I can't address the prop issues at higher altitudes, nor the turbocharging issues, nor the cooling issues.

However, purely from an IAS/TAS standpoint, 250 mph TAS at 15K ft at the temps generally seen around here is about 190 mph IAS. So _IF_ it can maintain 190 mph IAS all the way up to 50K ft., then TAS will be about 500 mph. Apparently they're derating things a bit.

The constant IAS calculation would be where the story becomes murky. Yes, constant IAS means 490-500 mph, but that is exactly why my view oscillates between confusion and skepticism. The thrust available decreases with TAS. The power required for a given IAS rises with altitude (proportional to 1/sqrt of density differences). This is why the IAS drops for a turbocharged prop driven airplane at constant BHP...resulting in a large deviation from the "constant IAS" calculation. To maintain constant IAS, BHP has to increase markedly.

For a practical example, look at a POH for the P210 or PA-31, and track the real TAS capability. Calculating the excess HP available at 2000 msl vs 20000 msl at constant BHP tracks the basic relationships very well and is actually less than the calculated capabilities probably due to prop efficiency loss or perhaps even pressurization demands.

The major prop manufacturers each acknowledge the deleterious effects of density altitude on propeller propulsive efficiency. And more than one turbo manufacturer acknowledges the issues associated with pressure losses in a compression ignition engine at high altitude. It is a fundamental risk.

All of these are large, big ticket problems to solve.

I don't get anything out of scribbling away to prove bumblebees can't fly, and it's a waste of time to do so. I am instead genuinely curious how these calculations could work out to be so different from what a team of engineers that are working on this every day seem to think. It would be nice to know what is behind that gap, that's all.

In the past, the answer has come in the form of bad faith and investment scheming. In this case, I'd like to keep an open mind and learn something newish instead.