> From: "Vivian Meazza" > > Jim wrote > > > Hi Andy, > > > > On the p51d fdm configuration, it looks like the substantial change was > > actually increasing the turbo multiplier from 2.0 to 5.5, and not > > reducing the cruise speed as stated in the CVS log of March 23. The > > cruise speed change does have an effect, but it is fairly small. > > > > The problem with putting the turbo multiplier up in that range is the > > manifold pressure output is directly multiplied by that number. So full > > throttle produces an output of 164 inHG manifold pressure. We should be > > seeing about 61 inHG at sea level for this engine. > > > > Setting this multiplier lower to get the correct manifold pressure with > > turbo at sea level should reduce the maximum flight level for the aircraft > > since the second stage turbo cannot currently be modeled. On the other > > hand, using this lower value should NOT produce incorrect lower altitude > > performance since all the data I'm using is for below the 20,000 ft > > altitude where the second stage kicks in. The drag numbers calculated by > > YASim should be more or less correct up to at least up to 20,000 ft where > > the second stage would be kicking in. > > > > If there is a problem that setting the multiplier to 5.5 fixes, I suspect > > it is in the FDM design and not the P51D configuration. Any ideas how we > > can fix or work around this? > > > > Where do you get your numbers from for the boost? There are some > contemporary figures around for the Rolls Royce built versions of the > Mustang engine which indicate that the turbo multiplier should indeed be > around 5: > > http://www.spitfireperformance.com/jf934climb.jpg > > Otherwise, you won't get the correct high-altitude performance. In the > figure above extrapolate the boost curves back to sea-level, ignoring the > effect of the Boost Controller (aka wastegate), and you will see that 5 is > about right - even a bit more for the later Merlins. Don't forget that us > Brits work in psi gauge, while the ex-colonies work in psi absolute: same > thing +- 1 atmosphere. > > Although there is a bug in the current code which gives an incorrect readout > of boost pressure (I forwarded a correction to Andy some weeks ago), the > existing code gives pretty good results if you plug in the numbers right out > of the book. I have just done it for the Merlin XX and was very impressed by > the accuracy. > > Of course, we still have to model the gear-driven supercharger, but again I > have forwarded some code to Andy which does this. We are still waiting to > finalise some curves to match supercharger output. I've got some good-enough > results here. >
Hi Vivian, This sounds very interesting. I think I'll wait and see what Andy does with your patches before "fixing" the P51D again. Thanks, Jim _______________________________________________ Flightgear-devel mailing list [email protected] http://mail.flightgear.org/mailman/listinfo/flightgear-devel 2f585eeea02e2c79d7b1d8c4963bae2d
