Re: [Flightgear-devel] Problem: unrealistic YASim stalls
Tony Peden writes: > On Wed, 2003-01-08 at 03:44, David Megginson wrote: > > Tony Peden writes: > > > > > Aside from restoring from a saved state, when do you need the trimming > > > routine to step aside? It seems to me you'd want it the rest of the > > > time. > > > > Probably, but it's easy to make sure it's called from FlightGear, > > then. It's just a matter of where we locate the logic -- if > > FlightGear explicitly requests trimming, then we can be consistent > > across FDMs and make the code more obvious. > > Wait a minute, though. FG already has the ability to turn trimming > on and off: the boolean /sim/presets/trim Yup, and it can specify ground vs. in-air trimming ... although it's up to the FDM to honor this flag when it initializes itself. Curt. -- Curtis Olson IVLab / HumanFIRST Program FlightGear Project Twin Cities[EMAIL PROTECTED] [EMAIL PROTECTED] Minnesota http://www.menet.umn.edu/~curt http://www.flightgear.org ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] Problem: unrealistic YASim stalls
Tony Peden writes: > > Probably, but it's easy to make sure it's called from FlightGear, > > then. It's just a matter of where we locate the logic -- if > > FlightGear explicitly requests trimming, then we can be consistent > > across FDMs and make the code more obvious. > > Wait a minute, though. FG already has the ability to turn trimming > on and off: the boolean /sim/presets/trim Right -- I'm suggesting that it might make the code simpler and clearer (and avoid bugs and misunderstandings) if FlightGear always invoked the trimming routine explicitly; I know that there are ways to manage it already. 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] Problem: unrealistic YASim stalls
On Wed, 2003-01-08 at 03:44, David Megginson wrote: > Tony Peden writes: > > > Aside from restoring from a saved state, when do you need the trimming > > routine to step aside? It seems to me you'd want it the rest of the > > time. > > Probably, but it's easy to make sure it's called from FlightGear, > then. It's just a matter of where we locate the logic -- if > FlightGear explicitly requests trimming, then we can be consistent > across FDMs and make the code more obvious. Wait a minute, though. FG already has the ability to turn trimming on and off: the boolean /sim/presets/trim > > > All the best, > > > David -- Tony Peden <[EMAIL PROTECTED]> ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] Problem: unrealistic YASim stalls
Arnt Karlsen writes: > ..I see 2 more or less fundamentally different cases here, fdm's > internal to FG, and fdm's external to FG, where FG communicates > externally over network, with several fdm's, or with standalone > jsbsim instances and where FG "only" does visualization and > "controls our own plane". > > ..what if _all_ fdm's are made external to FG, as in > interfacing to the FG framework? > (Yeah, I know, I ask way too late. ;-) ) This should not be a problem as long as the external FDM has it's own trimming routine, which it probably will have anyway. Regards, Curt. -- Curtis Olson IVLab / HumanFIRST Program FlightGear Project Twin Cities[EMAIL PROTECTED] [EMAIL PROTECTED] Minnesota http://www.menet.umn.edu/~curt http://www.flightgear.org ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] Problem: unrealistic YASim stalls
Tony Peden writes: > Aside from restoring from a saved state, when do you need the trimming > routine to step aside? It seems to me you'd want it the rest of the > time. Probably, but it's easy to make sure it's called from FlightGear, then. It's just a matter of where we locate the logic -- if FlightGear explicitly requests trimming, then we can be consistent across FDMs and make the code more obvious. 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] Problem: unrealistic YASim stalls
On Tue, 7 Jan 2003 17:39:58 -0500, David Megginson <[EMAIL PROTECTED]> wrote in message <[EMAIL PROTECTED]>: > Over all, I think it would be better if none of the FDMs trimmed > automatically. FlightGear, which provides the primary user > interface, is in a much better position to know when trimming is > required; for example, you do want to trim when an altitude and speed > are selected on the command line, but you do not want to trim when a > flight is being restored from a save file. If the FDMs simply retrim > for steady state when requested, we can make sure the request is > issued when needed. That should clean up the JSBSim/FlightGear > interface code a bit as well. ..I see 2 more or less fundamentally different cases here, fdm's internal to FG, and fdm's external to FG, where FG communicates externally over network, with several fdm's, or with standalone jsbsim instances and where FG "only" does visualization and "controls our own plane". ..what if _all_ fdm's are made external to FG, as in interfacing to the FG framework? (Yeah, I know, I ask way too late. ;-) ) -- ..med vennlig hilsen = with Kind Regards from Arnt... ;-) ...with a number of polar bear hunters in his ancestry... Scenarios always come in sets of three: best case, worst case, and just in case. ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] Problem: unrealistic YASim stalls
