Re: [Flightgear-devel] YASim piston engine
Alex Perry wrote: I've never noticed it, but that doesn't mean it doesn't happen. For most throttle transients, the combination of prop momentum, throttle pump and induction system effects will hide the blade stall transition. Especially true if you have a controllable prop. Have you checked whether the blade profile implies that the whole blade stalls and unstalls at the same time ? It may be gradual. It is gradual. In fact, if you think about it, it has to be. A propeller that presented the same AoA at every point along the blade would have to change its degree of twist as the advance ratio changed. I didn't mean to imply that YASim is actually modelling the airflow around the propeller; it doesn't. What it does do is try to mimick an idealized propeller torque and efficiency curves (functions of the advance ratio). These have a kink at some point -- they don't continue to increase as the advance ratio drops to zero, because the blades reach an AoA of maximum lift. Andy -- Andrew J. RossNextBus 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
Re: [Flightgear-devel] YASim piston engine
It is gradual. In fact, if you think about it, it has to be. A propeller that presented the same AoA at every point along the blade would have to change its degree of twist as the advance ratio changed. I was just wondering whether the twist happened to correspond to the AOA-plus-advance so that large fractions of the blade would stall all in one go ... which would certainly be disconcerting to a governor. I didn't mean to imply that YASim is actually modelling the airflow around the propeller; it doesn't. What it does do is try to mimick an idealized propeller torque and efficiency curves (functions of the advance ratio). These have a kink at some point -- they don't continue to increase as the advance ratio drops to zero, because the blades reach an AoA of maximum lift. Is the kink blunt, like the one for a main wing with twist in it, or is the kink sharp enough to be really conspicuous ? ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] YASim piston engine
If you look carefully, you'll actually see the RPM drop very slightly before it starts increasing. The physical reason for this is that the blades are unstalling. As the flow attaches to them, they experience a sharp increase in induced drag. I was pretty pleased to notice this little tidbit; it kinda validates the model in an obtuse way. That being said, I have *no* idea if this effect is noticeable in a real aircraft. Alex? I've never noticed it, but that doesn't mean it doesn't happen. For most throttle transients, the combination of prop momentum, throttle pump and induction system effects will hide the blade stall transition. Especially true if you have a controllable prop. Have you checked whether the blade profile implies that the whole blade stalls and unstalls at the same time ? It may be gradual. ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] YASim piston engine
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. RossNextBus 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
Re: [Flightgear-devel] YASim piston engine
Andy Ross writes: 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. I'm not sure that's right. By the time the plane is moving fast enough to take off, the propeller should be at full RPM. All the best, David -- David Megginson [EMAIL PROTECTED] ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] YASim piston engine
David Megginson wrote: Andy Ross writes: 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'm not sure that's right. By the time the plane is moving fast enough to take off, the propeller should be at full RPM. That's exactly what it does. The core problem is that the propeller's aerodynamic drag (more strictly, counter-torque), when matched to a cruise performance, ends up being much too high at zero speed. The reason for this is that the propeller at takeoff is essentially in a stalled condition -- the AoA on the blades results in separated flow, and much less induced drag. YASim handles this by providing a cap on the drag and thrust coefficients that works at high AoAs (real torque and thrust curves really do look like this, btw). That cap is specified as a takeoff RPM which it will attempt to match. The takeoff-rpm value you specify will only be matched, though, at takeoff. As you speed up, you'll see the RPMs grow appropriately, until they match the cruise-rpm value you specified (presuming you end up at cruise conditions, obviously). Andy -- Andrew J. RossNextBus 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
Re: [Flightgear-devel] YASim piston engine
Nope; you can fairly easily get the RPM over 2700 in a fast cruise descent, but there is no way you'll manage over 2500 at Vy, never mind Vx. Andy Ross writes: 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. I'm not sure that's right. By the time the plane is moving fast enough to take off, the propeller should be at full RPM. All the best, David -- David Megginson [EMAIL PROTECTED] ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] YASim piston engine
Alex Perry writes: Nope; you can fairly easily get the RPM over 2700 in a fast cruise descent, but there is no way you'll manage over 2500 at Vy, never mind Vx. We're modelling a C172R, which maxes out at 2400RPM. Would the same apply there? All the best, David -- David Megginson [EMAIL PROTECTED] ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] YASim piston engine
You should talk to Andy and look at the prop model; the faster you go, the faster the prop can turn. Fast cruise descents can overspeed. I'm not at home and can't look up the exact numbers, sorry. Alex Perry writes: Nope; you can fairly easily get the RPM over 2700 in a fast cruise descent, but there is no way you'll manage over 2500 at Vy, never mind Vx. We're modelling a C172R, which maxes out at 2400RPM. Would the same apply there? All the best, David -- David Megginson [EMAIL PROTECTED] ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] YASim piston engine
Alex Perry writes: You should talk to Andy and look at the prop model; the faster you go, the faster the prop can turn. Fast cruise descents can overspeed. I'm not at home and can't look up the exact numbers, sorry. In JSBSim, the prop maxes out between 2065 and 2165 RPM at 0 velocity, but does manage to hit 2400 RPM at Vx. You can push it over in a steep dive (as you mention). You mentioned that a 2700 RPM engine doesn't max out at Vx; I'm wondering if things are different for the 160HP 2400 RPM C172R that we're modelling. All the best, David -- David Megginson [EMAIL PROTECTED] ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] YASim piston engine
David Megginson wrote: I'd like to suggest a couple of changes to the YASim piston-engine model, keeping in mind Andy's past caveats that it is very simplistic. OK, I've hacked at the idle RPM code a bit to reduce the idle MP. This is, well, a hack; but it puts the idle in a saner range on the DC-3 and C-172 without affecting full-throttle performance. Try the following patch to c172.xml, which makes the takeoff-rpm modification I talked about*. Hopefully things will be more to your liking; the aircraft maxes out at 2100 RPM while stopped, then the speed increases to something more like cruise as the airspeed grows. If you look carefully, you'll actually see the RPM drop very slightly before it starts increasing. The physical reason for this is that the blades are unstalling. As the flow attaches to them, they experience a sharp increase in induced drag. I was pretty pleased to notice this little tidbit; it kinda validates the model in an obtuse way. That being said, I have *no* idea if this effect is noticeable in a real aircraft. Alex? Anyway, the patch to c172.xml: diff -u -r1.5 c172.xml --- c172.xml2002/03/01 07:27:27 1.5 +++ c172.xml2002/04/09 02:57:35 @@ -57,7 +57,7 @@ propeller radius=1.0 cruise-speed=86 cruise-rpm=2200 cruise-alt=1 cruise-power=99 - takeoff-power=160 takeoff-rpm=2500 + takeoff-power=134.5 takeoff-rpm=2100 eng-power=160 eng-rpm=2500 x=-.6 y=0 z=0 mass=400 moment=8 actionpt x=0 y=0 z=0/ Have fun, Andy * And changes engine power at zero speed to reflect the different RPM. Don't ask why I don't calculate this myself; the reason is ugly.** ** OK, it has to do with the parser design. At the time the element is parsed, I may not have a valid engine object yet and I was too lazy to build a parse tree to inspect. :) -- Andrew J. RossNextBus 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
