Re: [Flightgear-devel] Engine models: start-up and commonality betweenFDMs
David Megginson wrote: Julian Foad writes: Ah, glad you're there. If you're interested and have time to look, my current attempt is at http://www.btinternet.com/~julianfoad/fgfs/JSB_piston_engine.diff http://www.btinternet.com/~julianfoad/fgfs/engine_sound.diff What's the current status of these? Are the people who work with sound in general agreement that they're a good thing? I've looked at the sound stuff, but without a proper explanation I see no need to apply it. Even worse, it doesn't seem to work without some code changes. Erik ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] Engine models: start-up and commonality betweenFDMs
David Megginson wrote: Julian Foad writes: Ah, glad you're there. If you're interested and have time to look, my current attempt is at http://www.btinternet.com/~julianfoad/fgfs/JSB_piston_engine.diff http://www.btinternet.com/~julianfoad/fgfs/engine_sound.diff What's the current status of these? They're only ideas in progress, and are not right. Please don't put them in CVS as they are. - Julian ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] Engine models: start-up and commonality betweenFDMs
Julian Foad writes: Ah, glad you're there. If you're interested and have time to look, my current attempt is at http://www.btinternet.com/~julianfoad/fgfs/JSB_piston_engine.diff http://www.btinternet.com/~julianfoad/fgfs/engine_sound.diff What's the current status of these? Are the people who work with sound in general agreement that they're a good thing? All the best, David -- David Megginson, [EMAIL PROTECTED], http://www.megginson.com/ ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] Engine models: start-up and commonality betweenFDMs
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. 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. 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. - Julian ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] Engine models: start-up and commonality betweenFDMs
David Luff wrote: On 11/10/02 at 4:02 AM Julian Foad wrote: Ah yes, starting, I seem to recall a lot of hacking and kludging to get everything to work :-) There's a number of problems currently: ... Have fun :-) Ah, glad you're there. If you're interested and have time to look, my current attempt is at http://www.btinternet.com/~julianfoad/fgfs/JSB_piston_engine.diff http://www.btinternet.com/~julianfoad/fgfs/engine_sound.diff but, as I said, not finished. (Well, it will never be finished. I mean not completely satisfactory as a patch yet.) It removes some of the arbitrary bits - especially the non-linear bits like if RPM N then ... - and makes starting and idling nicer (especially at throttle less than the minimum allowed idle setting - it was fun running it below 500 RPM, on the unstable side of its power curve, after I put the friction always present but before I put the idle adjust constant in there). - Julian ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] Engine models: start-up and commonality betweenFDMs
Julian Foad writes: Well, I suppose it needs someone to show how the two aims can be compatible. But it's not easy; it would require becoming familiar with both implementations and re-arranging the interfaces a bit. While that's the sort of thing I do at work, I'm not yet in a position to do it here. We can already override parts of JSBSim's internal implementation (such as its weather model). There's no reason we couldn't rig up JSBSim and YASim to take engine parameters from properties as well. The engine model needs basic information like fuel available, outside air temperature and humidity, static and dynamic pressure, etc., all of which are accessible from outside the FDMs. It would have to feed back fuel and oil consumption, location and direction of thrust, amount of thrust, and information about gyroscopic effects. 3. The engine revs up and slows down very slowly. For example, when I cut the magnetos from 2000 RPM it takes over a minute to run down and stop. At a certain point, friction should take over. I added a kludge in JSBSim to make that happen. All the best, David -- David Megginson, [EMAIL PROTECTED], http://www.megginson.com/ ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] Engine models: start-up and commonality betweenFDMs
