John Denker a écrit : > On 11/22/2007 07:28 AM, LeeE wrote: > > > FWIW I also spotted that the coded hard-coded hud has problems > > around headings of 180 deg so that if you are heading slightly to > > one side of 180 deg and there's enough of a cross-wind to swing the > > direction indicator to the other side of 180 deg it disappears, > > probably due to a wrap-around type error. > > Sounds like a bug to me. It could be a bug in the FDM (which seems > kinda unlikely) or it could be a bug in how the HUD uses the > information. > > Bugs like this are par for the course when such things are quantified > in the Euler-angle representation or the axis-angle representation. > For a lumbering bomber you can get away with using a lame > representation, but for something more maneuverable you would be much > better off using the geometric algebra representation aka Clifford > algebra aka multivectors aka quaternions. If you think you've got > problems when the heading angle goes to 180 degrees, think about what > happens when the pitch angle goes to ±90 degrees. > > > Yaw, to me means the difference between the direction that the > > aircraft is pointing and the direction that it is moving. > > Moving? From what follows I gather that means moving relative to the > airmass (not moving relative to the ground). It is important to > think clearly and speak clearly, so as to not blur this distinction. > > > Flying directly into, or with, any wind would produce no yaw but > > flying in a cross-wind would produce yaw. When sitting stationary > > on the ground, any cross-winds would result in an effective > > airspeed, so yes, I would expect there to be a large yaw element > > under those circumstances. > > That's the slip angle. It is almost universally denoted by β (beta). > It's in the tree as /orientation/side-slip-deg > /orientation/side-slip-rad which are computed by the FDM. > > If that variable doesn't show a large slip angle when parked in a > crosswind, that's a nasty bug. > > You can easily enough check whether the FDM is calculating this > correctly by recomputing it yourself. It's just the angle between > the relative wind vector (projected onto the XY plane) and the > heading vector. It's trivial to calculate: u = > /fdm/jsbsim/velocities/u-aero-fps v = > /fdm/jsbsim/velocities/v-aero-fps beta = atan2(v, u) convert from > radians to degrees if you wish ... although (as mentioned above) for > most purposes you're better off using the (u,v) vector as a vector, > rather than converting it to an angle. > > If the FDM is not calculating u-aero and v-aero properly and/or not > calculating β properly and/or not putting β in > /orientation/side-slip-rad then it is a very serious bug. > > Please don't call it the yaw angle. Please call it either slip angle > or β. Note that the thing some pilots call a yaw string really > should be called a /slip string/. > http://www.av8n.com/how/htm/multi.html#sec-slip-string > > > As to what I would do with it - I assume you are asking out of > > curiosity - it's needed to set the steering in tandam/quadracyle > > landing gear aircraft so that they can take-off and land in > > cross-winds. > > For that you need the motion of the airplane relative to the *ground* > not relative to the airmass. That's different. That's why I asked > what it would be used for. > > I'm not sure what that quantity should be called. It's not the yaw > angle. I reckon it's just the difference between the slip angle and > the wind-correction angle. This item you need to calculate for > yourself, since it's not in the tree AFAICT. It's trivial to > calculate u = /fdm/jsbsim/velocities/u-fps v = > /fdm/jsbsim/velocities/v-fps angle = atan2(v, u) ... although (as > mentioned above) for most purposes you're better off using the (u,v) > vector as a vector, rather than converting it to an angle.
Just to add my two cents, I added a Doppler panel to the A-6E (on the navigator's right console) showing the angle between _heading_ and _true track_ which mostly corresponds to the wind induced drift. You can see the nasal code in data/Aircraft/A-6E/Nasal/doppler.nas. All the best, Alexis ------------------------------------------------------------------------- This SF.net email is sponsored by: Microsoft Defy all challenges. Microsoft(R) Visual Studio 2005. http://clk.atdmt.com/MRT/go/vse0120000070mrt/direct/01/ _______________________________________________ Flightgear-devel mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/flightgear-devel
