The first thing we must understand is that engines in nitro RC helicopters have different operating requirements than in RC planes or even cars and boats for that matter.
Generally with planes, cars, and boats the engine is set as lean as possible to get the most power out of the engine with out overheating it of course. This is not the case with RC helicopters.
You basically want to set your engine in your RC nitro helicopter to run rich (more fuel than is required). You will notice this if you have ever seen a nitro helicopter fly, or have seen pictures or videos of RC helis. They produce a lot of blue/white smoke – this is because they are all running rich and sometimes use higher oil to fuel mix ratios.
There are two reasons for this:
1. To keep the engine running cooler so it won’t overheat and quit.
2. To make sure the engine is always getting ample fuel so it won’t quit.
You will notice the end result of both these points is to ensure the engine stays running. Keeping an engine running in an RC helicopter is obviously more important than milking ever ounce of power out of it. A flame out while you are 100 feet in the air when you haven’t learned how to do an auto rotation is not a good thing.
The next operational difference between RC helicopters and planes, cars, boats, is at what throttle setting nitro RC helicopters run at. Throughout a flight, helicopters are generally running mostly in the mid to mid high throttle range.
This places different requirements on the carburetor and is why all good nitro helicopter engines come with heli specific carburetors. They are designed to give the most reliable performance in the mid throttle band and ensure the engine is neither over fueled or under fueled while going through different transitional phases of flight. This is discussed in greater detail in the nitro tuning tips section.
As you can see from the above points, nitro RC helicopters rarely get 100% power out of the nitro engine because we want to keep that little engine running at all costs, especially when you are learning. As you move up to higher performance flying, you will probably lean out the mixture setting a bit more to keep your rotor rpm high, but nitro helis are always run on the rich side.
Which brings us to the next point of engine sizing in our nitro RC helicopter? If we are purposely under powering the engine to ensure is stays running, we must get that power back by putting in a slightly larger engine than what is called for.
Fortunately most nitro engine manufactures have recognized this and supply sized up engines. The RC helicopter manufacturers also recognize this and will specify what size engine to use. For example if you have a 40 sized helicopter kit, you will put in a popular sized heli nitro engine like a 46 or 50. Same goes for 30, 50, 60, and 90 sizes. Nitro engines are always sized a bit larger. This will assure you always have plenty of power reserves.