tal surface), and weight,
horizontal flight can be maintained.

Angle of Incidence, Minimum--The smallest angle of incidence at which,
for a given power, surface (including detrimental surface), and weight,
horizontal flight can be maintained.

Angle of Incidence, Best Climbing--That angle of incidence at which an
aeroplane ascends quickest. An angle approximately halfway between the
maximum and optimum angles.

Angle of Incidence, Optimum--The angle of incidence at which the
lift-drift ratio is the highest.

Angle, Gliding--The angle between the horizontal and the path along
which an aeroplane at normal flying speed, but not under engine power,
descends in still air.

Angle, Dihedral--The angle between two planes.

Angle, Lateral Dihedral--The lifting surface of an aeroplane is said to
be at a lateral dihedral angle when it is inclined upward towards its
wing-tips.

Angle, Longitudinal Dihedral--The main surface and tail surface are said
to be at a longitudinal dihedral angle when the projections of their
neutral lift lines meet and produce an angle above them.

Angle, Rigger's Longitudinal Dihedral--Ditto, but substituting "chords"
for "neutral life lines."

Angle, Pitch--The angle at any given point of a propeller, at which
the blade is inclined to the direction of motion when the propeller is
revolving but the aeroplane stationary.

Altimeter--An instrument used for measuring height.

Air-Speed Indicator--An instrument used for measuring air pressures or
velocities. It consequently indicates whether the surface is securing
the requisite reaction for flight. Usually calibrated in miles per hour,
in which case it indicates the correct number of miles per hour at only
one altitude. This is owing to the density of the air decreasing with
increase of altitude and necessitating a greater speed through space
to secure the same air pressure as would be secured by less speed at a
lower altitude. It would be more correct to calibrate it in units of air
pressure.

Air Pocket--A local movement or condition of the air causing an
aeroplane to drop or lose its correct attitude.

Aspect-Ratio--The proportion of span to chord of a surface.

Air-Screw (Propeller)--A surface so shaped that its rotation about an
axis produces a force (thrust) in the direction of its axis.

Aileron--A controlling surface, usually situated at the wing-tip, the
operation of which turns an aeroplane about its longitudinal axis;
causes