Why still no dive acceleration difference?

[BlackBerry]

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.

[MadBlaster]

about this part of the equation, Propellerthrust/weight ->

Simply lower the rpms to bring the p47 tips speeds down to optimal range .7-.9? Won't that give you better acceleration? It seems to work that way in game. Starting at 250 mph, full throttle, and 100% pp, nose trim 2 notches down, rads closed. if I nose it down into dive and crank down the prop pitch to 0% quickly and then bring it back up to ~77% and gradually lower throttle to about 77% in a dive from 250 mph, the planes gets to ~ 400 mph ias very quickly.

[BlackBerry]

Originally Posted by MadBlaster

about this part of the equation, Propellerthrust/weight ->

Simply lower the rpms to bring the p47 tips speeds down to optimal range .7-.9? Won't that give you better acceleration? It seems to work that way in game. Starting at 250 mph, full throttle, and 100% pp, nose trim 2 notches down, rads closed. if I nose it down into dive and crank down the prop pitch to 0% quickly and then bring it back up to ~77% and gradually lower throttle to about 77% in a dive from 250 mph, the planes gets to ~ 400 mph ias very quickly.

Yes, that works. But even you can get same propeller efficiency as 190, your weight is too great to overcome, Propellerthrust/weight is still inferior to 190's.

We need to know if 190's airscrew tip compressibility loss is modelled or not at high mach number where 190's huge "advance ratio" making efficiency even worse than P47's.

[MadBlaster]

agree, but better to do it than not if your p47 guy. for 190 guy, you have revealed something that I think goes ignored. sometimes it is good to over ride the vdm auto prop pitch control and go to manual mode for same reason, to keep tip speeds at optimal ranges.

[mayshine]

Thanks to BlackBerry and everyone,

all your threads have done greatly to help me to understand more.

and may be there is a chance to involve the Prop calculation in IL2

to knock down Cliff of Dover and be the most perfect all the way?

Sorry for offensive language to someone l

I am tricky and want this topic to enjoy a long life and drag more men
to research in their full strenth,

So that I deliberately use somewhat offensive language.

sorry again JTD or so

[BlackBerry]

Originally Posted by MadBlaster

agree, but better to do it than not if your p47 guy. for 190 guy, you have revealed something that I think goes ignored. sometimes it is good to over ride the vdm auto prop pitch control and go to manual mode for same reason, to keep tip speeds at optimal ranges.

That dosn't help much, I'll show you how terribly drop of propeller efficiency for SMALLER propeller.

Again, NACA Figure24, Both fw190's 3 meter prop. and p47's 4-meter prop @19500ft , 1350rpm for propeller CSP,

1)A point=126m/s TAS=453km/h TAS=323km/h IAS

P47's advance ratio=126/(4*22.5)=1.4

fw190's advance ratio=126/(3*22.5)=1.86

2)B point=180m/s TAS=648km/h TAS=462km/h IAS

P47's advance ratio=2.0
fw190's advance ratio=2.66

3)C point=216m/s TAS=777km/h TAS=555km/h IAS=345m.p.h. IAS

P47's advance ratio=2.4

fw190's advance ratio=3.2

dive.JPG

B piont--->When both p47 and fw190 dive to 20000ft/462km/h IAS=287m.p.h. IAS, p47's efficiency is 70%, almost twice of fw190's 38%!

C point---->When dive to 20000ft/555km/h IAS=345 mph IAS, fw190 almost lost its propellerthrust while p47's remaining 55%.

So when speed building up @high altitude, the third part of acceleration formular will also be p47's advantage. Thus we could image how "Thunderbolt" really is!

P47's exhaust turbine boost leads to the lost of exhaust boost for propeller, but that can't help fw190 much.

Finally, we can completely understand why Republic's engineers want the biggest propeller as possible.

[MadBlaster]

interesting. I'm suggesting that at point A with a canister of mw50 on board that the 190 guy trade high rpms and prop efficiency while TAS is low, for thrust 'instantaneously' (limited by rate of change of the blade angle) by over-riding auto prop pitch (slower response). The prop would not be spinning at 1350 rpm. It has a load on now, so is spinning at say the lower end of the power curve for the bmw engine, guesstimate, 1050 rpm. Point C for 190 is irrelevant because dive speed limitations are upon him. The race is for point B. How long does it take for p47 turbo to spool up??? How long does it take to get power from mw50???

[BlackBerry]

Originally Posted by MadBlaster

interesting. I'm suggesting that at point A with a canister of mw50 on board that the 190 guy trade high rpms and prop efficiency while TAS is low, for thrust 'instantaneously' (limited by rate of change of the blade angle) by over-riding auto prop pitch (slower response). The prop would not be spinning at 1350 rpm. It has a load on now, so is spinning at say the lower end of the power curve for the bmw engine, guesstimate, 1050 rpm. Point C for 190 is irrelevant because dive speed limitations are upon him. The race is for point B. How long does it take for p47 turbo to spool up??? How long does it take to get power from mw50???

1) German tried MW50 on BMW801 but finally gave up. It's said that MW50 is harmful to BMW801's piston. 1945 version Dora's Jumo213A(liquid cooled) was equipped with MW50. However, 20000ft(6000m) is probably above 190's FTH, so there is little benifit from WEP.

At "A" point, if fw190 uses 1050rpm propeller, tip rotating speed is 165m/s, TAS is 126m/s, Mach number=207/310=0.67. Quite below 0.9mach. But I'd to say the propeller's has a certain optimised rpm, if you use full 100% throttle while reducing propeller's rpm incorrectly, you will lose efficiency. Your propeller probably couldn't absorb engine's output.

2) C point is above fw190's max allowable diving limit?

3)I don't know how long does it take for p47 turbo to spool up. I guess it's several seconds?

4)Last but not least, Do you have the information of Dora's 41276.16 V propeller? The size of it, the reduction ratio etc. As we all known before 4.11m, Dora flys so fast.