Andy Ross writes: I'd *love* to see good numbers for propeller acceleration, however. If one of the Real Pilots out there could go out with a stopwatch and get us graphs of RPM vs. time for full throttle acceleration and cut-power deceleration I'd be eternally grateful. :) I don't want to get banned from the flying club (or demonstrate my limited forced-landing skills), so I'm not going to abuse the engines by jamming the throttle in hard to full from idle and timing it. Here are some unofficial observations, however, from various 172's: 1. The prop accelerates and decelerates very quickly. You're supposed to move the throttle smoothly so that the changes don't happen too quickly. 2. During runup, I push the throttle in from 1000RPM to 1700RPM -- the lag between throttle movement and tach indication is barely perceptable (under 0.25 sec). 3. Next, I pull the throttle from 1700RPM to idle. The drop on the tach from 1700RPM to about 600RPM is nearly instantaneous (again, under 0.25sec). 4. At shutdown, I set the engine to 1000RPM, then pull the mixture to shut it down. The engine continues to fire for a couple of seconds until it burns off all its fuel, but once it stops firing, the propeller stops in well under a second. During takeoff, I have other things to worry about, but I'll guess that the lag between 1000RPM and max static (2200-2400RPM) takes less than 0.5sec, possibly again less than 0.25sec. All the best, David -- David Megginson, [EMAIL PROTECTED], http://www.megginson.com/ ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] Engine models: start-up and commonality betweenFDMs
David Megginson wrote: I like the idea as well: it would be nice if the engine were its own subsystem and we could mix-and-match engines and FDMs (let's try the J3 cub with 180HP). Unfortunately, the FDM people haven't been too enthusiastic: in particular, JSBSim is supposed to run standalone as well as inside FlightGear, so they need some kind of internal engine model. Well, I suppose it needs someone to show how the two aims can be compatible. But it's not easy; it would require becoming familiar with both implementations and re-arranging the interfaces a bit. While that's the sort of thing I do at work, I'm not yet in a position to do it here. As for the guts of how the engines are modelled ... I first worked on the starting and stopping behaviour of the JSBsim engine. The thermodynamic model of the engine is probably very good but there's lots of yucky stuff there to do with starting etc. I've done some stuff there, and in the sound configuration, but not finished. I'll go into that later. Then I looked at the YASim piston engine to see how that handles starting. Before I try to do anything in there I want to ask about some things (Andy?): 1. PistonEngine::calc says // Calculate manifold pressure as ambient pressure modified for // turbocharging and reduced by the throttle setting. According // to Dave Luff, minimum throttle at sea level corresponds to 6 // manifold pressure. Assume that this means that minimum MP is // always 20% of ambient pressure. (But that's too much idle // power, so use 10% instead!) But we need to produce _zero_ // thrust at that setting, so hold onto the output value // separately. Ick. _mp = pressure * (1 + _boost*(_turbo-1)); // turbocharger float mp = _mp * (0.1f + 0.9f * _throttle); // throttle _mp *= _throttle; What's that bit about the separate output mp? An engine doesn't produce zero thrust at idle, just a low thrust. And manifold pressure isn't supposed to be related to thrust in a simple way, is it? Sorry to peer into a nasty bit. The beauty of Open Source is ... we can see the whole thing, warts and all! :-) 2. At the end of the same function, _egt = corr * (power * 1.1f) / (massFlow * specHeat); if(_egt temp) _egt = temp; When I'm running this at idle, _egt comes out at 80 (kelvin); presumably this should be added to ambient temp (which is 288) rather than clamped to it: _egt = corr * (power * 1.1f) / (massFlow * specHeat); _egt += temp; 3. The engine revs up and slows down very slowly. For example, when I cut the magnetos from 2000 RPM it takes over a minute to run down and stop. There is a negative torque added that should make it stop quickly, and I can't see what's wrong with it (that would have this effect). Actually, as acceleration of the engine is equally slow, perhaps the problem is in the transfer of the torque to the external propulsion system code - perhaps the wrong units? 4. That negative torque: Interpolate it away as we approach cruise RPMs, though, to prevent interaction with the power computations. Ugly. Actually, the only way the variable power is used after that point is to compute the EGT, and that wants to know thermally developed power anyway (i.e. excluding the starter motor contribution and the friction reduction) so that should be fine. I think it would be correct to subtract the torque loss at all speeds - in fact, more loss at higher speeds because of gas flow turbulence etc. By the way, the experimental values here were with j3cub-yasim because c172-yasim gives a solution failure for elevator. I have some local changes, but nothing in YASim or anything that I think could affect it - just in the JSBSim engine, sound handling, joystick, etc. For all this, my original aim was just to get simple things like a realistic cranking speed of about 100 RPM, and some rotation sound while the engine is spinning down after being switched off! - Julian ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] Engine models: start-up and commonality betweenFDMs
