[Flightgear-devel] Airspeed (indicated vs. calibrated) question.
Anyone have any thoughts or ideas? 1. FlightGear has a property called /velocities/airspeed-kt; does anyone know if this was originally intended to be VCAS, VIAS, VTAS, or what? 2. The pitot system uses /velocities/airspeed-kt as it's input (and seems to assume this value is VIAS.) 3. We tie /velocities/airspeed-kt to v_calibrated_kts in the FDM structure, but the pitot/airspeed systems interprets this value as indicated airspeed. 4. I want to pass in velocity from an external FDM. I can feed in vcas or vias. The FDM structure only has a slot for VCAS, but this is tied to /velocities/airspeed-kt, which seems to be interpreted as VIAS by the rest of the sim. 5. Do we have any code on the FG side that computes VCAS given VIAS or visa versa? 6. If we split out /velocites/airspeed-kt into /velocities/airspeed-vcas-kt and /velocities/airspeed-vias-kt are we going to break a lot of stuff? Right now I have to do something ... either pass in VIAS from the external fdm and have the subsystems work correctly (but it hurts to do this because everything in the FDM structures are calling the value calibrated speed.) Or I try to fix this up and probably make a bigger mess of everything. ??? Curt. -- Curtis Olsonhttp://www.flightgear.org/~curt HumanFIRST Program http://www.humanfirst.umn.edu/ FlightGear Project http://www.flightgear.org Unique text:2f585eeea02e2c79d7b1d8c4963bae2d ___ Flightgear-devel mailing list Flightgear-devel@flightgear.org http://mail.flightgear.org/mailman/listinfo/flightgear-devel 2f585eeea02e2c79d7b1d8c4963bae2d
Re: [Flightgear-devel] Airspeed (indicated vs. calibrated) question.
Curtis L. Olson wrote: 1. FlightGear has a property called /velocities/airspeed-kt; does anyone know if this was originally intended to be VCAS, VIAS, VTAS, or what? 2. The pitot system uses /velocities/airspeed-kt as it's input (and seems to assume this value is VIAS.) This value looks like IAS to me. It's set from YASim via the FGInterface::_set_V_calibrated_kts() function. Note that the difference between IAS and CAS are due solely to gauge/pitot system issues. As far as the FDMs are concerned, they mean exactly the same thing. There is also an equivalent airspeed value that comes out, which is CAS without the mach corrections. I have no idea if this ever makes it to the property system, but FGInterface has a slot for it. A true TAS value* comes out via _set_V_rel_wind(), but I'm not sure where this ends up in properties. * Er... sorta. This is total airspeed velocity magnitude, whereas the pitot would really only measure velocity along its axis... If your suggestion is that we chuck all of this stuff and just pick some standard output props for the FDMs to fill out, then I agree. :) Andy ___ Flightgear-devel mailing list Flightgear-devel@flightgear.org http://mail.flightgear.org/mailman/listinfo/flightgear-devel 2f585eeea02e2c79d7b1d8c4963bae2d
Re: [Flightgear-devel] Airspeed (indicated vs. calibrated) question.
Would this be a valid way to explain away the problem: The conversion between VCAS and VIAS is aircraft specific and is often shown in a table in the POH (and depends on things like flap position.) So FG can't come up with a nice tidy formula to convert between the two values. Thus FlightGear leaves everything as VCAS internally and doesn't allow for a separate VIAS at this point in time? Curt. Curtis L. Olson wrote: Anyone have any thoughts or ideas? 1. FlightGear has a property called /velocities/airspeed-kt; does anyone know if this was originally intended to be VCAS, VIAS, VTAS, or what? 2. The pitot system uses /velocities/airspeed-kt as it's input (and seems to assume this value is VIAS.) 3. We tie /velocities/airspeed-kt to v_calibrated_kts in the FDM structure, but the pitot/airspeed systems interprets this value as indicated airspeed. 4. I want to pass in velocity from an external FDM. I can feed in vcas or vias. The FDM structure only has a slot for VCAS, but this is tied to /velocities/airspeed-kt, which seems to be interpreted as VIAS by the rest of the sim. 5. Do we have any code on the FG side that computes VCAS given VIAS or visa versa? 6. If we split out /velocites/airspeed-kt into /velocities/airspeed-vcas-kt and /velocities/airspeed-vias-kt are we going to break a lot of stuff? Right now I have to do something ... either pass in VIAS from the external fdm and have the subsystems work correctly (but it hurts to do this because everything in the FDM structures are calling the value calibrated speed.) Or I try to fix this up and probably make a bigger mess of everything. ??? Curt. -- Curtis Olsonhttp://www.flightgear.org/~curt HumanFIRST Program http://www.humanfirst.umn.edu/ FlightGear Project http://www.flightgear.org Unique text:2f585eeea02e2c79d7b1d8c4963bae2d ___ Flightgear-devel mailing list Flightgear-devel@flightgear.org http://mail.flightgear.org/mailman/listinfo/flightgear-devel 2f585eeea02e2c79d7b1d8c4963bae2d
Re: [Flightgear-devel] Airspeed (indicated vs. calibrated) question.
