nd 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 about 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.

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.

[Illustration: H.E., Horizontal equivalent. D., Dihedral angle.]

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.

THE MARGIN OF POWER is the power available above that necessary to
maintain horizontal flight.

THE MARGIN OF LIFT is the height an aeroplane can gain in a given time
and starting from a given altitude. As an example, thus: 1,000 feet the
first minute, and starting from