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Aviation Glossary

A glossary of aviation terms and abbreviations as related to experimental aviation, building and designing

This will be a "living document" that will be updated on a regular basis. Your comments and participation are necessary to maintain and grow this list as accurately as possible. Although we are also aviators, this list will pertain more to building and designing than flying.

ABSOLUTE CEILING - A less often used term – the highest altitude an airplane can sustain level flight, or altitude above which the cabin pressurization system can no longer maintain a sufficient oxygen level for passengers and crew, and where the pressure differential is so great as to put severe stress on the pressure cabin of the aircraft. Most commercial jetliners have a ceiling of about 42,000 feet (12,802 meters) while some business jets can reach 52,000 feet or higher (15,850 meters.) Also see SERVICE CEILING.

ACCELERATED STALL - Any stall made to occur at other  than 1g. Also see HIGH-SPEED STALL and SECONDARY STALL

ACCESSORIES - Mechanical and electrical units mounted on an engine necessary for its operation such as starter, magnetos, fuel pump or the operation of other systems such as an alternator, vacuum pump, etc.

ADVERSE YAW - Yaw generated when the ailerons are used. The lifting wing generates more drag, causing an airplane to yaw (turn) toward it.

AGL - Above Ground Level, as a measurement of altitude above a specific land mass, and differentiated from MSL.

AILERON - The movable areas of a WINGFORM that control or affect the roll of an aircraft by working opposite one another—up-aileron on the right wing and down-aileron on the left wing.

AIRSPEED INDICATOR – (ASI) An instrument or device that measures the air speed of an aircraft through an air mass, but not its ground speed. 

ALODINE - A non-anodic protective coating. Alodine {aka Iridite, aka Chromate Conversion} is a microscopic thin film commonly prescribed on aluminum to provide an excellent surface prep for paint, aid in corrosion resistance and to impose desired electrical resistance characteristics, (commonly prescribed for radio and amplifier cabinets.) The the process requires an initial acid etching before the AL applying the alodine conversion coating. Alodine is a strong oxidizer and produces a chemical conversion of the surface (oxidizes it). Paint is reported to bond to the alodine'd surface well. 

ALCLAD - Trademark of Alcoa used as a generic term to describe corrosion resistant Aluminum sheet formed from high-purity aluminum surface layers metallurgically bonded to high strength Aluminum Alloy core material. These sheets commonly used by the aircraft industry.

Described in NACA-TN-259, of August 1927, as "a new corrosion resistant aluminum product which is markedly superior to the present strong alloys. Its use should result in greatly increased life of a structural part. Alclad is a heat-treated aluminum, copper, manganese, magnesium alloy that has the corrosion resistance of pure metal at the surface and the strength of the strong alloy underneath. Of particular importance is the thorough character of the union between the alloy and the pure aluminum. Preliminary results of salt spray tests (24 weeks of exposure) show changes in tensile strength and elongation of Alclad 17ST, when any occurred, to be so small as to be well within the limits of experimental error."

ANGLE OF ATTACK – (AOA) The acute angle at which a moving airfoil meets the airstream.

ANGLE OF INCIDENCE – (AOI) The angle at which an airfoil is normally fixed in relation to the longitudinal axis of an aircraft.

ANHEDRAL - The downward angle of a wing in relation to a horizontal cross-section line; aka CATHEDRAL. See DIHEDRAL.

ANODIZE - or anodising, is an electrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts. Anodizing increases corrosion resistance and wear resistance, and provides better adhesion for paint primers and glues than bare metal. Anodic films can also be used for a number of cosmetic effects, either with thick porous coatings that can absorb dyes or with thin transparent coatings that add interference effects to reflected light. Anodizing is also used to prevent galling of threaded components and to make dielectric films for electrolytic capacitors. Anodic films are most commonly applied to protect aluminium alloys, although processes also exist for titanium, zinc, magnesium, and niobium.

ANTI-SERVO TAB - A small portion of a flight-control surface that deploys in such a way that it works to resist the motion of the entire flight-control surface from the direction that the pilot or other forces apply to it. An anti-servo tab, unlike a traditional trim tab, is a dynamic device that increases resistance as the control surface is deployed further. Its function has a stabilizing effect.

On some aircraft with all-flying stabilators an anti-servo tab acts as a trimming device. In this use some manufacturers term it a “balance tab” or “anti-balance tab.”

ARM - In aircraft weight and balance, as well as load distribution, it is the distance from the CENTER OF GRAVITY (CG or CofG) to some point. For computations, arms measured forward from the c/g are positive (+n) and those measured aft of the c/g are negative (-n).

ASPECT RATIO - The ratio of the span to the chord of an airfoil—a high-aspect ratio wing has wide span and narrow chord, and vice-versa for a low-aspect ratio wing.

AUTOGYRO, AUTOGIRO - An aircraft, often wingless, with unpowered rotary airfoil blades that auto-rotate and serve as wings as they move through the air when mounted on a powered aircraft (or, in some cases, a glider). The latter spelling is a trademark of the Autogiro Corporation. 

AUTO-ROTATION - Automatic rotation of rotary blades from a HELICOPTER in an unpowered glide or the forward movement of an AUTOGYRO

AVGAS - A high-octane aviation fuel used for aircraft. Avgas is a portmanteau for aviation gasoline, as distinguished from MOGAS (motor gasoline), which is the everyday gasoline used in cars.

Avgas is used in aircraft that use piston or Wankel engines. Gas turbines can operate on avgas, but typically do not. Turbine and diesel engines are designed to use kerosene-based jet fuel.

AXIAL - Motion along a real or imaginary straight line on which an object supposedly or actually rotates.

BALANCED CONTROL SURFACE - A movable control surface, as an aileron or rudder, having an added physical extension or weights forward of the hinge-point to reduce forces on a joystick or yoke and to lessen the chance for aerodynamic FLUTTER. See ELEPHANT EARS.

BERNOULLI EFFECT or LAW or THEOREM - Since the pressure of a fluid is proportional to its velocity, airflow over the upper surface of an airfoil causes suction [lift] because the airstream has been speeded up in relation to positive pressure of the airflow on the lower surface. 

BLEED AIR - Hot air at high pressure, usually from the bypass section of a gas turbine (or jet) engine for de-icing, heating, and other uses. 

BLIND RIVET - A hollow rivet, made from any one of a variety of materials, having a shank that is expanded by drawing (or pulling) a mandrel with a head diameter greater than the hole diameter, through the shank toward the head of the rivet. Once sufficient tension is reached, the mandrel is automatically broken off (usually inside the rivet) by the pulling action of the tool designed for the installation of this type of rivet. Developed for use where access to both sides of the working piece is not an option, the blind rivet is designed to be installed from the working side, where the installer is “blind” to the opposite, expanded end.

