All engines have intake ports that are for a lack of a better term, " Positioned " so that the engine makes it's peak power at a certain RPM. With a helicopter engine it is rather high. If we take a look at the math it may be easier to understand. For aerobatics, helicopters run a rotor RPM of 2,000 to 2,200. Most helicopters are geared around 9:1. so if you multiply 2,200 by 9 you get 19,800. This is the engines RPM while doing extreem 3D. So you can see how the engine is designed for high RPM use. When put on an airplane the engine must run a prop that allows it to operate at the RPM where it develops it's peak power. In most cases this prop would be too small to overcome the drag of a typical sport airplane.