Microlighters

I have put about 70 hours now on my Raptor with a Rotax 582 blue top. My consumption is around 12-13L per hour and I normally cruise around 5000-5100 rpm. This is much better than the Rotax manual states (20.5L per hour @ 75% continuous performance) so it is a pleasant surprise and makes me think Rotax is being ultra cautious (as opposed to the car industry which seems ultra optimistic). What are other peoples experiences on fuel consumption on the 582?

rcbgreen wrote:I have put about 70 hours now on my Raptor with a Rotax 582 blue top. My consumption is around 12-13L per hour and I normally cruise around 5000-5100 rpm. This is much better than the Rotax manual states (20.5L per hour @ 75% continuous performance) so it is a pleasant surprise and makes me think Rotax is being ultra cautious (as opposed to the car industry which seems ultra optimistic). What are other peoples experiences on fuel consumption on the 582?

The rotax manual is accurate i.t.o. fuel consumption - you are just using less than 75% power .

Then I am still missing something as at 5100 rpm I am outputting 35 of the 48 kW which is 73% (as per power output graphs in manual).

rcbgreen wrote:Then I am still missing something as at 5100 rpm I am outputting 35 of the 48 kW which is 73% (as per power output graphs in manual).

The graphs show maximum power at full throttle at a specific RPM - i.e. RPM is reduced by increasing propeller load. Once you start reducing throttle, these graphs go out the window.

See the last curve here:

http://www.rotaxservice.com/documents/582perf.pdf

This shows fuel consumption at full throttle (take-off curve), and with a propeller load. Of course, the propeller curve depends heavily on the propeller, and atmospheric conditions.

Thanks, I think the light has come on. I may be doing 5100 rpm but not at full throttle so the fuel consumption is lower. So that 75% figure in the manual has limited practical use. I was trying to compare it to adverts for a Rotax 912 based planes where they generally quote 12-14 l/hr (also backed up by friends who have 912 based planes). My 582 is in theory using a similar amount of fuel but at a lower power output and efficiency.

rcbgreen wrote:Thanks, I think the light has come on. I may be doing 5100 rpm but not at full throttle so the fuel consumption is lower. So that 75% figure in the manual has limited practical use. I was trying to compare it to adverts for a Rotax 912 based planes where they generally quote 12-14 l/hr (also backed up by friends who have 912 based planes). My 582 is in theory using a similar amount of fuel but at a lower power output and efficiency.

The 75% comes from the big(er) plane world, where typical cruise is specified as 75% power (i.e. full throttle at 8000' cruise altitude). Recreational fliers usually fly at significantly lower power settings.

Advice that I got from Avcom when I posted a similar query was -

- assuming that you have an accurate fuel flow meter
- take the fuel flow at full throttle
- divide the fuel flow at whatever setting you are flying by the full flow reading, and that will be the closest approximation to the % power that your engine is operating at.

Sorry, I should have added that the result will not necessarily give you % power of 64 HP, as this is dependant on prop pitch density altitude etc, but will rather indicate the % of available power.

I do have a fuel flow meter. It's not 100% accurate but when I fill up to around 75L and set my known fuel on the meter to the same, it shows me about 23L when the mark on the tank says 21L (3.8% error not bad). At take off with max revs 6500 it is bouncing around 25L,/hr then when I start cruising at 5000 rpm it hovers around 9 L/hr unless I pull the bar in and give a bit more throttle. 5000/6500 = 77%.

rcbgreen wrote:I do have a fuel flow meter. It's not 100% accurate but when I fill up to around 75L and set my known fuel on the meter to the same, it shows me about 23L when the mark on the tank says 21L (3.8% error not bad). At take off with max revs 6500 it is bouncing around 25L,/hr then when I start cruising at 5000 rpm it hovers around 9 L/hr unless I pull the bar in and give a bit more throttle. 5000/6500 = 77%.

And by fuel consumption 9/25 = 36% - this is a much better estimate of your cruise percentage power.

Remember that the prop is a wing. The torque required to turn the prop is equal to the drag of the 'wing' (1/2 . pho . v^2 . S). So torque required is proportional to the square of the RPM. Power is torque * RPM, so power required is proportional to the cube of the RPM.