On Thu, 2002-04-18 at 18:23, Andy Ross wrote: > John Wojnaroski wrote: > > Indicated Airspeed (IAS) > > Calibrated Airspeed (CAS) > > Next comes Equivalent Airspeed (EAS) > > Finally TAS > > > > Bottom line question for the FDM' rs: Would you take a few moments > > to comment/explain how your models handle the various airspeeds. > > 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. :) > > 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). > > CAS gets hairy. At low speeds, it's identical to EAS. At higher > speeds, it needs to be corrected for compressibility; and at > supersonic speeds it needs to be corrected for shock wave effects. I > have a good handle on the first two, but the shock stuff is beyond me. > JSBSim had code for doing this, so (after verifying that it agreed > perfectly at sub-mach numbers) I just used that. :)
That code assumes that the pitot probe is placed out in front of the aircraft, so that a cone shaped shock forms ahead of it. It is further assumed that the pitot probe opening is small enough that that portion of the shock in front of it can be assumed to be planar. After that, it's a straightforward application of 1D compressible flow theory. > > Andy > > -- > Andrew J. Ross NextBus 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 -- 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
