RE: [Flightgear-devel] c310 origin
Jon Berndt <[EMAIL PROTECTED]> said: > I have seen posts to the effect that there IS a problem with properly > communicating the origin, and that this problem deals with rotating the 3D > model as specified by the FDM Euler angles, but NOT accounting for the > fact that the FDM assumes rotation about the CG, and the 3D model rotates > about a different point, such that the CG moves improperly and perhaps > puts the gear underground or something. Is this correct? Jon, Thought I should go back to this question to clarify why I don't see a problem. A couple of geometric assumptions: 1) Roll, Pitch, and Heading are always the same for every position on the aircraft. It'll be the same at the variable CG as it is at a fixed point on the nose. 2) The position of the aircraft is always reported at the same location. In otherwords Lon/Lat/Alt is always reported from a fixed location on the airframe. If those two are true then there should never be a problem. The primarily effect of longitudinal CG movement will be on the reported Alt (Altitude) of the aircraft. For example if the CG moves toward the back and the Altitude is reported at the nose of the aircraft, then one degree of pitch change will have a greater effect on the change of altitude at the nose. Movement of the CG will also affect the reported lon/lat of the aircraft, including lateral movement. So all we need to do is rotate the 3D model at the same location as the lon/lat/alt is reported and we should be reasonably correct. Close enough to be visually correct. This could be a good argument for reporting position data at the nose, far away from CG: Greater precision for reflecting the CG variation effect. Best, Jim ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
RE: [Flightgear-devel] c310 origin
Jon Berndt <[EMAIL PROTECTED]> said: > I don't really think that the CG (or anything like it) should have > anything to do with a common reference point. I think it should be > something you can readily see. The nose/prop hub tip is about as > unambiguous as it gets. Due to the nature of defining the flight dynamic > model, and the nature of defining the 3D model - both people will know > where nose/prop hub are located. Documenting a distance back from the nose for each aircraft would work fine. Whoever does the first flightmodel gets to pick the spot. The standard doesn't have to be all that unambiguous. Using the nose standard you've got 3D modelers deciding where on the airframe the aircraft should appear to rotate... which... umm... I guess is ok :-) > A question: is all the rotational/translation stuff figured out so that if > a pilot eyepoint is defined that the view will be properly rendered with > that in mind - even when the CG is askew because of fuel burnoff? If the fuel burnoff is refelected in the reporting of position and orientation by the FDM then the answer is yes. I would assume that would be the case :-) The chase view is really the only place we get screwed up if the origin isn't back on the wing. It looks funny. Not wrong, just funny. And it's kind of nice if the 3D model is in the middle of the screen instead of off to one side. Best, Jim ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
RE: [Flightgear-devel] c310 origin
> Yes, I knew this would sound a little complicated with swept,delta,body wing > aircraft. But making it the nose really just puts the decision on to the 3D > Modeler where to some degree the flight model designer could have a better > idea. Isn't there some way of deriving an average or nominal c.g. that we > could just lock in as the fixed point? I don't really think that the CG (or anything like it) should have anything to do with a common reference point. I think it should be something you can readily see. The nose/prop hub tip is about as unambiguous as it gets. Due to the nature of defining the flight dynamic model, and the nature of defining the 3D model - both people will know where nose/prop hub are located. A question: is all the rotational/translation stuff figured out so that if a pilot eyepoint is defined that the view will be properly rendered with that in mind - even when the CG is askew because of fuel burnoff? Jon smime.p7s Description: application/pkcs7-signature
RE: [Flightgear-devel] c310 origin
Jon Berndt <[EMAIL PROTECTED]> said: > It's neither impractical nor complex. FWIW, there really is a standard > already out there, and we use it. That is, the structural frame, as I have > outlined before. The only problem I see is that the FDM and the 3D model > rendering code need to have a static common point of reference - that's > why I mentioned the farthest point forward, such as the nose or prop hub. > Using the wing leading edge is unsatisfactory: think of the F-16, or the > space shuttle. What do you use in those cases? In the case of the X-15, do > you use the point where the wing meets the fuselage as the "leading edge" > or the point where the leading edge would intersect the centerline? The > nose tip or prop hub is unmistakable. We would report this position to > FlightGear and you would then have intimate knowledge of what to rotate > about. Yes, I knew this would sound a little complicated with swept,delta,body wing aircraft. But making it the nose really just puts the decision on to the 3D Modeler where to some degree the flight model designer could have a better idea. Isn't there some way of deriving an average or nominal c.g. that we could just lock in as the fixed point? Best, Jim ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
RE: [Flightgear-devel] c310 origin
> Really we just need the FDMs to agree on something. Or maybe > not...maybe the > idea of sharing 3DModel configs between FDMs (c310-jsbsim, > c310-yasim pointing > to the same model.xml) is impratical or too complex? Certainly if one FDM > models gear compression and the other doesn't, that is an issue > when it comes > to configuring the 3d model. It's neither impractical nor complex. FWIW, there really is a standard already out there, and we use it. That is, the structural frame, as I have outlined before. The only problem I see is that the FDM and the 3D model rendering code need to have a static common point of reference - that's why I mentioned the farthest point forward, such as the nose or prop hub. Using the wing leading edge is unsatisfactory: think of the F-16, or the space shuttle. What do you use in those cases? In the case of the X-15, do you use the point where the wing meets the fuselage as the "leading edge" or the point where the leading edge would intersect the centerline? The nose tip or prop hub is unmistakable. We would report this position to FlightGear and you would then have intimate knowledge of what to rotate about. smime.p7s Description: application/pkcs7-signature
RE: [Flightgear-devel] c310 origin
