Hi, > The FDM interface treats CAS and IAS identically -- the calibration > error, as you point out, is poorly defined and will vary between > installations. All FDMs simply report a single "calibrated airspeed" > value; if other code wants to model the vagaries of a given ASI, then > it's welcome to. :)
Fair enough.. Modern avionics systems display CAS; i.e. the IAS passes thru an air data computer that applies installation/flight envelope corrections. You'll normally find a toggle switch labeled "Air Data Off/On" or something like that so the pilot can see raw airspeed and altitude, if desired > It's also worth pointing out that the FDMs work, internally, with a > real, 3D velocity. So TAS is what you get natively as the projection > of velocity along the aircraft's X axis; everything else is computed > from that. EAS is really simple -- it's just the true speed > multiplied by the square root of the density ratio; it corresponds > directly to a given dynamic pressure (which is the space in which > force constants like drag coefficients are measured). > > Airspeed is measured with respect to the relative wind which is the pitch angle ( horizon to the aircraft X axis) minus the AOA (assuming wings level) True airspeed vector is parallel to the relative wind since it is derived from pitot-static-stagnation pressures. Seems that all three velocities (u,v,w) need to be used to determine the relative wind and thus the airspeed vector. . If the FDMs don't worry about the aircraft's location then whatever ground speed is used for navigational purposes is outside the scope of the FDMs. But if the intent is to provide a weather model then the FDMs need to know the aircraft's location to determine local atmospheric conditions that correspond with the weather model. Then the navigator running in the FMC on the display side and the location used by the FDMs on the sim side must agree. So if the FDMs are computing and reporting EAS then networked entities like the glass displays need to know what densities (standard vs non-standard atmosphere?) were used to convert to EAS or have the FDMs report the TAS value, or some other handshaking scheme to keep the two in sync Thoughts and comments? John W. _______________________________________________ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
