Chapter XX: Explanation of Aeronautical Terms

Every development in human progress is marked by a concurrent development in language. To express the new ideas, new words appear, or new meanings are given to words already in use.

As yet, the vocabulary of aeronautics is in the same constructive and incomplete state as is the science to which it attempts to give voice, and the utmost that can be done at this time is to record such words and special meanings as are in use in the immediate present.

A

_Adjusting Plane_--A small plane, or surface, at the outer end of
a wing, by which the lateral (from side to side) balance of an
aeroplane is adjusted. It is not connected with the controlling
mechanism, as are the ailerons--nor with any automatic device.

_Aerodrome_--A term used by Professor Langley as a better
name for the aeroplane; but latterly it has been applied to
the buildings in which airships are housed, and also in a few
instances, as a name for the course laid out for aeronautical
contests.

_Aerofoil_--Another name for the aeroplane, suggested as more
accurate, considering that the surfaces are not true planes.

_Aeronef_--Another name for an aeroplane.

_Aeroplane_--The type of flying machine which is supported
in the air by a spread of surfaces or planes, formerly flat,
and therefore truly “plane,” but of late more or less curved.
Even though not absolutely accurate, this term has resisted
displacement by any other.

_Aerostat_--A free balloon afloat in the air.

_Aeronate_--A captive balloon.

_Aileron_--A small movable plane at the wing-tips, or hinged
between the main planes, usually at their outer ends, operated by
the aviator to restore the lateral balance of the machine when
disturbed.

_Air-speed_--The speed of aircraft as related to the air in which
they are moving; as distinguished from land-speed (which see).

_Alighting Gear_--Devices on the under side of the aeroplane to
take up the jar of landing after flight, and at the same time to
check the forward motion at that moment.

_Angle of Entry_--The angle made by the tangent to the curve of
the aeroplane surface at its forward edge, with the direction, or
line, of travel.

_Angle of Incidence_--The angle made by the chord of the arc of a
curved “plane,” or by the line of a flat plane, with the line of
travel.

_Angle of Trail_--The angle made by the tangent to the rear edge
of a curved plane with the line of travel.

_Apteroid_--A form resembling the “short and broad” type of the
wings of certain birds--as distinguished from the pterygoid
(which see).

_Arc_--Any part of a circle, or other curved line.

_Arch_--The curve formed by bending the wings downward at the
tips, leaving them higher at the centre of the machine.

_Aspect_--The view of the top of an aeroplane as it appears when
looked down upon from above.

_Aspiration_--The (hitherto) unexplained tendency of a curved
surface--convex side upward--to rise and advance when a stream of
air blows against its forward edge and across the top.

_Attitude_--The position of a plane as related to the line of its
travel; usually expressed by the angle of incidence.

_Automatic Stability_--That stability which is preserved by
self-acting, or self-adjusting, devices which are not under the
control of the operator, nor a fixed part of the machine, as are
the adjusting planes.

_Aviation_--Flying by means of power-propelled machines which are
not buoyed up in the air, as with gas bags.

_Aviator_--The operator, driver, or pilot of an aeroplane.

B

_Balance_--Equilibrium maintained by the controlling mechanism,
or by the automatic action of balancing-surfaces--as
distinguished from the equilibrium preserved by stabilizing
surfaces.

_Balancing Plane_--The surface which is employed either
intentionally, or automatically, to restore a disturbed balance.

_Biplane_--The type of aeroplane which has two main supporting
surfaces or planes, placed one above the other.

_Body_--The central structure of an aeroplane, containing the
machinery and the passenger space--as distinguished from the
wings, or planes, and the tail.

_Brace_--A construction member of the framing of aircraft
which resists a compression strain in a diagonal direction--as
distinguished from a “stay,” or “diagonal,” which supports a
pulling strain; also from a strut which supports a compression
strain in a vertical direction.

C

_Camber_--The distance from the chord of the curve of a surface
to the highest point of that curve, measured at right angles to
the chord.

