Menu
▾
▴
Re: [Jsbsim-devel] Jsbsim-devel Digest, Vol 136, Issue 3
|
From: Sean M. <se...@se...> - 2017-12-06 09:45:51
|
Hi In terms of Marta’s observations regarding the moments can anyone clarify whether the moments need to always be entered in the config file as body axes moments as it appears from the source code in FGAerodynamics.cpp? The JSBSim reference manual - http://jsbsim.sf.net/JSBSimReferenceManual.pdf explicitly mentions the option of body axes and wind axes for the forces but doesn’t explicitly mention the axes that can/should be used for the moments. Although my reading of the reference manual – “All three systems accept ROLL, PITCH, YAW axis definitions.” Implies that the moments should be in the same axes as the forces. But it doesn’t appear to be code that way, is it a bug in the code or missing documentation that should mention that the moments always have to be defined using body axes? Regards From: Marta Marimon [mailto:mar...@vt...] Sent: Friday, December 1, 2017 5:01 PM To: jsb...@li... Subject: Re: [Jsbsim-devel] Jsbsim-devel Digest, Vol 136, Issue 3 Hi Sean, Thank you very much for your clarifications. That is exactly what I have just realised, DRAG/SIDE/LIFT in wind axes, X/Y/Z in body axes, being body and wind axes related through both alpha and beta rotations, and then AXIAL/SIDE/NORMAL in body axes but being AXIAL = -X and NORMAL = -Z. So my assumption was wrong. There is no option for stability axes. It would actually be good to be able to input the aerodynamic data in stability axes since usually from wind-tunnel testing the data is in stability axes. Another important aspect to note, as stated in line 220 of FGAerodynamics.cpp, is that DRAG and LIFT values are positive up and aft (wind axes rotated 180deg about Y-axis). Now, regarding the moments, I realised that they are directly calculated from the input moments plus the contribution of the forces (r x F) using the forces defined in body axes. Therefore the total moments at CG are calculated in body axes, which means that the input (ROLL/PITCH/YAW) must be in body axes. Again, my previous understanding was wrong. So we must always input the aerodynamic data in body axes regarding the moments and for the forces we can choose between the three options aforementioned. Is that right? Kind regards, Marta Marimon Flight Mechanics & Aerospace Engineer VTOL Technologies Ltd From: jsb...@li...<mailto:jsb...@li...> Sent: 01 December 2017 14:10 To: jsb...@li...<mailto:jsb...@li...> Subject: Jsbsim-devel Digest, Vol 136, Issue 3 Send Jsbsim-devel mailing list submissions to jsb...@li... To subscribe or unsubscribe via the World Wide Web, visit https://lists.sourceforge.net/lists/listinfo/jsbsim-devel or, via email, send a message with subject or body 'help' to jsb...@li... You can reach the person managing the list at jsb...@li... When replying, please edit your Subject line so it is more specific than "Re: Contents of Jsbsim-devel digest..." Today's Topics: 1. Re: Aerodynamic Axis Definition (Sean McLeod) ---------------------------------------------------------------------- Message: 1 Date: Fri, 1 Dec 2017 14:08:23 +0000 From: Sean McLeod <se...@se...> To: Development issues <jsb...@li...> Subject: Re: [Jsbsim-devel] Aerodynamic Axis Definition Message-ID: <62F81E4F38FD2F47AD0A9CC722BEEF452F39E00D@mbx01> Content-Type: text/plain; charset="utf-8" Hi Marta In terms of your question regarding the differences between the DRAG,SIDE,LIFT axes and AXIAL,SIDE,NORMAL axes you assumed that the AXIAL,SIDE,NORMAL axes were possibly stability axes. Looking at the code in FGAerodynamics.cpp and in particular the Run method I don?t think the AXIAL,SIDE,NORMAL axes are stability axes. My understanding is Body (X,Y,Z) -> Wind (DRAG,SIDE,LIFT) involves rotating around both alpha and beta and that Body (X,Y,Z) -> Stability () involves only rotating around alpha, i.e. the difference between Wind and Stability axes being beta. Now if you look at the switch statement starting on line 226 you?ll see the differences between the 3 axes types in JSBSim, i.e. atBodyXYZ, atLiftDrag, atAxialNormal. Where atLiftDrag == Wind axes. http://jsbsim.sourceforge.net/JSBSim/FGAerodynamics_8cpp_source.html [cid:image001.png@01D36ABE.85DD7510] Tb2w ? is the transform from body to wind using alpha and beta. So both atAxialNormal and atBodyXYZ apply the same transform Tb2w to convert their forces into the wind axes. The only difference between atAxialNormal and atBodyXYZ seems to be a sign convention/difference for the X and Z axes. Cheers From: Sean McLeod [mailto:se...