Resend after getting bounced earlier ... On Tue, 2003-01-07 at 14:39, David Megginson wrote: > Curtis L. Olson writes: > > > - respect runtime property changes (i.e. new altitude or location) > > > > I would respectfully suggest that this one not be implimented or at > > least be put at a low priority. We tried to do this in JSBsim and it > > can really get messy. > > The problem was that the startup trimming routine and the properties > always ended up arm-wrestling. In principle, this should be very > easy: > > 1. Before each iteration, copy all state variables in from the >bus/property tree. > > 2. After each iteration, copy all state variables back out to the >bus/property tree. > > YASim already does #2; it simply needs to add #1. > > JSBSim retrims automatically when certain values are changed outside > the FDM unless certain properties are set, etc. -- it's all fairly > confusing. > > Over all, I think it would be better if none of the FDMs trimmed > automatically. FlightGear, which provides the primary user interface, > is in a much better position to know when trimming is required; for > example, you do want to trim when an altitude and speed are selected > on the command line, but you do not want to trim when a flight is > being restored from a save file. If the FDMs simply retrim for steady > state when requested, we can make sure the request is issued when > needed. That should clean up the JSBSim/FlightGear interface code a > bit as well. > > Tony: what do you think? Aside from restoring from a saved state, when do you need the trimming routine to step aside? It seems to me you'd want it the rest of the time. > > > I think it would be much cleaner to force a reset to a new location > > each time we warp to a new location. This allows us to delete the FDM > > instance and create a new one so it can be freshly inited and trimmed > > for the new conditions. Otherwise, it's really hard not to carry over > > some state from the previous location which can cause obscenely large > > forces and other wierdness. > > The problem is that when we restore a saved flight or > start a premade scenario, You realize that this situation is exactly what the trimming routine is good for, do you not? In what way does it create a problem? > we'll get bumps (etc.) from the trimming routine, when the > saved state was already (presumably) steady. It must not have been ... of course, it's highly unlikely that any human can trim an aircraft as precisely as the trimming routine. > I think we're pretty > close -- you want to force a reset, and I want to be able to request a > retrim. > > > All the best, > > > David -- Tony Peden <[EMAIL PROTECTED]> ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] Problem: unrealistic YASim stalls
Curtis L. Olson writes: > - respect runtime property changes (i.e. new altitude or location) > > I would respectfully suggest that this one not be implimented or at > least be put at a low priority. We tried to do this in JSBsim and it > can really get messy. The problem was that the startup trimming routine and the properties always ended up arm-wrestling. In principle, this should be very easy: 1. Before each iteration, copy all state variables in from the bus/property tree. 2. After each iteration, copy all state variables back out to the bus/property tree. YASim already does #2; it simply needs to add #1. JSBSim retrims automatically when certain values are changed outside the FDM unless certain properties are set, etc. -- it's all fairly confusing. Over all, I think it would be better if none of the FDMs trimmed automatically. FlightGear, which provides the primary user interface, is in a much better position to know when trimming is required; for example, you do want to trim when an altitude and speed are selected on the command line, but you do not want to trim when a flight is being restored from a save file. If the FDMs simply retrim for steady state when requested, we can make sure the request is issued when needed. That should clean up the JSBSim/FlightGear interface code a bit as well. Tony: what do you think? > I think it would be much cleaner to force a reset to a new location > each time we warp to a new location. This allows us to delete the FDM > instance and create a new one so it can be freshly inited and trimmed > for the new conditions. Otherwise, it's really hard not to carry over > some state from the previous location which can cause obscenely large > forces and other wierdness. The problem is that when we restore a saved flight or start a premade scenario, we'll get bumps (etc.) from the trimming routine, when the saved state was already (presumably) steady. I think we're pretty close -- you want to force a reset, and I want to be able to request a retrim. 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] Problem: unrealistic YASim stalls
David Megginson writes: > That makes sense -- thanks. > > So, here's my near-term YASim wishlist, which is pleasingly short: > > - wing washout > - turbulence > - steady-state starts with non-zero velocity and/or in-air > - respect runtime property changes (i.e. new altitude or location) My wish list for YASim is slightly different: - steady-state starts with non-zero velocity and/or in-air Andy: if you have questions about how resets and position changes are handled in flightgear, please ask. - respect runtime property changes (i.e. new altitude or location) I would respectfully suggest that this one not be implimented or at least be put at a low priority. We tried to do this in JSBsim and it can really get messy. I think it would be much cleaner to force a reset to a new location each time we warp to a new location. This allows us to delete the FDM instance and create a new one so it can be freshly inited and trimmed for the new conditions. Otherwise, it's really hard not to carry over some state from the previous location which can cause obscenely large forces and other wierdness. Regards, Curt. -- Curtis Olson IVLab / HumanFIRST Program FlightGear Project Twin Cities[EMAIL PROTECTED] [EMAIL PROTECTED] Minnesota http://www.menet.umn.edu/~curt http://www.flightgear.org ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
RE: [Flightgear-devel] Problem: unrealistic YASim stalls
