If iron, or steel, or aluminum were actually solid....heavy flywheels might not require more power to spin.
But they are not solid, and every bit of mass in them is trying to go in a straight line at whatever velocity it is at along its vector. It is being pulled into an arc by the atoms ahead of it who were trying to go straight....and so on. Even if it was an actual solid, it would still create additional bearing surface drag when operated in a gravitational field.
On top of that, the flywheel is not moving at a constant speed during rotation at steady RPM. It accelerates and decelerates multiple times per revolution. If you dont believe that....OBD II misfire detection wouldnt have been working on all our cars for the last 21 years.
I have seen people claim that the deceleration of the crankshaft (during compression) offsets the acceleration of the crankshaft during the power stroke. BS. If that was true, the engine would not run. Heavy flywheels are a diminishing return formula where you trade some power for the smoothing effect in the driveline components.
I dont know of any professional level racing choosing big heavy flywheels, over light and small. During dyno cell testing (water brake engine dyno), I watched one of my V6 engines pick up over 15 hp when we cut the flywheel from 26 lbs to 13 lbs. These guys built and tuned Indy car engines, off road engines, drag engines, and NASCAR engines.
I dont think they steered me wrong. They also taught me to run the flywheel as small diameter as I possibly can, regardless of what weight I have to use.
Which is exactly the same thing manufacturers do for racing programs. Our turbo roadster, 20 years ago, used the small diameter flywheel/clutch/pressure plate design from the championship winning WRC cars.
Either none of the top engineers, race teams, Formula 1 designers etc know what they are doing.....or,
what??
Sorry...getting cranky in my old age, but there are too many good reasons to run LIGHT flywheels for any kind of racing that needs pure, continous, peak horsepower.