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| FM/DM threads Everything about FM/DM in CoD |
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#1
04-18-2011, 10:18 AM
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Phew...this is heavy reading, but invaluable. Thank you very much.
I and others new to CEM are continually frustrated by CEM in this sim and the board is crying out for a CEM thread. Quick question: I have a constant shaking/shuddering effect in my 109, despite trying different ATA and RPM combinations and also playing around a lot with the 109's variable pitch propeller. The only way I can seem to eliminate the shuddering in the plane is to run it at full ATA constantly, but surely this isn't good. The minute I decrease throttle the shuddering returns, and sometimes it gets so bad it's difficult to concentrate on flying. All my temps are within safe zones, so it's not temprature related. (And yes, my undercarriage is raised.  )
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#2
04-19-2011, 02:27 PM
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Viper2000,
thank you for your knowledgeable input! Quote:
Artist
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#3
04-19-2011, 04:09 PM
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Quote:
The amount of engine power required to do this varies as (C*roh*(v^3)/2)*S This means that you need more and more power per unit prop area in order to keep the velocity triangles the same as you go faster. So you either have too much prop to maintain rpm and efficiency at high speed, or too little prop to absorb all your engine power at low speed. If you climb then you can reduce the air density to compensate for this (though this then causes further matching problems because lift & drag vary as roh*v^2). However, your ability to speed up the prop in order to keep the same velocity triangles is limited by the fact that this will increase your tip Mach number, which will eventually lead to shock losses. If you started out turning the prop at a low tip speed for the low speed case then it will be pretty inefficient because of its low dynamic pressure; if you started out with a high tip speed then the prop is smaller and lighter but you can't get much faster without shock losses. Since infinitely variable transmissions are a real pain (and tend to be quite inefficient, which increases your cooling requirements as well as reducing useful power output), you would probably end up with perhaps two or three fixed gear ratios, and you would then have a constrained set of running lines because if you were in the wrong gear you'd either overspeed the engine or suffer detonation due to excessive boost at low rpm. Changing gear requires a clutch (heavy) and it also kills power to the prop during the gear change. Gear boxes capable of handling high powers are not easy to design, even with modern technology; just look at the problems with certain helicopter gearboxes for example - and they're only single speed (albeit with much larger reduction ratios). Making a multi-speed gearbox capable of handling 1000 bhp+ for 300 hours safely at flight weight just wasn't within the reach of 1940s technology; supercharger gearboxes were hard work, and they handled about an order of magnitude less power. Changing the blade angle of the prop is so much easier, and gives the opportunity to continuously vary rpm and boost independently of one another. It also offers the opportunity to feather the prop for reduced drag in case of engine failure with relatively little additional effort. Finally, it allows you to actually unstall most of the blade at low speed & high power, which gives much better takeoff performance.
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#4
04-19-2011, 06:30 AM
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Quote:
It is analogous to the wing strength tests airliners go through these days. The max strength is predicted by modeling and small tests, then full up tests are done to show that it at minimum meets its spec. Generally it is close, because you want to get the most out of your things, but if you've estimated your design power too high, you'll get a string of bad test failures without getting your engine certified, and freaking out your regulatory body. I suspect this predict, test loop is why in WWII most engines tended to both be rated in multiples of 50hp, and increase in multiples of 50hp. That's not a natural thing. During WWII there were a number of engine that were uprated simply because additional testing showed they could be run at the higher boosts, without any design change at all, beyond upping the boost limit. That is also part of what War Emergency Power was all about. Proving that an engine can provide a continuous power setting is much harder than simply proving you can run it for 5-10 minutes at a setting, so for the demand of war, engine companies would run quick and dirty tests to show that an over-boost setting could be used for a little while, even when they had no idea if it was safe to use continuously, or even when it was known to be unsafe for constant use, while they worked on either proving that it was good for continuous use at that setting, or fixing why it wasn't. This continues in modern unlimited air racing, where teams will often simply unlimit the boost entirely, to get those ludicrous power outputs they do. And sometimes their engines blow up.