BLUEPRINTING or BALANCING AND BLUEPRINTING - The meticulous matching to factory specifications of all parts and/or components. Hand-fitting of parts to the absolute design callout or manufacturer’s specifications or the procedure of improving the performance of an engine by dismantling and then rebuilding the reciprocating parts so that they meet exact tolerances, matching the designer’s blueprints.

BOUNDARY-LAYER CONTROL - The design or control of slotted or perforated wings with suction methods to reduce undesirable aerodynamic effects caused by the boundary layer—that region adjacent to the boundary where shear stresses dominate in the airflow over a WINGFORM.

BOWDEN CABLE - A bowden cable is a type of flexible cable used to transmit mechanical force or energy by the movement of an inner cable (most commonly of steel or stainless steel) relative to a hollow outer cable housing. The housing is generally of composite construction, consisting of a helical steel wire, often lined with plastic, and with a plastic outer sheath.

BOXER OR BOXER ENGINE - Any horizontally opposed engine in which the corresponding pistons reach top dead center simultaneously, such as those found in most light general aviation aircraft. Examples include Lycoming, Continental, and Franklin engines, as well as Jabiru and Rotax. Examples of automobile engines that fall into the category and are commonly converted for aircraft use include the air-cooled Volkswagen engines, Porsche, Subaru, and Chevrolet’s Corvair. Lesser-known boxer engines include the Citroën 2CV for automobiles and BMW and Honda (Gold Wing) for motorcycles. Boxer engines should not be confused with the less popular opposed piston engine designs, (sometimes referred to as “180° V engines” and almost never found in engines with less than eight cylinders) in which corresponding pistons share a crank pin, and thus each will reach top dead center half a crankshaft revolution after the other.

BRAKE SPECIFIC FUEL CONSUMPTION - As with brake horsepower (BHP), brake specific fuel consumption (BSFC, or SFC) refers to the specific fuel consumption when the power is measured by an external brake - in other words, a dynamometer.

BSFC is the ratio between the engine’s fuel mass consumption and the crankshaft power it is producing. This makes it both a valuable fuel efficiency indicator and one more useful tool in gauging an engine’s state-of-tune.

In the United States, the fuel flow for BSFC calculations is normally expressed in pounds per hour (lb/hr) while the output units are in horsepower (hp). So, the U.S. standard formula for BSFC becomes lb/hp-hr. For an engine producing 200 hp, while burning 100 pounds* of gasoline per hour, the equation would yield 100/200 = 0.50 BSFC, which is universally considered to be average or normal, while some would argue that a more accurate 0.51 lb/hr is correct.

*With fuel weighing approximately 6 pounds per gallon, 100 pounds works out to be 16.7 gallons. Therefore, if an aircraft engine manufacturer claims fuel consumption better than 16.7 gallons per hour (gph) for a 200-hp engine, it is probably being less than truthful, unless it has the dynamometer records to prove both horsepower and BSFC.

BRAYTON CYCLE - A thermodynamic cycle consisting of two constant-pressure processes interspersed with two constant-entropy processes. Also known as complete-expansion Diesel cycle, or Joule cycle. It is named after George Brayton (1830–1892), the American engineer who developed it, although it was originally proposed and patented by Englishman John Barber in 1791.

BUMPED COWLING - An engine FAIRING, generally circular, with welts or compound shapes in its surface to accommodate cylinder heads. 

CABANE STRUT - One of several structural members, usually vertical and sometimes streamlined, that support or otherwise connect the wing center-section from the fuselage, typically found in high-wing or biplane open-cockpit configurations.

CALIBRATED AIRSPEED (CAS) - The indicated airspeed of an aircraft, corrected for position and instrument error. CAS is equal to true airspeed in standard atmosphere at sea level. Compare INDICATED AIRSPEED (IAS) and TRUE AIRSPEED (TAS).

CAMBER - The convex or concave curvature of an airfoil.

CANARD - An arrangement in which the horizontal stabilizer and elevators of an aircraft are mounted in front of the wing(s). An airframe configuration of fixed-wing aircraft in which the forward surface is smaller than the rearward, the former being known as the "canard," while the latter is the wing. In contrast, conventional aircraft have a small horizontal stabilizer behind the wing.

CARSON'S SPEED - The term Carson’s speed refers to the paper “Fuel Efficiency of Small Aircraft” (AIAA-80-1847, 1980) by Professor Bud Carson of the U.S. Naval Academy, which, using prior work by Gabrielli and von Karman, defines this speed as the maximum speed per unit of fuel burned. Carson’s speed can be calculated as 1.316 times the speed for maximum lift to drag ratio, which, in turn, is 1.316 times the speed for minimum power and minimum sink rate. Carson’s speed is also defined as the tangent point on a line that is tangent to the drag polar and passes through the origin. Reference: http://members.EAA.org/home/flight_reports/wittman_tailwind.html

CASTING FLASH - A thin irregular ridge of metal on the outer face of a casting, resulting from seepage of the molten metal into the joint between the separate components of the mold used in its manufacture. During the final cleaning and finishing of a cast object, the flash is usually knocked off and filed smooth.

CATHEDRAL  see  ANHEDRAL

CEILING - (1) The heights above the earth's surface of the lowest layer of clouds or obscuring phenomena that is reported as "broken," "overcast," or "obscuration," and not classified as "thin" or "partial". (2) The maximum height above sea level in STANDARD AIR attainable by an aircraft under given conditions—see ABSOLUTE CEILING, SERVICE CEILING. 

CENTER OF GRAVITY (CG or CofG) - The longitudinal and lateral point in an aircraft where it is stable; the static balance point.

CHORD - The measurable distance between the leading and trailing edges of a WINGFORM

COAMING - A padded, protective rim around an open cockpit.

COANDA EFFECT or WALL-ATTACHMENT EFFECT is the tendency of a moving fluid, either liquid or gas, to attach itself to a surface and flow along it. As a fluid moves across a surface, a certain amount of friction (called skin friction) occurs between the fluid and the surface, which tends to slow the moving fluid. This resistance to the flow of the fluid pulls the fluid toward the surface, causing it to stick to the surface. Thus, a fluid emerging from a nozzle tends to follow a nearby curved surface – even to the point of bending around corners – if the curvature of the surface or the angle the surface makes with the stream isn’t too sharp. Discovered in 1930 by Henri Coanda, a Romanian aircraft engineer, the phenomenon has many practical applications in fluidics and aerodynamics.

COLLECTIVE PITCH - A cockpit control that changes the PITCH of a helicopter's rotor blades; used in climbing or descending.