On Mon, 2003-01-06 at 17:54, Curtis L. Olson wrote: > Jon Berndt writes: > > > > There might be a yaw rate effect too. When the aircraft is yawing, > > > > the wingtip going "back" also sees a higher AoA and will drop if it > > is > > > > past the stall. Same deal. The snap roll needs a stalled "down" > > > > wingtip to get the divergence, in any case. > > > > > > That makes sense -- thanks. > > > > > > There's also the shadowing of an inboard portion of one wing as beta gets > > larger. Don't know if that is part of the effect, tho'. > > What about "virtual" yarn? > > X-Plane does something like this and it look like it would be a pretty > cool effect. > > http://x-plane.com/images/cola.jpg > > This doesn't mean we'd need to do exactly the same thing, but the idea > of graphically representing what the internal flight dynamics is > calculating is kind of nifty ... Hmm, this is getting dangerously close to that old dream of real-time CFD ... > > Curt. -- Tony Peden <[EMAIL PROTECTED]> ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] Problem: unrealistic YASim stalls
On Mon, 2003-01-06 at 15:26, Andy Ross wrote: > David Megginson wrote: > > I found it easier simply to picture different 2D sections of the > > wing stalling at different times, but I can see how your explanation > > might lead to a programmatic solution faster. > > Right, but what is it about different secions of the wing stalling at > different times that causes the aircraft to snap roll? That's the > part that was non-obvious to me. You get the snap roll effect even > when there is no dihedral or sweep, in situations where the wing > "should" be stalling symettrically. > > It happens because when the aircraft is rolling (even a little bit), > one wing is going down and the other up. This causes a difference in > AoA at the tips. If the wing tips are stalled, then the wing going > down is pushed farther into the stall, while the one going up sees a > lift recovery. So the net effect is that the aircraft tries to roll > *into* the existing roll direction. That's an unstable situation > (like a ball sitting at the top of a hill), and it will diverge if the > roll rate has any non-zero value. > > The more common explanation for washout (that the ailerons are > "masked" by the tip stall) is incorrect. Hmm, I think that depends mostly on how the flow separation progresses forward on outboad part of the wing. If it's very sudden, then you are probably right. If not, then preservation of roll control could well be the reason. > The snap roll doesn't happen > because of aileron authority, it happens because the aircraft is > literally unstable in roll. Aileron authority can obviously help to > recover, but the tendency to roll off into a divergent snap roll is > the real effect, and it is a different behavior from the normal roll > stability of the aircraft in typical conditions. > > > Isn't the snap roll usually uncoordinated? I've never done aerobatics > > myself. If it is, then I wonder what the role of the uncoordination > > is. > > If there is any wing sweep or dihedral, then a non-zero yaw angle also > changes the relative AoA of the wings to produce the same effect. If > you apply full rudder on such an aircraft, then you don't need to pull > up the nose as far to get the "down" wing tip stalled. > > There might be a yaw rate effect too. When the aircraft is yawing, > the wingtip going "back" also sees a higher AoA and will drop if it is > past the stall. Same deal. The snap roll needs a stalled "down" > wingtip to get the divergence, in any case. > > Andy -- Tony Peden [EMAIL PROTECTED] We all know Linux is great ... it does infinite loops in 5 seconds. -- attributed to Linus Torvalds ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] Problem: unrealistic YASim stalls
On Mon, 2003-01-06 at 11:31, Andy Ross wrote: > [Sorry for the delay. This one was hard, and had to wait for the > weekend for an investigation.] > > David Megginson wrote: > > Andy: unfortunately, none of your suggestions helped (details > > below). How are you modelling washout in YASim? From the violent > > roll that comes with every stall, it looks like all of the wing is > > stalling simultaneously, so the plane loses roll control from the > > ailerons at the same time as it loses lift from the wings. > > Wow, good call. The lack of washout modelling is indeed the problem. > And the details turn out to be great fun, to boot: > > What happens in the current (no washout) implementation is that the > aircraft nears a stall some (just a tiny bit) non-zero roll rate. The > wing going down therefore experiences a higher AoA than the upward > wing. In normal flight modes, this has a pro-stability effect. The > asymmetric lift distribution opposes the roll rate. Once the wings > are beyond the stall point, however, increasing AoA's decrease lift > and this causes a pro *roll* moment. That's a divergence, and the > aircraft very rapidly rolls off into a wild departure. > > I've always understood washout the way you explained it. It's there > to keep the ailerons unstalled and the plane controllable at high > AoAs. I figured this was something that could be "modelled away" by > simply increasing the stall width to reflect the fact that the whole > wing stalls gradually at different AoA's. But that is a demonstrably > minor effect; you can pump the aileron effectiveness up to ridiculous > magnitudes in YASim and you will still be doing snap rolls. > > The real reason for washout (or at least a better physical > explanation) is this: the washout that maintains the tips below stall > AoA keeps as much of the "stable" derivative as possible out on the > wing tips where the moment arm is long. If the early stall happens > near the fuselage on a short moment arm, then the overall behavior > will still be stable, not divergent, even past the peak of the "whole > aircraft" lift curve. > > It also explains why aircraft which are "normally" stable in the stall > can sometimes do divergent snap rolls (c.f. Luke and Ryan's > anecdotes). If you pull the AoA high enough to put the tips in the > stall too, then the divergent mode reasserts itself. You can do this > with an accelerated maneuver, for instance. Also, flying at aft > c.g. configurations results in more relative elevator authority which > could plausibly overwhelm the washout at the