Jon Berndt <[EMAIL PROTECTED]> said: > Tony wrote: > > > Are we sure we want to put the 3D model origin to cg offsets in the FDM > > config file. IIRC, having multiple 3D models for any one aero model is > > pretty standard fare in the MSFS world. > > The only thing we'd do different in JSBSim is to say where the nose/prop > tip is. This would give a *known* common reference point between the two - > and something that does not move over time as fuel is burned off. > > The question is: is this what the 3D modeler guys are looking for? That would work and would need to be different for various models of the c310 since the nose length varies. It would be easiest if the fdm simply reported the lon/lat/alt at the point on the same longitudinal axis as the nose/proptip but back where the leading edge of the wing was. Best, Jim ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
RE: [Flightgear-devel] c310 origin
Jon Berndt <[EMAIL PROTECTED]> said: > > IMHO it'd make the most sense to put the offset (from the nose or > > tail) to the > > 3D model origin location into the FDM's aircraft config xml file. This > > location should be on the leading edge of the wings and z axis > > centered on the > > nose as described above. The 3D modeler's could refer to that data for > > getting the model right, and the fdm should simply report lon/lat/alt > at > > *that* location. This is how the YASim 747 currently works and > > it seems fine. > > > Maybe I am off=base on this, or misunderstanding the problem, but in my > previous experience with 3D modeling and IRIXGL some years ago one had to > be careful where the origin of the 3D model was, and the order of > rotation/translation. This is what I interpret your concern to be about. Nope. We've got that covered. What I'd like to see is the reported position data (lon/lat/alt) be at a location in the aircraft which lines up with the leading edge of the wing and on the longitudinal axis. For example, the light grey lines in this image more or less illustrate the kind of location (although this example is maybe slightly aft): http://www.spiderbark.com/fgfs/u3aorigin.png That really could be developed as a standard, and specified in the terms you suggest. However, we can translate any origin you choose. That one just seemed to be the easiest as it'll work for the current rendering code without incorporating new offsets. Really we just need the FDMs to agree on something. Or maybe not...maybe the idea of sharing 3DModel configs between FDMs (c310-jsbsim, c310-yasim pointing to the same model.xml) is impratical or too complex? Certainly if one FDM models gear compression and the other doesn't, that is an issue when it comes to configuring the 3d model. Best, Jim ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
RE: [Flightgear-devel] c310 origin
Tony wrote: > Are we sure we want to put the 3D model origin to cg offsets in the FDM > config file. IIRC, having multiple 3D models for any one aero model is > pretty standard fare in the MSFS world. The only thing we'd do different in JSBSim is to say where the nose/prop tip is. This would give a *known* common reference point between the two - and something that does not move over time as fuel is burned off. The question is: is this what the 3D modeler guys are looking for? Jon smime.p7s Description: application/pkcs7-signature
Re: [Flightgear-devel] c310 origin
Jim Wilson writes: > Norman Vine <[EMAIL PROTECTED]> said: > > > In your example you may not see the error but . > > What I meant was that offseting to the varible center of gravity wouldn't be > visible either outside or inside the aircraft. The FDM already provides the > attitude effects, it is the change in axes that wouldn't be noticable. > Actually you know if we did offset the camera and model to center of gravity > it could have a very minor negative effect. Assuming that some day we have a > dialog to configure payload, we'll see the 3D model shift back and forth on > the tarmac as we add fuel, passengers and luggage and the 3D origin changes :-) OK but when in tet to be implemented external 'flier mode' similar to the existing static external view, which should be tied to the Model Origin, you should see the difference in the effects of a rotation of the plane when the plane's Center of Gravity moves > The question is this: If the FDM's agree to use the nose as the origin, > we'll have an offset to the 3D model's origin, right? Yes but we don't care where it is as long as Model explicitly states where it is in reference to the 'bounding sphere' or 'bounding box' of the model > Where would we put the > setting so that it was available to both the model and the viewer, defined > per aircraft type? in the Model-set.xml file, this way a FDM may be used by more then one model > IMHO it'd make the most sense to put the offset (from the nose or tail) to the > 3D model origin location into the FDM's aircraft config xml file. This > location should be on the leading edge of the wings and z axis centered on the > nose as described above. The 3D modeler's could refer to that data for > getting the model right, and the fdm should simply report lon/lat/alt at > *that* location. This is how the YASim 747 currently works and it seems fine. If we use a position relative to a 'bounding shape' then this can actually be determined at run time Norman ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
RE: [Flightgear-devel] c310 origin
On Sat, 2002-12-14 at 07:26, Jon Berndt wrote: > > IMHO it'd make the most sense to put the offset (from the nose or > > tail) to the > > 3D model origin location into the FDM's aircraft config xml file. This > > location should be on the leading edge of the wings and z axis > > centered on the > > nose as described above. The 3D modeler's could refer to that data for > > getting the model right, and the fdm should simply report lon/lat/alt > at > > *that* location. This is how the YASim 747 currently works and > > it seems fine. > > > Maybe I am off=base on this, or misunderstanding the problem, but in my > previous experience with 3D modeling and IRIXGL some years ago one had to > be careful where the origin of the 3D model was, and the order of > rotation/translation. This is what I interpret your concern to be about. > In the past day or two Tony, David, Andy, and myself have moved close (I > think) to what the FDM can do to help 3D modelers place the model > properly. If we can come up with a standard reference frame for the an > aircraft model (insofar as it deals with the FDM), would this help? And > since the CG can move during flight, we need a way to have a common > "anchored" reference point for both 3D modelers and FDMs. The suggested > standard reference frame for FDMs is: > > === > X positive out the tail > Y positive out the right side > Z positive to complete the system, upward > origin as desired or as specified in the manufacturers structural frame, > or whatever ... but: > KEY: The farthest forward point on the aircraft, i.e. nose, or prop hub, > must be specified in the FDM config file, so the relationship between the > FDM origin and 3D model origin can be obtained. > Also, all measurements are in inches. > === > > I have seen posts to the effect that there IS a problem with properly > communicating the origin, and that this problem deals with rotating the 3D > model as specified by the FDM Euler angles, but NOT accounting for the > fact that the FDM assumes rotation about the CG, and the 3D model rotates > about a different point, such that the CG moves improperly and perhaps > puts the gear underground or something. Is this correct? Are we sure we want to put the 3D model origin to cg offsets in the FDM config file. IIRC, having multiple 3D models for any one aero model is pretty standard fare in the MSFS world. > > Jon -- Tony Peden <[EMAIL PROTECTED]> ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