_Caster_, or _Castor_, _Wheel_--A wheel mounted on an upright
pivoted shaft placed forward of its axle, so that it swivels
automatically to assume the line of travel of an aeroplane when
landing: used in the alighting gear. To be distinguished from a
fixed wheel, which does not swivel.

_Cell_--A structure with enclosing sides--similar to a box
without top or bottom stood upon one side. The vertical walls
of the cell give lateral stability, and its horizontal walls
fore-and-aft stability.

_Centre of Gravity_--That point of a body where its weight
centres. If this point is supported, the body rests in exact
balance.

_Centre of Lift_--The one point at which the lifting forces of
the flying planes might be concentrated, and produce the same
effect.

_Centre of Resistance_--The one point at which the forces
opposing the flight of an air-craft might be concentrated, and
produce the same result.

_Centre of Thrust_--The one point at which the forces generated
by the revolving propellers might be concentrated, and produce
the same effect.

_Chassis_--The under-structure or “running-gear” of an aeroplane.

_Chord_--The straight line between the two ends of an arc of a
circle or other curved line.

_Compound Control_--A mechanical system by which several distinct
controls are operated through different manipulations of the same
lever or steering-wheel.

_Compression Side_--That side of a plane or propeller blade
against which the air is compressed--the under surface of a
flying plane, and the rear surface of a revolving propeller.

_Curtain_--The vertical surface of a cell--the wall which stands
upright.

D

_Deck_--A main aeroplane surface. The term is used generally in
describing biplanes; as the upper deck, and the lower deck; or
with aeroplanes of many decks.

_Demountable_--A type of construction which permits a machine to
be easily taken apart for transportation.

_Derrick_--A tower-shaped structure in which a weight is raised
and allowed to fall to give starting impetus to an aeroplane.

_Dihedral_--That form of construction in which the wings of an
aeroplane start with an upward incline at their junction with the
body of the machine, instead of stretching out on a level.

_Dirigible_--The condition of being directable, or steerable:
applied generally to the balloons fitted with propelling power,
or airships.

_Double Rudder_--A rudder composed of two intersecting planes,
one vertical and the other horizontal, thus enabling the operator
to steer in any direction with the one rudder.

_Double-Surfaced_--Planes which are covered with fabric on both
their upper and lower surfaces, thus completely inclosing their
frames.

_Down-Wind_--Along with the wind; in the direction in which the
wind is blowing.

_Drift_--The recoil of an aeroplane surface forced through the
air: also the tendency to float in the same direction as the wind.

E

_Elevator_--A shorter name for the elevating planes or elevating
rudder, used for directing the aeroplane upward or downward.

_Ellipse_--An oval figure outlined by cutting a cone through from
side to side on a plane not parallel to its base. Some inventors
use the curves of the ellipse in forming the wings of aeroplanes.
See Hyperbola and Parabola.

_Entry_--The penetration of the air by the forward edge of
aircraft surfaces. See Angle of Entry.

_Equivalent Head Area_--Such an area of flat surface as will
encounter head resistance equal to the total of that of the
construction members of the framework--struts, braces, spars,
diagonals, etc., of the aerial craft.

F

_Feathering_--A form of construction in which mounting on hinges,
or pivots, permits the surfaces to engage the air flatwise in one
direction and to pass edgewise through it in other directions.

_Fin_--A fixed vertical stabilizing surface, similar in form to
the fin on the back of a fish.

_Fish Section_--A term applied to the lengthwise section of
an aircraft when the outline resembles the general shape of a
fish--blunted in front and tapering toward the rear. This form is
believed to encounter less resistance than any other, in passing
through the air.

_Fixed Wheel_--A wheel in a fixed mounting, so that it does not
swivel as does a caster wheel.

_Flapping Flight_--Flight by the up-and-down beating of wings,
similar to the common flight of pigeons.

_Flexible Propeller_--A propeller in which the blades are frames
covered more or less loosely with a fabric which is in a measure
free to adjust its form to the compression of the air behind it
as it revolves.

_Flying Angle_--The angle of incidence of the main surface of an
aeroplane when in flight. See Ground Angle.

