Andy Ross wrote: > Vivian Meazza wrote: > > The attached diff models the output of a gear-driven > > supercharger > > I just now got a chance to sit down and puzzle this out. I see where > it's going: instead of ignoring the RPM contribution to boost, it adds > an extra factor that reduces the boost at lower RPMs. It works by > normalizing the "rpm" value to the range [0:1] (expressing it as a > fraction of the engine's solution RPM) and then applies this as an > extra factor on top of the turbo-mul value from the configuration > file: > -0.25*rpm^3 - 0.15*rpm^2 + 1.11*rpm
Right - this approach seemed to me to be consistent with the existing code. > Now, one mild criticism of the code here is the use of the pow() > function to compute integer powers. Just in case you don't know the > trick, you can express that equation in a C expression as something > like: > > rpm * (1.11 - rpm * (0.15 * rpm + 0.25)) > > But that's more or less irrelevant. I used the power form because it is easier to read, but if the other form produces a performance advantage, then of course we must use it. > My real confusion is that while > this function reaches its maximum at a normalized RPM of slightly more > than 1.0 (sounds about right), it maximum value is only about 0.7 or > so. This will have the effect of reducing turbo-mul by about 30% from > pre-existing aircraft configurations. Is that the intent? Or should > the function be re-scaled to equal 1.0 at maximum boost? I've made a typing error here. The equation should be: -0.25*rpm^3 + 0.15*rpm^2 + 1.11*rpm This does have a max value of 1.01 at the normalised rpm value of 1. Now I'm concerned that it all worked so well before. I'm going to have to test all over again. > The other issue is that this function drops off really fast in > overspeed situations. I'd intuitively expect some drop-off in MP at > high RPMs (compressor stall) before reaching a constant asymptote, but > this thing plunges like a rock, crossing zero (negative MP?) at about > 1.8. Remember that the normalization RPM isn't always guaranteed to be > the maximum engine speed, so this regime might actually be close to > what you can do in the cockpit in a fast dive, for example. I think that this is a very valid point, and I agree. I have no good evidence for how the curve should drop, except that at some point the compressor goes supersonic, and surges. I have an alternative curve which is more asymptotic in shape, but doesn't fit quite so well at low rpm. Problem is that the controller holds the rpm rock solid at 3000 rpm, whatever I do. Interestingly, this was also claimed for the controller in real life. Here's what I'm trying to do http://myweb.tiscali.co.uk/vmeazza/FlightGear/supercharge.pdf Hang on, I'll have to find out why it all worked so well with an error. It will take me till after the weekend to retest it all. I'll test the second curve as well and try to reach a conclusion as to which is better. Thanks for all this, I'll get back to you soonest Regards, Vivian _______________________________________________ Flightgear-devel mailing list Flightgear-devel@flightgear.org http://mail.flightgear.org/mailman/listinfo/flightgear-devel 2f585eeea02e2c79d7b1d8c4963bae2d
