Put an r/c car flywheel on your airplane engine and watch it spin up to 30000 rpm!

Car engines do not stay at full power all the time. They are revved up and down continuously. A car engine also wears out very quickly, due to the high rpm and other stresses. Trying to run even a car engine at peak rpm for an extended period of time, such as an airplane flight, would most likely wear it out or destroy it in short order.

As a side note, in the early days of r/c cars they were powered by airplane engines that had extra clamp-on cooling fins. They also revved much higher than when used with a propeller in airplanes.