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#5
04-19-2011, 01:33 PM
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Quote:
Rolls-Royce were the world experts in breaking their engines, since essentially their entire development strategy was to clear an engine for a given rating, then push it harder and strengthen whatever broke first. They also did the same sort of thing with overhaul life extensions; once more than about 30% of engines reaching maintenance organisations were reaching their full design life they'd go for a life extension; this allowed the life of fighter Merlins to rise from 240 hours in 1939 to over 300 hours (single engine) or 360 hours (twins) in 1944/5. I agree that there would inevitably be some scatter in the life of an engine, especially when we're talking about overloading and running to failure; that's why I said "15 minutes or so". Quote:
Quote:
After this most ratings were at the nearest integer value of boost in psi, which makes sense because it's quite hard to imagine a pilot trying to set boost to the nearest ¼ or ½ psi in combat. Meanwhile, on the other side of the pond, Americans were building Merlins under license and fitting them to Mustangs. The original Merlin Mustang was the Mustang X, built at Hucknall and fitted with the Merlin 65 (basically a Merlin 66 with a 0.42 reduction gear). This was rather an interesting machine; it featured a chin radiator to supplement the Allison Mustang's small belly radiator, and had Spitfire exhausts. North American subsequently came out with the Mustang III/P-51B/C and fitted it with the American equivalent of the Merlin 65, which was the V-1650-3. Since the Americans liked to rate their engines in terms of "Hg absolute manifold pressure, somebody obviously got out their slide rule and did a quick conversion. 1 standard atmosphere = 101325 Pa = 14.696 psi = 29.92 "Hg Therefore, if you convert +18 psi boost into "Hg you get: 29.92*(14.696+18 )/14.696 = 66.57 "Hg. So they rounded up and called it 67 "Hg, and the Americans got about an extra 0.65% power in MS gear: Quote:
Of course, since this is from a Rolls-Royce publication, the American manifold pressures have been converted back to the nearest ¼ psi boost.  [You can't really compare FS gear performance directly because the supercharger gear ratios are too different. The small difference in MS gear ratio is probably just because the Americans used different gear grinding machines. For much the same reason the V-1650-7 has a 0.479 reduction gear ratio vs 0.477 for Spitfire Merlins (almost everything else used 0.42 in order to swing a bigger prop). Anyway, I included the FS gear performance for completeness.] In peacetime, when developing engines for sale to a customer, obviously the power, FTH and SFC are the main parameters of interest. These would usually be backed up by a contractual performance guarantee of some sort. The boost and rpm required to get to the guaranteed performance are both entirely academic; the customer doesn't care as long as he gets the performance. The engine manufacturer predicts what he can make, knocks off a small margin for safety, and then tries to get a contract to build the engine. Once everybody has signed on the dotted lines, the engine manufacturer then needs to develop his product until it meets the guarantee, or else pay out as demanded by the penalty clauses. In wartime, things are different. The objective is simply to improve upon the performance currently available. So development doesn't stop when a target performance level is reached, and therefore the power achieved is academic. What matters is how much boost and rpm the engine can handle, the objective being to roll that performance out to as many engines in service as possible via whatever mods are required, whilst meanwhile attempting to develop future engines to handle more. Pretty much the only reason that the power is even specified in the rating is so that you can put a random sample of engines on the dyno for production or maintenance quality control purposes. Once the engine is in service it's going to be flown according to the boost, rpm and temperature limits; the actual installed power output is unknown. As for the Reno guys, they certainly push to high boost, but I think much of the extra performance probably comes from extra rpm, cropped supercharger rotors and copious ADI. The R.M.17.S.M. was actually tested at +36 psi and 3100 rpm in 1944 for about 2620 bhp; that's about 103" Hg. It was also the only "stock" Merlin to get extra power from more aggressive cams. What the Reno guys seem to do is run at or close to the overspeed limit of 3600 rpm, chuck in vast amounts of ADI and then up the boost until they're on the edge of detonation. Then they can then crop the supercharger rotors until they're riding the edge of detonation when running WOT on a hot day and call the job a good'un (boost varies as rpm^2, so 3600 rpm almost certainly gives them more boost than they can actually use - something like 140" Hg according to a quick back of envelope calculation, though how many of them actually crop their supercharger rotors I don't know; likewise I don't know if they've started playing about with the valve timing). They certainly often fit V-1710 conrods and various other things to strengthen the engine, and put spraybars in front of the cooling system so that they can undersize the intake for reduced cooling drag. I'd estimate that they're chucking out something close to 4000 bhp by the time they've finished. One day I'd like to go out there to see and hear the action before they all run out of spares and/or somebody from the American equivalent of the HSE decides that air racing is too dangerous... *The list of ratings dates from 1948 and therefore shows the final version of R.M.1.S.; R.M.2.S. is a fossilised rating which shows the old 87 octane version of R.M.1.S. but with increased takeoff power before the boost control before +12 psi operations with the boost control cutout mod were cleared.
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