Crumpp, thank your comments，I think I've found the answer！ the NACA report of Hamilton standard tells us everything：1350rpm propeller 13ft diameter CSP just like P47's with the exception of 3-blade vs 4-blade.
http://digital.library.unt.edu/ark:/...etadc62146/m1/
There were two airfoil being tested， Clark Y airfoil was before WWII, not laminar. NACA16 was during WWII, NACA16 airfoil is laminar flow profile, and the test shows that there is no advantage of NACA16 airfoil when propeller's tip speed is 1Mach and when "advance ratio" is above 2.0, but there is no more than 3% efficiency benefit from NACA16 when "advanc ratio" is between 1.2 and 2.0.
advance ratio= TAS/(rpm*diameter)
fw190:3 meter propeller, 1400 rpm
p47d: 4 meter propeller, 1400 rpm
You can see Figure 24, when tip speed is 1 Mach, the more advance ratio, the lower efficiency. So we now come to know why Repulic engineer wanted as big propeller as possible because they wanted smaller advance ratio! When fw190 and P47 dive to same high TAS, the P47 has smaller advance ratio and higher efficiency AS LONG AS BOTH PROPELLER'S TIP SPEED IS AROUND 1 MACH. Republic engineers were right: since high TAS diving(efficiency loss)is inevitable for P47, why not prefer low "advance ratio" while accepting the high mach number of 4 meters big propeller's tip?
The complete formular of diving 65 degree is below:
acceleration=g*cos(65)-dragcoefficent*(TAS)^2/weight+Propellerthrust/weight
A simple math question: if you are P4D7's pilot fighting against a fw190G(both arr 250mph TAS @10000ft ), how can you get higher dive acceleration? On the right of formular there are three parts:
1) g*cos(65)
You have nothing to do with it, every a/c shares same value.
2) dragcoefficent*(TAS)^2/weight
Your huge weight is your advantage, and the bigger TAS, the more important role this part plays. So you should build up speed ASAP.
3) Propellerthrust/weight
Unfortunately the third part is your enemy's advantage. Although P47's efficiency is almost same as fw190's, your huge weight is your shortcoming.The NACA report says when tip speed is above 0.9 mach, the drag coefficient of tip increase rapidly, and when tip speed is 1.0 mach and advance ratio is above 2.0, the efficiency of propeller drops sharply. @250 IAS ,p47's advance ratio is about 1.32, not high, but as speed slightly building up, efficiency drops quicker than 0.5-0.8 Mach curve.
http://digital.library.unt.edu/ark:/...dc62146/m1/43/
http://digital.library.unt.edu/ark:/...dc62146/m1/13/
http://digital.library.unt.edu/ark:/...dc62146/m1/11/
You are now 250mph TAS @10000ft, your tip speed is 1 mach, and you will suffer from compressibility loss while your enemy dose NOT. The fw190 has more thrust than you, his weight is less than you, therefore his thrust/weight is much greater than you so that he can overcome your advantage----the second part of formular.
What should you do ? The answer is very simple and as same as the conclusion we've got from 2nd part of formular:
BUILD UP SPEED ASAP.
Drag him down!!! Yes, your tip speed will be always above 1 mach but now fw190's is also around 1 mach, he is now suffering from compressibility just like you, furthermore, his advance ratio(J) will much bigger than you, so his propeller efficiency drops more sharply than you. Now, the third part of formular is NOT enemy's advantage any more. You've succeded in eliminating his advantage and retaining and enlarging yours.
Congratulations from Republic engineers! You now have energy advantage by diving to high TAS, you are extending your distant now, do what you want to do.
The last thing is that if il2 4.11 models the compressibility loss of propeller efficiency . If not, there are 2 probem with its FM.
1) every piston a/c dives faster than it shoud be @ high TAS.
2)For those a/c like P47, the advantage of high TAS diving acceleration has been ignored, so is it's Low TAS diving accelaration shortcoming.
Now we can perfectly explain the fact of 1943 Dec test between fw190G and P47D, and many other comparation such as spitfire vs bf109 initial and final diving difference.
BTW, it's stupid for P47 to dive in a shallow angle with zeke which demonstrates the 6 ton thunderbolt has only a littile advantage(100yards)to the "kite"----Zeke52. I can image those angry faces of the Republic's engineers.
As for P51d, He has similar high TAS diving acceleration with P47, but the reason is not much depending on huge weight, it is very low coefficient of laminar wing. pls look at 2nd part of formular.