RE: [Flightgear-devel] c310 origin
> IMHO it'd make the most sense to put the offset (from the nose or > tail) to the > 3D model origin location into the FDM's aircraft config xml file. This > location should be on the leading edge of the wings and z axis > centered on the > nose as described above. The 3D modeler's could refer to that data for > getting the model right, and the fdm should simply report lon/lat/alt at > *that* location. This is how the YASim 747 currently works and > it seems fine. Maybe I am off=base on this, or misunderstanding the problem, but in my previous experience with 3D modeling and IRIXGL some years ago one had to be careful where the origin of the 3D model was, and the order of rotation/translation. This is what I interpret your concern to be about. In the past day or two Tony, David, Andy, and myself have moved close (I think) to what the FDM can do to help 3D modelers place the model properly. If we can come up with a standard reference frame for the an aircraft model (insofar as it deals with the FDM), would this help? And since the CG can move during flight, we need a way to have a common "anchored" reference point for both 3D modelers and FDMs. The suggested standard reference frame for FDMs is: === X positive out the tail Y positive out the right side Z positive to complete the system, upward origin as desired or as specified in the manufacturers structural frame, or whatever ... but: KEY: The farthest forward point on the aircraft, i.e. nose, or prop hub, must be specified in the FDM config file, so the relationship between the FDM origin and 3D model origin can be obtained. Also, all measurements are in inches. === I have seen posts to the effect that there IS a problem with properly communicating the origin, and that this problem deals with rotating the 3D model as specified by the FDM Euler angles, but NOT accounting for the fact that the FDM assumes rotation about the CG, and the 3D model rotates about a different point, such that the CG moves improperly and perhaps puts the gear underground or something. Is this correct? Jon smime.p7s Description: application/pkcs7-signature
Re: [Flightgear-devel] c310 origin
Norman Vine <[EMAIL PROTECTED]> said: > In your example you may not see the error but . What I meant was that offseting to the varible center of gravity wouldn't be visible either outside or inside the aircraft. The FDM already provides the attitude effects, it is the change in axes that wouldn't be noticable. Actually you know if we did offset the camera and model to center of gravity it could have a very minor negative effect. Assuming that some day we have a dialog to configure payload, we'll see the 3D model shift back and forth on the tarmac as we add fuel, passengers and luggage and the 3D origin changes :-) > You certainly want to offset the camera to its real location > if you ever want to be able to move your head inside of the cockpit That works already. > It really is no big deal to do this just one translate per iteration of the > main loop, and once implemented it will just work for all view modes Thats right. We just need to vector to the offset location and come up with the adjusted lon/lat/alt at that location. I just think that 3D model origin should be a fixed location with the x axis at the leading edge of the wing and the z axis going through the nose. The question is this: If the FDM's agree to use the nose as the origin, we'll have an offset to the 3D model's origin, right? Where would we put the setting so that it was available to both the model and the viewer, defined per aircraft type? IMHO it'd make the most sense to put the offset (from the nose or tail) to the 3D model origin location into the FDM's aircraft config xml file. This location should be on the leading edge of the wings and z axis centered on the nose as described above. The 3D modeler's could refer to that data for getting the model right, and the fdm should simply report lon/lat/alt at *that* location. This is how the YASim 747 currently works and it seems fine. Best, Jim ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] c310 origin
David Megginson wrote: OK, finally I understand the problem. What we need to do, then, is apply the euler angles to the CG rather than the origin when rotating the 3D model. We need only two steps: 1. have the FDMs report the current CG relative to the origin (if they don't already); and 2. add a couple of transforms to acmodel.cxx so that we use the CG as the centre of rotation. This will be especially nice for modelling out-of-balance situations (like 500lb of baggage in the back of a 172) -- everything will come out looking right. It will also be valuable for aircraft that have massive balance shifts, like a military aircraft dropping ordnance. I don't expect that this will be a big job. What does everyone else think? Ultimately we can ask the aero designers to add just *one* other location, being the (pre defined) 3D model location of origin. This takes the weight off of the 3D designers and places it where it belongs, at the back of the aero designes. The 3D model location of origin should be available *inside* the aero config file, but should be propagated to FlightGear. (All, if you ask me). Erik ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] c310 origin
Jim Wilson writes: > > This same effect is less or more depending on exactly where the "origin" is. > We could eliminate it by offseting the lon/lat/alt and roll/pitch/hdg to the > current center of gravity for the camera, but that isn't necessary. It'll > look good enough (you can't visually see the error) if the origin is just > placed in a reasonable approximate location. In your example you may not see the error but . You certainly want to offset the camera to its real location if you ever want to be able to move your head inside of the cockpit It really is no big deal to do this just one translate per iteration of the main loop, and once implemented it will just work for all view modes norman ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] c310 origin