_Footpound_--The amount of force required to raise one pound to
a height of one foot.

_Fore-and-aft_--From front to rear: lengthwise: longitudinal.

_Fuselage_--The framework of the body of an aeroplane.

G

_Glider_--A structure similar to an aeroplane, but without motive
power.

_Gliding_--Flying down a slope of air with a glider, or with an
aeroplane in which the propelling power is cut off.

_Gliding Angle_--The flattest angle at which a given machine will
make a perfect glide. This angle differs with different machines.
The flatter the gliding angle the safer the machine.

_Ground Angle_--The angle of incidence of an aeroplane surface
when the machine is standing on the ground.

_Guy_--A wire attached to a more or less distant part of the
structure of any aircraft to prevent spreading. Also used to
denote controlling wires which transmit the movements of the
levers.

_Gyroscopic Action_--The resistance which a rotating wheel, or
wheel-like construction, exhibits when a disturbing force tends
to change its plane of rotation.

H

_Hangar_--A structure for the housing of aeroplanes.

_Head Resistance_--The resistance encountered by a surface moving
through the air.

_Heavier-than-air_--A term applied to flying machines whose
weight is not counterbalanced or buoyed up by the lifting power
of some gas lighter than air; and which weigh more than the
volume of air displaced.

_Helicopater_--A type of flying machine in which propellers
revolving horizontally lift and sustain its weight in the air.

_Horizontal Rudder_--The rudder surface which is used to steer an
aircraft upward or downward: so-called because it lies normally
in a position parallel to the horizon; that is, level.

_Horse-power_--An amount of work equivalent to the lifting of
33,000 footpounds in one minute. See Footpound.

_Hyperbola_--The outline formed by the cutting of a cone by a
plane passing one side of its axis at such an angle that it would
also intersect another cone placed apex to apex on the same axis.

K

_Keel_--A framework extending lengthwise under an aircraft to
stiffen the construction: usually employed on airships with
elongated gas-bags.

L

_Lateral_--From side to side; that is, crossing the length
fore-and-aft, and generally at right angles to it.

_Land-speed_--The speed of aircraft as related to objects on the
ground. See Air-speed.

_Landing Area_--A piece of land specially prepared for the
alighting of aeroplanes without risk of injury.

_Leeway_--Movement of a machine aside from the intended course,
due to the lateral drift of the whole body of air; measured
usually at right angles to the course.

_Lift_--The raising, or sustaining effect of an aeroplane
surface. It is expressed in the weight thus overcome.

_Lighter-than-air_--A term used to designate aircraft which,
owing to the buoyancy of the gas attached, weigh less than the
volume of air which they displace.

_Longitudinal_--In a lengthwise, or fore-and-aft direction.

M

_Main Plane_--The principal supporting surface of an aeroplane.
In the biplane, or the multiplane type, it denotes the lowest
surface, unless some other is decidedly larger.

_Main Landing Wheels_--Those wheels on the alighting gear which
take the shock in landing.

_Mast_--A vertical post or strut giving angular altitude to
guys or long stays. Also used (erroneously) to designate a spar
reaching out laterally or longitudinally in a horizontal position.

_Monoplane_--An aeroplane with one main supporting surface. A
Double Monoplane has two of such surfaces set one behind the
other (tandem) but on the same level.

_Multiplane_--An aeroplane having several main planes, at least
more than three (for which there is the special name of triplane).

N

_Nacelle_--The framework, or body, of a dirigible balloon or
airship.

_Negative Angle of Incidence_--An angle of incidence below the
line of travel, and therefore expressed with a minus sign.
Surfaces bent to certain curves fly successfully at negative
angles of incidence, and exhibit a comparatively large lift.

O

_Ornithopter_--A type of flying machine with wing surfaces which
are designed to raise and sustain the machine in the air by
flapping.

P

_Panel_--Another name for Curtain--which see.

_Parabola_--The form outlined when a cone is cut by a plane
parallel to a line drawn on its surface from its apex to its
base. Declared to be the correct scientific curve for aeroplane
surfaces, but not so proven, as yet.

