Converting a weedeater for use as a remote controlled airplane engine is actually quite simple, cost effective, and it can give a person a great sense of accomplishment. Unlike model airplane engines, weedeaters are cheap and easy to find and burn a gasoline and oil mix you can get at any gasoline station. There are many different weedeater engines, but the basic idea for the conversion is the same.
Selecting an engine
The best weedeater engines to use in a plane will have the highest power-to-weight ratio. The smaller weedeater engines have a lower power-to-weight ratio. You likely will need a 15cc engine to get the plane up in the air, and still better results from the 25cc and up weedeater engines. Some engines are also easier to strip down because they require less cutting.
To achieve the highest possible power-to-weight ratio, remove all unneccessary parts attached to the engine. Most engines will require some amount of cutting to do this-- some of this will be through metal. Keep in mind that you will need to counter every ounce you leave on the engine by adding weight to the tail. Everything that doesn't mount the engine to the firewall can go.
The engines exhaust will have a big impact on the performance of the engine. Several manufacturers make commercially produced mufflers for model airplanes. However, it is quite simple to fabricate a muffler by drilling a hole in the bottom of a can of styling mousse or cheese whiz and attach it with a few small and inexpensive parts you can find at hardware stores. The muffler quietens your engine while not inhibiting the overall performance. Position the weight of the exhaust to the rear of the plane as much as possible for balance while in flight.
The flywheel on the engine will work and will start the engine when spun at around 500 rpm. However, some wish to remove the flywheel and purchase an electronic ignition system. This will remove the flywheel weight from the nose and move the center of gravity toward the middle of the plane where the electronic ignition will be. The overall weight reduction of installing an electronic ignition is only about a half pound, but it will result in easier and more reliable starting as well as a smoother idle.
The propeller size appropriate for an engine will vary according to the engine's power output. A 15- 16-inch prop will be appropriate for most 25cc engines. Typically, propeller speeds of 7000-8000 at full power will result in the best performance, with higher RPMs resulting in higher speeds, while lower RPMs at maximum power will produce more thrust. Dimensions of propeller blades vary as longer and narrower blades produce higher speeds, and shorter wider blades produce the most thrust.