"There is small clue as Crummp said in 2005"

LOL! Seven years ago. No where to run, no where to hide.

German 3-broad chord blade prop plane lost 4% top level speed compared with 3-narrow-blade, according to drag fomular, new broad design lost roughly 8% efficiency around 5.5Mach. Why? Because the bad thing--more drag--become more noticable when speed building up?How about 0.7 Mach? Broad chord design will lose 15% efficiency than narrow chord?

There is evidence that 3-blade NACA16 on P47 gets 63% efficiency at 0.7 Mach, so let's assume that ClarkY and Gottingen(narrow chord) share the same performance with everyelse being equal.

But fw190a8's diameter is quite smaller than P47's, then much bigger advance ratio, guess fw190a8 get 45% efficiency at 0.7 Mach?

Allied 4-blade NACA16 outperforms 3-blade NACA16 at 0.4 Mach, with extra 7-10%, probably at 0.7 Mach, 4-blade NACA16 will get 75% efficiency.

45% vs 75%? This is critical for P51P47Temepest's tactics. I strongly suggest il2 developing team simulate the efficiency of WWII late prop at high speed using software such as ANSYS, otherwise, this game could not perfectly simulate western line where the "battles are not forgotten".

what about windmilling and internal friction /cylinder compression of engine? bmw801 only had 14 cyl. p47 had 18 cyl. so, maybe some inefficiency of 3 blade wide was offset by less internal friction in the 801 and that is why they went with 3 blade wide.:-P

bf109k4 ：3-blade prop
spitfire xiv：4-blade prop even 5-blade

Both are liquid cooling engine.

btw，r2800 and bmw801 share almost same front area.

you miss the point.

What a wide 3 blade may lose in "thrust" vector efficiency from more drag , it may gain in 'weight" vector efficiency when "thrust" vector efficiency is at zero in a dive and high TAS. The wider blade means higher tip speed from ram air verses thinner 3 blade. This means greater torque to overcome internal friction from the engine. The pilot can reduce internal friction by lowering manifold pressure, but not completely. The engine will still tend to over-speed and this is mitigated by lowering rpms/coarsening blade pitch. So, internal friction of the engine is more easily overcome by the weight vector with the wider blade I speculate.

One way to measure the internal friction of the engine. What was the cranking force required to start the 801? What was the cranking force required to start the r-2800? If two engines are identical, it takes more cranking force to start an engine with a 4 blade wider diameter prop attached than a 3 blade smaller diameter. The engines were not identical. The r-2800 had more cylinders than the 801. If you take off the props, I suspect it takes more cranking force to start the r-2800. I suspect the p47 had more internal friction to overcome than the fw190. This would hamper dive acceleration.

Inncrease the area of our lifting surface = more drag

If you know how to do sections, then you know the common expression in propeller blade element theory is Cb for chord length and our local section Drag is expressed as:

~D = 1/2pVb^2CbCD

OK Gents,

I've read this thread with fascination, but the details are WAAAAY over my head. Do you have a conclusion, ie, IL2 is way off or it is close enough for a $40 flight sim that is a decade old?

Cloyd

lol, I have no idea.

maybe add some sound effects when the tip speeds go mach.

maybe add some sound effects to p47 turbine spinning up.

I did notice in up3, rc4, there seems to be turbo lag in the dial indicator verses the engine rpm indicator. so somebody was thinking good thoughts. i think it was a mod p47, maybe p47B. felt a little more agile than the others too.

Read this article and you will know high-speed propeller was a leading edge issue in WWII when people didn't know much about the transonic aerodynamiacs(0.8-1.0 Mach). They even could NOT precisely test the propeller efficiency at high speed in fullscale wind tunnel.

http://aerade.cranfield.ac.uk/ara/19...report-640.pdf

Il2 is a very closed simulation when speed is below 0.8 Mach, but when speed of anything(wing, propeller tip,etc) is beyond 0.8 Mach, not very accurate.

Try to find a WWII fighter with cowl guns and a 4 bladed propeller. I cannot think of a single one.