On 13 Dec 2002 15:22:38 -0800 Tony Peden <[EMAIL PROTECTED]> wrote: What I'm proposing is that we don't have to agree to use the same point. We just need to provide a way to correct for the difference. Ah, yes. We will need to provide another point - location of nose tip (or whatever). In inches. X axis positive toward the tail. Jon ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] c310 origin
Tony Peden writes: > On Fri, 2002-12-13 at 12:40, Jon S Berndt > > >I think it's pretty clear that we aren't going to agree on one > > >coordinate system to use. > > > > > >Aside from that, agreeing on one takes away the freedom to choose a > > >location that makes sense for the aircraft you are working on (for both > > >the FDM and 3D modeling). > > > > I may not have made myself clear. I'd personally like to > > use structural frame. However, that's not always > > available, so we have to come up with something. We really > > only care about distances, anyhow, not so much exact > > locations. So, if we agree upon a convention (origin at > > the nose tip, normally), then we can simply report the CG > > location in that system and the Euler angles and the model > > comes out looking fine. So, I think we can agree on > > something. > > What I'm proposing is that we don't have to agree to use the same point. > We just need to provide a way to correct for the difference. Here Here !!! Norman ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] c310 origin
On Fri, 2002-12-13 at 12:40, Jon S Berndt wrote: > On Fri, 13 Dec 2002 12:09:34 -0800 (PST) > Tony Peden <[EMAIL PROTECTED]> wrote: > > >I think it's pretty clear that we aren't going to agree on one > >coordinate system to use. > > > >Aside from that, agreeing on one takes away the freedom to choose a > >location that makes sense for the aircraft you are working on (for both > >the FDM and 3D modeling). > > I may not have made myself clear. I'd personally like to > use structural frame. However, that's not always > available, so we have to come up with something. We really > only care about distances, anyhow, not so much exact > locations. So, if we agree upon a convention (origin at > the nose tip, normally), then we can simply report the CG > location in that system and the Euler angles and the model > comes out looking fine. So, I think we can agree on > something. What I'm proposing is that we don't have to agree to use the same point. We just need to provide a way to correct for the difference. > > Jon > > ___ > Flightgear-devel mailing list > [EMAIL PROTECTED] > http://mail.flightgear.org/mailman/listinfo/flightgear-devel -- Tony Peden <[EMAIL PROTECTED]> ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
RE: [Flightgear-devel] c310 origin
David Megginson <[EMAIL PROTECTED]> said: > Jim Wilson writes: > > > Actually, no, well yes this is a good idea. But I don't think it is > > necessary. AFAIK the model rotates correctly already. > > The FDMs give us lon/lat/alt and roll/pitch/hdg. FlightGear then has > to apply transformations to make the model appear in the right place > and with the right orientation. After reading your posting, I'm > trying to remember how, exactly, we do that -- I'm willing to be that > we just rotate the model around the origin by the roll, pitch, and > heading. That means that the view from inside will be wrong as well, > especially in a big plane. > Those position and orientation values are used to transform and rotate the camera. Here is a detailed description of what happens...extreme example (I'm thinking I did this once before :-)): 1) You are in chase view looking at the aircraft in flight from the side, a profile view. 2) The "origin" is set to the nose of the aircraft. 3) The aircraft model itself already rotates correctly about its c.g. even with the origin at the nose. 4) When you pitch down, the nose decreases in altitude by anywhere from a half to several meters depending on the aircraft and the degree of pitch down. 5) Since the camera is tied to the origin it always decreases in altitude equal to the nose. 6) This has the effect of making it look like the nose is stationary and the rest of the plane is wagging behind it (like a dog's tail). This same effect is less or more depending on exactly where the "origin" is. We could eliminate it by offseting the lon/lat/alt and roll/pitch/hdg to the current center of gravity for the camera, but that isn't necessary. It'll look good enough (you can't visually see the error) if the origin is just placed in a reasonable approximate location. Best, Jim ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] c310 origin
David Megginson writes: > Jim Wilson writes: > > > Actually, no, well yes this is a good idea. But I don't think it is > > necessary. AFAIK the model rotates correctly already. > > The FDMs give us lon/lat/alt and roll/pitch/hdg. FlightGear then has > to apply transformations to make the model appear in the right place > and with the right orientation. After reading your posting, I'm > trying to remember how, exactly, we do that -- I'm willing to be that > we just rotate the model around the origin by the roll, pitch, and > heading. Yup that's how its done once we have reestablished a Geocentric Cartesian Coordinate System and 'rolled' in our Local Tangent Space transformed so as to honor Andy's favorite our 'current scenery's tile center' based OpenGL space Cheers Norman ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
RE: [Flightgear-devel] c310 origin
Tony Peden writes: > Since this makes the 3D modeler's choice independent of the FDM > modeler's choice, it seems the most sensible to me. That wasn't my intention -- you'd still need the same origin for this to work. It's just a way for the 3D engine to know what point to pivot the model around when applying the Euler angles. 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] c310 origin
On Fri, 13 Dec 2002 14:56:39 -0500 David Megginson <[EMAIL PROTECTED]> wrote: Jim Wilson writes: > Actually, no, well yes this is a good idea. But I don't think it is > necessary. AFAIK the model rotates correctly already. If you are rotating the model in the same order as the FDM, then the *orientation* _will_ be correct, yes. However, if you rotate about the nose, the CG will translate. If you rotate about the CG, then you would be correct. The FDMs rotate the aircraft about the CG. You must translate the origin of the vehicle to the current CG and THEN rotate the vehicle in the same order the FDMs do. Jon The FDMs give us lon/lat/alt and roll/pitch/hdg. FlightGear then has to apply transformations to make the model appear in the right place and with the right orientation. After reading your posting, I'm trying to remember how, exactly, we do that -- I'm willing to be that we just rotate the model around the origin by the roll, pitch, and heading. That means that the view from inside will be wrong as well, especially in a big plane. The orientation should be correct, but the placement of the CG would be wrong, as I outlined above. Jon ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] c310 origin
