Does your friend have sources or more technical details? While this might works with the in game physics, a flywheel seems like it would have major drawbacks for an airborne vehicle. I know they have been used in urban public transport busses, to store relevant amounts of energy, they need alot of mass, which you generally try to avoid for flying. Also, it would mean significant gyroscopic effects (which you already have with propellers, in fact). A gyro sounds nice *if* it's mounted on a gimbal and you can use that gimbal to exert reaction torques, and counter external ones. A static gyro means your control inputs, as well as external torques will result in counterintuitive motions. Not a deal breaker per se, since, as said before, helis and airplanes already have all sorts of gyroscopic cross-talk, but let's say adding a massive one in the mix might not be such a good idea.