Andy Ross wrote: > Vivian Meazza wrote: > > Andy Ross wrote: > > >* For one, I still hate the boost function that goes negative at high > > > RPM > > > > I have revised the curve: now a Hoerl power function. It's a good fit > > over the rpm range up to x4 peak power rpm (unnecessary: x3 is too > > much imho) and tails off thereafter reflecting less output as more of > > the compressor stalls. The output remains positive for ALL values of > > rpm, and won't break under any circumstances. > > OK, check. This doesn't look bad at all. It's asymptote is zero, > which still sounds wrong to me*, but as you point out it doesn't reach > that regime until well beyond a meaningful operating range.
The graphs I have seen really don't explore the condition much above the normal operating regime. A stalled compressor won't pump much though. > * My wife, in graduate school, did work with very high vacuum gas > phase chemistry. The first line of pumps attached to the vacuum > chamber were "turbo pumps" which, as I understand it, were > essentially very high speed fans designed to whack any stray > molecules out of the chamber kinetically. This corresponds loosely > to a compressor operating in a very high supersonic regime, where > inter-molecule reactions are irrelevant -- there will still be some > "boost" just due to the kinetic operation of the compressor > rotation. Dunno, just intuition obviously. Hmmm. Pressure of a few molecules being smacked about - call it zero. But when it's the last few in a vacuum chamber worth doing. > > >+ Explain better why you want the new CUTOUT control and didn't just > > > make the wastegate setting modifiable at runtime (which simplifies > > > the engine model and seems more general, IMHO). > > > > The Merlin (Hurricane, Spitfire and P51d) had a Boost Control which > > acted on the throttle to control the boost pressure: I briefly > > considered modelling that, but it is adequately modelled by the > > wastegate in YASim. The CUTOUT control seems to me to be simple to > > implement, reflects the way it worked in reality, and is applicable > > to several models. > > OK, so the boost control was like a wastegate then: A spring-loaded > pressure release value that opened up to keep the manifold pressure > clamped to some fixed value? And it was analog-adjustible? It's actually nothing like a wastegate. Strictly speaking a wastegate stands between the exhaust valves and the turbo to adjust the inlet pressure to the power turbine by allowing more or less of the exhaust gas to bypass the power turbine. It may be controlled by a spring or in more sophisticated applications by sensors on the inlet side. The effect is to control manifold air pressure (MAP). What YASim models should more properly be called a MAP controller. > The reason I ask is because the Mustang POH I have doesn't (I don't > think) mention a control like this. What it did have is a lever to > select which of the three supercharger states were active: no > supercharging, first stage blower, or second stage blower. Literally, > it was just linked to valves that opened up around each of the two > gear-driven compressors. No analog control whatsoever -- just a > warning not to engage each blower below/above certain altitudes and > power settings . I'm confused - I'd like to see the actual words. All the references that I've seen say that the Merlin came with 3 basic varieties of supercharger: a. Single stage, one speed. E.g. Hurricane I, Spitfire II b. Single stage, 2 speed. E.g. Hurricane II, Spitfire III c Two stage, two speed. E.g. Spitfire IX, Mustang P51 There was only one blower - the 2 stage had 2 impellers on a single shaft. The output of the first stage fed to the input of the second stage via an intercooler and thence to the engine via an aftercooler. Where there were 2 speeds, known, for some reason obscured by time, as 'moderate' and 'full', these were selected by an electro-hydraulic clutch mechanism working in conjunction with a planetary gear train. I know of no reference to an unboosted or neutral position for this mechanism. I thought that the gear change was manual, but I've just written some nasal code for Jim Wilson which changed gear according to height, so perhaps not. (I haven't got a reference to that either - Jim?) > And it wouldn't have behaved like a turbo waste gate. The boost for a > given stage is more or less solely a function of RPM and doesn't have > the "clamping" behavior that the wastegate setting would show. The Boost Control clamped the supercharger output to the preset value (higher as time went by and superchargers improved and higher octane fuel became available*) by retarding the throttle as the limit was approached. It could only be adjusted by mechanics on the ground. * 6 psi to 9 psi at the start of the war. The introduction of 100-octane fuel allowed a rise to 18 lb, allowing considerable increases in power output. The introduction of 150-octane fuel in 1944 allowed further increases to 25 lb boost. > My guess is that the "cutout" switch in the UK planes is just a > different user interface for the same underlying ductwork, and that > using a new control axis is not needed. Instead, how about using the > existing BOOST input and setting its value to (for example) 0.0, 0.5 > and 1.0 based on the position of the cutout lever and the blower stage > lever (or whatever it's called in the Hurricane). No it didn't - the Boost Control Cutout did what it said it did - cut out (overrode) the Boost Control to allow the full output of the supercharger to be developed. In some aircraft that was done by a separate lever, in others it was a gate on the throttle. Usually it was secured by a seal so that the ground crew could see that it had been operated. Excessive use severely shortened the life of a Merlin. Here's a good reference for all the foregoing: http://www.unlimitedexcitement.com/Pride%20of%20Pay%20n%20Pak/Rolls-Royce%20 Merlin%20V-1650%20Engine.htm > I'm willing to be wrong on this; I certainly don't claim to be an > expert on engines in general or the Merlin specifically. > > But even so, here's a software engineering justification for ejecting > the CUTOUT control. The existing code tries really hard to be > general, and useful for many aircraft. Adding a new control axis > corresponding to every control idiom in every aircraft we want to > model is going to get out of hand really quickly. Regardless of which > one of us is correct above, the CUTOUT axis can already be implemented > by using the existing engine code unchanged: the BOOST control is > already there, and all that would be needed for the wastegate hack > would be to make it configurable (i.e., the engine code doesn't > change). It would be possible to simulate the Boost Control Cutout by adjusting the wastegate on the fly to a very high number effectively disabling it (I take that it is possible to do that). It's a hack, I don't like it, but ... > This is preferable, IMHO, to mucking with the internals of the engine > model. I know your change was a one-liner bit over override logic, > but the problem is that the engine code already has a bunch of stuff > like that, and it's already getting hard to follow. Your change might > look simple right now, but add three more one-liners and it won't be > quite so obvious which one has precedence and subtle bugs will get > introduced; exactly this kind of thing happened with the gear ratio > code. It literally took almost a year for that (conceptually simple!) > change to stabilize and work its bugs out. Yeah, I remember that struggle ... > The cleanest solution at this point, IMHO, would to split out the > x-charger implementation and do it twice: once for gear-driven > superchargers and again for exhaust-driven turbochargers. Hmmm, the only difference is in the shape of the curve: it's only a couple or so lines of code: if supercharger curve A, else curve B I have a curve B in mind, but it's not tested. I have no live data against which to test. Josh might have something for the B29? The rest stands, I think, subject to a bit more analysis. I'm not thinking of attempting turbo-lag: it's only really significant in automobile engineering. Regards, Vivian _______________________________________________ Flightgear-devel mailing list Flightgear-devel@flightgear.org http://mail.flightgear.org/mailman/listinfo/flightgear-devel 2f585eeea02e2c79d7b1d8c4963bae2d
