On Sat, 2008-12-20 at 04:46 -0700, John Denker wrote: > On 12/20/2008 02:44 AM, Ron Jensen wrote: > > John, > > > > You've just convinced me you don't have a clue what is happening inside > > this code. > > You keep saying that. Even if it were true (which I > doubt), it wouldn't be important. > > I call attention again to the scenario I posted: > > >> The Sim World c182rg is sitting on > >> the runway at KSFO. The propeller control is pulled back, so that the > >> engine is operating at relatively low revs, about 1750 RPM in > >> contrast to redline which is 2400 RPM. The pilot can observe that if > >> the throttle is open anywhere between 52% and 100% open, moving the > >> throttle has no effect on the MAP. Similarly, it has no effect on the > >> shaft power of the engine, as you can confirm by looking at the > >> property tree. This insensitivity to throttle setting is dramatically > >> unlike what is seen in a Real World Cessna 182RG. > > If you would kindly take a couple of minutes, you could > verify that what I say about the SW c172rg is true. If > you had the slightest experience in real aircraft, or if > you thought about the physics for a moment, you would > know that what I say about the RW C172RG is true.
John, I did look at the SW c182rg, and it behaves as you describe. And I will stipulate that behavior is not correct compared to a real-world c182rg. > Think about it: If the first half of the throttle motion > has *no* effect on the MAP, what reason is there to think > that the second half of the motion will have any effect? > It's just a mechanical valve. It's not psychic. The throttle is a valve. In the instant case, when it opens past about 60% it is flowing 100% of the engine demand. Or more specifically the function doMAP() converts ambient pressure, throttle position, and RPM into a manifold pressure between the ambient pressure and some smaller value, generally around 6 inHg. It does that as coded, yes we could change the algorithm, but that would not and should not change the behavior we are seeing. > You keep saying that the code is working as designed. So > what? That just means that the design is unrealistic. No, it means the c182rg propeller configuration and/or the engine configuration need to be adjusted. The engine model is putting out ~175 hp at 1750 RPM. According to the propeller configuration data this just enough power to turn the prop, with no torque left over to accelerate it faster. So, do we need more horsepower from the engine or less power absorption from the propeller? > The performance of the model is unrealistic. I said that > before looking at the code. Obviously you didn't look at the code. > > It is definitely not because code section you seem to > > love going on about. > > It was unrealistic before I looked at the code, and it is > still unrealistic. > > Do you really expect anybody to believe that things are OK > when pulling the throttle halfway back has absolutely no > effect on the MAP? Yes. > Explaining (again) how the code makes this happen this will > not help. The fact remains that it should not happen. The > RW aircraft does not do this, and the SW aircraft should not > do it either. Neither will blindly blaming sections of code from FGPiston.cpp and drawing pictures of idealized pressure curves. Lets look instead at finding out the real reasons why the output behavior is not as it should be. First question, prop_81in2v.xml gives a minimum and maximum propeller pitch of 12.0 and 31.8. Any idea if these numbers are right or are they just guesses? Second question, prop_81in2v.xml gives a minimum and maximum rpm of 900 and 2400 rpm. Any idea if these numbers are right or are they just guesses? Thanks, Ron ------------------------------------------------------------------------------ _______________________________________________ Flightgear-devel mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/flightgear-devel
