ally operates the ailerons and elevator. The
"joy-stick".

Cavitation, Propeller--The tendency to produce a cavity in the air.

Distance Piece--A long, thin piece of wood (sometimes tape) passing
through and attached to all the ribs in order to prevent them from
rolling over sideways.

Displacement--Change of position.

Drift (of an aeroplane as distinct from the propeller)--The horizontal
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 "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.

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.

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

Edge, Trailing--The rear edge o