tips. > > Cool. I learned something this weekend. A "snap roll" is a > physically well-defined thing: it is a roll executed in a post-stall > environment where the roll-moment-due-to-roll-rate coefficient is > divergent. > > So anyway, YASim needs to model washout. In principle, this should be > pretty easy. Each wing segment (Surface object, as currently > implemented) gets its own orientation already. We just need to decide > on a way to specify it to the solver. Would a linear interpolation > between "base" and "tip" incidences work? I don't know much about > washout design as implemented on typical aircraft. A fancier > mechanism would allow you to specify washout as an interpolated curve > per-station curve along the span, but that sounds like it might be > overkill to me. Does anyone have a preference? I'm sure any twist is hard enough to manufacture, much less that which is non-linear, so I'm sure linear will be just fine. > > Andy -- Tony Peden <[EMAIL PROTECTED]> ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] Problem: unrealistic YASim stalls
David Megginson writes: > Norman Vine writes: > > > Easiest way I know of to learn about airfoil stall is to stick a bunch > > of yarns into an iceboat sail and go for a ride :-) > > That's a lot of work, when you can just go flying in light snow and > watch the flakes around the wings. I haven't tried that yet, though. Believe me, being fixed at one end the yarns tell a *much* better story, and at speed, the iceboats sail is operating at a comparable angle of attack of a small plane :-) Cheers Norman ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
RE: [Flightgear-devel] Problem: unrealistic YASim stalls
Jon Berndt writes: > > > There might be a yaw rate effect too. When the aircraft is yawing, > > > the wingtip going "back" also sees a higher AoA and will drop if it > is > > > past the stall. Same deal. The snap roll needs a stalled "down" > > > wingtip to get the divergence, in any case. > > > > That makes sense -- thanks. > > > There's also the shadowing of an inboard portion of one wing as beta gets > larger. Don't know if that is part of the effect, tho'. What about "virtual" yarn? X-Plane does something like this and it look like it would be a pretty cool effect. http://x-plane.com/images/cola.jpg This doesn't mean we'd need to do exactly the same thing, but the idea of graphically representing what the internal flight dynamics is calculating is kind of nifty ... Curt. -- Curtis Olson IVLab / HumanFIRST Program FlightGear Project Twin Cities[EMAIL PROTECTED] [EMAIL PROTECTED] Minnesota http://www.menet.umn.edu/~curt http://www.flightgear.org ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
RE: [Flightgear-devel] Problem: unrealistic YASim stalls
> > There might be a yaw rate effect too. When the aircraft is yawing, > > the wingtip going "back" also sees a higher AoA and will drop if it is > > past the stall. Same deal. The snap roll needs a stalled "down" > > wingtip to get the divergence, in any case. > > That makes sense -- thanks. There's also the shadowing of an inboard portion of one wing as beta gets larger. Don't know if that is part of the effect, tho'. Jon smime.p7s Description: application/pkcs7-signature
Re: [Flightgear-devel] Problem: unrealistic YASim stalls
David Megginson writes: > Norman Vine writes: > > > Easiest way I know of to learn about airfoil stall is to stick a bunch > > of yarns into an iceboat sail and go for a ride :-) > > That's a lot of work, when you can just go flying in light snow and > watch the flakes around the wings. I haven't tried that yet, though. Yeah, and trust me, the "I'm buying the yarn for my Grandma" excuse only works so many times... Curt. -- Curtis Olson IVLab / HumanFIRST Program FlightGear Project Twin Cities[EMAIL PROTECTED] [EMAIL PROTECTED] Minnesota http://www.menet.umn.edu/~curt http://www.flightgear.org ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] Problem: unrealistic YASim stalls
Norman Vine writes: > Easiest way I know of to learn about airfoil stall is to stick a bunch > of yarns into an iceboat sail and go for a ride :-) That's a lot of work, when you can just go flying in light snow and watch the flakes around the wings. I haven't tried that yet, though. 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] Problem: unrealistic YASim stalls
Andy Ross writes: > > Isn't the snap roll usually uncoordinated? I've never done aerobatics > > myself. If it is, then I wonder what the role of the uncoordination > > is. > > If there is any wing sweep or dihedral, then a non-zero yaw angle also > changes the relative AoA of the wings to produce the same effect. If > you apply full rudder on such an aircraft, then you don't need to pull > up the nose as far to get the "down" wing tip stalled. > > There might be a yaw rate effect too. When the aircraft is yawing, > the wingtip going "back" also sees a higher AoA and will drop if it is > past the stall. Same deal. The snap roll needs a stalled "down" > wingtip to get the divergence, in any case. That makes sense -- thanks. So, here's my near-term YASim wishlist, which is pleasingly short: - wing washout - turbulence - steady-state starts with non-zero velocity and/or in-air - respect runtime property changes (i.e. new altitude or location) 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] Problem: unrealistic YASim stalls
David Megginson writes: >> > I found it easier simply to picture different 2D sections of the wing > stalling at different times, but I can see how your explanation might > lead to a programmatic solution faster. Easiest way I know of to learn about airfoil stall is to stick a bunch of yarns into an iceboat sail and go for a ride :-) Norman ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] Problem: unrealistic YASim stalls
