Aerodynamic question for you engineers

wrote in news:c3beb705-9fa5-424c-a65e-
:

On Jan 27, 5:12 pm, Tina wrote:
Sorry. Rigid bodies do NOT rotate around their cg if an external
force
is applied whose vector goes thru it.

Drop a yardstick, cg at the 18 inch mark, so that its zero inch edge
hits a table. The center of rotation as a reaction to that force is
the table edge.

You may write an equation that descibes rotation around its cg, and
another that describes translation, but a center of rotation, to many
who deal with such things, is that point on a rotating body whose
translational motion does not include rotation, the body appears to
rotate around it.

In the case I just described, such a point is at the end of the
yardstick.

You are obviously defining center of rotation differenrtly than I am,
but my American Institute of Physics Handbook on page 2-9 talks about
rotation "in which some axis or point remains fixed in space". That
is
the center of rotation. In the several examples I've given that axis,
the center of rotation, is not at the center of gravity.

I am sure the math and classical physics folks use the same
definition. It's perfectly fine to talk abou other ways of
describing
rotation, but engineers who think about it a little, even if they are
pilots, would tend, I expect, tend to agree with AIP handbook if they
are trying to communicate with other engineers.
.
As I claimed earlier, if allowed thusters on a rigid body, I can make
it rotate around ANY point. The table edge in my example could be
replace by such a thruster.

Now, if the forces are removed, you will get no argument from me that
rotation is about the CG. The forces are not removed in the OP's
question.

I think I see where Bertie's coming from. Rotation is
about the CG, while that CG is moving along some line due to external
forces.


What I mean is, is tht the CG is the CG.no matter what. While other
forces may alter the center of rotation ( my spin example, for example)
by application of an eccentric force, the CG is stil the center of that
mass's universe and is ultimately the governer of the rotation.
The spin is a perfec example. the airplane is not sliding down it's CG,
but a point smewhere out along the stalled wing, more than likely.
However, it is still, at the same time, rotating about it's CG.
The earth rotates about it's axis, bvut it also rotates along many
others dictated by their mass. We wobble consderably because of the
moon's pull, for instance, but we're still hinged on our own CG.

Sorry, I;m making it sound more compicated than it is.



Bertie