"Teacherjh" wrote in message
...
To add to this answer (about how an airplane behavs during turns), most
airplanes are designed with dihedral. This means that the wings point up
a
little bit. If you looked at a plane from in front of the nose, the wings
will
form a slight V shape. This makes the "lift" that each wing produces
point a
little bit inward, towards the center of the plane, rather than straight
up.
Now, if the plane goes into a shallow bank, the wing that is lowered will
become more level, and the lift will point straight up, while the wing
that is
raised will become more tilted, and the lift will point more towards the
center
of the plane. More of the lift on this "tilted" wing is "wasted" (in the
sense
of not holding the airplane up). So, since the other wing exerts more
upwards
force, it causes the plane to return to level flight. This is one of the
things that makes an airplane inherently stable in flight.

Separate from this, when an airplane is banked in coordinated flight and
turning, the outer wing (which is the one that is raised to bank the plane
to
make the turn) is actually travelling faster than the inner wing. It has
to,
because that wing is further from the center of the turn. (to see this,
imagine the plane turning so sharply that it's just about pivoting on one
wingtip) The faster wing will produce more lift, and cause the airplane
to
bank in the direction of the turn. This is called "overbanking tendency".

So, there are two opposite tendencies. Dihedral is more important with
shallow
banks (and gentle turns), and the overbanking tendency is more important
with
steeper turns and banks. Somewhere in the middle, they cancel out.

Looking at the nose of the plane, we see the V-shape of the wings, the
dihedral angle. Lift is always perpendicular to the wings. So in flight,
the two lift vectors tilt in and "cross" over the plane itself. In level
flight the horizontal components of the two lift vectors are equal and
opposite. Thus, they cancel and the plane flies straight. Now, if you
bank the plane so that one wing is horizontal, that wing will have no
horizonal lift vector component. But, the other wing will have double. The
result is a big, net horizontal force on the plane. This forces the plane
to the center of the turn the way the force in a string swinging a rock
keeps the rock in a circle. That's what makes a plane able to go in a
circle, not the rudder.