COLLECTOR RING - A circular duct on a radial engine into which exhaust gases from its cylinders are safely discharged.

CONSTANT-SPEED PROPELLER - A hydraulically or electrically controlled propeller that can change its blade pitch to take better advantage of the power supplied by an engine in much the same way that a transmission in a car takes better advantage of its power source. The mechanism varies depending on the aircraft, but the desired effect is to change the angle of attack of the propeller blades to take a smaller or larger "bite" of air as it rotates.

CONTACT! Magazine - An independant, non-profit experimental aviation magazine started by Mick Myal in 1991. It's published bimonthly by it's editor, Patrick Panzera. Read More

COWL, COWLING - A removable FAIRING around an aircraft engine for the purposes of streamling or cooling. 

COWL FLAP - A controllable louver to regulating airflow through an engine's COWLING

DEADSTICK - Descending flight with engine and propeller stopped.

DECALAGE - The difference in angular settings [angles of attack] of the wings of a biplane or multiplane.

DELTA-WING - A triangularly-shaped aircraft wing having a low aspect ratio, a sharply-tapered leading edge, a straight trailing edge, and a pointed tip.

DENSITY ALTITUDE - The pressure altitude corrected for temperature deviations from the standard atmosphere. Density altitude bears the same relation to pressure altitude as true altitude does to indicated altitude.

DEPARTURE STALL - A stall in the takeoff configuration with power.

DIHEDRAL - The acute angle, usually upward, between the wing of an airplane and a horizontal cross-section line. Opposite of ANHEDRAL.

DOPE - Preservative and pigmented coloring for fabric aircraft covering and paints, generally nitrate lacquer but generically used to denote all early shellac and coal-tar mixtures on up to present-day acrylics.

DORSAL FIN - A lateral fin/rudder extension on the top of a fuselage. Opposite of VENTRAL FIN

DOWNWASH - The air deflected perpendicular to the direction of movement of an airfoil.

DOWNWIND TURN - Long a point of contest among pilots, there is in reality no such thing as far as the airplane is concerned. Proponents claim that airplanes lose air speed and gets pushed from behind (potentially causing a stall) when turning downwind, while opponents (and the laws of physics) argue that an airplane, like a boat in a river whose speed is only relative to the water and not the shore, is unaffected within the movement of an air mass and that it gains only ground speed.

DRAG - The resisting force exerted on an aircraft in its line of flight opposite in direction to its motion. Compare THRUST.

DRAG WIRE - A wire designed to resist DRAG forces, usually running from a forward inboard point to an outboard aft point.

DRIFT PIN - Typically used in metalworking, a drift pin, drift pin punch, or simply drift is the name for a tool used in the alignment of adjoining holes prior to bolting or riveting metal parts together. Generally made from tool steel, a drift pin can be of virtually any length or diameter but sized appropriately for the task at hand. Drift pins are usually mildly tapered and act like a wedge, coercing the two (or more) pieces into alignment prior to being replaced by the fastener.

DRY WEIGHT - The weight of an engine exclusive of any fuel, oil, and coolant.

DURAL - Originally a tradename for a wrought aluminum-copper alloy created by Bausch Machine Tool Co, now fallen into generic use as any aluminum alloy containing 3.0-4.5% copper, 0.4-1.0% magnesium, and 0.1-0.7% manganese. Alcoa's version is commonly referred to as "Duraluminum," popularly used in aircaft manufacture.

DYNAFOCAL ENGINE MOUNT - An aircraft engine mount system in which straight lines projected through the mounting bolts intersect at the center of gravity (or the center of mass) of the engine and propeller assembly. Dynafocal is a registered trademark owned by Lord Corporation and is credited for a reduction of engine vibration over that of any other design.

DZUS FASTENER - ("Zoose") A generic term for any number of quick-acting, screw-type fasteners typically used for the securing of cowlings, inspection plates and other panels, requiring no special tool other than a slotted screwdriver or a coin, usually only requiring a quarter-turn to lock or unlock.

EAA - (The Experimental Aircraft Association) is an international organization of aviation enthusiasts based in Oshkosh, Wisconsin. Since its inception, it has grown internationally with over 170,000 members.

ECU - (engine control unit) is an electronic unit that controls various aspects of an internal combustion engine’s operation.
The simplest ECUs control only the quantity of fuel injected into each cylinder each engine cycle. More advanced ECUs found on most modern cars also control the ignition timing, variable valve timing (VVT), the level of boost maintained by the turbocharger (in turbocharged cars), and other peripherals.

ELEPHANT EAR - (1) An air intake characterized by twin inlets, one on each side of the fuselage. (2) A type of balanced aileron in which the outer edges are noticeably larger than the control itself. See BALANCED CONTROL SURFACE and example Travel Air 4000.

ELEVATOR - The movable part of a horizontal airfoil which controls the pitch of an aircraft, the fixed part being the STABILIZER.

ELEVON - A hinged device on the rear portion of an aircraft wing combining the functions of an elevator and an aileron. Usually found on delta-wing aircraft, it can be moved in the same direction on either side of the aircraft to obtain longitudinal control, or differentially to obtain lateral control. Also see FLAPERON.

ELT - Emergency Locator Transmitter, a type of distress beacon used in aircraft

EMPENNAGE - An aircraft's tail group, includes rudder and fin, and stabilizer and elevator. Old French: empenner, to feather an arrow, from Latin penna, feather.

EQUIVALENT AIRSPEED (EAS) is calibrated airspeed (CAS) corrected for compression of air in the pitot tube (same as CAS in standard atmosphere at sea level; less than CAS at higher altitudes and faster airspeeds). EAS is an airspeed that is not normally used in general aviation.

ETA - Estimated Time of Arrival.

ETD - Estimated Time of Departure.

EXHAUST AUGMENTER - A tube or pipe, sometimes one of several, through which the exhaust gases from an aircraft reciprocating engine are directed to provide additional thrust or to assist the removal of engine cooling air as it exits the rear of the engine compartment.

FAIRING - An added streamlining structure or auxiliary member, most often of light metal, whose only purpose is to reduce drag. Fairings are not load-bearing and, therefore, are not meant to carry any principal air loads placed on the airplane structure.

FBO - Fixed-Base Operator (or Fixed Base Operation). A commercial operator supplying fuel, maintenance, flight training, and other services at an airport. 

FEATHERING - In the event of engine failure, the process of adjusting a controllable-pitch propeller to a pitch position where the blade angle is about 90° to the plane of rotation in order to stop its windmilling and lessen drag.