David Megginson wrote: > I found it easier simply to picture different 2D sections of the > wing stalling at different times, but I can see how your explanation > might lead to a programmatic solution faster. Right, but what is it about different secions of the wing stalling at different times that causes the aircraft to snap roll? That's the part that was non-obvious to me. You get the snap roll effect even when there is no dihedral or sweep, in situations where the wing "should" be stalling symettrically. It happens because when the aircraft is rolling (even a little bit), one wing is going down and the other up. This causes a difference in AoA at the tips. If the wing tips are stalled, then the wing going down is pushed farther into the stall, while the one going up sees a lift recovery. So the net effect is that the aircraft tries to roll *into* the existing roll direction. That's an unstable situation (like a ball sitting at the top of a hill), and it will diverge if the roll rate has any non-zero value. The more common explanation for washout (that the ailerons are "masked" by the tip stall) is incorrect. The snap roll doesn't happen because of aileron authority, it happens because the aircraft is literally unstable in roll. Aileron authority can obviously help to recover, but the tendency to roll off into a divergent snap roll is the real effect, and it is a different behavior from the normal roll stability of the aircraft in typical conditions. > Isn't the snap roll usually uncoordinated? I've never done aerobatics > myself. If it is, then I wonder what the role of the uncoordination > is. If there is any wing sweep or dihedral, then a non-zero yaw angle also changes the relative AoA of the wings to produce the same effect. If you apply full rudder on such an aircraft, then you don't need to pull up the nose as far to get the "down" wing tip stalled. There might be a yaw rate effect too. When the aircraft is yawing, the wingtip going "back" also sees a higher AoA and will drop if it is past the stall. Same deal. The snap roll needs a stalled "down" wingtip to get the divergence, in any case. 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] Problem: unrealistic YASim stalls
Arnt Karlsen wrote: > ..some planes use distinctly different airfoils, leading edge cuffs, > slots etc, to"wash out". Some different-airfoil wings transform > gradually towards the tip, and not neccesarily in a linear fashion, > some of these can get really weird. Actually, this is supported already, for those who have the patience to work with such minutia. You can chop the wing up into as many separate "vstab" objects as you like, each with an independant configuration. The point of handling washout as a separate concept is that (counter to my original thought) it turns out to be important for all aircraft with high aspect wings, not only those with hyper-realist authors. :) In fact, adding a "tip section" to the cessna wing with a different incidence value would be an immediately useful way to test washout effects. 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] Problem: unrealistic YASim stalls
On Mon, 06 Jan 2003 11:31:46 -0800, Andy Ross <[EMAIL PROTECTED]> wrote in message <[EMAIL PROTECTED]>: > So anyway, YASim needs to model washout. In principle, this should be > pretty easy. Each wing segment (Surface object, as currently > implemented) gets its own orientation already. We just need to > decide on a way to specify it to the solver. Would a linear > interpolation between "base" and "tip" incidences work? I don't know > much about washout design as implemented on typical aircraft. A > fancier mechanism would allow you to specify washout as an > interpolated curve per-station curve along the span, but that sounds > like it might be overkill to me. Does anyone have a preference? ..some planes use distinctly different airfoils, leading edge cuffs, slots etc, to"wash out". Some different-airfoil wings transform gradually towards the tip, and not neccesarily in a linear fashion, some of these can get really weird. Start with a linear thing and drop in all sorts of weird handles to keep us future eaa types happy. ;-) -- ..med vennlig hilsen = with Kind Regards from Arnt... ;-) ...with a number of polar bear hunters in his ancestry... Scenarios always come in sets of three: best case, worst case, and just in case. ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] Problem: unrealistic YASim stalls
Andy Ross writes: > The real reason for washout (or at least a better physical > explanation) is this: the washout that maintains the tips below > stall AoA keeps as much of the "stable" derivative as possible out > on the wing tips where the moment arm is long. If the early stall > happens near the fuselage on a short moment arm, then the overall > behavior will still be stable, not divergent, even past the peak of > the "whole aircraft" lift curve. I found it easier simply to picture different 2D sections of the wing stalling at different times, but I can see how your explanation might lead to a programmatic solution faster. > Cool. I learned something this weekend. A "snap roll" is a > physically well-defined thing: it is a roll executed in a > post-stall environment where the roll-moment-due-to-roll-rate > coefficient is divergent. Isn't the snap roll usually uncoordinated? I've never done aerobatics myself. If it is, then I wonder what the role of the uncoordination is. > So anyway, YASim needs to model washout. In principle, this should be > pretty easy. Each wing segment (Surface object, as currently > implemented) gets its own orientation already. We just need to decide > on a way to specify it to the solver. Would a linear interpolation > between "base" and "tip" incidences work? I don't know much about > washout design as implemented on typical aircraft. A fancier > mechanism would allow you to specify washout as an interpolated curve > per-station curve along the span, but that sounds like it might be > overkill to me. Does anyone have a preference? Start linear -- the real-world is probably not predictable enough that anyone would notice increased realism from an interpolated curve. On a separate note, I make no claim to understand how flaps and washout interact, but perhaps that's more obvious to others. 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] Problem: unrealistic YASim stalls
