P-51 MP vs Speed Question and Concerns about Power/Thrust

[USARStarkey]

So a while back Yo-Yo posted a chart showing DCS Final P-51 at 61inches Military Power vs the same ww2 report I posted at 61inches. The chart can be seen here.

 

http://forums.eagle.ru/showpost.php?p=1577405&postcount=7

 

 

To confirm the chart, I went and did some tests, because I feel that the P-51 is under-performing at heights over 20,000ft. I flew as level as is humanly possible. Do to this, I placed a AI aircraft in front of me as a reference, and found that placing the sight on the plane allowed for extremely accurate level flight.

 

The test was conducted at 24,000ft, above which the SC can no longer maintain 67inches of boost. My goal was to determine max speed at FTH at 67inches so I could compare it to Yo-Yo's data for Military power. I presumed that I might be able to reach a realistic top Speed at WEP. What I found was weird. Top speed at FTH was 421mph. Only 1mph faster than the 420mph listed as the top speed for Military power at FTH. (This is far short of the 437-443mph often quoted for the Mustang at 24-26k.) So either the chart Yo-Yo posted is a typo regarding mil power, which I find unlikely since he compared to to the mil power curve from the AAF report, or something is wrong with P-51 power production in High blower. As a side note, I tested Mil power as well, and I was no longer able to keep up with the AI and stop speed fell to mere 413mph- which is 10mph less than the AAF test yo-Yo compared it to. At 67inches, the DCS 51 is 2-3mph slower than the AAF test at MIL power---not WEP. Compared to the WEP rating, it is 22mph slower(at 26k). In DCS FTH is 24,000-24300 or so. AAF reports show between 24,500 and 26,000. Top speeds at these heights at WEP are rated at 437-443mph. Top speed on the posted chart appears to be 431-2mph at 27000 and some change. I will post my track below, and encourage others to reproduce the results. However, please be careful on how you do level flight. I did the test as follows:

 

I started at 365knots, due to the delay in power onset and the need to stabilize, speed increased to 374knots due to a slight dive. I then placed the piper on the AI and flew for a extended period of time waiting to see when the plane stopped decelerating for long period of time. I periodically checked the variometer to ensure that the piper being on the AI was indeed level flight.

 

 

Sidenote: I realize the Dora is in beta etc. But some friends and I did some speed runs between the Dora and 51 and found that between 24,000 and as high as 30,000 the Dora was able to overtake and outpace the P-51. According to every piece of German and American data I have been able to find, at the power settings we have in game the Dora should be quite a bit faster between 14,000 and its FTH at 18,000 and over 23,000 the P-51 should start to gain the advantage. Once again, I ask that other people do the same tests and post tracks showing the results.

speed run24000.trk

[Pilum]

Looking from external view F2 it looks like the radiator is quite open in the track you posted. While this is expected in the climb condition, I would not have expected that in the high speed case. At high speed I would have expected the radiator to be more closed since the high speed ram pressure will ensure a high flow of air through the radiator anyway.

 

 

The ram effect will do two things: one being to raise the pressure of the air in the duct which is good since it promotes more heat transfer due to the higher air density, the other being a raised temperature due to the compression of the air in the duct which of course is not good. However, since the ambient temperature at 24 kft would be in the order of 240 deg K (-33 deg C) there should still be quite good cooling since the ram air temperature would still be low:

 

 

To/T= 1 + (1.4-1)/2*M**2

 

 

To= 240(1 + 0.2*M**2)

 

 

To=240(1+ 0.2*0.63**2)=259 K or -13.7 deg C

 

 

Po/P= (To/T)**3.5=1.31

 

 

Po=1.31*0.37=0.48 bar

 

 

So while air pressure is only 48% of sea level pressure, the effective cooling temperature goes down to -14 deg C at 24 kft 440 mph (Assuming 15 deg C ambient at sea level).

 

 

I did a run a sea level (attached track) and the top speed was quite close to the expected: around 580 Km/h or 360 mph. However, if you look at the external here you will see that the radiator is more closed than at the 24 kft run.

 

 

So while the historical sea level speed of 360 mph seems to be attainable in DCS with radiator in auto, the speed at 24 kft seems a bit lower than historical data would imply and one explanation for the discrepancy could then be that the radiator is in DCS modeled as more open at 24 kft than 0 kft for extended periods at WEP. How realistic this is I don't know and I guess that depends on the thermal modeling.

 

 

However, I think the important thing here is the relative performance: If the P51D is now a bit slower than historical at 24 kft I think that is OK as long as the Fw190D9 is also slower than historical, i.e. that the relative performance is maintained. If this is so then a few percent deviation is quite OK I think. We have to remember that DCS is a model, not reality and as a model it is by far the best yet! :thumbup:

 

 

Attached is my C++ simulation estimate of the relative performance and while the Dora is faster at nearly all altitudes with these power settings, the Pony should be in the order of 10 mph faster at 24 kft.

speed run 0 kft ver 1.trk

Edited September 20, 2014 by Pilum