With proper gearing, you can run a flywheel safely, Step it down do it isn't going like a BOOH.  An 8:1 ratio puts the flywheel at no more than 5K at top speed.  That and since you wont really want to run it WOT, it would be lower than 5K. It also doesn't need to be very heavy either, it just needs to be balanced.  When you think about it a prop isnt very heavy, but the shape of the blade is your resistance, so the flywheel can be something like a ducted fan. Hmm now that would make a good setup. You can use compressed air blown into the discharge to simulate load under varying throttle settings too. <div>
</div><div>Sorry, just throwing ideas out. This thread has got me thinking, which sometimes leads to expensive projects. </div>