component of the reaction produced by the action of driving through the
air a surface inclined upwards and towards its direction of motion
_plus_ the horizontal component of the reaction produced by the
"detrimental" surface _plus_ resistance due to "skin-friction."
Sometimes termed "head-resistance."

=Drift, Active=--Drift produced by the lifting surface.

=Drift, Passive=--Drift produced by the detrimental surface.

=Drift= (_of a propeller_)--Analogous to the drift of an aeroplane.
It is convenient to include "eddies" and "cavitation" within this term.

=Drift, to=--To be carried by a current of air; to make leeway.

=Dive, to=--To descend so steeply as to produce a speed greater than the
normal flying speed.

=Dope, to=--To paint a fabric with a special fluid for the purpose of
tightening and protecting it.

=Density=--Mass of unit volume; for instance, pounds per cubic foot.

=Efficiency=--

Output
------
Input.

=Efficiency= (_of an aeroplane as distinct from engine and propeller_)--

Lift and Velocity
---------------------------
Thrust (= aeroplane drift).

=Efficiency, Engine=--

Brake horse-power
----------------------
Indicated horse-power.

=Efficiency, Propeller=--

Thrust horse-power
--------------------------------
Horse-power received from engine
(= propeller drift).

NOTE.--The above terms can, of course, be expressed in foot-pounds. It
is then only necessary to divide the upper term by the lower one to find
the measure of efficiency.

=Elevator=--A controlling surface, usually hinged to the rear of the
tail-plane, the operation of which turns an aeroplane about an axis
which is transverse to the direction of normal horizontal flight. [8]

=Empennage=--See "Tail-plane."

=Energy=--Stored work. For instance, a given weight of coal or petroleum
stores a given quantity of energy which may be expressed in foot-pounds.

=Extension=--That part of the upper surface extending beyond the span of
the lower surface. [9]

=Edge, Leading=--The front edge of a surface relative to its normal
direction of motion. [10]

=Edge, Trailing=--The rear edge of a surface relative to its normal
direction of motion. [11]

=Factor of Safety=--Usually taken to mean the result found by dividing
the stress at which a body will collapse by the maximum stress it will be
called upon to bear.

=Fineness= (_of stream-line_