[Sorry for the delay. This one was hard, and had to wait for the weekend for an investigation.] David Megginson wrote: > Andy: unfortunately, none of your suggestions helped (details > below). How are you modelling washout in YASim? From the violent > roll that comes with every stall, it looks like all of the wing is > stalling simultaneously, so the plane loses roll control from the > ailerons at the same time as it loses lift from the wings. Wow, good call. The lack of washout modelling is indeed the problem. And the details turn out to be great fun, to boot: What happens in the current (no washout) implementation is that the aircraft nears a stall some (just a tiny bit) non-zero roll rate. The wing going down therefore experiences a higher AoA than the upward wing. In normal flight modes, this has a pro-stability effect. The asymmetric lift distribution opposes the roll rate. Once the wings are beyond the stall point, however, increasing AoA's decrease lift and this causes a pro *roll* moment. That's a divergence, and the aircraft very rapidly rolls off into a wild departure. I've always understood washout the way you explained it. It's there to keep the ailerons unstalled and the plane controllable at high AoAs. I figured this was something that could be "modelled away" by simply increasing the stall width to reflect the fact that the whole wing stalls gradually at different AoA's. But that is a demonstrably minor effect; you can pump the aileron effectiveness up to ridiculous magnitudes in YASim and you will still be doing snap rolls. The real reason for washout (or at least a better physical explanation) is this: the washout that maintains the tips below stall AoA keeps as much of the "stable" derivative as possible out on the wing tips where the moment arm is long. If the early stall happens near the fuselage on a short moment arm, then the overall behavior will still be stable, not divergent, even past the peak of the "whole aircraft" lift curve. It also explains why aircraft which are "normally" stable in the stall can sometimes do divergent snap rolls (c.f. Luke and Ryan's anecdotes). If you pull the AoA high enough to put the tips in the stall too, then the divergent mode reasserts itself. You can do this with an accelerated maneuver, for instance. Also, flying at aft c.g. configurations results in more relative elevator authority which could plausibly overwhelm the washout at the tips. Cool. I learned something this weekend. A "snap roll" is a physically well-defined thing: it is a roll executed in a post-stall environment where the roll-moment-due-to-roll-rate coefficient is divergent. So anyway, YASim needs to model washout. In principle, this should be pretty easy. Each wing segment (Surface object, as currently implemented) gets its own orientation already. We just need to decide on a way to specify it to the solver. Would a linear interpolation between "base" and "tip" incidences work? I don't know much about washout design as implemented on typical aircraft. A fancier mechanism would allow you to specify washout as an interpolated curve per-station curve along the span, but that sounds like it might be overkill to me. Does anyone have a preference? 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] Problem: unrealistic YASim stalls
I understand that many higher-performance aircraft can have quite violent (and even unrecoverable) stalls, as can some trainers like the Traumahawk I have to disagree with that quote.. I did the first 40 hours of my training in a Tomahawk, and I was never able to get a wing to drop.. I know they added some little strips on the leading edge of the wings about half way down to help cause a mild stall before a full fledged stall would occur. The worst stall I have been involved with was in a Cessna 172 where the right wing dropped extremely quickly to about 75 degrees, while my instructor was trying to show me how docile the 172 is. I am not sure if he had to much left rudder or something, but it wasn't very docile. I personally have not had any problems in the following aircraft during any type of stall. Just remember to keep them coordinated and you should never have any problems. Piper Tomahawk Warrior Archer Arrow Cessna 172 172RG Ryan ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] Problem: unrealistic YASim stalls
Andy: unfortunately, none of your suggestions helped (details below). How are you modelling washout in YASim? From the violent roll that comes with every stall, it looks like all of the wing is stalling simultaneously, so the plane loses roll control from the ailerons at the same time as it loses lift from the wings. As you know (but others might not), real wings on GA aircraft are twisted so that the incidence angle at the wing root is higher than the incidence angle at the wing tips -- that way, the wing root always has a higher AOA and will stall first, dropping the nose while the wing tips (and ailerons) are still effective. Because of the washout, a typical gentle stall in a Skyhawk or Cherokee (for example) has no noticeable roll component, even if there is a sharp nose drop. Perhaps there's a problem with the washout code in YASim right now. [specific comments follow] Andy Ross writes: > Try modifying the "flap" setting on the hstab (the effectiveness of > the stabilizor flaps) until full elevator is just barely enough to > acheive stall AoA. You are helped in this because (I think) the > "approach" configuration in the file actually represents a stall. The > solver prints out the elevator required to trim for approach in the > solution report, make this as close to 1.0 as you can. > > This might be