GLOSSARY OF TERMS
© 2003 World Hovercraft Organization

Acceleration
The rate of change of velocity.

ACV
Abbreviation for air cushion vehicle(s), a family of vehicles that travel on a cushion of air. The hovercraft belongs to the ACV family.

Air density
A measurement of the mass of air per unit volume for a given air temperature.

Airfoil
An airfoil or aerofoil is a part or surface, such as a wing, propeller blade, or rudder, whose shape influences control, direction, thrust, lift, or propulsion.

Air gap
Also called daylight clearance, air gap refers to the distance between the bottom of the hovercraft skirt and the surface beneath it.

Air pressure
Measurement of the force exerted by air above, at, or below atmospheric pressure on a unit of area.

Angle of attack
The angle between an airfoil or wing and the direction of the wind relative to it.

Angular speed
The rate at which an object rotates or spins.

Angular velocity
Angular speed in a certain direction, clockwise or counter-clockwise.

Arc length
The length of an arc, or a section of a circle's perimeter.

Archimedes' Principle of Buoyancy
The principle stating that when a body is immersed in a fluid at rest it experiences an upward or buoyant force equal to the weight of the fluid displaced by the body.

Atmospheric pressure
The pressure exerted by the atmosphere at the surface of the earth due to the weight of the air above.

Bag skirt
A type of hovercraft skirt consisting of a flexible fabric tube that surrounds the perimeter of the hovercraft.

Bernoulli's Principle
Bernoulli's Principle states that an increase in the velocity of a fluid is always accompanied by a decrease in pressure.

Body
The top surface of the hovercraft; usually attached to the hull.

Bow
The front of a boat or hovercraft.

Buoyancy
The upward force on an object immersed in a fluid; equal to the weight of the fluid displaced by the object (Archimedes' Principle).

Centerline
A line of symmetry along the axis of an object.

Centrifugal "force"
Not an actual force but, rather, the result of an object's inertia trying to maintain motion along a straight line when the object is forced to travel along a curve.

Centripetal force
When traveling in a circle or curve, the force that pulls an object towards the center of the circle or curve.

CFM
Abbreviation for "cubic feet per minute," a rate of fluid flow.

Chalk line
A tool consisting of an enclosed spool of string with powdered chalk inside, allowing the user to stretch the string to a particular length then pluck or snap the string to create a straight chalk mark on the surface.

Charge
An electromagnetic property of matter that can be positive or negative and will cause either an attractive or repulsive force on another charge (Coulomb's Law).

Chemical Potential Energy
Energy in the chemical bonds of matter.

Circumference
The distance around the outside of a circle.

Clearance
The space between two objects, allowing for free movement.

Coanda Effect
The Coanda Effect states that a moving stream of fluid in contact with a curved surface will tend to follow the curvature of the surface rather than continue to travel in a straight line.

Cockpit
The area where the pilot or passenger(s) sit(s); also called a cabin.

Coefficient of friction
A quantity representing the extent to which friction develops between two objects in contact as the normal force changes.

Conductor
There are two kinds of conductors: electrical and thermal. An electrical conductor is a material that allows an electric charge to move through it. A thermal conductor is a material that allows heat to flow through it.

Contact force
A force between objects in contact with each other. A contact force can be attractive (as in a tension force), repulsive (as in a normal force), resistive (as in friction), restoring (as in a spring force), or arbitrary (someone pushing and pulling on things).

Contact line
A theoretical line around the perimeter of a hovercraft where the lower edge of the skirt makes surface contact. Jupe and segmented finger skirts have complicated contact lines. The contact line can move as the skirt flexes and changes shape.

Control surface
Any movable surface, usually the rudders, designed to deflect air and cause the resulting force to change the direction of the hovercraft.

Coulomb's Law
Coulomb's Law describes the interaction between two objects that have electric charges. It states that the force between two charged bodies is equal to a constant k approximately 8.99×109 N m2/C2 times the product of the two charges, all divided by the square of the distance between the bodies. If the two charges are both either positive or negative, the force will be positive and the bodies will repel each other. If one is positive and one is negative, the force will be negative and they will attract each other.

Cross-sectional area
The area of a two dimensional slice of a three dimensional object.

