Helicopter Terminology Definitions

Absolute Ceiling

The maximum altitude at which level flight can be maintained. The greatest height that can be achieved by that aircraft, except by zoom climbing.

Acceleration

The rate of change of velocity (both speed and direction). Can mean changing direction without change in velocity.

Advancing Blade

The blade which is moving forward into (or against) the oncoming airflow.

Aerodynamic Centre

Point about which there is no change in moment with change in angle of attack.

Air Taxiing

Movement of the helicopter by hovering at speeds less than 60 KIAS, at low height above the ground.

Airfoil (or Aerofoil)

A surface which is shaped to derive lift from air passing it. A body designed to move through the air and obtain a useful reaction other than drag.

Airspeed

Motion relative to the air.

Altitude

Height measured against the datum of Mean Sea Level.

Angle of Attack

The angle between the relative airflow and the chord of the airfoil.

Angle of Incidence

An old-fashioned (and incorrect) term for angle of attack. More correctly, the angle of the fuselage of a fixed wing airplane, or the angle of the horizontal stabilizer on helicopter tail-boom.

Anticipator

A device for sending a signal forward.

Approach

The flight path from cruise to hovering or landing.

Articulated Rotor

A rotor system with hinges for flapping, leading / lagging and feathering of the rotor blades.

Autogiro

An aircraft which has a rotor which operates in autorotation to produce lift and has a power source to produce thrust. May also be towed.

Autorotation

The process of turning the rotor from air passing up from below also the maneuvers associated with flying and landing without power from an engine.

Axis

An imaginary straight line passing through a point which is used to define motion or position.

Axis of Rotation

A line about which a body rotates.

Azimuth

The angle taken from a reference.

Bank Angle

The angle between the lateral axis and the horizon.

Blade Flapping

The movement of rotor blade about the flapping hinge, that is up and down, lead and lag is taken by the structure of the hub.

Blade Pitch Angle

The angle between the chord line of the blade and a reference line on the rotor hub.

Blow Back

The movement of the rotor blades away from the relative wind without any control input. More commonly called flapback.

Camber

The degree of curvature of the surface of an airfoil.

Ceiling

The maximum altitude that can be obtained. There are various types of ceiling, namely: Hover ceilings (in and out of ground effect), service ceiling and absolute ceiling.

Center of Gravity

A point where the weight forces in the body may be considered to be concentrated.

Center of Pressure

The point on the chord line where the resultant of all the aerodynamic forces on the airfoil section may be said to be concentrated.

Centrifugal Force

The force created by the tendency of a body to follow a straight line path against the force which causes it to move in a curve, resulting in a force which tends to take it away from the axis of rotation.

Chip Detector

A device for detecting the presence of small pieces of metal in a lubrication system.

Chord

The straight line between the leading edge and trailing edge of an airfoil.

Co-efficient of Drag

The non-dimensional value for the amount of drag produced by an airfoil section.

Co-efficient of Lift

The non-dimensional value of the amount of lift produced by an airfoil section.

Co-relator

Mechanical linkage between the cockpit throttle and engine throttle which is supposed to change the engine throttle to collectively control the pitch of the blades. Designed to reduce the cockpit throttle adjustments needed due to collective movement.

Coanda Effect

The effect of a jet of air blowing out from an airfoil or other shape and reacting with the air passing.

Collective

The method of controlling the amount of blade pitch where all blades are rotated in pitch an equal amount simultaneously.

Collective Bounce

A phenomenon where the collective is in resonance with the vertical motion of the helicopter, making the motion worse.

Confined Area

A smaller than normal area for landing, or maneuvering a helicopter in.

Coning Angle

The angle the rotor blades make with the horizontal, due to the combined action of lift on the blades and centrifugal force due to rotation.

Coriolis Effect

The tendency of a mass to increase or decrease its angular velocity when its radius of rotation is changed. More correctly called conservation of angular momentum.

Couple

Two equal and opposite parallel forces tending to produce pure rotation.

Cyclic

The common pitch in which the pitch of the rotor blades changes during each cycle of rotation. Used to control the tilt of the tip path plane.

Damper

A device to slow the rate of motion. Can be used in hubs to slow the angular motion due to lead and lag, or in landing gear. A shock absorber on a car is a damper.

Deadman's Curve

A misnomer for the height velocity curve.

Density Altitude

The pressure altitude corrected for temperature and humidity.

Departure

Leaving an area or condition - i.e. testing a loss of controlled flight.

Disk

The area swept by the tips of the rotor blades.

Disk Loading

The weight per unit area of the blade. Expressed in pounds per square foot, or kilograms per square meter.

