SolarWinds

Hi all,
I am flying a CMpro SWALLOW-EX with an OS FS-91 and 14X6 APC prop (10% nitro CoolPower). The plane is under-powered possibly due to angine not performing well. The RPM is 8800 static at full throttle. Is it normal or do I make a mistake? The engine is (hopefully) well breaked and until now I have burn more than 2 gallons of fuel.
Can naybody help me? I do not believe that the OS FS91 is so powerless.

Thanks
Yannis Solaris
Greece

Bax

My Feedback: (11)

8,800 RPM is a bit low for the O.S. FS-91 Surpass engine. While we normallly use an APC 12 x 10 prop, we also get performance in the middle 9,000's. While your prop may not seem to be too much load, it may be that the airfoil and blade profile adds more load than the APC equivalent.

Try some different propellers. You want to have the engine running in the middle-9,000's or higher (you can go into the low-10,000's with no problems!) when it's leaned to just rich of peak RPM on the ground. If your engine can't do that, even with other props, then something's wrong.

SolarWinds

Thanks, BAX,
The fact is that I have not seen rpm more than 9000 with any prop, so something is wrong. Since fuel system is ok (fuel, tank, fuel lines...) the problem must be in the engine itself. What is your opinion? What should I check? compression, valve clearence?


Thanks again,
Yannis

Bax

My Feedback: (11)

Any number of things can be contributing to low RPM. Since you're using a good-quality fuel, it's not the problem as long as it's reasonably fresh. Old fuel that's been opened a long time will pick up moisture, which will compromise running.

Please realize that the RPM number we quote are the peak RPM numbers we get on our test stand. We use APC propeller and Omega 10% fuel in all the engines we run in our service department.

Low RPM can be caused by poor compression, bad valve seals, poorly-adjusted valves, wrong prop, not enough breakin, and so on.

The breakin procedure can affect top-end power a lot. Normally, you'd want to break in a ringed fours-stroke engine by running it at full throttle as rich as possible and still keep it running. Do this for about the first 15-20 minutes, and then gradually work the mixture towards peak RPM by leaning slightly for a few seconds and then richening back. Do this gradually, working leaner as you lean, and not richening back as much each time. Eventually, you should be able to lean the engine to the point where it's just rich of peak RPM and will hold the needle setting through a full tank of fuel.

You may want to check how you have the engine leaned. It may not be lean enough to develop full power. Usually, you can't do as good a job by listening...you really need to use a tach. From your note, it sounds like you do use a tach, so use that as your guide to finding the peak RPM. Richen 300-400 RPM off peak, and you should be good to fly.

SolarWinds

Thank you BAX, i will try to break-it during the weekend and see the results.

Thanks,
Yannis