Cushion pressure
Liquid or gas pressure measured between the hull of the hovercraft and the earth's surface.

Daylight clearance
Also called air gap, daylight clearance is the distance between the bottom of the hovercraft skirts and the surface beneath it.

Density
The ratio of mass to volume of an object.

Diameter
The distance across the center of a circle, from one side to the other. The diameter of a circle is equal to twice its radius.

Drag
Any force that creates resistance to motion.

Driving gear
A gear that rotates while in contact with another gear, causing the other gear to rotate in the opposite direction.

Driven gear
A gear in contact with a rotating gear; it will rotate in the opposite direction.

Dynamic friction
Friction between two objects in contact that are moving. Dynamic friction is always less than static friction.

Dynamic pressure
The pressure of a fluid in motion, measured by the pressure it exerts on a flat surface.

Efficiency
The ratio of output work compared to input work. Efficiency is sometimes expressed as a percent, in which case the ratio is multiplied by 100 to give a percentage value.

Elastic potential energy
Energy stored in a spring or elastic object due to its being stretched or compressed.

Electric force
The force resulting from the difference between existing charges.

Electricity
The area of physical phenomena dealing with the behavior of electric charges. There are two main branches of electricity: electrodynamics, the study of charged particles in motion; and electrostatics, the study of charged particles at rest.

Electron
A negatively charged particle that is a small part of an atom.

Energy
The capacity for doing work. An important property of energy is that in a system with no external influences, the total amount of energy can never change. This is called the Law of Conservation of Energy.

Epoxy
A two-part compound of resin and hardener used as a very strong waterproof glue. It is often used in conjunction with fiberglass cloth or tape.

Fan
A rotating, multi-bladed device (usually 4 or more blades) for moving volumes of air in ducts with only a small pressure increase.

Fiberglass
Fine filaments of glass, usually made into a mat or cloth. When combined with epoxy or polyester resin, fiberglass creates a strong, long-lived, rigid, and waterproof material.

Finger skirt
A finger skirt, also called a segmented skirt, is a type of skirt consisting of several segments that press together when inflated.

Fluid
A liquid or gas that flows and assumes the shape of its container.

Foot-pound
A unit of work equal to the work done by a force of one pound acting through a distance of one foot in the direction of the force. Also a unit of torque equal to the amount of torque exerted by a force of one pound at a distance of one foot.

Form
A pattern or mold used to give shape to something else.

Form drag
Form drag, also called profile drag or wind resistance, is the drag force created on a hovercraft as it displaces the fluid through which it moves. If moving forward, form drag results in greater air pressure at the front of the hovercraft than at the rear.

FPS
Abbreviation for the term "feet per second".

Frame of reference
The perspective from which a system is observed. There are two kinds of reference frames: inertial and non-inertial. An inertial reference frame is one in which Newton's First Law of Motion holds. In a non-inertial reference frame, such as a rotating carousel or a moving car, things appear to accelerate without the forces to cause it.

Friction
A force that opposes the motion between two objects in contact with each other. The force of friction between two objects is determined as the product of the normal force between the two objects and the coefficient of friction between them.

Gear ratio
The ratio of radii, diameters, or circumferences of two gears. When two gears are meshed, the gear ratio is equal to the ratio of the torques exerted by the gears and to the reciprocal of the ratio of their angular velocities.

Gravitational potential energy
Potential energy of an object due to its height above the earth.

Gravity
An attractive force between any two objects. The strength of the force varies with the mass of each object and the distance between them. Scientists today do not know at all how gravity works!

Heave
The "up and down" motion experienced by a boat or hovercraft.

Hover height
The distance between the bottom of the hovercraft's hull and the surface over which it is hovering.

Horsepower
A unit of power, equal to 33,000 ft lb/min or 550 ft lb/s or 745.7 Watts.

Hovercraft
A member of the Air Cushion Vehicle (ACV) family that is amphibious; it can travel on water as well as on land.

Hub
A metal disk that makes a mechanical connection between a propeller or lift fan and the axle or shaft turning it.

Hull
The bottom of the hovercraft, which usually contains buoyancy foam and which will usually float like a boat.

