On Monday 19 April 2004 23:15, Vivian Meazza wrote: > Andy Ross wrote > > > Vivian Meazza wrote: > > > The "takeoff" values. Are these the power absorbed by the > > > > propeller at > > > > > propeller rpm, or the engine output at engine rpm, super- or > > > un-supercharged? > > > > Un-supercharged. And the equations are solved such that both > > power values are the same. Basically, don't sweat this one; > > it affects performance only at the very start of the takeoff > > roll. Leave it out until you get things working, and then > > start fiddling with it to get the initial RPM right. > > > > > Finally, I've had some difficulty understanding the concept > > > > of using > > > > > absolute pressure for the Boost Control Valve (BCV). In the > > > > real world > > > > > a BCV comprises [...] and is thus corrected for altitude. > > > > Actually, everything I've read indicates that wastegate > > designs are calibrated to absolute pressure, not relative > > pressure (which makes sense, obviously, because what you are > > trying to regulate is the force on the engine parts, not the > > overpressure in the manifold which is a non-critical structural part). > > > > Measuring absolute pressure is mechanically more difficult > > but not impossible. It doesn't have to be as simple as a > > single spring valve. > > > > > I suppose that we should update the documentation to reflect these > > > misinterpretations. > > > > Roger. See if what's there now makes more sense. > > > > Andy > > I think I might be getting somewhere. > > First I started with these values: > > eng-power="1140" eng-rpm="2850" > turbo-mul="4" wastegate-mp="48" > cruise-alt="17500" cruise-power="1140" > cruise-speed="308" cruise-rpm="2850" > takeoff-power="900" takeoff-rpm="2500" > manual-pitch="true" > gear-ratio = "0.477" > > AS before, YASim converges nicely > > However, eng-power should be the un-supercharged max power, so I reduced > eng-power value, while holding all others constant. YASim only converged > with a eng-power >= 1130. This cannot be the un-supercharged max power? > > I then set eng-pwr to 1140, and tried reducing turbo-mul. YASim only > converged with turbo-mul >=3.5 > > I next set turbo-mul to 4, and tried reducing cruise-rpm. YASim only > converged with cruise-rpm > 2820. I then tried increasing cruise-rpm. YASim > only converges with cruise-rpm < 6799. Is it possible that "reduction > gearing" reduces engine revs for a given propeller rpm? I thought it was > the other way around. So assuming this to be the case, I set cruise-rpm to > 5975, and repeated the above sequence. YASim converges when: > > eng-power >=575 > turbo-mul >=0.0001 > > In each case the other value is set with the initial value as above. Now, > assuming a turbo-mul value of 2 as a reasonable guess, and 750 HP as the > un-supercharged output, we get this: > > eng-power="750" eng-rpm="2850" > turbo-mul="2" wastegate-mp="48" > cruise-alt="17500" cruise-power="1140" > cruise-speed="308" cruise-rpm="5975" > takeoff-power="725" takeoff-rpm="5000" > manual-pitch="true" > gear-ratio = "0.477" > > YASim converges nicely. Problem solved? > > Regards > > Vivian Meazza
As I understand it, everything apart from the 'eng-' prefix values relate to the props... However, I've noticed the same sort of things when trying to get the Comper Swift working properly - this has a similarly geared 0.47 eng/prop ratio. I've been able to get close to the correct performance with the Pobjoy R in the Swift by using some 0.47*proper numbers in the definition but that has in turn adversely effected the mixture and fuel consumption figures, by approx 0.47... LeeE _______________________________________________ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
