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Cubic inches vs. Weight

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Cubic inches vs. Weight

Old 02-10-2019, 04:13 PM
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stearman70's Avatar
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Default Cubic inches vs. Weight

Hello. Although I have many years building radio control airplanes and I have a general idea about which engine to put a ship, I wonder if there is a more or less accurate rule to know that weight lifts a cylinder engine, regardless of its wing profile. For example, a .46 C.I., 2-Stroke for 5 to 7 lbs, etc. Anticipated thanks
Old 02-11-2019, 08:20 AM
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The problem with a question like this is there are simply too many variables. Take the engine displacement. Is it a 2 stroke or a 4? Can the 4 with a slightly bigger prop generate the same thrust the 2 stroke does? And even among the 2 strokes you can find a varying range of HP. And this holds true through all sizes.

Then there's the plane itself. How is it supposed to fly? You can put a 46 on a pretty heavy J-3 Cub and have a great flying Cub. But that Quickie 500 pylon racer with the same engine is a whole different animal.

I've flown 11 pound war birds on a 61. But my last "beater" plane had a 61 where a 46 was intended. But, I made changes to it too. I also have an 11 pound Citabria flying on a 1.20-4 stroke. And then my 11 pound Jenny uses a 4 stroke 70 and does just fine. And it goes on.

Bottom line is, decide how you want it to fly? Do you want your Cub to fly like a Cub? Or a P-51? And engine accordingly.

Of course you then have the "Install the biggest engine that will fit" crowd. Seen a lot of those types crash.
Old 02-14-2019, 06:12 PM
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It does depend on the airplane type, expected/required performance and skill level. There are no set rules. If you have the building skills and pilot skills to construct an airplane with a 2:1 power to weight ratio then by all means go for it. On the other side of the coin if your thing is to putt around the sky on the wing then go for that as well. Don't allow anyone to decide for you what you are capable of.
Old 02-15-2019, 04:26 AM
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I have to agree with the others here. There isn't a formula for this. The best you can do is go by the manufacturers recommendations and read a thread or two about what others have used.
Example: I have a fairly heavy .40 size Cub. It's heavy because I painted it and did some scale detailing. The Evolution .46 that it came with flew it fairly well. I then went to a Magnum .52 that just wasn't powerful enough. I stayed at full throttle all the time with that engine.Then I put in a Saito .72, which has less horsepower than the Evolution, but the plane flies better with it due to the bigger prop. So there's a lot more to it than just getting the right amount of power.

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