Andy Ross > Vivian Meazza wrote: > > Back from a holiday in France. The modified software tests OK > > here. One small snag: the MP is not ambient when the engine is not > > running. Strictly, this should be turning, I suppose, because a > > wind-milling supercharger will produce some pressure. Near enough > > though. > > Yeah, good point. But it's more complicated than running vs. not > running I think. If a normally aspirated engine is turning with the > throttle closed (whether or not it is burning fuel), it will try to > suck a vacuum in the manifold. Superchargers will produce the same > RPM-to-boost curve as they do when the engine is running, because > their impeller is being driven at the same speed.
Already fixed in the patch. > But turbochargers > don't, because their impeller is driven by exhaust pressure; in a > windmilling situation, the exhaust pressure is going to be *less* than > ambient and the turbo will want to turn backwards! (No idea if this > actually happens or not -- is there a relief system to prevent this?) No - a windmilling engine is an air pump pushing air from the inlet manifold to the exhaust manifold, to the extent allowed by the throttle. The near vacuum in the inlet manifold is a symptom, not a cause, and air will not flow in the reverse direction, unless the engine suddenly stops pumping. (perhaps in the event of catastrophic failure). A windmilling turbo engine will provide a small positive pressure to the power turbine, but not to the same extent as a gear-driven supercharger. > Anyway, the bug here is obvious: playing with the throttle on the > ground with the engine shut down cause the manifold pressure gauge to > move up and down, which clearly doesn't happen in reality. But a fix > is going to be subtle and hard, so I'm willing to live with this until > we can come up with a realistic set of requirements. Fix is already in cvs. Are you using the right version???? > While we're listing obvious shortcomings of the current model, note > that it also doesn't handle intercooling, which I'd argue is more > important than getting idle/windmilling manifold pressure correct. :) I've been thinking about intercooler/aftercooler (there is a subtle historical difference) recently. I concluded that you can regard the supercharger (1st stage/intercooler/2nd stage/aftercooler, or any combination) as simply producing an output pressure (or temperature, I think). If you know the full throttle (aka critical) height, you can set the turbo-mul parameter accordingly, and that seems to produce an accurate enough simulation. Trying to simulate the effects of any charge cooling (and I forgot - the fuel is introduced upstream of the supercharger in the Merlin to aid cooling and mixing) supposes a level of detailed knowledge which I don't think we have available. You might come to a different conclusion. Idle manifold pressure is important - otherwise we'll never get the aircraft to land! Anyway - that was fixed by your cvs input. > > I've wasted a couple of days coming to the conclusion that the > > published graphs for the Merlin show the so-called "boost pressure" > > in psi absolute, with zero at 1 atmosphere. > > You mean "gauge", right? Normally "absolute" pressure connotes > interpreting zero as vacuum. Yes. I meant pressure measured against a standard, not against ambient. The current parameter mp-inhg is all that I need (suitably manipulated to turn it into Brit units). No problem: all works well, once I understood what the graphs that I have were using. > > Moving on, the Boost Control/Boost Control Cutout has been written > > in Nasal. This works well, but YASim needs to be told to ignore the > > wastegate. I tried setting an improbable value for the wastegate, > > but of course, YASim uses the wastegate value when solving, so this > > doesn't work. > > I'm a little confused. The boost cutout works by mapping a property > name to the wastegate setting. You can set that property to any value > you like (say, 1000000) during solution. Note that the "improbably > high wastegate" mechanism is actually the way that YASim implements > this internally. For a normally aspirated engine, the _maxMP value is > set to 1.0e06, so I know that this will work. You certainly are! The Boost Control works by adjusting the _throttle_ (in accordance with reality and your earlier suggestion). If the Boost Control is to work, then the wastegate must be non-op. I thought I could do this by using, say, 300 inhg for the wastegate setting, but this doesn't work because YASim uses this value in the solution. So, while YASim needs the accurate wastegate (or more correctly - max MP) for its calculation, it has to skip applying this so that the Boost Control (which includes the Boost Control Cutout) can function. No problem - fix is in the patch. > The supercharger Boolean isn't a bad idea in principle, as it will be > needed eventually to handle the difference between turbocharging and > supercharging. But I'd rather wait until we have an actual > implementation in mind before adding it. I'm pretty sure your current > requirements can be met with existing code. The patch in my previous email, combined with your recent cvs input does all that is required, both now and in preparation for the supercharger/turbo charger thing. If you can suggest a way of doing it using the existing code, then I'm listening. Josh Babcock ought to be asking for the turbo charger for the B29 now, but hasn't yet (perhaps he's now using JSBSim?). I've been unable to find much available on the web for the Wright R-3350. I'm doing some work on the aircraft carrier right now, but I've done some preparation for the turbo simulation. Vivian _______________________________________________ Flightgear-devel mailing list [email protected] http://mail.flightgear.org/mailman/listinfo/flightgear-devel 2f585eeea02e2c79d7b1d8c4963bae2d
