when the max recommended engine size in 2stroke for a plane is a 55 and 4stroke is 82... does this mean that one would get the same speed from them in that specific plane with the same prop?

I dont think that is true at all. A 4 cycle needs a bigger prop to be efficient and produce the same output as a 2 cycle, as far as speed goes. That is why you have the golden rule. If a plane needs a 40 2 cycle, it will need a 60 4 cycle. Good Luck, Dave

Not at all......even if you had 2 airplanes with everything the same I doubt you'd get the same feel or the same speed....close but different enough you could tell.....
A 4 stroker will give you more torque, but that does not mean more speed.....

If either engine can turn the same prop the same rpm, they have the same torque.

But 4C engines don't turn up the rpm that a 2C does, so they are propped different.

The speed that a model can achieve is proportional to the power output of the engine (which varies with the operating rpm), but it's not a linear relationship. The relationship of speed to power is (power ratio)^.333

It takes 8 times the power to double the speed.

yeah I have figured... was just a question... I will get both engines... If I feel like speed it would be the 2stroke and if I feel like 3D 4stroke...

Might be a nuisance switching back and forth. The 4-stroke will weigh much more and might have a different "footprint" of mounting holes. The bug in the pudding is that you may have to add 4 oz of weight to the 2-stroke set-up . . . reducing it's speed in climbs. Whatever you can do with the battery may help: forward with the 2 & moved aft with the 4.

Have fun.

You never did tell us the model. Scale? Sport? Aerobatic? Very aerobatic? Not all airframes are born equal. No matter which sneakers you put on a pig it will never run fast.

If you're like me you won't be using the two-stroke in the plane much. Use the right prop and the four will likely wring out the model well enough.

I am also a fan of 4stroke... its the Extra 300SP from GP... think I will be getting myself the 82a SAITO with a 3bladed 13x6 and 2 bladed 14x6 prop and experiment with those... will get a different plane and run a 2stroke in there for speed... I am swapping my FA65 for a Rossi 66 2stroker with claimed 3.5HP...

The Rossi 65 makes that claim at 17,000 rpm. That means about an 11" prop of maybe 7" of pitch to get the engine into it's power band (it might not even turn that big of prop). For instance, a racing quickie engine (Nelson or Jett) makes close to 3 hp at about 22K in the air. They are only turning 8 3/4" diameter with around 8 pitch, with very narrow blades. On a draggy airplane, you will not like the results. In many cases the better choice is the lower revving torquer with a big prop.

the Rossi 65 is almost double the price here as to an OS61... and makes 1.5 or so more horses at the same RPM... it must have some power and potential... I am thinking of using it with a Sundowner 50 from Hangar9...

Oh Yeah! That might be a good combination if you can shoehorn it into the front end and the airplane is strong enough.


If not strong enough, that will be interesting too.