FEDERAL AIR REGULATION (FAR)
FAR Part 91  -  General Aviation (portions apply to all operators)
FAR Part 103 - Ultralight Vehicles
FAR Part 105 - Parachute Jumping
FAR Part 108 - Airplane Operator Security
FAR Part 119 - Certification: Air Carriers and Commercial Operators
FAR Part 121 - Domestic, Flag, and Supplemental Air Carriers and Commercial Operators of Large Aircraft
FAR Part 123 - Travel Clubs
FAR Part 125 - US Civil Airplanes, seating 20 or more passengers or a maximum payload capacity 6,000# or more
FAR Part 127 - Air Carriers using helicopters for scheduled interstate flights (within the 48 contiguous states)
FAR Part 129 - Foreign Air Carrier and Foreign Operators of US registered aircraft engaged in common carriage
FAR Part 133 - Rotorcraft External Load Operations
FAR Part 135 - Air Taxi Operators and Commercial Operators
FAR Part 137 - Agricultural Aircraft Operations
FAR Part 141 - Pilot School

FERRY FLIGHT - A flight for the purpose of (1) returning an aircraft to base; (2) delivering an aircraft from one location to another; (3) moving an aircraft to and from a maintenance base. Ferry flights, under certain conditions, may be conducted under terms in a special flight permit.
FIN - The fixed part of a vertical airfoil that controls the yaw of an aircraft; the movable part being the RUDDER. Sometime referred to as Vertical Stabilizer.

FIREWALL - A fire-resistant bulkhead that isolates the engine from other parts of an airplane's structure.

FISHTAILING - A rudder-controlled side-to-side [yawing] motion to reduce air speed, generally prior to landing.

FLAP - A movable, usually hinged AIRFOIL set in the trailing edge of an aircraft wing, designed to increase LIFT and/or DRAG by changing the CAMBER of the wing or used to slow an aircraft during landing by increasing lift. Also see FOWLER FLAP, SLOTTED FLAP, and SPLIT FLAP.

FLAPERON - A control surface combining the functions of a FLAP and an AILERON.

FLARE - A simple maneuver performed moments before landing in which the nose of an aircraft is pitched up to minimize the touchdown rate of speed.  

FLIGHT ENVELOPE - An aircraft's performance limits, specifically the curves of speed plotted against other variables to indicate the limits of speed, altitude, and acceleration that a particular aircraft cannot safely exceed. 

FLOATPLANE - A water-based aircraft with one or more mounted pontoons, as differentiated from a hulled SEAPLANE [Flying Boat], but often used generically. 

FLYING WIRES - Interplane bracing wires that help support wing loads when the plane is in flight. Direction of travel is upward and outward from the fuselage to the interplane struts. Also known as LIFT WIRES, the opposite of LANDING WIRES

FLUTTER - A self-starting and potentially destructive vibration where aerodynamic forces on an object couple with a structure's natural mode of vibration to produce rapid periodic motion.

FOWLER FLAP - Trademark name of a flap attached to a wing's trailing edge with a system of tracks and rollers to slide backwards before hinging downwards, thereby increasing both camber and chord, creating a larger wing surface better tuned for lower speeds. Named for its inventor, USAF engineer Harland D Fowler. 

FRISE AILERON - A type of aileron that has a beveled or contoured leading edge projecting beyond its inset hinges. When the trailing edge is lowered, it forms an extension of the wing surface; when raised, its nose protrudes below the wing surface, protruding into the airflow increasing drag.

DRAG and reducing YAW. Named for its inventor, British engineer Leslie George Frise. Unfortunately, as well as reducing adverse yaw, Frise ailerons will increase the overall drag of the aircraft, and therefore they are less popular in aircraft where minimizing drag is important (e.g. in a glider)

FUSELAGE - An aircraft's main body structure housing the flight crew, passengers, and cargo and to which the wings, tail and, in most single-engine airplanes, engine are attached. French: fuselé, tapering.

g or G -  see  LOAD FACTOR

GAP - The distance between two adjacent wings of a biplane or multiplane.

GAS TURBINE - An internal-combustion engine consisting essentially of an air compressor, combustion chamber, and turbine wheel that is turned by the expanding products of combustion.

GLASS COCKPIT - Said of an aircraft's control cabin which has all-electronic, digital and computer-based, instrumentation.

GLIDER - An unpowered aircraft capable of maintaining altitude only briefly after release from tow, then gliding to earth. Compare SAILPLANE.

GROSS WEIGHT - The total weight of an aircraft when fully loaded, including fuel, cargo, and passengers; aka Takeoff Weight.

GROUND CUSHION  see  GROUND EFFECT 

GROUND EFFECT - Increased lift generated by the interaction between a lift system and the ground when an aircraft is within a wingspan distance above the ground. It affects a low-winged aircraft more than a mid- or high-winged aircraft because its wings are closer to the ground; aka GROUND CUSHION.

GROUND LOOP - Usually defined as a rapid rotation of a fixed-wing aircraft in the horizontal plane while on the ground. In powered aircraft, the ground loop phenomenon is predominantly associated with aircraft that have conventional landing gear and is typically blamed on the center of gravity being positioned behind the main wheels.

GROUNDSPEED - The actual speed that an aircraft travels over the ground—its "shadow speed"; it combines the aircraft's AIRSPEED and the wind's speed relative to the aircraft's direction of flight. 

GULL-WING - Descriptive of wing in frontal view bent as the wing of a seagull; a distinctive shallow, inverted "V" shape—see Stinson SR-10 or inverted gull-wing Vought F4U.

GYROPLANE - A rotorcraft whose rotors are not engine-driven, except for initial starting, but are made to rotate by action of the air when the rotorcraft is moving and whose means of propulsion, usually a conventional propeller, is independent of the rotor system. Similar to AUTOGYRO.

HALL EFFECT - Generation of an electric potential perpendicular to both an electric current flowing along a conducting material and an external magnetic field applied at right angles to the current upon application of the magnetic field.

HELICOPTER - A wingless aircraft acquiring its lift from revolving blades driven by an engine about a near-vertical axis. A ROTORCRAFT acquiring its primary motion from engine-driven rotors that accelerate the air downward, providing a reactive lift force, or accelerate the air at an angle to the vertical, providing lift and thrust.

HIGH BLOWER - A blower-type SUPERCHARGER set at high rpm.

HIGH-SPEED STALL - Any stall made to occur at more than 1g, such as pulling out of a dive or while turning. Also see SECONDARY STALL or ACCELERATED STALL.