@se...] Sent: Friday, December 1, 2017 2:48 PM To: Development issues <jsb...@li...> Subject: Re: [Jsbsim-devel] Aerodynamic Axis Definition Hi Marta In terms of your question regarding how JSBSim decides which axis type is in use in the config file take a look at the FGAerodynamics::DetermineAxisSystem method in FGAerodynamics.cpp. http://jsbsim.sourceforge.net/JSBSim/FGAerodynamics_8cpp_source.html It does more than simply look at the first axis name encountered. It runs through all axis elements in the config file and checks to confirm that you are using a consistent set. So just make sure that you use a consistent set. Cheers From: Marta Marimon [mailto:mar...@vt...] Sent: Friday, December 1, 2017 12:32 PM To: jsb...@li...<mailto:jsb...@li...> Subject: [Jsbsim-devel] Aerodynamic Axis Definition Hello everyone, I would like to clarify the definition of the aerodynamics axis systems. I have seen similar posts discussing about this but I couldn?t get a clear answer. For the three group sets available is the following correct? * DRAG, SIDE, LIFT : Wind axes --> X aligned with the total relative wind vector (True Airspeed) (pointing forward in normal flight), Z perpendicular to the X and remaining within the aircraft symmetry plane (pointing downward in normal flight), Y completes a right hand coordinate system (pointing to the right wing in normal flight). * AXIAL, SIDE, NORMAL: In the manual it says body axes, but I assume they are stability axes. If that is correct --> X pointing into the relative wind vector projected onto the aircraft XZ symmetry plane, Z perpendicular to the X and remaining within the aircraft XZ symmetry plane, Y pointing out the right wing. * X, Y, Z: Body axes --> X aligned with the aircraft longitudinal axis pointing forward, Z in the symmetry plane pointing downward, Y towards the right wing. So this is basically the typical convention in aeronautics, but might not be the one used in JSBSim. Could anyone clarify me that? Specially the DRAG/SIDE/LIFT and AXIAL/SIDE/NORMAL group sets? Then, I understand that depending on the first name encountered/read in the config file, one system or another is used to load the aerodynamic data. However, what about the moments? Because the names (ROLL/PITCH/YAW) are unique. Are they also associated with the same coordinate system as the forces? For example: * If we input DRAG/SIDE/LIFT, the moments ROLL/PITCH/YAW will be in wind axes * If we input AXIAL/SIDE/NORMAL, the moments ROLL/PITCH/YAW will be in whatever axes (stability?) * If we input X/Y/Z, the moments ROLL/PITCH/YAW will be in body axes Which means that we have to define first the forces than the moments for JSBSim to know what axes to use. Or is there only one single coordinate system for input the ROLL/PITCH/YAW data? If so which one? I read that somewhere and I am confused. This is very important to build the model right! If you could explain me, that will be great. Thank you in advance. Regards, Marta Marimon Flight Mechanics & Aerospace Engineer VTOL Technologies Ltd ________________________________ -------------- next part -------------- An HTML attachment was scrubbed... -------------- next part -------------- A non-text attachment was scrubbed... Name: image001.png Type: image/png Size: 38452 bytes Desc: image001.png ------------------------------ ------------------------------------------------------------------------------ Check out the vibrant tech community on one of the world's most engaging tech sites, Slashdot.org! http://sdm.link/slashdot ------------------------------ Subject: Digest Footer _______________________________________________ Jsbsim-devel mailing list Jsb...@li... https://lists.sourceforge.net/lists/listinfo/jsbsim-devel ------------------------------ End of Jsbsim-devel Digest, Vol 136, Issue 3 ******************************************** ________________________________ [Image removed by sender. AVG logo]<http://www.avg.com/internet-security> This email has been checked for viruses by AVG antivirus software. www.avg.com<http://www.avg.com/internet-security> |