On Fri, 13 Dec 2002 12:09:34 -0800 (PST) Tony Peden <[EMAIL PROTECTED]> wrote: I think it's pretty clear that we aren't going to agree on one coordinate system to use. Aside from that, agreeing on one takes away the freedom to choose a location that makes sense for the aircraft you are working on (for both the FDM and 3D modeling). I may not have made myself clear. I'd personally like to use structural frame. However, that's not always available, so we have to come up with something. We really only care about distances, anyhow, not so much exact locations. So, if we agree upon a convention (origin at the nose tip, normally), then we can simply report the CG location in that system and the Euler angles and the model comes out looking fine. So, I think we can agree on something. Jon ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] c310 origin
--- Norman Vine <[EMAIL PROTECTED]> wrote: > David Megginson writes: > > > Jim Wilson writes: > > > > > 2). On a fixed wing aircraft, if origin is anywhere but between > the > > > wings the external model views that follow along with the > aircraft > > > (e.g. chase view) will not look correct. The camera is tied to > the > > > origin. If the origin is at the nose or tail then the plane > > > "looks" like it's wagging when it pitches. And if the origin is > > > too high or too low again the model can "appear" to be swinging > > > like a pendulum or sliding when it should simply be rolling. > > > > OK, finally I understand the problem. What we need to do, then, is > > apply the euler angles to the CG rather than the origin when > rotating > > the 3D model. We need only two steps: > > > > 1. have the FDMs report the current CG relative to the origin (if > they > > don't already); and > > > > 2. add a couple of transforms to acmodel.cxx so that we use the CG > as > >the centre of rotation. > > This is exactly why there was, and I think still is, a > cg == center of geometry > and a > CG == Center of Gravity > > Both are needed and both are 'aptly' named > > Can we decide which one gets the UPPER CASE > once and for all now ? No matter the case, cg will always be center of gravity to me ... > > Norman > > > ___ > Flightgear-devel mailing list > [EMAIL PROTECTED] > http://mail.flightgear.org/mailman/listinfo/flightgear-devel > > __ Do you Yahoo!? Yahoo! Mail Plus - Powerful. Affordable. Sign up now. http://mailplus.yahoo.com ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] c310 origin
--- Jon S Berndt <[EMAIL PROTECTED]> wrote: > On Fri, 13 Dec 2002 13:45:11 -0500 > David Megginson <[EMAIL PROTECTED]> wrote: > > >OK, finally I understand the problem. What we need to do, then, is > >apply the euler angles to the CG rather than the origin when > rotating > >the 3D model. We need only two steps: > > > >1. have the FDMs report the current CG relative to the origin (if > they > > don't already); and > > Isn't this the way it used to work? Or, how does it work, > now? I think we track the initial CG, as well as the > current CG. > > >2. add a couple of transforms to acmodel.cxx so that we > > use the CG as the centre of rotation. > > > >This will be especially nice for modelling out-of-balance situations > >(like 500lb of baggage in the back of a 172) -- everything will come > >out looking right. It will also be valuable for aircraft that have > >massive balance shifts, like a military aircraft dropping ordnance. > I > >don't expect that this will be a big job. What does everyone else > >think? > > Of course, the best way *is* to use the same origin for > the FDM and the 3D model. If we all agree on an "FDM > Frame", then we'll all know where the CG is if we report > that number based on the FDM frame (and by FDM frame I > mean the nose-tip thing we've been talking about). Euler > angles from the FDM should be able to be used directly by > the model rendering code I would think, instead of having > to do rotations *and* translations for the 3D model if we > have different frames of reference. I think it's pretty clear that we aren't going to agree on one coordinate system to use. Aside from that, agreeing on one takes away the freedom to choose a location that makes sense for the aircraft you are working on (for both the FDM and 3D modeling). > > Jon > > ___ > Flightgear-devel mailing list > [EMAIL PROTECTED] > http://mail.flightgear.org/mailman/listinfo/flightgear-devel > > __ Do you Yahoo!? Yahoo! Mail Plus - Powerful. Affordable. Sign up now. http://mailplus.yahoo.com ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
RE: [Flightgear-devel] c310 origin
--- David Megginson <[EMAIL PROTECTED]> wrote: > Jim Wilson writes: > > > 2). On a fixed wing aircraft, if origin is anywhere but between > the > > wings the external model views that follow along with the aircraft > > (e.g. chase view) will not look correct. The camera is tied to > the > > origin. If the origin is at the nose or tail then the plane > > "looks" like it's wagging when it pitches. And if the origin is > > too high or too low again the model can "appear" to be swinging > > like a pendulum or sliding when it should simply be rolling. > > OK, finally I understand the problem. What we need to do, then, is > apply the euler angles to the CG rather than the origin when rotating > the 3D model. We need only two steps: > > 1. have the FDMs report the current CG relative to the origin (if > they > don't already); and > > 2. add a couple of transforms to acmodel.cxx so that we use the CG as >the centre of rotation. > > This will be especially nice for modelling out-of-balance situations > (like 500lb of baggage in the back of a 172) -- everything will come > out looking right. It will also be valuable for aircraft that have > massive balance shifts, like a military aircraft dropping ordnance. > I > don't expect that this will be a big job. What does everyone else > think? > Since this makes the 3D modeler's choice independent of the FDM modeler's choice, it seems the most sensible to me. > > 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 > > __ Do you Yahoo!? Yahoo! Mail Plus - Powerful. Affordable. Sign up now. http://mailplus.yahoo.com ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
RE: [Flightgear-devel] c310 origin
