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