enough to fix your problem -- you could still get a > viscious asymettric stall with violent control input, but gentle > motion of the yoke wouldn't be able to pull the nose high enough. Unfortunately, it seems to make no difference. I modified the value of hstab/flap0/@lift until I got the following in the solution: YASim solution results: Iterations: 1181 Drag Coefficient: 22.8378 Lift Ratio: 101.002 Cruise AoA: -0.842131 Tail Incidence: 0.597357 Approach Elevator: -0.994847 CG: -2.337, 0.000, -0.132 A gentle, power-off stall still results in a violent roll. > The problem with the stalls themselves should be fixeable with the > stall subtag on the wing. There is a "width" parameter that controls > the sharpness of the lift curve peak. It's roughly twice the radius > of curvature of the top of the peak, in degrees (not exactly, because > I'm using a cubic interpolant, but close enough). It's currently set > to six, which I would think would be pretty gentle. But you could try > a higher value and see what you get. Is it possible I have a unit bug > in there? > Another tunable you could play with is the "peak" number. This > controls how high the "normal" lift peak is in relation to the more > fundamental one at 45° generated by the underlying surface model. > Setting this value higher results in a sharper lift drop past the > stall. Lower values produce more gentle curves. I have *no* idea > what the right value for this is; I haven't seen any data on wing lift > though the full 360° of AoA. :) I changed wing/stall/@width to 12 and wing/stall/@peak to 1.1, and saw no significant difference in the stall behaviour -- a gentle, power-off stall still results in a violent roll. 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] Problem: unrealistic YASim stalls
Andy: Thanks for the suggestions -- I will try them all out, especially the elevator adjustments. It is worth noting, however, that even when I have succeeded in getting a sharp nose drop in a power-off stall on a 172, I have not seen a wing drop. You get a bit of roll with a power-on stall, and you can get a strong wing drop sometimes in a departure (banked) stall, but even when the nose drops like a roller coaster in a 172, the wings stay pretty-much level. I know that in a real plane the wing root stalls first because it has a higher incidence angle than the wing tips -- would that account for the roll stability in a stall, even with the sharp pitching down? 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] Problem: unrealistic YASim stalls
On Thu, 2003-01-02 at 13:35, Andy Ross wrote: > This might be enough to fix your problem -- you could still get a > viscious asymettric stall with violent control input, but gentle > motion of the yoke wouldn't be able to pull the nose high enough. That sounds about right to me. In the Cessna 172 that I fly, you can get some exciting nose-drop behavior in a power-on stall, or with a more abrupt control movement. But, if the aircraft is lightly loaded (200lb pilot, 30gal fuel) and you do a power-off stall gently, you just hear it go in and out of the buffet every few second while you descend smoothly. I've flown several incipient spins (with an instructor, un/cross-coordinated power-on stall) in the Cessna 172 and they were quite exciting. :-) -Luke -- Luke Scharf, Jack of Several Trades http://www.ccm.ece.vt.edu/~lscharf ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] Problem: unrealistic YASim stalls
[Sorry for the delay. I was out of town last week.] David Megginson wrote: > What can we do to prevent the over-eager wing drop in YASim? Is > there something we can change in the config files, or is it a C++ > code problem? Barring bugs, this is a pure configuration problem. It sounds like there are two issues with the model. The first is that the asymmetry in wing stalls is too high, and that the stalls themselves are too viscious. The second is that it is too easy to put the aircraft into a stall in the first place. The second is the easiest to explain. From your description, it sounds like full elevator in the real plane puts the aircraft at an AoA just barely past the stall -- you feel the buffet, but don't lose much lift. This makes an awful lot of design sense to me, I'm sure they intended it that way. My guess is that the YASim elevator is capable of pulling the AoA well past stall, so you get nastier behavior. Try modifying the "flap" setting on the hstab (the effectiveness of the stabilizor flaps) until full elevator is just barely enough to acheive stall AoA. You are helped in this because (I think) the "approach" configuration in the file actually represents a stall. The solver prints out the elevator required to trim for approach in the solution report, make this as close to 1.0 as you can. This might be enough to fix your problem -- you could still get a viscious asymettric stall with violent control input, but gentle motion of the yoke wouldn't be able to pull the nose high enough. The problem with the stalls themselves should be fixeable with the stall subtag on the wing. There is a "width" parameter that controls the sharpness of the lift curve peak. It's roughly twice the radius of curvature of the top of the peak, in degrees (not exactly, because I'm using a cubic interpolant, but close enough). It's currently set to six, which I would think would be pretty gentle. But you could try a higher value and see what you get. Is it possible I have a unit bug in there? Another tunable you could play with is the "peak" number. This controls how high the "normal" lift peak is in relation to the more fundamental one at 45° generated by the underlying surface model. Setting this value higher results in a sharper lift drop past the stall. Lower values produce more gentle curves. I have *no* idea what the right value for this is; I haven't seen any data on wing lift though the full 360° of AoA. :) 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] Problem: unrealistic YASim stalls
