Weathervaning

"Robert Moore" wrote in message
. 7...
But I think that the discussion was weathervaning into the
actual wind, not the relative wind.

That's not how I read it. Alex was asking about flight on final approach in
a crosswind. In that situation, aileron by itself to maintain groundtrack
along the extended centerline will definitely cause the relative wind to not
be aligned with the longitudinal axis. That lack of alignment then results
in imbalanced forces on the vertical stabilizer, which then results in
weathervaning.

If I bank away from the
wind, does the airplane then weathervane into or away from
the wind?

Away from the the Earth-relative wind, but into the airplane-relative wind.

The only thing that can cause the airplane to
weathervane into the actual wind is for the wheels to be in
contact with the ground.

If you mean "the only thing that can cause the airplane to weathervane into
the actual wind without any other control inputs", then yes...I'd have to
agree with that. I don't think that's the question originally posed though.

(Nitpicking

Without the qualification I mention, banking into the actual wind causes a
slip in the direction of the actual wind, which causes the relative wind to
come from the same direction as the actual wind, which would cause
weathervaning into the actual wind. Without enough aileron input, the plane
wouldn't actually weathervane all the way around to align the longitudinal
axis with the actual wind, but the airplane certainly would have
weathervaned toward that direction.

It's all about your frame of reference and your other assumptions. Lacking
the necessary assumptions, one is free to imagine situations where the
statement is not true.

(End nitpicking)

Without the pivot, a weathervane doesn't weathervane.

True. However, even when not on the ground, there is a pivot. It just
happens to be at the CG rather than the landing gear. Also, the "wind"
that's relevant for the question of weathervaning changes from being the
wind relative to the Earth, to being the wind relative to the airplane.

Pete