Hump drag
Also called wave drag, hump drag occurs when cushion air pressure under the hull displaces the surface an amount equal to the cushion pressure. One inch of displacement yields around 5.2 lb/sq. ft., or 1 cm of displacement yields 1411.9 Pa. This depressed surface has the same resistance as that of a boat hull of the same displacement. Usually maximum engine power is required to climb out of this depression and fly above the water surface.

Hump speed
Hump speed, also called planing speed, is the speed at which a hovercraft traveling over water begins to lift out of the depression it makes. It is the speed at which this transition occurs and it is the point of maximum resistance.

Impact drag
Friction caused when the hovercraft skirt impacts waves, rocks, ice, snow, grass, or sand.

Imperial Units
A system of units commonly referred to as English Units, British Units, or U.S. Customary Units. Outside the United States and in a few Caribbean countries, Imperial Units have, for the most part, been replaced by SI Units. Some base Imperial Units include the foot (length), the slug (mass), the second (time), and the pound (force).

Impulse
A change in the momentum of an object.

Inertia
Similar to mass, inertia is the property of an object that opposes any change in motion. Inertia is measured using the same units as those used to measure mass.

Inertial mass
The measurement of an object's resistance to changes of motion. The inertial mass of an object is the mass that is used in Newton's Second Law of Motion.

Insulator
There are two types of insulation: electrical and thermal. An electrical insulator is a material that does not allow electric charge to move through it. A thermal insulator is a material that does not allow heat to flow through it.

Integrated hovercraft
An integrated hovercraft uses only one propeller or fan for both lift and thrust.

Joule (J)
A unit of work or energy equal to one Newton meter (N m).

Joystick
A stick to which steering and throttle cables are often attached. In a hovercraft, the joystick's left-to-right movement moves the rudder, thus steering the craft.

Jupe skirt
A jupe skirt, also called a cell skirt, is a type of hovercraft skirt consisting of a number of cells in the form of cones with their tops cut off and the bases attached to the bottom of the hovercraft.

Kill switch
A switch designed to shut off the thrust or lift engine, or both, usually attached to the pilot with a cord or lanyard which automatically "kills" the engine if the pilot is thrown out of the craft.

Kinetic energy
Energy due to the motion of an object.

LCAC
LCAC is a military acronym for "Landing Craft Air Cushion". LCACs are high-speed, amphibious landing craft used for transporting weapons systems, equipment, cargo and personnel from ship to shore and across beaches; they can carry a 60-70 ton payload.

Lever arm
A lever arm, also called a moment arm or bell crank, is a rigid bar or beam free to turn around a fixed point (fulcrum). Also, when applying torque, lever arm refers to the distance between the point where the force acts and the point of rotation.

Lift
Aerodynamic forces that support a vehicle solely due to airflow or pressure.

Lift air cushion
The region of pressurized air under the hovercraft, which provides lift.

Lift duct
The circular, horizontally mounted duct surrounding the lift fan that is used to create lift air in a hovercraft with separate engines, or the duct that supplies air to the cushion in an integrated hovercraft.

Lift engine
The engine used to drive a lift fan. The lift engine is usually mounted near the bow on small hovercraft.

Lift fan
Usually a multi-bladed fan, mounted close to the horizontal to force air downwards to provide the air cushion on which the hovercraft rides.

Lift skirt
The flexible fabric component surrounding the bottom edge of a hovercraft. Its purpose is to trap or seal lift air under the hovercraft. The design of the skirt determines the hover height. Skirts are usually of the bag, finger, or jupe type.

Lift surface
The total area of the hovercraft's hull bottom inside the skirt contact line. The cushion pressure acts directly on this surface to create lift.

Linear acceleration
The rate of change of linear speed, where the final speed is greater than the initial speed, as opposed to linear deceleration.

Linear deceleration
The rate of change of linear speed, where the final speed is less than the initial speed, as opposed to linear acceleration.

Manometer
A device that measures pressure.

Mass
The amount of matter in an object. Common units for mass are kilograms (SI) and slugs (Imperial).

Measurement uncertainty
An estimated range of values in which the true value of the measurement lies; an indication of how exact a measurement is.