Dissymmetry of Lift

The unequal lift across the disk resulting from the difference in relative airspeed between the advancing and retreating blades.

Downwash

The mass of air displaced by the rotor system.

Drag

The force acting on an object as it passes through the air.

Droop Stops

The stops in the rotor hub which prevent the rotor blades from drooping when at low speed or static.

FAA

Federal Aviation Administration. The governing body for regulation and certification in the United States of America.

Feathering

The pitch change of the rotor blade which is due to collective or cyclic movement.

Flap Back

The act of the rotor disk flapping away from the relative wind.

Flap Restrainers

The restraining devices on the rotor hub which prevent the rotor blades from flapping to too high an angle when the blades are stopped or rotating slowly.

Flapping

The act of the rotor blade moving vertically about the flapping axis.

Flare

The change in flight path from descent to level or a smaller rate of descent. Typically done close to the ground, and if done correctly results in zero rate of descent. Zero groundspeed is a secondary aim.

Freewheel Unit

A device placed in the drivetrain to permit the rotor to continue to turn after an engine failure, or to permit the rotor to turn faster than the driving part of the engine.

Gravity

The force.

Ground Effect

The effect of the ground on the downwash from the rotor. Normally acts to decrease the power required to hover.

Ground Resonance

A resonance in the helicopter made worse by contact with the ground.

Gyroscopic Precession

When a force is applied to a rotating mass, parallel to the axis of rotation, the rotating body will tilt in the direction of the applied force, but 90° farther around in the direction of rotation. Applies to all rotating bodies.

Height

The distance above a datum, typically the ground.

Height Velocity Curve

The curve defining the area within which it is difficult or impossible to make a safe landing following an engine failure.

helicopter

An aircraft which obtains its lift by the powered rotation of rotor blades, and is capable of maintaining position with respect to the earth with no forward airspeed.

hingeless rotor

A rotor hub with no physical hinges in flapping or lead-lag. All motion relating to flapping, and lead-lag is taken by the structure of the hub. A feathering hinge may or may not be present.

horizontal stabilator

A horizontal wing mounted in such a fashion as to assist in the stability of the pitch axis.

Hover Ceiling

The maximum altitude at which it is possible to hover using takeoff power.

hover

Maintaining zero speed with respect to the ground (zero groundspeed hover) or the air, (zero airspeed hover).

hover taxiing

The act of moving across the ground at a slow speed. Typically a legal definition used by Air Traffic Control.

IGE

In Ground Effect. Flight where the proximity to the earth has an effect on the performance or handling of the aircraft.

induced drag

The drag due to producing lift.

induced velocity

The air moving down caused by the rotor blades producing lift.

ISA

(International Standard Atmosphere) The standard agreed properties for the atmosphere, against which deviations are compared.

Knot

Rate of speed meaning one nautical mile per hour.

lead - lag

The back and forth motion of the rotor blades about the vertical hinge.

lift

The vertical force used to support the helicopter.

manifold pressure

The pressure in the intake manifold of a piston engine, a measure of the power of the engine.

mass

Mean chord line halfway between the upper and lower surfaces of an airfoil.

moment

The product of a force and its perpendicular distance from its axis.

nautical mile

Measure of distance. One nautical mile is 1.15 statute miles, or 6,080 feet, or 1.85 kilometers.

Nr

A shorthand for the rotational RPM of the rotor blades.

OGE

Out of Ground Effect. Flight where the proximity to the earth has no effect on the performance or handling of the aircraft.

parasite drag

The drag due to the fuselage passing through the air.

pitch angle

The mechanical angle between the chord line of the blade and the main rotor hub.

pitch attitude

The angle between the longitudinal axis of the fuselage and horizon.

Plane of rotation

The plane about which the tips of the rotor is rotating.

pressure altitude

The altitude measured against a standard pressure setting (29.92" of mercury, or 1013 millibars).

profile drag

The drag caused by the shape of a body as it passes through the air.

rear wing

A wind coming from the rear of the helicopter.

rearward flight

Strictly speaking it would be only be rear groundspeed.

reciprocating engine

An engine which uses a 2 or 4 cycle system, pistons and so on to produce power.

relative wind

The wind speed and direction relative to the nose of the helicopter.

retreating blade

The blade which is going away from the direction of relative wind.

retreating blade stall

The stall of the retreating blade, due to high forward airspeed, angle of attack or G loading.

roll or rolling

Angular motion about the longitudinal axis of the helicopter.

rotational energy

The energy possess by the rotor system due to rotation.

rotational velocity

The rate of rotation of a body.

rotor

A system of rotating airfoils.

rotor disc

The area of the rotating blades.

RPM

Revolutions per minute.