HORSEPOWER – (HP) The motive energy required to raise 550# one foot in one second, friction disregarded. With any rotating engine HP can be determined by multiplying the torque and the revolutions per minute (RPM) and dividing by 5,252.
           Torque (ft. lbs) x RPM
HP =   --------------------------------
                      5252 

HYPERSONIC - Speed of flight at or greater than Mach 5.0, exceeding SUPERSONIC

INDICATED AIRSPEED (IAS) - A direct instrument reading obtained from an air speed indicator uncorrected for altitude, temperature, atmospheric density, or instrument error. Compare CALIBRATED AIRSPEED and TRUE AIRSPEED.

INDUCED DRAG - is caused by that element of the air deflected downward which is not vertical to the flight path but is tilted slightly rearward from it. As the angle of attack increases, so does drag; at a critical point, the angle of attack can become so great that the airflow is broken over the upper surface of the wing, and lift is lost while drag increases.

INDUCTANCE - (1) The property of an electric circuit by which a varying current in it produces a varying magnetic field that induces voltages in the same circuit or in a nearby circuit. It is measured in henrys. Inductance symbol: L. (2) The capacity of an electric circuit for producing a counter electromotive force when the current changes.

INERTIA FORCE - A force due to inertia, or the resistance to acceleration or deceleration.

JOYSTICK or STICK - A single floor- or roof-mounted control stick—sideways movement produces ROLL, and forward/backward movement produces PITCH (rudder pedals produce YAW). 

KNOT - One nautical mile, about 1.15 statute miles (6,080'); eg: 125kts = 143.9mph.

LAMINAR-FLOW AIRFOIL - A low-drag airfoil designed to maintain laminar (smooth, continuous) flow over a high percentage of the CHORD about itself. Often relatively thin, especially along the leading edge, with most of its bulk near the center of the chord.

LANDING WIRES - Interplane bracing wires that help support wing loads when the plane is on the ground. Direction of travel is downward and outward from the fuselage. Opposite of FLYING WIRES.

LIFT - The force exerted on the top of a moving airfoil as a low-pressure area [vacuum] that causes a WINFGFORM to rise. AIRFOILs do not "float" on air, as is often assumed—like a boat hull floats on water—but are "pulled up" [lifted] by low air pressures trying to equalize.

LIFT COEFFICIENT - A number that aerodynamicists use to model all of the complex dependencies of shape, inclination, and some flow conditions on lift. CL is the non-dimensional coefficient of lift. If the subscript used is a lowercase l, then the section or airfoil (2D) lift coefficient is meant. If the subscript used is an uppercase L, then the lift coefficient for an entire wing or other body (3D) is meant.

CL = L / (1/2 x ρ x V2 x S), where

CL is the lift coefficient, non-dimensional
L is the lift force
Ρ is the fluid density (typically for air)
V is the free-stream velocity (that is, the speed of the wing in question)
S is the area of the wing in question

Make sure that all units cancel.

Using such non-dimensional coefficients allows us to test small-scale wings in a tunnel and then scale them up (or down) to the desired size later.  

LIFT-DRAG RATIO - The lift coefficient of a wing divided by the drag coefficient, as the primary measure of the efficiency of an aircraft; aka L/D Ratio (L over D).

LIFT WIRES - Interplane bracing wires that help support wing loads when the plane is in flight. Direction of travel is upward from the bottom of the fuselage to the top of the interplane struts. Also known as FLYING WIRES, the opposite of LANDING WIRES.

LIGHT SPORT AIRCRAFT - Special FAA certification class (LSA) for an aircraft other than a helicopter or powered-lift—single-engine aircraft, airship, balloon, GLIDER, GYROCOPTER, ROTORCRAFT, weight-shift-control aircraft. While limiting the types of aircraft that could be flown by a SPORT PILOT, it simplified requirements for a obtaining a pilot license and did not require a medical examination.

LOAD FACTOR (g) - The proportion between lift and weight commonly seen as g (sometimes capitalized)—a unit of force equal to the force of gravity times one. 

LOFTING - Design or fabrication of a complex aircraft component, as with sheet metal, using actual-size patterns or plans, generally laid out on a floor. The term was borrowed from boat builders.

LONGERON - A principal longitudinal member of a fuselage's framing, usually continuous across a number of supporting points.

LTA - Lighter-than-air craft, generally referring to powered blimps and dirigibles, but often also includes free balloons.

MACH or m. - A number representing the ratio of the speed of an object to the speed of sound in the surrounding air or medium in which it is moving.

MAGNAFLUX - The Magnaflux process is a method of testing ferrous metals for surface and subsurface flaws, most often used on industrial tools and engine parts during maintenance inspections. It works by applying a magnetic field to the component causing a high concentration of magnetic flux at surface cracks, which can be made visible by dusting iron powder or a similar magnetic material over the component, using either wet or dry methods. The wet method consists of bathing the part(s) in a solution containing iron oxide particles while placed in the magnetic field and inspecting it with a black light (ultraviolet light). The particles flux around the imperfections, and the patterns are visible under the black light. The dry method is based on the same principle. Parts are dusted with iron oxide particles and charged using a yoke. The particles are attracted to the discontinuities and are visible by black light.

MAGNETO, MAG - An accessory that produces and distributes a high-voltage electric current for ignition of a fuel charge in an internal combustion engine.

MAGNUS EFFECT - The effect on a spinning cylinder or sphere moving through a fluid, in which force acts perpendicular to the direction of motion and to the direction of spin. This is used to advantage in baseball, in which the trajectory of a pitched ball is a distinct curve. Applied to aeronautics in experimental wingforms, the Magnus Theory states that if air is directed against a smooth, revolving cylinder, whose circumferential speed is greater than that of the air current, a force is directed against one side of the cylinder—air compressed on one side and vacuum formed on the other—creating lift. Named for physicist Heinrich Gustav Magnus (1802-70). 

MEAN SEA LEVEL  see  MSL

MOGAS – An aviation nickname for everyday automobile gasoline intended by the purchaser  for aircraft use. Mogas is a portmanteau for autoMObile gasoline, as distinguished from AVGAS (aviation gasoline) which is specifically blended for aviation use and not intended nor legal to use in land-based internal combustion engines.

MONOCOQUE - Type of fuselage design with little or no internal bracing other than bulkheads, where the outer skin bears the main stresses; usually round or oval in cross-section. Additional classifications are (1) Semi-Monocoque, where the skin is reinforced by LONGERONS or BULKHEADS, but with no diagonal web members, and (2) Reinforced Shell, in which the skin is supported by a complete framework or structural members. French: monocoque, single shell. 

MSL - Mean Sea Level. The average height of the surface of the sea for all stages of tide; used as a reference for elevations, and differentiated from AGL.