Julian Foad wrote: David Megginson wrote: it would be nice if the engine were its own subsystem and we could mix-and-match engines and FDMs Well, I suppose it needs someone to show how the two aims can be compatible. But it's not easy; it would require becoming familiar with both implementations and re-arranging the interfaces a bit. Well, for the record, I think this is a great idea. It really wouldn't be hard at all to wire up an interface between YASim's Thruster class and an external engine/propeller representation. What's that bit about the separate output mp? An engine doesn't produce zero thrust at idle, just a low thrust. And manifold pressure isn't supposed to be related to thrust in a simple way, is it? I should have written power instead of thrust, of course. The real point is that the output power of an ideal engine at peak mixture is proportional to the mass input, which is proportional to the product of RPM and manifold pressure. So an ideal engine will produce zero power only at zero MP. ...which is ridiculous, of course. In the real world, engines have a minimum MP when the throttle is at idle, and produce near-zero usable power at that MP. This is because of all kinds of complicated internal losses (pressure drop across the intake valves, internal friction, you name it...). I couldn't simply assume that an engine that produced 200 HP at 29 inches of MP would produce 41 HP (6/29 of 200) at idle; nor could I come up with a good/broad/general thermodynamic argument for what those losses should be. However, I had an empirical number from Dave Luff that engines idle at about 6 inches. So I cheated and interpolated from that point instead. :) When I'm running this at idle, _egt comes out at 80 (kelvin); presumably this should be added to ambient temp (which is 288) rather than clamped to it: That sounds like a bug. I'd have to look more carefully at the physics to be sure. But adding the temperature to ambient is almost guaranteed to be wrong. There are exactly zero adiabatic processes that are symmetric with change of temperature; that's what the second law of thermodynamics is all about. More likely, I'm converting the units incorrectly or using a miscalibrated value for power due to the hack above. 3. The engine revs up and slows down very slowly. For example, when I cut the magnetos from 2000 RPM it takes over a minute to run down and stop. One issue is that the moment of inertia for the Cub looks too big. The current value is the same as the one for the 172, which has a larger prop. All other things being equal, it should scale with the mass of the propeller; I'd try a value about half of the Cessna's. This will result in too much stored energy in the prop that has to be dissipated, and thus a longer run-down (and run-up) time. You could also try increasing the 10% of cruise torque value that gets used for internal friction. I don't have any good numbers for what is appropriate, so I guessed. This is the kind of value that could/should be made tunable via configuration. I'd *love* to see good numbers for propeller acceleration, however. If one of the Real Pilots out there could go out with a stopwatch and get us graphs of RPM vs. time for full throttle acceleration and cut-power deceleration I'd be eternally grateful. :) 4. That negative torque: Interpolate it away as we approach cruise RPMs, though, to prevent interaction with the power computations. Ugly. Actually, the only way the variable power is used after that point is to compute the EGT, and that wants to know thermally developed power anyway (i.e. excluding the starter motor contribution and the friction reduction) so that should be fine. The engine power numbers are also used to handle the propeller solution too. My thinking here appears to have been that I didn't want an extra value interfering with the solver in funny ways. It's possible that all the changes cancel out later; just realize that the numbers get used outside the function, too. By the way, the experimental values here were with j3cub-yasim because c172-yasim gives a solution failure for elevator. Oh dear, is there another solver issue? This works fine for me. Are you absolutely sure you have current YASim code and base package? Platform/compiler? This is the solution result that I see: YASim solution results: Iterations: 362 Drag Coefficient: 18.0245 Lift Ratio: 90.6643 Cruise AoA: -0.131865 Tail Incidence: 0.582091 Approach Elevator: -0.920007 CG: -2.404, -0.033, 0.218 For all this, my original aim was just to get simple things like a realistic cranking speed of about 100 RPM, and some rotation sound while the engine is spinning down after being switched off! The sound issue is trivial -- just have one sound for the propeller noise that scales with RPM, and another for engine noise that is gated on /engines[0]/running. The realistic cranking speed is just a
