David Megginson wrote: > The most important one is idle RPM. Currently, the YASim c172 idles > at about 1,350 RPM. From what I've found, recommended RPM for taxiing > a C172 is only about 900 RPM, and some checklists I've see state that > the engine should be idling at over 575 RPM, so I'd guess that 600-750 > RPM is probably the right range.
This is actually an interaction. The idle RPM is, by definition, the speed at which propeller drag is equal to engine power output. I agree, it is currently too high. The original YASim model simply dropped MP to zero at zero throttle. This had a "zero" idle, which wasn't much better. (The problem was masked because the model clamped RPM to be 500 RPM or higher). These days, it uses 6" as the minimum manifold pressure, on advice from Dave Luff. The problem is, that the power output is still calculated as linear with MP, so there's too much power at idle. Real engines have internal friction & such. The best thing to do would probably be to allow the user to specify an "idle" RPM and simply force the power to be "right" at zero throttle. This is a little hairy, but not awful. > The second one has to do with static RPM. Under "Propeller Limits", > the C172 TCDS lists a maximum static RPM at full throttle of > 2,065-2,165. That's well under the maximum 2,400 RPM for a C172R in > motion. Currently, the YASim C172 prop goes right up past 2,400 RPM > even when the plane is not moving. This part is actually tunable. The propeller model does a two-point fit to the parameters specified. It makes sure that at "cruise" speed and power, the engine torque at the specified RPM matches the propeller counter-torque. It also makes sure that the propeller torque at takeoff (i.e. zero-speed) conditions is at least low enough to produce the specified takeoff-rpm. The reasoning behind this is complicated, unfortunately -- it has to do with the idealized shape of a propeller torque curve that YASim is using internally. (That is, I can't remember it well enough to explain it right now, sorry.) Basically, if I understand the request, it should be sufficient to set the takeoff-rpm value to 2100 or so, while leaving the cruise value alone. That should do what you want. I didn't bother to look up real numbers when I did most of the YASim planes, so this is going to be a common type of problem with them. > YASim could interpolate between static RPM/power and takeoff RPM/power > to find the effect of velocity, and between idle RPM/power and static > RPM/power to find the effect of the throttle. The engine model would > work much more realistically with only a little more work. Sadly, it's actually a lot more complicated than that. The interpolation you want is already done, and it's behavior is constrained by the efficiency curve that YASim uses. While it sounds like it should be this simple, getting things right in reality is a lot harder -- you want the propeller efficiency to drop smoothly off as speed increases, while simultaneously making sure that the drag on the engine isn't too high at takeoff, while making sure that the peak of the efficiency curve goes in the right spot to insure sufficient takeoff thrust. I think the bulk of the problems you're experiencin are due to my inability to expose the model in a sensible and understandable way, rather than a core shortcoming. Andy -- Andrew J. Ross NextBus Information Systems Senior Software Engineer Emeryville, CA [EMAIL PROTECTED] http://www.nextbus.com "Men go crazy in conflagrations. They only get better one by one." - Sting (misquoted) _______________________________________________ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
