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#1
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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#2
05-01-2011, 09:21 PM
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Quote:
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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#3
05-02-2011, 05:39 PM
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Hi Vip,
My quick formula sketched above was to show by the absurd that you can't boost anytime anywhere anyhow. In my example if P=0 (outside air pressure) there is no compression at all. Secondly the compressor is actioned by the engine and such take a non-negligible power out of the engine for the compression of the intake air. The higher you climb, the more the air need to be compressed but as the air is less dense the at a cte fuel/air mixture, the less fuel would be injected lowering the subsequent power available for the mechanical compression of the air. This is why as you know Compressor are rated for a given alt. Now let's get back to the higher octane fuel. Let says that the M engine (this is an example) give 8lb at 15000. As the engine need to be fed by more air at any given altitude to outsource the power of the higher graded fuel, at 15000 the compressor would hve run earlier (e-g at a lower alt) it's full potential. Hence you max boost would hve been at 3000ft lower hence the max power wld hve been available at a denser air hence your max speed hve some chance to be less spectacular than with a slide rule (the ^3 effect of power/drag as you hve rclled us earlier - where drag means everything and not only external body drag) SO it's more balanced that saying that if I hve got 100hp more out of an engine powering a giving airframe I would hve a top speed increased by 100/initial power * top speed of airframe Ok Ok most of this you hev taken it into account. But what I mean is that only by changing the fuel, the SPit won't hve automatically a better top speed. Many thing hve to be adapted like the carb inlet (what you hev mentioned) or the pulley, the size of the compressor wheel (the volumetric ratio) etc... It is doubtfoul that BoB Spitfire where converted to the final standard due to historical events. Yes the 100 wld hve had a better accel and this is confirmed by the combat report man can read (I am thinking abt that top leading ace that was shot down racing deck with spit in in 6 sure enough that he was safe in his faster 109 - need to dig out his name) Material ? to put it short : the amount of E of a system [du] equate somewhat the amount of caloric E (Q in joules) and the amount of work [dW] (1st thermodynamic low -  damn always forgot the right nbrs )If you put in more internal U you also got more Q to get a slight amount of extra W (as it's easier for the E to be drawn out as extra Q than to be converted in more W). And don't forget that the more Q you had in any piping (I mean a closed syst) the more T is raising hence the more the viscous effect you hve to deal with (air then drag (see above)) Take for example the compound series of eng from the Germans with for example two 601 mated together on the same shaft . Did the output power was increased by a factor of two... no ! This was a silly example as well but it was meant to show by the extreme my purpose. Here is what I mean : higher grade means higher strain on the eng (and I mean direct mechanical works too) with the consequence of a lower eng life. Yes engineer knew in 1940 how to pull out 2600Hp out of a merlin ... but for how long ? Would it fit any fighters pilot expectation ? To drive  a comparaison with cars : I can pull 1000HP out of a Nissan Primera but would I out speed a Porshe ? (Plsd post it on Youtube )Sry this is a rather long post ~S Last edited by TomcatViP; 05-02-2011 at 05:44 PM.
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#4
05-02-2011, 06:31 PM
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More boost = more power
You might not be able to get more boost (i.e. because you're already running WOT at max rpm). But if you could get more boost then it would result in more power. If you want to get technical then the chain was something like IHP>SHP>BHP IHP would just be a function of boost & rpm SHP is the actual mechanical power produced by the piston engine itself BHP is the power available to do useful work after the power required to drive the supercharger has been subtracted. As a general rule, if you improve the fuel available and use this improvement to increase the maximum boost of an engine, the maximum TAS that the aeroplane can achieve doesn't change much. What happens is that the minimum altitude at which it can attain that maximum TAS decreases to the new FTH associated with the increased maximum boost. So you basically drop a vertical line on the TAS/altitude diagram from the old FTH to the new FTH. This approach makes all manner of implicit assumptions and is therefore somewhat quick & dirty. But it's generally quite close to the truth for WWII fighters. Obviously, the IAS at the new FTH will be considerably higher. The extra power will also dramatically improve acceleration and rate of climb at or below the new FTH. The increased ROC was one of the main attractions of 100 octane fuel for fighters during the Battle because it reduced the amount of warning time required to make a successful interception. If I put a bigger supercharger onto the engine but maintain constant boost, then SHP stays basically the same (it probably goes down a little due to increased compressor delivery temperature, but piston engines aren't generally explicitly T4 limited like gas turbines, so this is a second order effect). FTH obviously goes up. BHP at FTH goes down, because of the increased power required to drive the supercharger. However, neglecting transonic dragrise and assuming reasonable compressor efficiency, the max TAS will always go up, because the increased FTH means that the density of the air that the aeroplane is trying to move out of its way has decreased. power required = 0.5*roh*v^3*Cd So long as roh decreases faster than BHP, you're winning. Eventually, if you take the process to the extreme, you find that at some very high FTH, the BHP available is only just sufficient to sustain the aircraft in flight at its maximum endurance speed; at this point you need to start looking to the airframer for further improvements. Additionally, low altitude performance will be extremely marginal due to throttling losses, but this may be improved simply adopting a multi-speed supercharger drive system of some sort (or possibly by using VIGVs in the supercharger if that's your bag). You also find that simply bolting a bigger supercharger onto the engine stops working well once the pressure ratio required exceeds 3-4ish for a single centrifugal stage due to the fact that the tipspeed required becomes mechanically challenging. It's therefore expedient to use multiple stages.
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#5
05-02-2011, 05:07 AM
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#6
05-26-2011, 04:45 PM
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The C3 optimized eng was a real relief on that point. Regarding the quality of the materials used in german engines, I would only point out the respective wet weight of Allison, DB605 and Jumo engines. Pls remind that late vers of all those 3 had nearly the same output power. It remind me the engine mount designed for the Allison powered D9 flying today (warbird). The goal of the engineer was to add weight to get back the right balance (usually you work with a contrary objective). ~S! Note : sry to dig out tht one. :-p Last edited by TomcatViP; 05-26-2011 at 04:59 PM.
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