MULTI-PORT FUEL INJECTION (MPFI or MFI) –Multi-port fuel injection is a system that injects fuel into the intake port just upstream of the intake valve rather than at a central point within an intake manifold. MPFI (or just MPI) systems can be sequential, in which injection is timed to coincide with each cylinder’s intake stroke; batched, in which fuel is injected to the cylinders in groups, without precise synchronization to any particular cylinder’s intake stroke; or simultaneous, in which fuel is injected at the same time to all the cylinders. Typical fuel pressure runs between 40 to 60 pounds per square inch.

NACELLE - A streamlined enclosure or housing to protect something such as the crew, engine, or landing gear. French: nacelle, from Latin, navicella, little ship.

NITRIDING - Gas nitriding is a case-hardening process whereby nitrogen is introduced into the surface of a solid ferrous alloy by holding the metal at a suitable temperature and in contact with a nitrogenous gas, usually ammonia. Nitrogen released by the decomposition of ammonia reacts with the metal to make iron nitride, a hardening substance. Process methods for nitriding include: gas (box furnace or fluidized bed), liquid (salt bath), and plasma (ion) nitriding.

PANTS - A popular word for streamlined, non-load bearing fairings to cover landing wheels. Also sometimes called Spats or, when fully enclosing the wheel struts, Skirts.

PARASITIC DRAG (also called parasite drag) is drag caused by moving a solid object through a fluid medium—or in the case of aerodynamics, a gaseous medium. Parasitic drag is made up of many components, the most prominent being form drag, directly related to the general size and shape of the body. Skin friction and interference drag are also major components of parasitic drag.

PATTERN - The path of aircraft traffic around an airfield, at an established height and direction. At tower-controlled fields the pattern is supervised by radio (or, in non-radio or emergency conditions by red and green light signals) by air traffic controllers.

PAYLOAD - Anything that an aircraft carries beyond what is required for its operation during flight, theoretically that from which revenue is derived, such as cargo and passengers.

PILOT IN COMMAND (PIC) - The pilot responsible for the operation and safety of an aircraft during flight time.

PILOT-INDUCED OSCILLATION: When the pilot of an aircraft inadvertently commands an often-increasing series of corrections (typically in pitch) in opposite directions, each as an attempt to inhibit the aircraft’s reaction to the previous input with a correction in the opposite direction.

PITCH - (1) Of the three axes in flight, this specifies the vertical action, the up-and-down movement. Compare ROLL and YAW. (2) The angle of a propeller or rotor blade in relation to its arc; also the distance advanced by a blade in one full rotation.

PITOT TUBE - More accurately but less popularly used, Pitot-Static Tube, a small tube most often mounted on the outward leading edge of an airplane wing (out of the propeller stream) that measures the impact pressure of the air it meets in flight, working in conjunction with a closed, perforated, coaxial tube that measures the static pressure. The difference in pressures is calibrated as air speed by a panel instrument. Named for French scientist Henri Pitot (1695-1771).

PLANFORM - or plan view is a vertical orthographic projection of an object on a horizontal plane, like a map. In aviation, a planform is the shape and layout of an airplane's wing and fuselage. Of all the myriad planforms used, they can typically be grouped into those used for low-speed flight, found on general aviation aircraft, and those used for high-speed flight, found on many military aircraft and airliners.

POBEREZNY, PAUL HOWARD (b. September 14, 1921 in Leavenworth County/Kansas) is a US aviator and aircraft designer famous for his work in establishing the Experimental Aircraft Association (EAA) in 1953 and promoting homebuilt aircraft.

POWER LOADING - The GROSS WEIGHT of an airplane divided by the rated horsepower, computed for Standard Air density.

PROPELLER SPEED REDUCTION UNIT (PSRU) - A gearbox or a belt and pulley device used to reduce the output rotational speed (rpm) when compared to the higher input rpm of the powerplant. These arrangements allow the use of relatively small' displacement, high-revving, internal combustion automotive engines to turn propellers within an efficient speed range. Certified aircraft engines, where the propeller most commonly is fastened directly to the engine crankshaft, develop peak power near the peak safe and efficient speed for the propeller--2,500 to 3,000 rpm. This speed is considered the typical maximum rpm for a single engine aircraft propeller. Note that there are examples of factory certified aircraft that have used a PSRU. The Cessna 170 used a a geared unit during its production run the 1970s.  

PUSHER - A propeller mounted in back of its engine, pushing an aircraft through the air, as opposed to a TRACTOR configuration.

QUADRAPLANE, QUADRUPLANE - An aircraft having four or more WINGFORMS

RAMJET - An aerodynamic duct in which fuel is burned to produce a high-velocity propulsive jet. It needs to be accelerated to high speed before it can become operative.

REYNOLDS NUMBER - In fluid mechanics, a number that indicates whether the flow of a fluid (liquid or gas) is absolutely steady (in streamlined, or laminar flow) or on the average steady with small, unsteady changes (in turbulent flow; see turbulence). The Reynolds number, abbreviated NRe or Re, has no dimensions (see dimensional analysis) and is defined as the size of the flow — as, for example, the diameter of a tube (D) times the average speed of flow (v) times the mass density of the fluid (r) — divided by its absolute viscosity (m). Osborne Reynolds demonstrated in 1883 that the change from laminar to turbulent flow in a pipe occurs when the value of the Reynolds number exceeds 2,100.

ROCKWELL SCALE "HRC" - The Rockwell scale is a hardness scale based on the indentation hardness of a material. The Rockwell test determines the hardness by measuring the depth of penetration of an indenter under a large load compared to the penetration made by a preload. There are different scales, which are denoted by a single letter, that use different loads or indenters. The result, which is a dimensionless number, is noted by HRX where X is the scale letter.

There are several alternative scales, the most commonly used being the "B" and "C" scales. Both express hardness as an arbitrary dimensionless number.

ROGALLO WING - A flexible, delta-wing plan in which three rigid members are shaped in the form of an arrowhead and joined by a flexible fabric, which inflates upward under flight loads. Originally specific to paragliders, but now found on some powered aircraft. 

ROLL - Of the three axes in flight, this specifies the action around a central point. Compare PITCH and YAW.

ROTARY ENGINE - A powerplant that rotates on a stationary propeller shaft. An American invention by Adams-Farwell Co (1896), it was first used for buses and trucks in the US (1903), then copied by French engineers for early aircraft engines (1914). 
Other rotary engines: Besides the configuration described above, with cylinders moving around a fixed crankshaft, several other very different engine designs can also be described as rotary engines. The most notable pistonless rotary engine, the Wankel rotary engine has also been used in cars (notably by Mazda in cars such as the RX-7 and RX-8), as well as in some experimental aviation applications. Although the GAS TURBINE produces rotary motion directly, it is not generally considered a rotary engine.

