Thread: Engine rpm

I plan on burning 91 oct mogas including cross countries. (collapsible 5 gal "cans"). Does anyone know what is the best rpm for cruising? The CTLS mentions 4800 rpm for best cruise and max endurance at 4300 rpm. I have heard it said that you should always run 5000+ rpm. But I have also heard it said that the 5000+ rpm applies to running with 100LL. If I want to get 1500+ hrs before rebuild. what should I cruise at?

Hi Grumpy,

Happy to have you onboard!

You may have misread on some of those rpm's. I am a Rotax Service Center. Taught in the Rotax schools and fairly well known is that the best all around rpm for your 912uls engine is 5500 max rpm WOT straight and level. This gives you the best of all performance for a ground only adjustable prop with the 912ULS with the best overall torque to hp ratio. The best of all worlds would be an "in flight" adjustable prop, but LSA's in the US can't use those prop systems. The rpm for cruise should be between 4800-5200rpm with 5000 rpm being around the best and max torque. You can only run over the 5500 mark up to 5800 for max hp for only 5 minutes. The torque starts down after 5000 rpm and really heads down after 5200 rpm, but the hp will keep climbing. This engine was specifically designed to run between 4800-5200 rpm for best vibration, fuel and power to weight ratio and you should easily see your 1500 hrs. If you go on what they call "on condition" inspections some of my friends have 2800-3000 hours on their engines and never an overhaul. Most CT's and I'm not sure about the LS yet have prop pitches set to run at 5200 rpm WOT and that is a little course. Rotax says to run the prop engine combo at 5500 rpm. You can cause issues with a Rotax engine if you try to have too course prop pitch not to mention you lose hp, climb, fuel economy and just overall performance. 5500rpm will not take you over the 120 knot limit for LSA aircraft.
If you have any doubts just wait until the rest of this board chimes in.
I have been running Rotax engines now for a number of years and I would say a fairly large number of CT pilots like 5500 rpm, of course that is just speculation and my opinion.

Anybody else care to help grumpy see the light.

Let me know and I'll be happy to help you any way I can.

Originally Posted by grumpy1

... If I want to get 1500+ hrs before rebuild. what should I cruise at?

5,500

Shame on you Ed.
You know you live up closer to God than the rest of us.

i set my cruise rpm for 10,000' so it doesn't matter that i live higher. i think best performance altitude is 7,500' MSL so that would be a good altitude to pitch for as well.

my favorite pitch is 5,600 WOT. this allows the engine to run easy at 5,500 while i am throttled back and provides some extra available climb performance.

Hi Ed,

For your elevation and altitudes your rpm's are set good. I only give you a hard time because you have a very unique flying situation compared to the rest of us.

We find the 912S very happy cruising at 5200 - less noise, less fuel burn, not much less speed.

Doug

Hi Doug,

Is your max rpm set fro 5500? 5200 rpm the Rotax can fly all the time. If your max is 5500 rpm you will burn a little less fuel at 5000, but not enough to be a big deal. 5200 should put you between 5-5.5 gal. hr. in real numbers. Rotax engine are usually very close in their fuel and gal. per hour burn rates. It can be a little different depending if the plane is really drag dirty or nice and sleek like the CT. The fuel burn is different from the guys who have props set to achieve 5200 WOT verses 5500 rpm WOT. The 5500 rpm guy will burn a little less.

Thanks for all of your imputs. The CTLS is set for 5500, 5 minutes at 5800 as Roger stated. What is expected burn at 5200 and 4800rpm cruise at a particular altitude? I would like to know if my Dynon fuel burn is accurate. Thanks
Grumpy

Hi,

The Dynon is accurate for the most part.