On Thu, 2002-04-18 at 21:02, John Wojnaroski wrote: > 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
Are you sure that isn't to switch between air data and data from the inertial system? > > > 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) No. What you describe is much closer to flight path angle, or the angle at which the aircraft is descending or ascending. For steady state conditions, flight path angle can be computed exactly as you describe. Angle of attack is, by definition, the angle between the aircraft X axis and the relative wind in the pitch axis. > 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. . They are indeed. Vt = sqrt ( u*u + v*v + w*w ) > > If the FDMs don't worry about the aircraft's location We don't. 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. The weather data should come from outside the FDM's. We need the local temperature, pressure, and density along with wind information so that the aircraft dynamics can be computed correctly. > > 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 JSBSim and (I believe) YASim can accept externally supplied atmosphere data but also have internal models of the standard atmosphere. We only need to agree (during FG startup) who the source is going to be. At any rate, the data available on the bus should be the same as what's used by the FDM's. > > Thoughts and comments? > John W. > > > _______________________________________________ > Flightgear-devel mailing list > [EMAIL PROTECTED] > http://mail.flightgear.org/mailman/listinfo/flightgear-devel -- 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