ROTORCRAFT - A heavier-than-air aircraft that depends principally for its support in flight on the lift generated by one or more rotors. Includes helicopters and gyroplanes.

RUDDER - The movable part of a vertical airfoil which controls the YAW of an aircraft; the fixed part being the FIN.

RUDDERVATOR / RUDDERVATORS - The control surfaces on an airplane with a V-tail configuration. They are located at the trailing edge of each of the two airfoils making up the tail of the plane. The name derives from a combination of the word rudder and elevator. In traditional aircraft tail configurations, the rudder provides horizontal (yaw) control and the elevator provides vertical (pitch) control. Ruddervators provide the same control effect, albeit through a more complex control system by mixing the control inputs. Yaw is achieved through deflecting both ruddervators in the same direction (left or right) as viewed from either the front or the rear of the plane - one toward the centerline of the craft and the other away from the centerline. Pitch is achieved through deflecting both ruddervators in opposite directions - both toward centerline for up elevator and both away from centerline for down elevator.

SAILPLANE - An unpowered, soaring aircraft capable of maintaining level flight for long periods of time after release from tow and of gaining altitude using wind currents, as opposed to a GLIDER. 

SCIMITAR PROPELLER - A scimitar propeller is any propeller that is shaped like a scimitar sword, with increasing sweep along the leading edge. Fixed-pitch versions that have an exaggerated sweep are reported to behave similarly to a constant-speed propeller flexing to change pitch as the prop is loaded and unloaded.

SCRAMJET - Acronym for supersonic combustion ramjet, in which combustion occurs at supersonic air velocities through the engine. 

SEAPLANE - A water-based aircraft with a boat-hull fuselage, often amphibious. The term is also used generically to define a similar Flying Boat and a pontoon FLOATPLANE

SECONDARY STALL - Any stall resulting from pulling back too soon and too hard while recovering from any other stall. Usually a HIGH-SPEED or ACCELERATED STALL.

SERVICE CEILING - The density altitude at which flying in a clean configuration, at the best rate of climb airspeed for that altitude and with all engines operating and producing maximum (available) continuous power, will produce a 100 feet per minute climb. Margin to stall at service ceiling is 1.5g.

The one engine inoperative (OEI) service ceiling of a twin-engine, fixed-wing aircraft is the density altitude at which flying in a clean configuration, at the best rate of climb airspeed for that altitude with one engine producing maximum continuous power and the other engine shut down and feathered, will produce a 50 feet per minute climb. However some performance charts will define the service ceiling as the pressure altitude at which the aircraft will have the capability of climbing at 50 fpm with one propeller feathered. Also see ABSOLUTE CEILING 

SERVO TAB - A small portion of a flight-control surface that deploys in such a way that it helps to move the entire flight-control surface in the direction that the pilot wishes it to go. A servo tab is a dynamic device that deploys to decrease the pilot’s workload and destabilize the aircraft.

SESQUI-WING - A lesser-span additional wingform, generally placed below the main planes of an aircraft, generally a biplane.

SHOULDER-WING - A mid-wing monoplane with its wing mounted directly to the top of the fuselage without use of CABANE STRUTs.

SINK, SINKING SPEED - The speed at which an aircraft loses altitude, especially in a glide in still air under given conditions of equilibrium.

SLATS - Movable vanes or auxiliary airfoils, usually set along the leading edge of a wing but able to be lifted away at certain angles of attack.

SLIPSTREAM - The flow of air driven backward by a propeller or downward by a rotor. Compare DOWNWASH

SLOT or SLAT- A long, narrow, spanwise gap in a wing, usually near the leading edge, to improve airflow at high angles of attack for slower landing speeds. A higher coefficient of lift is produced as a product of angle of attack and speed, so by deploying slats an aircraft can fly slower or take off and land in a shorter distance. They are usually used while landing or performing maneuvers which take the aircraft close to the stall, but are usually retracted in normal flight to minimize drag.

SLOTTED FLAP - A flap that, when depressed, exposes a SLOT and increases airflow between itself and the rear edge of the wing. 

SMOH - "Since Major Overhaul," an acronym seen in reference to the operating hours, or time remaining, on an engine.

SPLIT FLAP - A FLAP built into the underside of a wing, as opposed to a Full Flap wherein a whole portion of the trailing edge is used. 

SPOILER - A long, movable, narrow plate along the upper surface of an airplane wing used to reduce lift and increase drag by breaking or spoiling the smoothness of the airflow.

SPORT PILOT - Special FAA certification enabling "budget" pilotry; see LIGHT SPORT AIRCRAFT and LSA feature.

SPONSON - A short, wing-like protuberance on each side of a seaplane fuselage to increase lateral stability.

STABILATOR - A movable horizontal tail that combines the actions of a stabilizer and elevator, increasing longitudinal stability while creating a pitching moment.

STABILIZER - The fixed part of a horizontal airfoil that controls the pitch of an aircraft; the movable part being the ELEVATOR

STAGGER - The relative longitudinal position of the wings on a biplane. Positive Stagger is when the upper wing's leading edge is in advance of that of the lower wing [eg: Waco YKS], and vice versa for Negative Stagger [eg: Beechcraft D17]. 

STALL - (1) Sudden loss of lift when the angle of attack increases to a point where the flow of air breaks away from a wing or airfoil, causing it to drop. (2) A maneuver initiated by the steep raising of an aircraft's nose, resulting in a loss of velocity and an abrupt drop.

STANDARD DAY (Standard Atmosphere) - An arbitrary atmosphere established for calibration of aircraft instruments. Standard Air Density is 29.92 inches of mercury and temperature of 59° F, equivalent to an atmospheric air pressure of 14.7# per square inch.

STATIC WIRE - A clip-on wire used to ground an aircraft by drawing off static electricity, a potential fire hazard, during refueling.

STOICHIOMETRIC - or theoretical combustion is the ideal combustion process during which a fuel is burned completely. A complete combustion is a process that converts all carbon (C) to carbon dioxide (CO2), all hydrogen (H) to water (H2O), and all sulfur (S) to sulfur dioxide (SO2). If there are unburned components in the exhaust gas such as C, H2, or CO, the combustion process is uncompleted.

SUPERCHARGER - An air pump or blower in the intake system of an internal combustion engine. Its purpose is to increase the air-charge weight and therefore the power output from an engine of a given size. In an aircraft engine, the supercharger counteracts the power loss that results from the decrease of atmospheric pressure with increase of altitude. Various types of pumps and compressors may be used as superchargers, which are either mechanically driven by the engine crankshaft or powered by the engine exhaust gas. Also see TURBOCHARGER

SUPERSONIC - Speed of flight at or greater than Mach 1.0; literally, faster than the speed of sound.