Curtis L. Olson wrote: The conversion between VCAS and VIAS is aircraft specific and is often shown in a table in the POH (and depends on things like flap position.) So FG can't come up with a nice tidy formula to convert between the two values. Thus FlightGear leaves everything as VCAS internally and doesn't allow for a separate VIAS at this point in time? Sounds good to me, although I'd still argue for renaming all the properties and removing the FGInterface code (just make the FDMs set them properties themselves). Andy ___ Flightgear-devel mailing list Flightgear-devel@flightgear.org http://mail.flightgear.org/mailman/listinfo/flightgear-devel 2f585eeea02e2c79d7b1d8c4963bae2d
Re: [Flightgear-devel] Airspeed (indicated vs. calibrated) question.
From: Curtis L. Olson snip 6. If we split out /velocites/airspeed-kt into /velocities/airspeed-vcas-kt and /velocities/airspeed-vias-kt are we going to break a lot of stuff? This has been there so long I'd almost suggest depricating it and removing airspeed-kt sometime in the distant future. A massive edit in the base package and c source would take care of FlightGear, but I'd guess it might break some things for external interface users. Can we have the vcas and vias properties and continue to write the vcas value to airspeed-kt? An after thought...maybe we should NOT write airspeed-vias-kt unless it is actually being modeled. Then change all the current references from airspeed-kt to airspeed-vcas-kt so people (modelers, autopilot designers) know what they are actually using. If at some point the FDM models IAS for a particular aircraft, then the gauges and autopilot configurations can be changed accordingly. Best, Jim ___ Flightgear-devel mailing list Flightgear-devel@flightgear.org http://mail.flightgear.org/mailman/listinfo/flightgear-devel 2f585eeea02e2c79d7b1d8c4963bae2d
Re: [Flightgear-devel] Airspeed (indicated vs. calibrated) question.
On Dienstag 15 März 2005 22:10, Andy Ross wrote: Curtis L. Olson wrote: The conversion between VCAS and VIAS is aircraft specific and is often shown in a table in the POH (and depends on things like flap position.) So FG can't come up with a nice tidy formula to convert between the two values. Thus FlightGear leaves everything as VCAS internally and doesn't allow for a separate VIAS at this point in time? Sounds good to me, although I'd still argue for renaming all the properties and removing the FGInterface code (just make the FDMs set them properties themselves). This is a good thing. I think that an FGInterface class should get a base property node which should be the root for this aricraft. Currently this is just the root property node for /. What we need to write then is a document which property nodes are required and which should, if present, be named with a specific name. Also having a consistent naming scheme for units might be a good idea. Where I like the approach we have now is for the position (lat/lon/alt). Since most FDM's do not compute in geodetic coordinates we can keep an interface where the FDM sets the the position in geocentric, geodetic or cartesian coordinates. Greetings Mathias -- Mathias Fröhlich, email: [EMAIL PROTECTED] ___ Flightgear-devel mailing list Flightgear-devel@flightgear.org http://mail.flightgear.org/mailman/listinfo/flightgear-devel 2f585eeea02e2c79d7b1d8c4963bae2d
