It is not practicable to have a gap of much
more than a distance equal to the chord, owing
to the drift produced by the great length of struts
and wires such a large gap would necessitate.
By staggering the top surface forward, however,
it is removed from the action of the lower surface
and engages undisturbed air, with the result that
the efficiency can in this way be increased by
about 5 per cent. Theoretically the top plane
should be staggered forward for a distance equal
to about 30 per cent. of the chord, the exact
distance depending upon the velocity and angle
of incidence; but this is not always possible to
arrange in designing an aeroplane, owing to difficulties
of balance, desired position, and view of
pilot, observer, etc.
6. Horizontal Equivalent.--The vertical component of
the reaction, i.e., lift, varies as the horizontal
equivalent (H.E.) of the surface, but the drift
remains the same. Then it follows that if H.E. grows
less, the ratio of lift to drift must do the same.
A, B, and C are front views of three surfaces.
A has its full H.E., and therefore, from the point
of view from which we are at the moment considering
efficiency, it has its best lift-drift ratio.
B and C both possess the same surface as A,
but one is inclined upwards from its centre and
the other is straight but tilted. For these reasons
their H.E.'s are, as illustrated, less than in the
case of A. That means less vertical lift, and,
the drift remaining the same (for there is the
same amount of surface as in A to produce it),
the lift-drift ratio falls.
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