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Re: [Jsbsim-devel] Converting aerodynamic coefficients from AeroRP to cg
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From: Tony P. <ton...@co...> - 2014-10-23 15:32:37
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Correct. Nit: aero center and aerorp are not the same. The aerodynamic center on the wing moves around with LE config, TE config, Mach number, and probably other things as well. The aerorp is chosen as one, constant point to take the moments around to facilitate data collection and reduction. Now, it happens that 25% MAC is pretty close to the aero center for a clean wing at low Mach numbers so it's a convenient choice for aerodynamicists. In reducing collected data that way, real movement of the aero center gets reflected in the measured coefficients and that's the reason you typically won't find any explicit accounting of the change in aero center in aero models. -- Tony Follow your heart, don't give up, and try like hell to not be stupid. Repeat when, not if, the latter occurs. > On Oct 23, 2014, at 7:11 AM, Sean McLeod <se...@se...> wrote: > > Thanks Jon and Tony > > So all the entries in the pitch axis (Cmalpha, Cmde, Cmq and Cmadot) are moments about the aero center (AeroRP). > > Has anyone else tried using the text book examples of the short period approximation and the resulting transfer functions and compared the output from those with the output from JSBSim to see how closely they match during the short period timeframe? > > Whether I make the cg and AeroRP coincident or not I don't get a very good match. > > Cheers > > -----Original Message----- > From: Jon S. Berndt [mailto:jon...@co...] > Sent: Thursday, October 23, 2014 3:24 PM > To: 'Development issues' > Subject: Re: [Jsbsim-devel] Converting aerodynamic coefficients from AeroRP to cg > >> Based on my work with my F-14 model which has CM Basic (alpha) I don't >> think that you can use the built in AeroRP approach if you have values >> for CM(alpha) >> >> JSBSim works out the pitching moment due to Cl based upon the AeroRP - >> CG moment arm; if your CM Basic is the usual "Pitch moment due to >> alpha" >> (see NASA CR-1756 for an example) that is balanced by the offset of >> the centre of pressure (AERORP) from the CG; i.e. building up the >> yaw/pitch moments from a base value (CLBAS, CYBAS, CNBAS). If you use >> the normal JSBSim AeroRP then it will double up the pitching moment as >> it is effectively the same force that JSBSim is calculating based on >> the offset of the Cl from the CoP. >> >> --Richard > > I'm not sure I follow this. In the case where wind tunnel data is used as a basis for an aerodynamic model, the wind tunnel data will be delivered both in a specified reference frame, and given a specified aerodynamic reference point. The force coefficients are independent of location, of course. But the moments specified in the wind tunnel data report will be highly dependent on the moment reference center, because the moment that acts on the aircraft in flight is found this way (not sure of the sign convention at the moment): > > Moment = Moment_about_aero_center + force * (CG_position - AeroRP_position) > > Moment_about_aero_center is what you get from the wind tunnel report, couple with the AeroRP_position. The other part is calculated by JSBSim and summed with the first part, and it has been validated to work correctly (for quite a long time). > > This is essentially all there is for the calculation of aerodynamic moments. > Sometimes in tech reports there is a lot less information given than would be desired - maybe that is what has happened in your case. In the case where you are given only a total aerodynamic moment, it is possible to prevent the force X moment_arm value from being summed into the mix. You would specify that the force should only be supplied at the CG. This is done with an attribute in the aerodynamic force calculation in an axis element. For > instance: > > <axis name="LIFT"> > > <!-- Lift axis coefficients functions --> > > <function name="aero/force/Lift_wbh" apply_at_cg="true"> > <description> Lift due to alpha </description> > <product> > <property>aero/function/ground-effect-factor-lift</property> > <property>aero/qbar-area</property> > <table> > <independentVar lookup="row">aero/alpha-rad</independentVar> > <independentVar > lookup="column">aero/stall-hyst-norm</independentVar> > <tableData> > 0.0 1.0 > -0.09 -0.22 -0.22 > 0.34 1.3 1.05 > 0.36 1.15 1.15 > </tableData> > </table> > </product> > </function> > > .... > > The above would cause the lift force to NOT be summed with the native moment calculated later. Note that I am not sure if this code has made it into FlightGear yet. > > I hope this helps. > > Jon > > > > ------------------------------------------------------------------------------ > _______________________________________________ > Jsbsim-devel mailing list > Jsb...@li... > https://lists.sourceforge.net/lists/listinfo/jsbsim-devel > _______________________________________________ > The JSBSim Flight Dynamics Model project http://www.JSBSim.org _______________________________________________ > > > ------------------------------------------------------------------------------ > _______________________________________________ > Jsbsim-devel mailing list > Jsb...@li... > https://lists.sourceforge.net/lists/listinfo/jsbsim-devel > _______________________________________________ > The JSBSim Flight Dynamics Model project > http://www.JSBSim.org > _______________________________________________ > |