Andy Ross wrote: > Vivian Meazza wrote: > > However (and there's always a however), I can't land the > > thing. Closing the throttle and pulling back the propeller pitch > > control doesn't reduce the power enough. I reasoned that there was too > > much boost with the throttle closed, (currently set at 10%, > > AFAICS). 10% of the supercharger output at sea level before the > > wastegate is a big number > > Yeah, that's a good point. The idle tuning of the engine was done > before turbocharging was added, and doesn't really map well to the new > regime. As a near-term workaround, you can always pull the mixture > way back to reduce power to near-zero. > > I'm wary of adding a magic number to the configuration files for this, > though. I wonder if there's a saner way of calculating an appropriate > idle power dynamically from the input values...
I can't find one right now. Empirically, piston engines in general seem to be set to idle at around 750-850 rpm, although I have read just one report of a Merlin idling at 600. 750-850 rpm implies a minimum throttle setting of about 6%. This number works fine in practice. The attached diff includes a hack which works for all YASim piston engines currently in the inventory. It would be better if we could close the throttle, and then let the idle function settle at 750-850 rpm. Perhaps you can come up with a more elegant solution? > A related issue that stands to be fixed is that the current code > doesn't really model a gear-driven supercharger properly. With a > centrifugal compressor like this, the output pressure gain is a direct > funcition of RPM. The existing code tries to model an exhaust-driven > turbo charger by simply multiplying the input MP by the turbo-mul > factor, which isn't the same thing. The behavior will match at > maximum power output, but be off in the middle and at the low end. The attached diff models the output of a gear-driven supercharger. I've used a 3rd order polynomial which matches the few examples I have been able to find, giving near-linear output up to the max-power rpm of the engine, with a little tail-off below this point, and a larger one above. More importantly, it produces realistic results. > > One other 'however': the property mp-inhg seems to be bound to the > > supercharger output before the wastegate is applied. > > Really? PistonEngine.cpp:112 looks like it is setting the current MP > based on the turbo output. Maybe there's a different bug somewhere > (units conversion, maybe?) I may be misinterpreting the code, but PistonEngine.cpp:114 " _mp *= _throttle;" seems to be modifying the turbo output incorrectly, and thus the output to the property browser. This line may be redundant? In any case, it's the pressure after the 'wastegate' i.e. 'mp' which we need for the cockpit gauge, either in inhg absolute (US) or psi-gauge (UK). The attached diff provides the output pressure ( _mp) and the manifold pressure (mp). Regards, Vivian
YASim.diff
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