I too agree that 4-strokes run cooler.....as I see it in my engines. A 4 stroke would be great in a heli if you could get a bigger cylinder head on it (like 2-strokes) but the valve systems kinda get in the way.... (but a s a side note... I remembering seeing in the Tower catalog a 4-stroke engine for RC Cars.....I wonder how that thing works in that high heat environment?)

As for saying that using 100% synthetic oil in a 4-stroke is bad... I totaly disagree. From first hand experience I find that using 100% synthetic fuel works fantastic...cool engines, smooth idles, and loads of power. Specificaly YS's and Saito prefer the synthetic mix....and the use of any more than 2% Castor in your YS will just give you fuel system problems. As for engine longevity..... I don't believe that using Castor is going to make your engine last longer.... I use and have been using synthetic oil in my fuel for over 5 years and all my engine still run like swiss clocks, and the only parts I've bought are glow plugs...... Making sure you run all the fuel out of the engine after a days flying, using after run oil, and not running lean are much more imprtant to your engines lonfg term health .

I think the belief amongst many modelers that 4 strokers run hotter is due to the fact that they have a higher exhaust temperature. The exhaust of a 2 stroke is cooler because of the fact that the incoming cold intake charge mixes with the exhaust somewhat before the exhaust port closes.

Jeremy

I don't think either 2 strokes or 4 strokes are necessarily better for this application. I think people forget that torque is just a function of power and rotational velocity (rpm). Since a heli rotor is typically a constant rpm system, we can find the power needed to drive the rotor at this rpm at max pitch and then take this power as a constant in our drive system design. We can then calculate a gear reduction ratio that will give us the right torque from any engine that can output the peak power we need.


Torque, power and rpm are described by:

Power = Torque x rpm


So as an example, say it takes 1.6 hp to turn your heli rotor at 1600 rpm. We can take these values as constants and then work out the other values.


We know from published data that a certain .46 2 stroke makes 1.6hp. This is the right amount of power so we will use this engine. We also know that it makes this power at 16000 rpm. So we can calculate that we need a 10:1 gear reduction. Obviously the torque at the rotor head is 10 times that at the motor output shaft, but the power is constant so everything works fine.

Now, we find a 4 stroke to do the job. A .91 will fit the bill, but it makes 1.6hp at, say, 9000 rpm. Now the math says we need 5.625:1 gear reduction.

Note that the peak torque values for these 2 engines never entered into the equation.

Now how to decide which motor to use? That depends on what your goal is - highest power/weight, greatest fuel efficiency, lowest noise, best reliability, mechanical simplicity etc...

Has any one ever used an infrared heat gun to measure the actual temps? This would tell the true story. I have access to one. Will try it some time.