SWEEPBACK - A backward inclination of an airfoil from root to tip in a way that causes the leading edge and often the trailing edge to meet relative wind obliquely, as WINGFORMs that are swept back.

SWING-WING - A wing whose horizontal angle to the fuselage centerline can be adjusted in flight to vary aircraft motion at differing speeds.

TAILDRAGGER  see  CONVENTIONAL GEAR.

TARMAC - (1) A bituminous material used in paving; a trade name for Tar MacAdam. (2) An airport surface paved with this substance, especially a runway or an APRON at a hangar.

TAS - True Air Speed. Because an air speed indicator indicates true air speed only under standard sea-level conditions, true air speed is usually calculated by adjusting an Indicated Air speed according to temperature, density, and pressure. Compare CALIBRATED AIR SPEED and INDICATED AIR SPEED.

THRUST - The driving force of a propeller in the line of its shaft or the forward force produced in reaction to the gases expelled rearward from a jet or rocket engine. Opposite of DRAG.

THRUST HORSEPOWER - The force-velocity equivalent of the thrust developed by a jet or rocket engine. The thrust of an engine-propeller combination expressed in horsepower; it differs from the shaft horsepower of the engine by the amount the propeller efficiency varies from 100 percent.

(Naval architecture) The product of the speed of advance of a marine propeller through the water, in feet per second, and the thrust delivered by the propeller, in pounds, divided by 550.

TORQUE - A twisting, gyroscopic force acting in opposition to an axis of rotation, such as with a turning propeller; aka Torsion.

TRACTOR - A propeller mounted in front of its engine, pulling an aircraft through the air, as opposed to a PUSHER configuration.

TRAILING EDGE - The rearmost edge of an AIRFOIL.

TRIKE - Nickname for a weight-shift-control aircraft, such as a paraglider. 

TRIM TAB - A small, auxiliary control surface in the trailing edge of a WINGFORM, adjustable mechanically or by hand, to counteract ("trim") aerodynamic forces on the main control surfaces. 

TRUE AIRSPEED (TAS)- The speed of an aircraft along its flight path, in respect to the body of air (air mass) through which the aircraft is moving. Also see CALIBRATED AIRSPEED, GROUND SPEED, and INDICATED AIRSPEED

TURBOCHARGER – (Turbo) An air compressor or SUPERCHARGER on an internal combustion piston engine that is driven by the engine exhaust gas to increase or boost the amount of fuel that can be burned in the cylinder, thereby increasing engine power and performance. On an aircraft piston engine, the turbocharger allows the engine to retain its sea-level power rating at higher altitudes despite a decrease in atmospheric pressure. Also see SUPERCHARGER

TURBOJET - An aircraft having a jet engine in which the energy of the jet operates a turbine that in turn operates the air compressor.

TURBOPROP - An aircraft having a jet engine in which the energy of the jet operates a turbine that drives the propeller.

ULTRALIGHT - An aeronautical vehicle, operated for sport or recreational purposes that does not require FAA registration, an airworthiness certificate, or pilot certification. Primarily single-occupant vehicles, although some two-place vehicles are authorized for training purposes. Operation of an ultralight vehicle in certain airspaces requires authorization from ATC.

UNDERCARRIAGE - The landing gear of a land-based aircraft, including struts, frames, and wheels. A very British word that has limited use in the USA.

UPWASH - The slight, upward flow of air just prior to its reaching the leading edge of a rapidly moving airfoil.

USEFUL LOAD - The weight of crew, passengers, fuel, baggage, and ballast, generally excluding emergency or portable equipment and ordnance.

V-SPEED – V, Velocity, as used in defining specific air speeds at specific configurations or conditions: Read More

VARIOMETER - (also known as a vario, rate of climb and descent indicator [RCDI], rate of climb indicator, vertical speed indicator [VSI], or vertical velocity indicator [VVI]) is one of the flight instruments in an aircraft (mostly used in sailplanes, hang gliders, paragliders, etc.) used to inform the pilot of the near instantaneous (rather than averaged) rate of descent or climb in order to detect the presence of a thermal or other forms of lift for unpowered aircraft.

VENTRAL FIN - A fin/rudder extension on the bottom of a fuselage. Opposite of DORSAL FIN.

VENTURI TUBE - A small, hourglass-shaped metal tube, usually set laterally on a fuselage facing into the slipstream to create suction for gyroscopic panel instruments. Now outdated by more sophisticated means.

VFR - Visual Flight Rules that govern the procedures for conducting flight under visual conditions. The term is also used in the US to indicate weather conditions that are equal to or greater than minimum VFR requirements. Also used by pilots and controllers to indicate a specific type of flight plan.

VFR ON TOP - Flight in which a cloud ceiling exists but modified VISUAL FLIGHT RULES are in effect if the aircraft travels above the cloud layer. 

VISUAL METEOROLOGICAL CONDITIONS (VMC) - Meteorological conditions expressed in terms of visibility, distance from clouds, and ceiling equal to or better than specified minima. 

VORTILONS - Small fencelike surfaces extending in front of the wing and attached to the undersurface. They are particularly useful in preventing spanwise flow at high angles of attack, by shedding a vortex, similar to that of a wing fence. Vortilons generate a vortex over the upper surface at high angles of attack.

VSI - Vertical Speed Indicator. A panel instrument that gauges rate of climb or descent in feet-per-minute (fpm). Also Rate Of Climb Indicator. 

WASHOUT - The terms 'wing twist' and 'washout' refer to wings designed so that the outboard sections have a lower ANGLE OF INCIDENCE, 3 or 4° or so, and thus lower AOA than the inboard sections in all flight conditions. One reason for wing twist is to reduce INDUCED DRAG, the other is to improve the stall characteristics of the wing. With twist, the sections near the wing root reach the stalling AOA first, thus allowing effective aileron control even as the stall progresses from inboard to outboard. This is usually achieved by building a geometric twist into the wing structure by rotating the trailing edge so providing a gradual decrease in AOA from root to tip, but of course washout reduces the total lift capability a little but this disadvantage is more than offset by the wing twist improving elliptical lift distribution and thus decreasing induced drag.

WINGFORM - A wingform is the shape and layout of an airplane's wings as vied from above or below. Also see PLANFORM

WINGLET - A small, stabilizing, rudder-like addition to the tips of a wing to control or employ air movement.

WING LOA
DING
- The maximum take-off gross weight of an aircraft divided by its wing area.

YAW - Of the three axes in flight, this specifies the side-to-side movement of an aircraft on its vertical axis, as in skewing. Compare PITCH and ROLL.

YOKE - The control wheel of an aircraft, akin to an automobile steering wheel. 

 
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