us, I think, that the
greater the span, the greater the mass of undisturbed air engaged, and,
as already explained, the reaction is partly the result of the mass of
air engaged. I say "undisturbed" advisedly, for otherwise it might be
argued that, whatever the shape of the surface, the same mass of air
would be engaged. The word "undisturbed" makes all the difference, for
it must be remembered that the rear part of the underside of the surface
engages air most of which has been deflected downwards by the surface
in front of it. That being so, the rear part of the surface has not the
same opportunity of forcing; the air downwards (since it is already
flowing downwards) and securing there from an upwards, reaction as has
the surface in front of it. It is therefore of less value for its area
than the front part of the surface, since it does less work and secures
less reaction--_i.e._, lift. Again, the rarefied area over the top of
the surface is most rare towards the front of it, as, owing to eddies,
the rear of such area tends to become denser.

[Illustration]

Thus, you see, the front part of the surface is the most valuable from
the point of view of securing an upwards reaction from the air; and so,
by increasing the proportion of front, or "span," to chord, we increase
the amount of reaction for a given velocity and area of surface. That
means a better proportion of reaction to weight of surface, though the
designer must not forget the drift of struts and wires necessary to
brace up a surface of high aspect ratio.

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 higher 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, a