"Michael" wrote in message
om...
"Gary L. Drescher" wrote
Two differences. First, the ball is not given a shove to keep it
oscillating back and forth even when the plane is going straight (any
such
oscillation damps out almost instantly). Second, the ball is
constrained to
move in only one plane. The idea of the pendulum is to start it
swinging
side to side (say, east-west if you're heading north) but let it move
freely; then, if the plane yaws, the pendulum is still swinging
east-west,
so the yaw is detectable. At least in principle.

OK, now I get it. You're using the principle that the plane of
oscillation in a pendulum is rigid in space, just like the plane of
rotation of a gyroscope. Sure, that makes a lot of sense. Should
work just like a gyro. Only issue is how you're going to keep it
swinging. You would need a drive system and an escapement.

Michael

I think the pendulum's 'rigidity in space' is a qualified
circumstance, and depends upon the accelerations imposed
upon it.

Consider the typical bob-on-a-string pendulum. At the top of
the swing the bob has zero velocity, maximum potential energy,
at the bottom, maximum velocity and minimum potential energy.

Potential energy with respect to what? - with respect to
whatever net forces are involved, which are also the forces
that cause the bob to accellerate to maximum, and back to 0.

What is there about the pendulum bob that will cause it to
swing back in the same plane as it was formerly swinging?
Normally it will be its position relative to the pendulum
anchor and with respect to the external forces upon it.

There are other ways to make pendulums, of course,
but I think you'll find they all share this limitation.

Move the anchor point in some direction perpendicular
to the direction of the forces, or change the direction of
the forces, and you will in general change the plane in
which the pendulum swings. Both of these can be
expected to occur in a moving aircraft, which should
make the pendulum useless.