Jim Wilson writes: > Actually, no, well yes this is a good idea. But I don't think it is > necessary. AFAIK the model rotates correctly already. The FDMs give us lon/lat/alt and roll/pitch/hdg. FlightGear then has to apply transformations to make the model appear in the right place and with the right orientation. After reading your posting, I'm trying to remember how, exactly, we do that -- I'm willing to be that we just rotate the model around the origin by the roll, pitch, and heading. That means that the view from inside will be wrong as well, especially in a big plane. 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] c310 origin
On Fri, 13 Dec 2002 13:45:11 -0500 David Megginson <[EMAIL PROTECTED]> wrote: OK, finally I understand the problem. What we need to do, then, is apply the euler angles to the CG rather than the origin when rotating the 3D model. We need only two steps: 1. have the FDMs report the current CG relative to the origin (if they don't already); and Isn't this the way it used to work? Or, how does it work, now? I think we track the initial CG, as well as the current CG. 2. add a couple of transforms to acmodel.cxx so that we use the CG as the centre of rotation. This will be especially nice for modelling out-of-balance situations (like 500lb of baggage in the back of a 172) -- everything will come out looking right. It will also be valuable for aircraft that have massive balance shifts, like a military aircraft dropping ordnance. I don't expect that this will be a big job. What does everyone else think? Of course, the best way *is* to use the same origin for the FDM and the 3D model. If we all agree on an "FDM Frame", then we'll all know where the CG is if we report that number based on the FDM frame (and by FDM frame I mean the nose-tip thing we've been talking about). Euler angles from the FDM should be able to be used directly by the model rendering code I would think, instead of having to do rotations *and* translations for the 3D model if we have different frames of reference. Jon ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] c310 origin
David Megginson writes: > Jim Wilson writes: > > > 2). On a fixed wing aircraft, if origin is anywhere but between the > > wings the external model views that follow along with the aircraft > > (e.g. chase view) will not look correct. The camera is tied to the > > origin. If the origin is at the nose or tail then the plane > > "looks" like it's wagging when it pitches. And if the origin is > > too high or too low again the model can "appear" to be swinging > > like a pendulum or sliding when it should simply be rolling. > > OK, finally I understand the problem. What we need to do, then, is > apply the euler angles to the CG rather than the origin when rotating > the 3D model. We need only two steps: > > 1. have the FDMs report the current CG relative to the origin (if they > don't already); and > > 2. add a couple of transforms to acmodel.cxx so that we use the CG as >the centre of rotation. This is exactly why there was, and I think still is, a cg == center of geometry and a CG == Center of Gravity Both are needed and both are 'aptly' named Can we decide which one gets the UPPER CASE once and for all now ? Norman ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
RE: [Flightgear-devel] c310 origin
David Megginson <[EMAIL PROTECTED]> said: > Jim Wilson writes: > > > 2). On a fixed wing aircraft, if origin is anywhere but between the > > wings the external model views that follow along with the aircraft > > (e.g. chase view) will not look correct. The camera is tied to the > > origin. If the origin is at the nose or tail then the plane > > "looks" like it's wagging when it pitches. And if the origin is > > too high or too low again the model can "appear" to be swinging > > like a pendulum or sliding when it should simply be rolling. > > OK, finally I understand the problem. What we need to do, then, is > apply the euler angles to the CG rather than the origin when rotating > the 3D model. We need only two steps: > > 1. have the FDMs report the current CG relative to the origin (if they > don't already); and > > 2. add a couple of transforms to acmodel.cxx so that we use the CG as >the centre of rotation. > > This will be especially nice for modelling out-of-balance situations > (like 500lb of baggage in the back of a 172) -- everything will come > out looking right. It will also be valuable for aircraft that have > massive balance shifts, like a military aircraft dropping ordnance. I > don't expect that this will be a big job. What does everyone else > think? Actually, no, well yes this is a good idea. But I don't think it is necessary. AFAIK the model rotates correctly already. But with the camera tied to the origin as it is in the chase views, you can get the "appearance" that the model is not rotating correctly. You could tie the camera to the CG offset, but really I don't think it will be noticable for the added effort and math. (The only time it might be noticable is when flying in formation otherwise the background is too far off). It is much easier to just put the origin itself somewhere near where the aircraft would seem to pivot and roll about (approx cg, normally balanced) and leave it at that. The 747 looks fine since Andy made that very change to it a couple weeks ago. Best, Jim ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
RE: [Flightgear-devel] c310 origin
Jim Wilson writes: > 2). On a fixed wing aircraft, if origin is anywhere but between the > wings the external model views that follow along with the aircraft > (e.g. chase view) will not look correct. The camera is tied to the > origin. If the origin is at the nose or tail then the plane > "looks" like it's wagging when it pitches. And if the origin is > too high or too low again the model can "appear" to be swinging > like a pendulum or sliding when it should simply be rolling. OK, finally I understand the problem. What we need to do, then, is apply the euler angles to the CG rather than the origin when rotating the 3D model. We need only two steps: 1. have the FDMs report the current CG relative to the origin (if they don't already); and 2. add a couple of transforms to acmodel.cxx so that we use the CG as the centre of rotation. This will be especially nice for modelling out-of-balance situations (like 500lb of baggage in the back of a 172) -- everything will come out looking right. It will also be valuable for aircraft that have massive balance shifts, like a military aircraft dropping ordnance. I don't expect that this will be a big job. What does everyone else 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] c310 origin
