OK, let's back up and take another look at this. In previous messages -- mine and others -- it has not always been clear what remarks refer to real-world aircraft and what remarks refer to the model. Such remarks are OK if we think the model is faithful to reality, but since it has become clear that the model is not faithful, we need to be more scrupulous about distinguishing model from reality.
Abbreviations: RW or RL -- Real World or Real Life SW -- Sim World Unless otherwise specified, SW refers to the rc2 package. Let's start with some basic science. Consider a RW piston engine with a _fixed_ impedance in the induction system. That means no governor and some fixed throttle position, fixed for the duration of the experiment. It is an exercise in elementary algebra to prove (subject to mild assumptions) that as the engine rotation speeds up, the engine power output goes up. To first order in rotation rate, the power is proportional to rotation rate. To second order, the power is less than proportional, because (due to the impedance) the MAP goes down ... but this second-order term never overtakes the first-order term. This result is robust and insensitive to details such as the design of the propeller. This result agrees with long-established pilot lore and standard operating procedures. Now, contrast that with the following SW scenario: I used the rc2 c172p on the runway at KSFO. Sea level, standard conditions. I set the throttle to 0.7 and left it there. As the SW aircraft accelerated down the runway, the airspeed increased from 0 to 80. The engine rotation rate increased from 2212 to 2341. So far so good. The odd thing is that the SW power _decreased_ from 75% to 66%. That is IMHO very weird ... not the sort of thing that would ever happen in RL ... not even close. ===== In a previous note I conjectured that the SW o-320 had some sort of unrealistic "governor" that kicked in under some high-load conditions. I need to heavily modify that conjecture, because in the SW takeoff (as described above) I made sure that the power was never more than 75% of rated power. So whatever you want to call this "governor" thing, it is producing highly unrealistic effects under a wide range of conditions, wider than I previously suspected. Please don't tell me that the SW implementation is working as designed. If so, that just means that the design is broken. ============================= Things to try: The first thing I would try would be a) remove all vestiges of this "governor" thing. b) add some nonzero impedance to the SW induction system, to represent the air cleaner, carburetor, manifold plumbing, et cetera. This is nontrivial. With the RW engine running, there is no way that RW MAP ever equals ambient static pressure, even at full throttle ... in contrast to the SW engine where we see SW MAP=static routinely. I suspect the RW engine was designed to develop 100% power at roughly 27" at 2700 RPM (sea level, full throttle). Maybe 28/2700. Somewhere in there. I would add enough SW impedance so that 2700 RPM corresponded to 27" (sea level, full throttle) just to get into the right ballpark ... and then see what further refinements were needed. ------------------------------------------------------------------------------ SF.Net email is Sponsored by MIX09, March 18-20, 2009 in Las Vegas, Nevada. The future of the web can't happen without you. Join us at MIX09 to help pave the way to the Next Web now. Learn more and register at http://ad.doubleclick.net/clk;208669438;13503038;i?http://2009.visitmix.com/ _______________________________________________ Flightgear-devel mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/flightgear-devel
