Why still no dive acceleration difference?

[Crumpp]

P47D which has better dive acceleration.

This is wrong. You are confusing terminal velocity with aceleration.

At Vmax:

FW190

9418lbs * sin 45 = 6660lbs excess thrust

a = F/m

m = 9418lbs/32.2 = 292 lb-s^2/ft

a = 6660lbs/292lb-s^2/ft

a = 22.8 ft/s^2 for the Focke Wulf

P47D22:

13500lbs * sin 45 = 9546lbs

a = F/m

m = 13500lbs/32.2 = 419 lb-s^2/ft

a= 9546lb/419lb-s^2/ft

a = 22.78 ft/s^2 for the P47D22

That is the best case scenario for the P47D22.

If we dive from say, 260 KEAS, then we see 25.35 ft/s^2 from the FW190 and 24.4ft/s^2 from the P47.

The FW190 has a .95ft/s^2 advantage in aceleration rate.

[fruitbat]

anyone know what the Vne limits are for these planes?

[Crumpp]

anyone know what the Vne limits are for these planes?

Yes, they are listed in the Operating Notes....

At low altitude:

FW190 - 466 mph IAS

P47 - 500 mph IAS

[fruitbat]

Originally Posted by Crumpp

Yes, they are listed in the Operating Notes....

At low altitude:

FW190 - 466 mph IAS

P47 - 500 mph IAS

Not everyone has the operating notes for these planes! I knew they would be in them though!!

thanks for the figures, appreciated.

[BlackBerry]

Originally Posted by Crumpp

This is wrong. You are confusing terminal velocity with aceleration.

At Vmax:

FW190

9418lbs * sin 45 = 6660lbs excess thrust

a = F/m

m = 9418lbs/32.2 = 292 lb-s^2/ft

a = 6660lbs/292lb-s^2/ft

a = 22.8 ft/s^2 for the Focke Wulf

P47D22:

13500lbs * sin 45 = 9546lbs

a = F/m

m = 13500lbs/32.2 = 419 lb-s^2/ft

a= 9546lb/419lb-s^2/ft

a = 22.78 ft/s^2 for the P47D22

That is the best case scenario for the P47D22.

If we dive from say, 260 KEAS, then we see 25.35 ft/s^2 from the FW190 and 24.4ft/s^2 from the P47.

The FW190 has a .95ft/s^2 advantage in aceleration rate.

In your opinion, when dive from Vmax at 45 degree(from SL to a deep valley),

excess thrust=weight vector=weight*sin(45)

for any aircraft with mass=m, there is

a=weight*sin(45)/mass=32.2*sin(45)=22.8ft/s^2

for a 10lb plane, a=22.8
for a 100lb plane,a=22.8
for a 1000lb plane, a=22.8

In my opinion, when dive from Vmax at 90 degree(from SL to a deep valley),

excess thrust=weight vector+ engine thrust - drag force

at Vmax:
For full loaded fw190A8:

a = 9.8+ 7155/4272-0.2778*(160.5 ^2)/4272= 9.8+1.67- 1.67=9.8m/s^2= 32.2ft/s^2

For full loaded P47D:

a = 9.8+ 8767/6152-0.3681*(154.3 ^2)/6152= 9.8+1.425- 1.425=9.8m/s^2= 32.2ft/s^2

when speed building up to 720km/h=200m/s.......

For full loaded fw190A8:

a = 9.8+ 7155*(578/720)/4272-0.2778*(200 ^2)/4272= 9.8+1.35- 2.6=8.55m/s^2


For full loaded P47D:

a = 9.8+ 8767*(555/720)/6152-0.3681*(200 ^2)/6152= 9.8+1.10- 2.39=8.51m/s^2


when speed building up to 800km/h=222m/s......


For full loaded fw190A8:

a = 9.8+ 7155*(578/800)/4272-0.2778*(222 ^2)/4272= 9.8+1.21- 3.2=7.81m/s^2


For full loaded P47D:

a = 9.8+ 8767*(555/800)/6152-0.3681*(222 ^2)/6152= 9.8+0.989- 2.95=7.84m/s^2


when speed building up to 850km/h=236m/s......


For full loaded fw190A8:

a = 9.8+ 7155*(578/850)/4272-0.2778*(236 ^2)/4272= 9.8+1.14- 3.62=7.32m/s^2


For full loaded P47D:

a = 9.8+ 8767*(555/850)/6152-0.3681*(236 ^2)/6152= 9.8+0.93- 3.33=7.4m/s^2


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.

However, if fw190A8 propeller efficiency drops from 80% to 50% at 850km/h,

For full loaded fw190A8:

a = 9.8+ (50%/80%)7155*(578/850)/4272-0.2778*(236 ^2)/4272= 9.8+0.71- 3.62=6.89m/s^2 quite smaller than 7.4m/s^2 of P47D.

if fw190A8 propeller efficiency drops from 80% to 50% at 800km/h,

For full loaded fw190A8:

a = 9.8+ 50%/80%*7155*(578/800)/4272-0.2778*(222 ^2)/4272= 9.8+0.76- 3.2=7.36m/s^2 quite smaller than 7.84m/s^2 of P47D.

