On Thu, 2002-04-18 at 17:57, John Wojnaroski wrote: > > ----- Original Message ----- > From: John Wojnaroski > To: [EMAIL PROTECTED] > Sent: Thursday, April 18, 2002 3:02 PM > Subject: Airspeeds? > > > Hi, > > On the subject of airspeeds: > > Indicated Airspeed (IAS) is calculated based on the difference between > stagnation pressure in the pitot tube and the static pressure. formula is > square root of 2 x [ stag - static ] / air density, if I recall correctly. > > now to get to true airspeed (TAS) we pass through a couple of "filters" > > Calibrated Airspeed (CAS) is IAS corrected for installation and position of > the sensors which can be somewhat > complex and it also varies during high AOAs and beta's.
Variation with alpha and beta is a product of the pitot-static system, so these errors will be apparent in IAS as well. > Either you have an > air data computer that handles the correction > for you or a few numbers called out in the flight manual to apply depending > on your flight regime. For light aircraft it > is probably just a couple of knots to add or subtarct from the indicated. > > Next comes Equivalent Airspeed (EAS) which is CAS corrected for > compressibily (F factor).Which can be ignored if you plan to stay below 200 > kts and 10,000 feet. You can find a table on most flight computer wheels. > the result is to reduce > the TAS. If your trying to do precise navigation at higher altitudes and > airspeeds you need to worry about this. > > Finally TAS is EAS corrected for temperature and pressure (density > altitude). Now this is important even for those in a c172 since you use TAS > to compute your flight plan and adjust ground speeds for the winds at your > flight altitude. > > So for the more complex equipment : IAS=>CAS=>EAS=>TAS =>Mach number > > For the smaller and lighter: IAS=>TAS will work the majority of the time For a simulator, IAS is impossible to compute in a generic way since the variation from CAS will be different from airplane to airplane. > > A general formula for the conversion is 1.7 * density-altitude /1000 gives > percent of change. Say at 10,000 feet an IAS of 100knots yields a TAS = 100 > * (1.0 + .17) = 117. The factor 1.7 increases to about 2.1 at 30,000 feet. > Note it is density altitude ( temp and pressure ) used to make the > conversion. > > Bottom line question for the FDM' rs: Would you take a few moments to commen > t/explain how your models handle the various airspeeds. > Is the calibrated for a c172? Is the EAS to TAS done outside the FDMs? > Are there installation corrections for the other aircraft models (dc-3, 747, > x-15, c310, etc)? We do not do installation corrections at all, just give CAS as IAS. (This is usually what the instrument is trying to show anyway) All the forces and moment calculations depend on TAS, so that is done inside the FDM's. We compute EAS as well. > > Ultimately, the FMC in the glass displays has to have the same set of > environmental data as the flight models by way of a common database or > pressure and temp parameters used in the calculations. Right now, it looks > like the values identified are not all "reported" across the FDM interface. > > Regards > 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
