In the case of B, you will note that the R R is not directly
opposed to gravity. This results in the appearance of M,
and so the resultant direction of motion of the aeroplane
is no longer directly forward, but is along a line the resultant
of the thrust and M. In other words, it is, while flying
forward, at the same time moving sideways in the direction M.
In moving sideways, the keel-surface receives, of course,
a pressure from the air equal and opposite to M. Since
such surface is greatest in effect towards the tail, then the
latter must be pushed sideways. That causes the aeroplane
to turn; and, the highest wing being on the outside of the
turn, it has a greater velocity than the lower wing. That
produces greater lift, and tends to tilt the aeroplane over
still more. Such tilting tendency is, however, opposed by
the difference in the H.E.'s of the two wings.
It then follows that, for the lateral dihedral angle to
be effective, such angle must be large enough to produce,
when the aeroplane tilts, a difference in the H.E.'s of the
two wings, which difference must be sufficient to not only
oppose the tilting tendency due to the aeroplane turning,
but sufficient to also force the aeroplane back to its original
position of equilibrium.
It is now, I hope, clear to the reader that the lateral
dihedral is not quite so effective as would appear at first
sight. Some designers, indeed, prefer not to use it, since its
effect is not very great, and since it must be paid for in loss
of H.
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