WRK

My Feedback: (2)

I have a suspicion that on our bigger two stoke engines say 80 CC and up, most of the engines we use on our planes produce their peak power
at a higher RPM than we usually run them, of course the exhaust system has a lot to say about that.

I wonder if a change in port timing (and possibly other changes) would benefit some of the engines, that is bring down the peak power point to the low 6000 RPM range, where it could be better utilized. I would bet that a lot of the engines peak at 7000 RPM or better.

Since many of the engines we use have major parts in them that come from engines not designed for R/C some of the manufacturers are probably
handicapped as far as changing things too much.

Just changing the exhaust port timing can really move the power curve around.

av8tor1977

My Feedback: (6)

I just don't prop mine so large and load them down so much. While everyone else is running 6000 ish, I run my engines at around 8000 and sometimes close to 10,000 on certain engines. We always ran high rpms with the glow engines right? Of course, great big props work better for 3D flying, which is why it is so common. I haven't checked the port timing, but I think a lot of the purpose built engines have lower timing more consistent with lower rpm operation. I fly mostly engines converted from chain saws and such, and they do like to rev!

AV8TOR

pe reivers

Most two stroke gas engines can be made to run very wel around 6000 rpm. However, there is a rule of thumb that any engine will run well at about a piston mean speed of 15 m/sec. Above that, and metallurgy and lubrication will create a narrow operating window.
As Jody would say, now go play outside, or do the math. VVVBG

On props the story is different. You should not run them at tip speeds above 0.6 Mach. You can get more thrust out of them exceeding this figure, but propulsion efficiency goes to pots if you do. To much energy will be wasted converting valuable engine energy into noise.

The goal will always be to match the engine to a prop that fits the plane and flying style + local field requirements. To that end, on the engine side, there will be muffler adaption and carb selection. (smaller/larger carbs often allow a desirable shift in operating rpm window)

All this is not for the faint of heart. A lot of books have been written about the subject, and getting it all together can be a real PITA.

karolh

So very true, especially the very last bit.

Karol

w8ye

My Feedback: (16)

Most of my chain saws run right at 14000 when you rev them up and stay a little above 9000 rpm in the cutting of the wood.

But my 50cc and larger gasoline model airplane engines run between 6500 and 7200 with the correct prop and regular muffler on the ground. Some are even chainsaw engine cylinders but prop efficiences drop off when the props turn too fast and make the ripping noises.

pe reivers

My first gas engine was a quadra 35 I bought with a plane. It had the standard muffler. The propeller fitted allowed the engine to run 8500 rpm. Performance was less than promising. It barely flew the plane.
Reverting to a 18x10 prop which reduced rpm to about 6000 made the plane very nice to fly and allowed slight aerobatics when using energy management.