On Fri, 13 Dec 2002 10:09:06 -0800 Andy Ross <[EMAIL PROTECTED]> wrote: Whatever convention we pick should be an easily explainable and identifiable from the *shape* of the airframe only. Not everyone has a POH handy, very few people have W&B or C.G. numbers, and even things like the centerline are subject to argument on some aircraft. True. Referencing the ground plane is especially bad, since the gear are going to compress differently depending on load. *Very* true. I don't think anyone really thinks we should use the ground plane for anything other crashing into. I continue to like "the origin is at the tip of the nose". You'd have to look really, really hard to find an aircraft without an identifiable nose. And finding an aircraft author who doesn't understand the concept is literally impossible. Even for those of us who have access to the structural frame, it wouldn't be hard to bias our config file numbers to reflect an origin at the nose or farthest forward projection. The structural frame numbers are just nice to have and use (when you have them). The X-axis should probably be coincident with the thrust axis centerline for single engine props; for and for non-single-engine aircraft: coincident with a line through the tip of the nose of the fuselage and parallel to the fuselage as it sits "level" (not necessarily as it sits on the ground - think of the DC-3 - but perhaps as it is "level" in cruise, where one might expect the cabin floor to be roughly horizontal). In both cases the Z axis would extend out from the tip of the nose/prop, upward. Jon ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] c310 origin
Andy Ross writes: > Whatever convention we pick should be an easily explainable and > identifiable from the *shape* of the airframe only. Not everyone > has a POH handy, very few people have W&B or C.G. numbers, and even > things like the centerline are subject to argument on some > aircraft. Referencing the ground plane is especially bad, since > the gear are going to compress differently depending on load. > > Remember that many/most 3D model authors aren't particularly > interested in aerodynamics, and may very well be working from > photographs and 3-views only. Forcing these people to look up a > reference datum from an unfamiliar source is only going to > discourage them. Someone has to write an aerodynamic model before a 3D model is any use. The aero designer needs to publish the reference datum she used in a README, and then the 3D designer can simply follow it. Actually, all of the aero designers will have to agree on the datum. I can live with a modified version of Jon's suggestion: x-origin as specified by the TCDS (free online) and/or POH, defaulting to the tip of the nose/spinner when not available. y-origin at the centreline. z-origin at the height of the nose tip. > And, quite honestly, what's the actual advantage to using a > reference datum anyway? No one does weight and balance > calculations in Blender. :) Well, it does give you the location of (say) the back seat on the X-axis. More importantly, once we implement dynamic W&B more easily in FlightGear, it will be nice to have the model mesh up; even now, using the published W&B datum (from the TCDS or the POH) gives you a few numbers more easily in the aero modelling (like where to put the fuel tank) -- if you're using the same datum, errors will be easier to spot and the file will be easier for others to understand. 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] c310 origin
This brings back the discussion a couple weeks ago on the 747 origin. The following is what I got from that thread: 1). The reported "origin" is arbitrary in relation to the FDM's internal workings...ie how pitch/roll/yaw is calculated. The exact position of the 3D model origin can be calculated to be anywhere on the aircraft. 2). On a fixed wing aircraft, if origin is anywhere but between the wings the external model views that follow along with the aircraft (e.g. chase view) will not look correct. The camera is tied to the origin. If the origin is at the nose or tail then the plane "looks" like it's wagging when it pitches. And if the origin is too high or too low again the model can "appear" to be swinging like a pendulum or sliding when it should simply be rolling. In "reality" the model rotates correctly no matter where the origin is defined. This is only a visual effect of translating the camera position from the origin and there's pretty much no way to avoid it other than placing the origin as suggested above. It seems that after the now numerous past discussions, that we should be considering the visual effect first when it comes to 3D representations. This is only about the 3D model. Also, this location will differ substantially with different aircraft (consider the C172 with its wing forward and high, the swept centrally located ERJ-135 wings, or the Concorde's delta wing. It would be hard to make a general statement other than "usually center of the fuselage, and somewhere on the wings" or "just so the model's behavior looks reasonable". I hate to bother the engineers with this appearances stuff. Having grown up in a family of engineers I know how hard it can be to pick out a matching tie in the morning (which explains why most don't wear ties these days :-)). Best, Jim Jon Berndt <[EMAIL PROTECTED]> said: > > That's the hard part. The POH and the TCDS give the standard origin > > on the X axis (the weight and balance reference datum), and the origin > > on the Y axis can be assumed to be the centreline of the plane, but > > where do you put the Z origin? For a single, the thrustline of the > > propeller might be a good choice; > > Yes. > > > alternatively, the cabin floor at the firewall (right in front of the > > rudder pedals) should work for any aircraft, > > eh ... > > > and the ground sort-of works, as long as you're consistent > > about strut compression. > > yecch. > > > I'm not big on rules and standards, but we > > could publish a list of friendly suggestions for all aero and 3D > > modellers: > > > > 1. Put the X axis origin at the published weight-and-balance reference > >datum. > > 1) Respect the manufacturer's structural coordinates, if known. If not, > place the X axis origin at the published weight and balance reference > datum. If not known, place the X axis origin at the tip of the aircraft > nose or propeller hub - whichever is farther forward (not including probes > or antennas). > > > 2. Put the Y axis origin at the centreline of the plane. > > Yes. > > > 3. Put the Z axis origin [where? the ground?]. > > Place the Z axis origin as specified by the manufacturer's structural > coordinate system, if known. If not, place the Z axis origin at the tip of > the nose or propeller hub that the X axis origin is located at. This would > place the origin for the entire axial system at the nose or propeller hub. > > Maybe there should be a few sensible systems and we can specify which one > is to be used. For the FDM, as is known, its the relative distances that > need to be known, with the stipulation that the X axis is positive > backwards, the Y axis is out the right wing, and the Z axis completes that > using the RHR. > > Perhaps the above have problems, too, but it I agree that some kind of > agreement should be reached. > > Jon > -- Jim Wilson - IT Manager Kelco Industries PO Box 160 58 Main Street Milbridge, ME 04658 207-546-7989 - FAX 207-546-2791 http://www.kelcomaine.com ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] c310 origin
