"Mxsmanic" wrote in message
...

How much of a movement in the little ball in the turn indicator
corresponds to something you can feel in a real aircraft?

Pretty much everyone can sense large deflections -- they'll
feel as they're being pushed to one side of the plane. The
more experienced you are, the smaller movements you can
detect without looking.

And how
much of a movement represents an error large enough to affect flight
safety or proper maneuvering?

It is considered to be good style, and it is generally more
efficient, to maintain coordinated flight in most situations.
Better for passengers & easier not to spill your drink.
But in a slip -- used to deliberately increase air resistance
when you are too fast or too high, or to align the plane with
the runway in a crosswind landing -- the plane will be
uncoordinated on purpose; the ball on the inside of the turn
doesn't indicate "an error" at all, nor does it make the
flight any less safe.

A skid -- which doesn't have any purpose outside of
training AFAIK -- is more problematic. This tends to be a
issue on base-final turns when pilots are tempted to use
too much rudder to tighten up the turn if they've
underestimated wind drift or otherwise miscalculated. When
you are in a turn, the inside wing is always moving through
the air a bit slower than the outside wing and so is closer
to stalling. A skidding turn increases this airspeed difference,
and if you're too slow turning base to final, the danger is that
it'll make it more likely that the inside wing will stall and start
a spin. Just where the ball might be when this happens would
depend on your airspeed, so there's no simple "red line" beyond
which you can't push the ball if that's what you're looking for.