Re: [Flightgear-devel] Fw: Airspeeds?
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
Re: [Flightgear-devel] Fw: Airspeeds?
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. RossNextBus 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
Re: [Flightgear-devel] Fw: Airspeeds?
Hi, Bottom line question for the FDM' rs: Would you take a few moments to comment/explain how your models handle the various airspeeds. Just a quick thank you for the info put. To make the FG/OpenGC interface effective will require a little handshaking on both ends to sync up. Back in a few weeks with some ideas and options. Regards John W. ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] Fw: Airspeeds?
On 19 Apr 2002 05:25:24 -0700, Tony Peden [EMAIL PROTECTED] wrote in message 1019219125.18988.16.camel@raptor: 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. ... For a simulator, IAS is impossible to compute in a generic way since the variation from CAS will be different from airplane to airplane. ... We do not do installation corrections at all, just give CAS as IAS. (This is usually what the instrument is trying to show anyway) ..most planes have some info on IAS - CAS conversion, placarded or in manuals etc, these form workable starting points? ..for magnetic compass manouvering errors: compare it with a gyro in standard rate turns, double rate turns etc, both righthand and lefthand. Video tape the pair. And chances are we want this tape for all magnetic latitudes... ;-) -- ..med vennlig hilsen = with Kind Regards from Arnt... ;-) ...with a number of polar bear hunters in his ancestry... Scenarios always come in sets of three: best case, worst case, and just in case. ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
[Flightgear-devel] Fw: Airspeeds?
- 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. 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 For the smaller and lighter: IAS=TAS will work the majority of the time 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)? 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
