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#11
04-30-2011, 08:24 AM
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#12
04-30-2011, 12:01 PM
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Here, once again is the Mk IIa Spitfire CoD speeds compared to real Spitfire speeds.  As can be seen the Mk IIa speeds are spot on when compared to a +12 lbs boost Spitfire Mk Ia. The speeds are around 20-25mph faster than the 9 Lbs boost Mk II a speeds which would put them about right for 12 lbs boost. Also at the full throttle height of around 18,000 ft the boost there should be no speed difference between 9 lbs or 12 lbs boost. The Mk IIa in CoD (at least in speed for altitude) is definately a 12 lbs boost version. The guage in CoD is broken.
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#15
04-30-2011, 11:09 PM
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#16
04-30-2011, 11:19 PM
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More boost means more power. FTH for some boost pressure P is the height at which this boost is delivered with a wide open throttle. As you climb with the throttle wide open, whatever height you're at is the FTH for whatever boost you've got. What you mean is that above the FTH for +12 psi boost, being allowed to use +12 is academic, because the supercharger can't deliver it. Quote:
The Spitfire II has a Merlin XII; supercharger gear ratio 9.089 The Merlin III at +12 psi delivers 1310 bhp at 9000'. The Merlin XII at +12 delivers 1280 bhp at 10500'. AFAIK both figures exclude ram. In any case, the two aeroplanes are not identical. The last time I tested the Spitfire II, I got the distinct impression that it was faster than it should be. I think that operating the boost cutout really does give about +8 boost; if you look at the instruments in the no-cockpit view then you'll see that they read up to +12, but only show +8. I think that we actually have the R.M.2.S. rating; +8¼ psi boost takeoff power, which gives 1000 bhp at 3000 rpm at sea level, but with +12 power levels. This means that we have too much power at altitude, and the brief testing I have conduced at 18000' gave me a top speed in the region of 380 mph... The alternative explanation would be reduced airframe drag. Anyway, R.M.2.S. was an interim rating for the Merlin III prior to the introduction of the +12 psi combat rating; the Merlin XII was a somewhat different animal designed for +12 boost from the start and would deliver about 1165 bhp at sea level and +12 psi, and takeoff power of 1175 bhp at +12½ psi boost. It seems to me that more testing is required, both with CEM on and off, especially given that the sim has been quite extensively patched in the last 20 days... It is also important to remember that whatever results we get from the sim need to be corrected to standard conditions before they can be directly compared with historical test reports.
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#17
05-01-2011, 11:26 AM
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http://forum.1cpublishing.eu/showpos...8&postcount=12 Quote:
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#18
05-01-2011, 04:05 PM
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More boost does not mean more power : 0+0x12 = 0
 Temp might be the limiting factor to consider (tht's where MW50 (with water) was so important for German eng with their poor quality materials). Early war Merlins shld be weaker than their late counterparts. Hence drawing a comparisons with later Merlins even at the same boost level is risky ~S! Last edited by TomcatViP; 05-01-2011 at 04:15 PM.
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#19
05-01-2011, 09:17 PM
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Historically a lot of these limits were not strictly set because of blowing a motor, or overheating ie. within 1 or 5 mins. Most of the time engine life was also a serious consideration factored into these time limits. So, if you could track a CoD pilots time in the air, in all his flights in that plane type, over how ever long he survives in that plane, then you could really trigger a 'realistic' blown engine from the 'consistent' over use of WEP. IMHO the direct link to heating would be a good/simple way to enforce the time limits short of anything better, but it's not strictly realistic..... On a side note the below may be of interest on this topic of over boost: Allisons in P40s were very resilient to over use and abuse of over boost, RAAF pilots did it routinely and they did not seem to suffer from this practice. See the below document: http://www.raafwarbirds.org.au/targe...39%20abuse.pdf Those interested in P40 Data may be interested in my online collection here. http://www.raafwarbirds.org.au/targe...40_archive.htm Last edited by Peril; 05-01-2011 at 09:33 PM.
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#20
05-01-2011, 09:21 PM
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A Merlin III with the R.M.2.S. rating could achieve +6¼ psi boost at 15500', which would deliver about 1000 bhp. A Merlin XII with an improved supercharger intake elbow and AVT40 carburettor could achieve +9 psi boost at 15750', which would deliver 1150 bhp. More boost = more power. Alternatively, compare the takeoff power of Merlin III engines at different ratings. The R.M.1.S. rating was +6¼ psi boost, 3000 rpm, giving 880 bhp at sea level. The R.M.2.S. rating was +8¼ psi boost, 3000 rpm, giving 1000 bhp at sea level. The engines were physically identical, with the extra power allowed by the use of 100 octane fuel. More boost = more power. Indeed, you can see that an extra +2 psi is worth about 120 bhp, whilst an extra +2.75 psi is worth about 150 bhp. To a first order approximation, you can see that running a first generation Merlin at +21 psi absolute gives about 1000 bhp unthrottled if the supercharger gear ratio is about 8.5. So in round numbers, that would be roughly 50 bhp for every extra psi boost. Now, this is very rough and ready stuff, but it's quite a good first order guess; a Merlin 66 on 150 grade fuel gives a little over 2000 bhp in MS gear unthrottled at +25 psi boost. 1000*(25+15)/20 ~ 2000. Basically what we're saying here is that the amount of power produced by the engine is proportional to its air consumption, which is limited by the physical size of its intake & exhaust valves. Supercharging increases the air consumption by increasing the charge density in the intake manifold. P*V = roh*R*T So actually the error associated with drawing a comparison between early and late Merlins is smaller than you might perhaps expect. Thermodynamically, the piston engine at the heart of the Merlin doesn't change much after the ramp head combustion chamber was discarded c.1938. The vast majority of its power development came from improvements to the supercharger. Valve timing was only changed for the prototype R.M.17.S.M engine. Mechanically there were considerable changes devoted to improving life at high power, but they didn't greatly impact upon the thermodynamics of the machine. Indeed, although the Griffon is mechanically very different from the Merlin, if you take the Merlin model developed by Hooker et al and plug in Griffon numbers then you'll find that the agreement is impressive, because thermodynamically they're extremely similar machines. /// ICDP, My high speed testing of the Spitfire II was indeed conducted with CEM disabled. I was actually trying to tackle the prop pitch change controversy at the time, rather than to investigate the performance of the aeroplane itself. But I was struck by just how much faster the Spitfire II was than the Spitfire I, and I suspect that what's happened is that 1c have done the same thing with the early Merlin that they did with the later Merlin when the Mustang III was introduced to IL2, namely increased the boost without changing the FTH appropriately. However, it's very difficult to be sure at this stage given the various bugs and our lack of knowledge of the atmosphere model etc.
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