer) probably require a higher throttle setting to get the same speed thereby using more fuel.

 

Drag racers over here in the west talked about a freak weather phenomenon where a super-rich (in oxygen) sea breezewould allow the extra power required to break records.

 

I believe I’ve had this happen twice to me this winter.

 

In the late afternoon, on climb out (normally at 60kts/5500), I was starting to over rev. I pulled the nose up and slowed to 50kts but still pulled to 5500 quite easily.

 

Under normal conditions the revs/speed would degrade somewhat with the increased climb angle but not on these two occasions.

 

I think the Rotax main jet (after 5000 rpm) is fairly rich and this allowed me to take advantage of the extra ambient oxygen.

 

On a side note, has anyone else noticed the exhaust temps getting cooler after 5000rpm? I think this is where you come off the needle and onto the main jet.

 

Of coarse fuel quality is a great variable too....

 

 

[Dafydd Llewellyn]

If you have an EMS that gives % power, try plotting fuel consumption against density altitude, for several %power settings; you may need to use a prayer wheel to get the density altitude, unless the EMS also provides this. That should give much less scatter in the results. The CD carbies used on the 912 and Jabiru motors are not fully altitude compensated, so the mixture may get a little richer at altitude - but the taper schedule on the carbie needle gets into the act as well.

 

 

[pmccarthy]

Great graph. I haven't worked out my 912's consumption to that degree but it is a lot higher than yours, about 20 to 22 lphI cruise between 5200 and 5400rpm what is your rpm range

I cruise at 4800 rpm for 65% power and 5000 rpm for 75% as in understand it. Rarely push it to 5200 to get home against a strong headwind. The high consumptions would be those few occasions.

 

 

[eightyknots]

thanks alf I will have a look at the figures from my last trip and do a comparison My skyview fuel flow reads a little high normally and a bit higher again if I do a lot of taxiing or idling as it measures fuel to the engine but not the return fuel

I wonder how you could avoid that 'error' caused by the return fuel being measured.

 

 

[crashley]

you can install another flow cube in the return line which then makes it read the correct amount but they are $250 so haven't installed one just gives me a bit more reserve

 

 

[eightyknots]

you can install another flow cube in the return line which then makes it read the correct amount but they are $250 so haven't installed one just gives me a bit more reserve

Does it somehow subtract the return flow from the total measured?