# velocity vector question

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i had been using a simple methid of computing velocity vector for a hud display; (A:INCIDENCE ALPHA, degrees)(A:INCIDENCE BETA,degrees)when a read a couple of posts by more talentented hud designers using; (A:Velocity body X, m/s) (A:Velocity body Z, m/s) atg2 rddg (A:Velocity body Y, m/s) (A:Velocity body Z, m/s) atg2 rddg i ran both vectors together ( a test before switching to the new method) and the vectors rode ontop of each other all the way, so what is the difference? why use the more complex 'new' method?

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There are a number of different velocities identified in Parameters.doc. These include velocities relative to aircraft body VELOCITY BODY (VB) and relative to the world VELOCITY WORLD (VW). I've tried to illustrate these in the following figure for an aircraft climbing at a speed V, and at an angle PLANE PITCH DEGREE (Theta), with an angle of incidence INCIDENCE ALPHA (Alpha) - ignoring the angle Beta for clarity.http://forums.avsim.net/user_files/139181.jpgVBx = V * sin(Alpha)VBz = V * cos(Alpha)therefore tan(Alpha) = VBx/VBz (because tan = sin/cos)Alpha = atan(VBx/VBz) and atan2(VBx, VBz)so that I believe the two snippets will give the same result.I suppose these question is the precise velocity vector you want - relative to the aircraft's body or relative to earth?

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There was some confusion about the difference between beta and sideslip at some point, but I agree that there's no reason why you shouldn't be able to use the beta variable instead of the more complex version. At one point I was using complex version and then switched to beta myself.--Jon

thanks!

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