David Megginson wrote: > 1. Put the X axis origin at the published weight-and-balance reference >datum. > 2. Put the Y axis origin at the centreline of the plane. > 3. Put the Z axis origin [where? the ground?]. I'll just state my opinion again, and then keep my head down until someone tells me where to move the origin. :) Whatever convention we pick should be an easily explainable and identifiable from the *shape* of the airframe only. Not everyone has a POH handy, very few people have W&B or C.G. numbers, and even things like the centerline are subject to argument on some aircraft. Referencing the ground plane is especially bad, since the gear are going to compress differently depending on load. Remember that many/most 3D model authors aren't particularly interested in aerodynamics, and may very well be working from photographs and 3-views only. Forcing these people to look up a reference datum from an unfamiliar source is only going to discourage them. I continue to like "the origin is at the tip of the nose". You'd have to look really, really hard to find an aircraft without an identifiable nose. And finding an aircraft author who doesn't understand the concept is literally impossible. And, quite honestly, what's the actual advantage to using a reference datum anyway? No one does weight and balance calculations in Blender. :) 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] c310 origin
On Fri, 13 Dec 2002 10:34:00 -0500, David Megginson <[EMAIL PROTECTED]> wrote in message <[EMAIL PROTECTED]>: > Jon Berndt writes: > > > > 1. Put the X axis origin at the published weight-and-balance > > > reference datum. > > > > 1) Respect the manufacturer's structural coordinates, if known. If > > not, place the X axis origin at the published weight and balance > > reference datum. If not known, place the X axis origin at the tip > > of the aircraft nose or propeller hub - whichever is farther > > forward (not including probes or antennas). > > The C310 provides an interesting case here, since the later versions > have a longer nose -- that's why the firewall might make more sense as > a default X origin (or the leading edge of the wings at the root). ..the tradition (at least for single prop planes) I believe is put the origin at the firewall _plus_ an offset, X pointing aft, so that "new longer" powerplants remain "positive". -- ..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] c310 origin
Jon Berndt writes: > > 1. Put the X axis origin at the published weight-and-balance reference > >datum. > > 1) Respect the manufacturer's structural coordinates, if known. If not, > place the X axis origin at the published weight and balance reference > datum. If not known, place the X axis origin at the tip of the aircraft > nose or propeller hub - whichever is farther forward (not including probes > or antennas). The C310 provides an interesting case here, since the later versions have a longer nose -- that's why the firewall might make more sense as a default X origin (or the leading edge of the wings at the root). > > 2. Put the Y axis origin at the centreline of the plane. > > Yes. OK, that's a no-brainer. > > > 3. Put the Z axis origin [where? the ground?]. > > Place the Z axis origin as specified by the manufacturer's structural > coordinate system, if known. If not, place the Z axis origin at the tip of > the nose or propeller hub that the X axis origin is located at. This would > place the origin for the entire axial system at the nose or propeller hub. Sounds reasonable. 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] c310 origin
> That's the hard part. The POH and the TCDS give the standard origin > on the X axis (the weight and balance reference datum), and the origin > on the Y axis can be assumed to be the centreline of the plane, but > where do you put the Z origin? For a single, the thrustline of the > propeller might be a good choice; Yes. > alternatively, the cabin floor at the firewall (right in front of the > rudder pedals) should work for any aircraft, eh ... > and the ground sort-of works, as long as you're consistent > about strut compression. yecch. > I'm not big on rules and standards, but we > could publish a list of friendly suggestions for all aero and 3D > modellers: > > 1. Put the X axis origin at the published weight-and-balance reference >datum. 1) Respect the manufacturer's structural coordinates, if known. If not, place the X axis origin at the published weight and balance reference datum. If not known, place the X axis origin at the tip of the aircraft nose or propeller hub - whichever is farther forward (not including probes or antennas). > 2. Put the Y axis origin at the centreline of the plane. Yes. > 3. Put the Z axis origin [where? the ground?]. Place the Z axis origin as specified by the manufacturer's structural coordinate system, if known. If not, place the Z axis origin at the tip of the nose or propeller hub that the X axis origin is located at. This would place the origin for the entire axial system at the nose or propeller hub. Maybe there should be a few sensible systems and we can specify which one is to be used. For the FDM, as is known, its the relative distances that need to be known, with the stipulation that the X axis is positive backwards, the Y axis is out the right wing, and the Z axis completes that using the RHR. Perhaps the above have problems, too, but it I agree that some kind of agreement should be reached. Jon smime.p7s Description: application/pkcs7-signature
re: [Flightgear-devel] c310 origin
Jim Wilson writes: > Between YASim, JSBsim, and the U-3A 3d-model we've got three > "origins" that represent the approximate position of the aircraft. > JSBsim is 0.2m higher than the 3D model and YASim is 0.2m lower. > It'd be nice if the two FDM's agreed with each other, at least on > the height above ground for the origin. That way I could adjust > the model to work correctly with both. That's the hard part. The POH and the TCDS give the standard origin on the X axis (the weight and balance reference datum), and the origin on the Y axis can be assumed to be the centreline of the plane, but where do you put the Z origin? For a single, the thrustline of the propeller might be a good choice; alternatively, the cabin floor at the firewall (right in front of the rudder pedals) should work for any aircraft, and the ground sort-of works, as long as you're consistent about strut compression. I'm not big on rules and standards, but we could publish a list of friendly suggestions for all aero and 3D modellers: 1. Put the X axis origin at the published weight-and-balance reference datum. 2. Put the Y axis origin at the centreline of the plane. 3. Put the Z axis origin [where? the ground?]. 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