_Pilot_--A term widely used for an operator, or driver, of any
form of aircraft.

_Pitch_--The distance which a propeller would progress during one
revolution, if free to move in a medium which permitted no slip
(which see); just as the thread of a bolt travels in the groove
of its nut.

_Plane_--Speaking with exactness, a flat spread of surface; but
in aeronautics it includes also the curved sustaining surfaces of
aeroplanes.

_Polyplane_--Another term for Multiplane.

_Port_--The left-hand side of an aircraft, as one faces forward.
See Starboard.

_Projected Area_--The total area of an irregular structure as
projected upon a flat surface; like the total area of the shadow
of an object cast by the sun upon a plane fixed at right angles
to its rays.

_Propeller Reaction_--A force produced by a single revolving
propeller, which tends to revolve the machine which it is
driving, in the contrary direction. This is neutralized in
various ways in the machines driven by single propellers. Where
two propellers are used it is escaped by arranging them to move
in opposite directions.

_Pterygoid_--That type of the wings of birds which is long and
narrow--as distinguished from the apteroid type.

_Pylon_--A tower-shaped structure used as a derrick (which see);
also for displaying signals to aeronauts.

R

_Radial Spoke_--A wire spoke extending from the hub of an
alighting wheel straight outward from the centre to the rim of
the wheel. See Tangent Spoke.

_Rarefaction Side_--A correct term for the incorrect “vacuum
side,” so-called. The side opposite the compression side: the
forward side of a revolving propeller blade, or the upper side
of a flying surface, or the side of a rudder-surface turned away
from the wind.

_Reactive Stratum_--The layer of compressed air beneath a moving
aeroplane surface, or behind a moving propeller blade.

_Rib_--The smaller construction members used in building up
surfaces. Generally they run fore-and-aft, crossing the spars or
wing-bars at right angles, and they are bent to form the curve of
the wings or planes.

_Rising Angle_--Technically, the steepest angle at which any
given aeroplane will rise into the air.

_Rudder_--A movable surface by which the aeronaut is enabled to
steer his craft in a desired direction. See Horizontal Rudder and
Vertical Rudder.

_Runner_--A construction similar to the runners of a sleigh, used
for alighting on some machines, instead of the wheel alighting
gear; a skid.

S

_Screw_--Another term for propeller; properly, screw-propeller.

_Single-surfaced_--A term used to designate wings or planes
whose frames are covered with fabric only on the upper side. See
Double-surfaced.

_Skid_--Another name for runner.

_Skin Friction_--The retarding effect of the adherence of the
air to surfaces moving rapidly through it. It is very slight
with polished surfaces, and in case of slow speeds is entirely
negligible.

_Slip_--The difference between the actual progress of a moving
propeller, and the theoretical progress expressed by its pitch.
It is much greater in some propellers than in others, due to the
“churning” of the air by blades of faulty design and construction.

_Soaring Flight_--The sailing motion in the air achieved by some
of the larger birds without the flapping of their wings. It is
to be distinguished from gliding in that it is in an upward
direction. Soaring has never been satisfactorily explained, and
is considered to be the secret whose discovery will bring about
the largest advance in the navigation of the air.

_Spar_--A stick of considerable length used in the framing of the
body of aeroplanes, or as the long members in wing structures.

_Stabilize_--To maintain balance by the automatic action
of adjunct surfaces, as distinguished from the intentional
manipulation of controlling devices.

_Stabilizer_--Any surface whose automatic action tends to the
maintaining of balance in the air.

_Stable Equilibrium_--That equilibrium which is inherent in the
construction of the machine, and does not depend upon automatic
or controlling balancing devices.

_Starboard_--The right-hand side of an aircraft as one faces
forward. See Port.

_Starting Area_--An area of ground specially prepared to
facilitate the starting of aeroplanes into flight.

_Starting Device_--Any contrivance for giving an aeroplane a
powerful impulse or thrust into the air. See Derrick.