> On another note, is there any possibility of adding a way to change the > loading of an airplane? It would be interesting to be able to do > something like: > fgfs--aircraft-type=c172r-3d-yasim \ > --aircraft-loading=fuel=38gal,frontseat=200lb,baggage=400lb > and then do crazy things in the simulated aircraft. You can do this with the JSBSim planes using the POINT_MASS item in the config file. Jon smime.p7s Description: application/pkcs7-signature
Re: [Flightgear-devel] Problem: unrealistic YASim stalls
On Wed, 2003-01-01 at 09:35, David Megginson wrote: > Luke Scharf writes: > > > I've had the same experience in the Cessna 172E Skyhawk that I fly. > > I can add this to Dave's observations: I haven't been able to cause > > the nose to drop in an attempted descending power-off turn stall. > > Some at Cessna did a GREAT job with this aircraft! > > Did you try the stall cross-controlled? Note that I'm not > recommending that, since it can put you inverted. Unless this maneuver is already known to be fairly benign, it seems to me that this is the sort of thing that should only be done: 1) by a flight test pilot experienced in type 2) with a full set of tunnel data available for review 3) in increments of pedal input: 25%, 50%, etc. > > > BUT, I've never tried to stall a C-172E fully loaded -- I fly in the > > utility category most of the time. So, our observations may not be > > valid, depending on how the simulated aircraft is loaded. > > > > How is the model in question balanced? > > We have it loaded and balanced in or near utility, I think. > > > All the best, > > > David -- Tony Peden <[EMAIL PROTECTED]> ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] Problem: unrealistic YASim stalls
Luke Scharf writes: > At my current level of piloting skill, I'm not going to intentionally > spin an airplane without a graybearded instructor or a parachute! The Piper Warrior isn't even spin-certified, unlike the 172. > Cool - so it should be fairly close to the way I fly the aircraft. > > On another note, is there any possibility of adding a way to change the > loading of an airplane? It would be interesting to be able to do > something like: > fgfs --aircraft-type=c172r-3d-yasim \ > --aircraft-loading=fuel=38gal,frontseat=200lb,baggage=400lb > and then do crazy things in the simulated aircraft. Yes, it is possible, but it's complicated and a big FDM-specific right now. I agree that it would be a good learning experience to let people try flying with, say, the CG way too far aft. 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] Problem: unrealistic YASim stalls
On Wed, 2003-01-01 at 12:35, David Megginson wrote: > Luke Scharf writes: > > > I've had the same experience in the Cessna 172E Skyhawk that I fly. > > I can add this to Dave's observations: I haven't been able to cause > > the nose to drop in an attempted descending power-off turn stall. > > Some at Cessna did a GREAT job with this aircraft! > > Did you try the stall cross-controlled? Note that I'm not > recommending that, since it can put you inverted. I kept it as well coordinated as I possibly could! At my current level of piloting skill, I'm not going to intentionally spin an airplane without a graybearded instructor or a parachute! > > BUT, I've never tried to stall a C-172E fully loaded -- I fly in the > > utility category most of the time. So, our observations may not be > > valid, depending on how the simulated aircraft is loaded. > > > > How is the model in question balanced? > > We have it loaded and balanced in or near utility, I think. Cool - so it should be fairly close to the way I fly the aircraft. On another note, is there any possibility of adding a way to change the loading of an airplane? It would be interesting to be able to do something like: fgfs --aircraft-type=c172r-3d-yasim \ --aircraft-loading=fuel=38gal,frontseat=200lb,baggage=400lb and then do crazy things in the simulated aircraft. -Luke -- Luke Scharf, Jack of Several Trades http://www.ccm.ece.vt.edu/~lscharf ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] Problem: unrealistic YASim stalls
Luke Scharf writes: > I've had the same experience in the Cessna 172E Skyhawk that I fly. > I can add this to Dave's observations: I haven't been able to cause > the nose to drop in an attempted descending power-off turn stall. > Some at Cessna did a GREAT job with this aircraft! Did you try the stall cross-controlled? Note that I'm not recommending that, since it can put you inverted. > BUT, I've never tried to stall a C-172E fully loaded -- I fly in the > utility category most of the time. So, our observations may not be > valid, depending on how the simulated aircraft is loaded. > > How is the model in question balanced? We have it loaded and balanced in or near utility, I think. 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] Problem: unrealistic YASim stalls
On Wed, 2003-01-01 at 08:34, David Megginson wrote: > In a real Skyhawk, it's 50:50 whether the nose will even drop much if > I pull the yoke all the way back; often, I just get mild buffetting as > the nose drops a couple of degrees, picks up speed, and lifts up again > and the plane mushes on forward. There's never any roll in a > power-off stall, though a power-on stall can sometimes cause a slight > incipient spin. I've had the same experience in the Cessna 172E Skyhawk that I fly. I can add this to Dave's observations: I haven't been able to cause the nose to drop in an attempted descending power-off turn stall. Some at Cessna did a GREAT job with this aircraft! BUT, I've never tried to stall a C-172E fully loaded -- I fly in the utility category most of the time. So, our observations may not be valid, depending on how the simulated aircraft is loaded. How is the model in question balanced? Thanks, -Luke -- Luke Scharf, Jack of Several Trades http://www.ccm.ece.vt.edu/~lscharf ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