Meniscus
The curved surface of a liquid in a narrow tube, caused by the tendency of a liquid to "stick" to itself and to the tube. In most cases, the liquid adheres to the tube, resulting in a concave meniscus, where the liquid is higher at the edges than in the middle. In some liquids, like mercury, however, the liquid sticks to itself more strongly, and produces a convex meniscus, where the liquid is "domed" so that it is higher in the middle. When measuring liquids, you read from the middle of the meniscus, either the bottom of a concave or the top of a convex one.

Metric System
A system of units developed in France at the time of the French Revolution (1789-1799), the Metric System was designed with several features in mind. First, that there would be only one unit for any given quantity; for example, length would be measured in meters instead of in feet, inches, rods, ells, hands, or any number of other specialized measures that may or may not be readily expressed in terms of each other. Second, that there would be a system in place for dealing with different scales, where units could be expressed as powers of ten instead of numerous arbitrary ratios between scales. The third principle of the metric system is that there would only be a few base units that can be used to define all the others. The most common set of base units, the one used by the SI units, is based on meters, kilograms, and seconds, although a system based on centimeters, grams, and seconds is used in some situations, such as astronomy and some electromagnetic applications.

Moment of inertia
The tendency of an object to resist changes in its angular velocity.

Momentum
The mass of an object multiplied by its velocity. One important property of momentum is that in a system where there are no outside forces, the total momentum of the masses in the system cannot change, although momentum can be transferred between masses.

Momentum drag
Drag caused from having to turn the cushion lift air until it escapes from the cushion. It is equal to the mass of the lift air times the velocity of the hovercraft.

MPH
Abbreviation for miles per hour.

Net force
The directional sum of all forces acting on an object.

Neutron
A subatomic particle with no charge that is usually found in the nucleus of an atom.

Newton
A unit of force equal to one kilogram-meter per second (kg m/s).

Normal force
A repulsive contact force between two objects, acting perpendicular to their surface of contact to oppose a force tending to press them together.

Nucleus
The extremely small, but very dense, central region of an atom, made up of protons and neutrons held together by particles called gluons.

Parallax Error
Measurement error that occurs when the object being measured is too far away from the measuring device.

Parallel
The relationship between lines, planes, etc. that lie at a 0° angle and are aligned in the same direction.

Payload
The useful load from which payment can be received. Payload does not include fuel, accessories, tools, or safety gear.

Perpendicular
At a 90° angle.

Pi
An arithmetical ratio equal to 22/7, or 3.1415927?

Pitch
The angle at which propeller or fan blades are set. Pitch determines how much energy the fan or propeller can absorb. Pitch can also refer to the motion of a boat or hovercraft where the fore and aft ends of the ship rise or fall relative to each other.

Planing speed
Planing speed, also called hump speed, is the speed at which a hovercraft traveling over water begins to lift out of the Archimedean depression. It is the speed at which this transition occurs and it is the point of maximum resistance.

Polar
A property of molecules in which one side of the molecule has a slight negative charge while the other side of the molecule has a slight positive charge.

Port
The left side of a boat or hovercraft when looking toward the bow.

Potential energy
Energy stored in an object due to position or some other aspect of its configuration. Some common forms of potential energy are gravitational, elastic, and chemical.

Power
The rate of work done per unit time.

Pressure
The force exerted on a surface per unit area of the surface.

Propeller
A twisted airfoil that rotates about its center of mass to provide thrust. A propeller is usually limited to four blades; with more it is called a fan.

Proton
A positively charged subatomic particle usually found in the nucleus of an atom.

Radian
A unit of angular measurement in which the arc length it cuts out of a circle is equal to the radius of the circle; equal to (180 ÷ ?)°, or approximately 57.3°.

Radius
The distance from the center of a circle to the outside of the circle. A circle's radius is equal to half its diameter.

Rate
A displacement-like quantity divided by the time it takes for the quantity to change. Examples: velocity, acceleration.

Reciprocal
The number one divided by any other number.

Relief cut
A cut or series of cuts, usually parallel, made to allow material to bend or flex.

Roll
The motion of a boat or hovercraft where the port and starboard sides move up and down relative to each other.

RPM
Abbreviation for revolutions per minute.

Rudder
A movable airfoil placed behind the propeller or thrust duct of a hovercraft to allow steering of the craft. The rudder's movement redirects airflow, which causes the hovercraft to turn.

