What 912uls power/rpm setting gets the best range?

[3kdscf]

It seems like an obvious question but I could not find the answer on google, the rotax-owner forum, nor here. What 912uls power/rpm setting gets the best range and fuel economy in a Kitfox? Has anyone collected data on this?

I know the rpm will vary from ship to ship because props vary. I have a whirlwind GA-RW3B set for best cruise and usually cruise at 5200rpm. 4000rpm is about what it takes to hold level flight with flaps off. My best guess is that my best fuel economy will probably be around 4300rpm. If nobody has any data I will probably do three two hour flights in no wind at 4200rpm, 4500rpm, and 4700rpm and see my fuel consumed and distance traveled.

I'm really interested in calculating my best fuel economy (mpg) for bragging rights with my continental powered friends :)

[jiott]

Best Range speed is at max. L/D for your aircraft. Probably somewhere near the Best Glide speed-around 65 mph for a Kitfox SS7. It is not a practical speed, much too slow.

[Eric Page]

...two hour flights in no wind...
This is the part I'm interested in! ;)

[3kdscf]

LOL! True! I know in many parts of the country, a "wind free day" is a hypothetical concept :) In the central valley of california, its not that hard to get wind free mornings :)

[3kdscf]

Best Range speed is at max. L/D for your aircraft. Probably somewhere near the Best Glide speed-around 65 mph for a Kitfox SS7. It is not a practical speed, much too slow.

65 is definitely the speed for best aerodynamic efficiency, the question is how efficient the engine at that low rpm. My gut feel is that the engine is more efficient at higher rpms so the best range will be at a speed somewhat higher than 65. How much over, I have no idea. Guess there is only one way to tell, do lots of flying! Shucky darn! :)

[avidflyer]

Doesn't Rotax recommend against extended run time at slower rpm? Seems I remember something about that. To lazy to go look it up just now though. JImChuk

[jiott]

Yes its not a recommended engine speed range; that's one reason its not practical. Also max L/D speed only gives max range with no headwind. A 65 mph headwind gives zero groundspeed and you would go nowhere but could try for a long time. Prop pitch will also affect the results, especially a constant speed prop. A lot of interesting testing could be done here, with different setups of props.

[3kdscf]

LOL! Very true! There was a time on my USA tour when I had to use 5300rpm to get a groundspeed of 60mph. Had I throttled back to "best range", my groundspeed would have been less than a bicycle.

[3kdscf]

OK, my first data! 20.75mpg

I know best glide is 65mph so that is the most effeciently aerodynamically but the engine is more effecient at higher power settings than that so my first guess was that the best range would be at 80mph indicated.

I had a 3-6mph headwind during this test:

Altitude: 2500 feet
Indicated airspeed: 80mph
Groundspeed: 84mph
RPM: 4600
Distance: 166 miles
Fuel: 8.0 gallons

Note that during this 166 miles I landed twice. Had I flown the 166 miles nonstop without the headwind, I probably could have gotten 22mpg or so. My next attempt will be at 75mph.

[Delta Whisky]

Not exactly the answer to your question but very appropriate to the issue: https://www.aopa.org/news-and-media/all-news/2010/december/01/technique-cheap-speed

There's more to the Carson speed then covered in the article; recommend you download a copy of his paper for all the details.

[3kdscf]

Not exactly the answer to your question but very appropriate to the issue: https://www.aopa.org/news-and-media/all-news/2010/december/01/technique-cheap-speed

There's more to the Carson speed then covered in the article; recommend you download a copy of his paper for all the details.

THAT is the article I was looking for, THANK YOU!!!!


In summary:
"A typical piston airplane’s maximum-range speed is about 10 percent above L/D max"

"Simply multiply your airplane’s best-glide speed by 1.316—or add 32 percent—and that’s the Carson number. "


This means for the Kitfox with its best glide speed of 65mph :
best range: 72mph ( 65*1.1)
Carson speed (best value): 85mph (65*1.316)


I now have the next two airspeeds to measure fuel economy! :)