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On Sun, Feb 17, 2002 at 04:15:01PM -0600, Jon S. Berndt wrote: > We are talking about simulating an aircraft, about an > aircraft-centric phenomena, and about a phenomena that is normally reported > by humans in a particular way. Sitting on the runway, pointed with a heading > of 20 degrees, and with a wind of 20 knots from 20 degrees (as is normally > reported, that is, the *from* direction is given), the aircraft sees 20 > knots headwind. This equation in FGTranslation shows this nicely: > > vAeroUVW = vUVW + State->GetTl2b()*Atmosphere->GetWindNED(); You are right. It is a meteorological convention to report wind with a 'from' heading. That's just what it is: a convention. In the same vein, it is a convention to report vertical winds as 'to' headings. Although vertical heading is binary, it is reported as 'to-up', not 'from-down'. So if you want to stick to the meteo conventions, your horizontal components should be 'from' components, while the vertical component should be 'to'. Getting confused yet? :-) Mathematically speaking, it is just a sign issue. Neither way is more 'mathematically correct' than the other. That's why it is possible to debate this topic for years without reaching consensus. My view is simply this: air is a moving body, just like every other moving body. It has a heading and velocity, just like every other moving body. I have no hard arguments for this, it just seems to me the logical way to think of it. I doubt that physicists use from-headings when they model wind as a three-dimensional vector, but I could be wrong. If there is a convention, it is not one used in general meteorology. At least I've never seen any of our local weathermen report wind as North and East components. :-) > I know that the wind vector that originates in the NED axis points *at* 200 > degrees and that the north component of this vector would be a negative > number. But, what happens if you add the aircraft velocity and the wind > velocity vectorially? You sure don't get the aircraft total sensed velocity > vector. Depends how you define the aircraft's velocity vector. In terms of aerodynamic modelling, the aircraft is 'mounted' on the air, and moves relative to it. Add the wind vector, and you get true ground speed. If you want to go back from true ground speed to relative speed, you subtract the wind. Does that seem awkward to you? > Of course you can do it a couple of ways. It just needs to be documented. We > have fallen short there, apparently. As a last resort you could rename and clone the wind properties: /environment/wind-down-fps becomes both wind-from-down-fps and wind-to-up-fps. :-) - -- Regards, "I RADIS, do you?" =Martin= http://www.iradis.org/ PGP: FE87448B DDF8 677C 9244 D119 4FE0 AE3A 37CF 3458 FE87 448B From: Martin van Beilen <[EMAIL PROTECTED]> To: [EMAIL PROTECTED] Subject: Re: [Flightgear-devel] Wind confusion. In-Reply-To: <005501c1b800$8c154700$[EMAIL PROTECTED]>; from [EMAIL PROTECTED] on Sun, Feb 17, 2002 at 04:15:01PM -0600 X-S-Issue: [EMAIL PROTECTED] 2002/02/18 11:20:58 a789bb439a5d67ea956cbb1380001bc5 -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.0.6 (GNU/Linux) iEYEARECAAYFAjxw1ZIACgkQN880WP6HRIuQEgCeMGsKbfd2O7Ao9B5TAQSv2nNJ blIAnRqyvQR3gEAW69X4Skxd0HEtF110 =FM8s -----END PGP SIGNATURE----- _______________________________________________ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
