It is obvious, I think, that the greater
the span, the greater the mass of air engaged,
and, as already explained, the reaction is partly
the result of the mass of air engaged.
Not only that, but, PROVIDED the chord is not
decreased to an extent making it impossible to
secure the best camber owing to the thickness of
the surface, the greater the aspect ratio, the better
the lift-drift ratio. The reason of this is rather
obscure. It is sometimes advanced that it is
owing to the ``spill'' of air from under the wing-
tips. With a high aspect ratio the chord is less
than would otherwise be the case. Less chord
results in smaller wing-tips and consequently less
``spill.'' This, however, appears to be a rather
inadequate reason for the high aspect ratio producing
the high lift-drift ratio. Other reasons
are also advanced, but they are of such a contentious
nature I do not think it well to go into them
here. They are of interest to designers, but this
is written for the practical pilot and rigger.
5. Stagger.--This is the advancement of the top surface
relative to the bottom surface, and is not, of course,
applicable to a single surface, i.e., a monoplane.
In the case of a biplane having no stagger, there
will be ``interference'' and consequent loss of
Efficiency unless the gap between the top and bottom
surfaces is equal to not less than 1 1/2 times the
chord. If less than that, the air engaged by the
bottom of the top surface will have a tendency
to be drawn into the rarefied area over the top
of the bottom surface, with the result that the
surfaces will not secure as good a reaction as would
otherwise be the case.
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