Safety wire
Wire that passes through holes drilled into a nut and bolt in order to prevent the nut and bolt from loosening due to vibration.

Scalar
A quantity that has a magnitude but no direction. Speed is an example of a scalar quantity.

SI Units
Abbreviation for the French term Système International d'Unités, or the International System of Units. As it is the most prevalent form of the metric system, the two names are often used interchangeably.

Skirt
The fabric that inflates around the perimeter edge of the hull of a hovercraft, entrapping the lift air cushion.

Skirt drag
Drag caused by the friction between the hovercraft skirt and the earth's surface.

Speed
The distance traveled per unit time, a scalar quantity.

Splitter
On a hovercraft, a splitter is any device designed to divide the flow of air, either from the lift duct or thrust duct, usually to provide lift and skirt inflation air.

Spring force
A contact force that acts between two bodies through a spring or other elastic medium. The spring force is a restoring force, meaning that it tends to force the distance to a certain length. If the distance is greater than the natural length, then the force is attractive. If the distance is less than the natural length, then the force is repulsive. If the distance is equal to the natural length, then the force is zero.

Starboard
The right side of a boat or hovercraft when looking toward the bow.

Static charge
An electric charge due to an imbalance between the number of electrons or negative charges and the number of protons.

Static friction
Static friction, also called starting friction, is the friction between two objects that are in contact but are not moving. Static friction is always greater than dynamic friction.

Static pressure
The pressure of a stationary fluid.

Static thrust
The measured thrust produced when movement is prevented.

Stern
The rear of a boat or hovercraft; also called a transom.

Stress
The pressure acting on a material or part that is trying to change its dimensions. Stress is the ratio of the force applied to the area of the material or part resisting the force.

Stroboscope
A device used to measure motion by using an adjustable flashing light to make moving devices appear to be stationary.

Strut
Any structural member that sustains tension or compression loads along its length axis.

System
A collection of one or more objects that interact with each other.

Tachometer
A device that measures the angular speed of an engine, usually in rpm (revolutions per minute).

Tension
An attractive contact force acting, often through a string or beam, between two objects to counteract a force tending to pull the two apart.

Throttle
The control used by a hovercraft pilot to increase or decrease engine speed.

Thrust
A force that produces motion. Thrust can result from the displacement of a fluid.

Thrust duct
A circular duct on a hovercraft that surrounds the propeller or fan that provides thrust. The thrust duct is usually constructed with an airfoil profile to improve thrust output.

Thrust engine
The engine that provides thrust for a hovercraft, usually by spinning a propeller or fan.

Thrust-to-weight ratio
The ratio of thrust produced to the gross weight of a vehicle. Increasing this ratio usually leads to better performance.

Torque
A turning or twisting force acting at the end of a lever arm that causes rotation.

Transmission
The means to transmit power from the hovercraft engine to the propeller.

Tuft
A piece of string or yarn that is used to observe the direction of airflow at various places throughout the hovercraft.

Vector
A quantity that has both magnitude and direction. These are illustrated as arrows whose length gives the magnitude of the vector. Force, velocity, and acceleration are all examples of vector quantities.

Velocity
The speed in a given direction; a vector quantity.

Volume
The amount of space occupied by an object, a substance, or by matter.

Watt (W)
A unit of power equal to one Joule per second (J/s) or one Newton meter per second (N m/s)

Wave Drag
Wave drag, also called hump drag, occurs when cushion air pressure under the hull of a hovercraft displaces the surface by an amount equal to the cushion pressure. 1 in. of water depressed equals a pressure of 5.2 lb/sq.ft. This depressed surface offers the same resistance as if it were a boat hull of the same displacement and shape. Maximum engine power is usually required for a hovercraft to climb out of this depression and fly above the water surface.

Wave impact drag
Drag caused by a hovercraft's lift skirt or hull being struck by waves when crossing rough water. This drag can severely limit speed in rough conditions.

Wetted resistance
Wetted resistance occurs only over water, when the bottom of a hovercraft's lift skirt occasionally touches the water.

Weight
A measure of the gravitational force acting on a mass.

Wind resistance
Drag produced when an object moves through air.

Work
The force applied to an object multiplied by the distance which the force moves the object; a vector quantity.