_Starting Impulse_--The thrust with which an aeroplane is started
into the air for a flight. Most machines depend upon the thrust
of their own propellers, the machine being held back by force
until the engines have worked up to flying speed, when it is
suddenly released.

_Starting Rail_--The rail upon which the starting truck runs
before the aeroplane rises into the air.

_Starting Truck_--A small vehicle upon which the aeroplane rests
while it is gaining sufficient impulse to take flight.

_Stay_--A construction member of an aeroplane sustaining a
pulling strain. It is usually of wire.

_Straight Pitch_--That type of pitch (which see) in a propeller
blade in which every cross-section of the blade makes the same
angle with its axis of revolution.

_Strainer_--Another term for Turnbuckle--which see.

_Strut_--An upright, or vertical, construction member of an
aeroplane sustaining a compression strain; as distinguished from
a brace which sustains a diagonal compression strain.

_Supplementary Surface_--A comparatively small surface used as an
adjunct to the large surfaces for some special purpose; as, for
instance, the preserving of balance, or for steering.

_Sustaining Surface_--The large surfaces of the aeroplane
whose rapid movement through the air at a slight angle to the
horizontal sustains the weight of the machine.

T

_Tail_--A rear surface on an aeroplane designed to assist in
maintaining longitudinal stability. It is in use principally on
monoplanes, and is often so arranged as to serve as a rudder.

_Tail Wheel_--A wheel mounted under the rear end of an aeroplane
as a part of the alighting gear.

_Tangent_--A straight line passing the convex side of a curved
line, and touching it at one point only. The straight line is
said to be tangent to the curve at the point of contact.

_Tangential_--In the position or direction of a tangent.

_Tangent Spoke_--A wire spoke extending from the outer edge of
the hub of a wheel along the line of a tangent until it touches
the rim. Its position is at right angles to the course of a
radial spoke (which see) from the same point on the hub.

_Tie_--A construction member connecting two points with a pulling
strain.

_Tightener_--A device for taking up the slack of a stay, or tie;
as the turnbuckle.

_Tractor Propeller_--A propeller placed in front, so that it
pulls the machine through the air, instead of pushing, or
thrusting, it from behind.

_Triplane_--An aeroplane with three main surfaces, or decks,
placed in a tier, one above another.

_Turnbuckle_--A device with a nut at each end, of contrary pitch,
so as to take a right-hand screw at one end, and a left-hand
screw at the other; used for drawing together, or toward each
other the open ends of a stay, or tie.

U

_Uniform Pitch_--That varying pitch in a propeller blade which
causes each point in the blade to move forward in its own circle
the same distance in one revolution.

_Up-wind_--In a direction opposite to the current of the wind;
against the wind; in the teeth of the wind.

V

_Vertical Rudder_--A rudder for steering toward right or left; so
called because its surface occupies normally a vertical position.

W

_Wake_--The stream of disturbed air left in the rear of a moving
aircraft, due mainly to the slip of the propeller.

_Wash_--The air-currents flowing out diagonally from the sides of
a moving aeroplane.

_Wing Bar_--The larger construction members of a wing, running
from the body outward to the tips. The ribs are attached to the
wing bars, usually at right angles.

_Wing Plan_--The outline of the wing or main plane surface as
viewed from above.

_Wing Section_--The outline of the wing structure of an aeroplane
as it would appear if cut by a plane passing through it parallel
to the longitudinal centre of the machine.

_Wing Skid_--A small skid, or runner, placed under the tip of
the wings of an aeroplane, to prevent damage in case of violent
contact with the ground.

_Wing Tip_--The extreme outer end of a wing or main plane.

_Wing Warping_--A controlling device for restoring disturbed
lateral balance by the forcible pulling down or pulling up of the
tips of the wings, or of the outer ends of the main surface of
the aeroplane.

_Wing Wheel_--A small wheel placed under the outer end of a wing
or main plane to prevent contact with the ground. An improvement
on the wing skid.

THE END

[Transcriber’s Note:

Inconsistent spelling and hyphenation are as in the original.]