From fw190A8, wide chord propeller was used in fw190, the new wide propeller outperforms old narrow chord cousin at low -medium speed, but is inferior to old one at high speed(>= vmax).

If we assume that there is 0.5 m/s^2 difference between Fw190A8 and P47D along the 20 seconds dive, in the end of dive, P47D is 10m/s=36km/h faster. In fact, there is quite some air compressibility at 800kmh(500mph) where aircrafts need 200HP+ to overcome the increase of air darg coefficient which is NOT a constant any more. The more air darg coefficent, the more dive acceleration advantage for P47D due to bigger weight.


Conclusion, the il2 FM(without Mach number) is NOT suitable for simulating the high speed dive because of air compressibility which influences the propeller efficiency and air-wing drag coefficient.

[Crumpp]

Duh...

damn Ti89 and my old brain.

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.

Go back and read the flight test results. This gives a good approximation of the performance you will see in the air.

However, if fw190A8 propeller efficiency drops from 80% to 50% at 850km/h,

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.

[BlackBerry]

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.

My estimate is below, please correct me if I'm wrong.

Efficiency: assume 3-blade naca16= 3-blade clark= 3-blade gotingen @0.7Mach

4-Blade naca of P47=88% at 0.4 Mach

3-Blade naca of P47=82% at 0.4 Mach
(See P47 data I'v posted)

3-Blade naca of P47d=63% at 0.7 Mach (out of envelop,so <80%) see data posted

3-Blade naca/gottingen of fw190A5= 50% at 0.7 Mach (3.3 m diameter, bigger advance ratio)

3-Blade naca/gottingen of fw190A8= 40% at 0.7 Mach(wide blade even lose 8% at Vmax)

4-Blade naca of P47=?????% at 0.7 Mach????? It should be bigger than 63% or not? It seems that US has never unclassified the 4-blade NACA16 high Mach number wind tunnel data. Do they want to cover something? But they did make the decision of choosing 4-blade In WWII.

If 70%, there is 30% efficiency advantage over fw190A8, this could explain a lot of 45 degree dive test in history.

[RegRag1977]

Originally Posted by BlackBerry

But they did make the decision of choosing 4-blade In WWII. [/B]

Thanks again for this interesting topic.

I just want to remind that 4-blade propeller fighter aircraft were used before WW2, so perhaps it is not really something as revolutionary as we may think?

Some german design (not produced in great series or prototypes) using 4 blade prop in the 30ies are for instance DO C1 nightfighter (1931) , or Arado 64 experimental (1931).

The oldest fighter design i know using a 4-blade propeller is the german WW1 fighter Siemens Schuckert SSW DIII series (1917-1918 ), that was considered as a very capable fighter at the time, once its cooling problem solved.

[BlackBerry]

Originally Posted by RegRag1977

Thanks again for this interesting topic.

I just want to remind that 4-blade propeller fighter aircraft were used before WW2, so perhaps it is not really something as revolutionary as we may think?

Some german design (not produced in great series or prototypes) using 4 blade prop in the 30ies are for instance DO C1 nightfighter (1931) , or Arado 64 experimental (1931).

The oldest fighter design i know using a 4-blade propeller is the german WW1 fighter Siemens Schuckert SSW DIII series (1917-1918 ), that was considered as a very capable fighter at the time, once its cooling problem solved.

The key is airfoil. IMO, at high speed above Vmax:

3-blade Gottingen=3-blade RAF=3-blade ClarkY=3-blade NACA16=4-blade Gottingen/RAF/ClarkY


However, 4-blade NACA16 is an exception. NACA16 was developed after 1939. There are three disadvantages of German propeller edficiency in late WWII AT HIGH SPEED ABOVE Vmax,ie 0.7Mach.

1)smaller diameter with same rpm as allied, so bigger advance ratio, less efficiency(maybe 10-15%). This is confirmed easily.

2) late period wide chord airfoil gives better performance within Vmax, but worse above Vmax(maybe 10%). This is also comformed.

3)allied 4-blade Naca16 outperforms 3-blade naca16 above Vmax. This has NOT been confirmed yet, but probably. maybe 7-10% efficiency improvement.


The third issue is very important, if it is confirmed, allied may have 30% efficiency advantage over German/Russia when above Vmax. If not, or 4-blade naca16 even worse than 3-blade naca16 at high speed. The allied advantage will not be profound.

30% efficiency advantage is around 500-600 HP in late WWII, hugh difference, key role in high speed dive/flight, vital for P47P51 high speed tactics.

[Crumpp]

I just want to remind that 4-blade propeller fighter aircraft were used before WW2, so perhaps it is not really something as revolutionary as we may think?

Yeah I don't understand the fixation on 4 blades. It is not some magical performance enhancement or brand new technology. For some reason, people like to think that because some of the late war allied fighters went to a 4 bladed propeller that there was something wrong or obselete about a 3 bladed propeller.

You are making a tradeoff between power absorbtion and efficiency. It does not make any difference if you load more power but lose efficiency to convert it to thrust. It also does not make any difference if you increase power absorbtion by adding a fourth blade or widening the chord of the existing three.

WWII really reached the limits of piston engine technology and there is not much to choose or accurately depict in propeller designs. There is a reason n=.85 is valid.