David Luff wrote: > Jon Berndt wrote: > > > Well, to rotate the aircraft realistically the refference point should > > > be known by the 3D modellers, but that aside. > > > > The rigid body rotates about the CG, not the aero ref. pt. > > What about rotation (the taking-off one)? Surely in that case it rotates > about the axles?
You're both right. Any point can be used as an origin, and any motion of a rigid body can be decomposed into a set of translations through space and rotations about that origin. There's really nothing magic about the center of gravity. It's just another point in the body's coordinate system. It's useful as a teaching aid because, in the absence (!) of any interaction, continuous (!) unaccelerated (!) motion of a rigid body can be captured by a constant velocity and a constant rotation speed. That's not true of other reference points (where "unacclerated" motion requires changing speeds and rotations). In the real world, bodies interact and accelerate, so the c.g. isn't quite as useful as a magic reference point (although it still simplifies the math internally). Here's a really quick thought experiment to make the point: Even if the aircraft is "motionless" on the ground, it's still rotating with a period of 24 hours about the center of the earth. The center of the Earth is most certainly *not* the c.g. of the aircraft. :) [This is not to say that current FDMs bother to model coriolis effects. I know for a fact that YASim doesn't.] Andy -- Andrew J. Ross NextBus 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
