On 11/13/02 at 12:16 AM Julian Foad wrote: >David Luff wrote: >> >> It looks to me like you've >> got 2 too many curly brackets in doEnginePower, although I could be >> misunderstanding what you're doing there. > >Yes, I have got too many. This is the friction that was applied only >when starting; I was making it permanent but haven't finished with it. >Do you agree that it should be permanently in? Does a constant torque >sound about right? That sounds more likely than constant power (which >means decreasing torque), to me. Conventional friction would give >constant torque; I'm not sure how oil and air viscosity behave, but I'd >expect the torque to increase at higher speeds rather than decrease.
Yes, friction mean effective pressure (basically torque non-dimensionalised by engine volume) goes up with speed. Typically it could be about 3 times as great at 2500rpm as at idle. IO360.cxx in LaRCsim has data from a published friction model in it that reflects this, and this ought to be put into FGPiston.cpp to provide resistance to windmilling operation and cranking instead of the crappy -x HP that I had in there as a hack. In fact, I'm not entirely sure why it got left out - I think it went into IO360.cxx after the JSBSim port and then never made the jump across. In the long term I'd like to see it go in during running as well, together with a model of how efficiently the fuel energy is converted to mechanical energy, which should be quite portable across most GA piston engines. However, the present power correlation we have is *brake* power, from Phil's Cessna flight management computor (Phil being the guy who originally started writing the file). Hence we can't put the friction in during normal running, or the power at the prop will be wrong, which will be noticable in the normal flight envelope. > >I don't understand how it could have worked with no resistance >implemented. A propeller hardly provides any resistance at low speeds, >so I would have thought you would have needed to tweak the developed >power down to almost zero at idle. Yes, it was hard to get the engine to idle slowly - in fact I'm not what we did in the end! I do recall that my first guess at starter motor torque had to be revised downwards considerably since the imbalance caused the speed to go unstable in the first time-step. > > >> What I am concerned about is the throttle minimum being set to 0.2. ... > >Ah, thank you for explaining this. I had not understood the mapping >onto manifold pressure and the power correlation. It certainly sounds >like the power correlation is the thing to un-tweak instead! > > >This puzzled me: the manifold pressure seems to be modelled as (for a >given throttle position) independent of speed. When a real engine is >running fast and you cut the throttle, the fast air flow will cause a >very low manifold pressure which will then rise to its new steady value >as the engine slows down. Without this effect, throttle changes will >not take effect as quickly as they should and the speed variation with >load changes will not be right. Maybe the effect is too small to be >important? > > >I might be attempting too much here; I know how car engines work but >don't have data to work from (or a lab), and I don't have experience of >modelling them either. I will tread carefully and check with you again >when I make some more progress. During engine running, manifold pressure is primarily a function of throttle position, but yes, also affected by engine speed. However, the throttle position effect is considerably larger, and is currently all we have in the model. The effect with speed should be modelled though, in order to make it harder to set a given engine speed and man pressure with a variable pitch prop. Obviously, in the limit when the engine speed is cut to zero, the effect of speed dominates completely, and the man pressure goes to ambient. This is implemented. Cheers - Dave _______________________________________________ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
