Duh...
damn Ti89 and my old brain.
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Conclusion, P47D has slightly better dive acceleration when reaching 750-850km/h if both propeller efficiency=80%. I think this is the il2 FM method, if you test both in il2 4.11m, You'll find slightly dive acceleration difference.
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Go back and read the flight test results. This gives a good approximation of the performance you will see in the air.
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However, if fw190A8 propeller efficiency drops from 80% to 50% at 850km/h,
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And the P47's CSP is going to drop too, blackberry. I like the idea you present with modeling efficiency. Problem is you are making the same mistake the NACA did in adopting the 16 series. The calculations just did not bear out and give good agreement with the air.
Just a thought, a generic modification of efficiency based on some basics of propeller design might enhance things. Problem is we don't have enough information on the propellers to implement any kind of detail.