Helicopter Terms, Expressions and Definitions

A change in velocity; a change in either speed or direction or both.

The rotorblade that is moving forward through 180° in the direction of flight and opposite to the relative wind.

Relating to the flow of air around a body and that body’s reaction to that flow. An aerodynamic shape is one that allows air to flow smoothly.

A rotor blade; specifically, a rotor blade defined by it’s surface curvature or outline, as viewed in cross section, and designed to provide lift. The profile of a rotor blade (or wing) that would be present if the blade was sliced from the leading edge to the trailing edge. The term “airfoil” also is often used to refer to the actual airfoil or lifting surface itself.

The structure of an aircraft. The frame itself, often made of tubes, or the entire fuselage. Typically, in discussions about rotorcraft, the “airframe” refers to the entire helicopter – less the rotors.

The tendency of an aircraft to self-restore airspeed to the “trimmed” airspeed upon an airspeed disturbance. Typically, the horizontal stabilizer is configured with an amount of down-lift proportional to free airspeed so as to balance the center of gravity located forward of the thrust vector (of the rotor or a wing).

As airspeed decreases below the “trimmed” airspeed, the down-lift of the horizontal stabilizer would also decrease, allowing the nose to lower and restore airspeed to the initial “trimmed” airspeed. Vice-versa upon an increase of airspeed. The amount of down-lift (or the incidence angle) of the horizontal stabilizer typically helps determine the “trimmed” airspeed.

The flow of air past a reference point. Airstream is normally depicted as a vector with amplitude and direction.

A term borrowed from physics which is used to indicated the height of a curve or the maximum value of some variable under measurement. Amplitude often refers to the maximum deviation of an oscillation.

The angle formed between the chord of a rotor blade and the relative wind; the angle between the chord line of an airfoil (rotor blade) and the relative wind or airflow acting on that airfoil. The AOA and airspeed on an airfoil determine the life and drag of that airfoil.

AOA may be positive or negative – positive refers to an AOA that creates an upward lift on the airfoil. Sometimes AOA can also refer to the angle between the longitudinal axis of an object (aircraft, enclosure, etc.) and the relative wind.

Same as angle of attack; sometimes defined as pitch.

A small rotor mounted on one side of the tail boom of piston-engine helicopters having a single main rotor. Its sideward push or pull counteracts torque. Varying the pitch of this rotor provides directional control of the fuselage.

A rotor whose blades are pivoted for flapping, dragging and pitch change.

Air Speed Indicator; an instrument used for measuring speed relative to airflow, not relative the ground.

The ratio of a rotor blade’s span to its chord.

The action where by a pilot puts a helicopter into a controlled decent without the aid of an engine. Usually the helicopter would have to be within a predefined (by manufacturer) height/velocity atittude to be successful.

Though the complete explanation is quite lengthy, basically the pilot lowers the collective control and puts the main rotor blade pitch into a negative angle to maintain rotor RPM during decent before feathering out (pulling the collective back up) to complete a touch down/landing.

Spinning of the main rotor by airflow over the blades during conditions of power off or power failuer. Sufficient lift is generally provided to allow controlled descent and safe landing.

A straight line, real or imaginary, that describes a center of rotation, as of the rotor disc.

The complete wood, metal or composite structure of an airfoil.

A helicopter’s gross weight divided by the total area of its main rotor blades, expressing the load supported by the blades.

Indicated airspeed of an aircraft, corrected for installation and instrumentation errors.

The top or bottom curve of an airfoil. If the top and bottom curves are equal (mirror images), the airfoil is symmetrical; if the top has more curvature than the bottom, the airfoil is asymmetrical.

The point in a helicopter where its total weight is centered for a given position. – The theoretical point where the entire weight of the helicopter is considered to be concentrated. – The point where the (usually) forward and aft balance is calculated depending on weight and balance calculations of both the helicopter/aircraft and it’s load including fuel and passengers.

The point on the chord where the resultant force of all aerodynamic forces is centered. – The point where the resultant of all the aerodynamic forces acting on an airfoil intersects the chord.

A clutch that enguages as a result of an outward force caused by turning motion. A device that mechanically separates, and engages drive transmission between the helicopters engine and combined rotor system. This system is usually automated without any pilot input. Such systems can reduced start-up and shut-down workload requirements for pilots.

A hypothetical force exerting an outward pull on the rotor blades away from the axis of rotation. – The apparent force that an object moving along a circular path exerts on the body constraining the object and that acts outwardly away from the center of rotation.

The force that attracts a body toward its axis of rotation. It is opposite centrifugal force.

A warning device that alerts you to any abnormal wear in a transmission or engine. It consists of a magnetic plug located within the transmission. The magnet attracts any metal particles that have come loose from the bearings or other transmission parts. Most chip detectors have warning lights located on the instrument panel that illuminate when metal particles are picked up.

The width of the rotor blade measured from the lead edge to the trailing edge. Chords can taper from root to tip in some designs. Also known as a straight, imaginary line from the leading to the trailing edge of an airfoil. – An imaginary straight line between the leading and trailing edges of an airfoil section.

A term used in reference to semirigid rotors describing the Happing or teetering axis of the rotor.

A system whether automatic or manual, of engaging the engine drive to the main flight transmission. eg. Centrifugal Clutch or clutchable V – belts to main rotor gearbox to drive the rotors.

A rotor system utilizing two rotors turning in opposite directions on the same centerline. This system is used to eliminated the need for a tail rotor.

The collective pitch control, or collective lever, is normally located on the left side of the pilot’s seat with an adjustable friction control to prevent inadvertent movement changes the pitch (angle of attack) of the main rotor blades collectively to achieve ascent, decent and hover. It is the equal and simultaneous altering of the pitch of all main rotor blades; controls vertical flight (height) of a helicopter.

The control for changing the pitch of all the rotor blades in the main rotor system equally and simultaneously and, consequently, the amount of lift or thrust being generated.

The result of the combination of centrifugal force pulling outwards on the rotor blades and the forward pull of the helicopters weight at the center of the rotor against gravity; this forms a shallow cone, or “V” shape in the plane of rotation.

A helicopter-airplane hybrid with advantages of both: vertical take-off and landing, hovering and high-speed flight.

The tendency of a rotor blade to increase or decrease its velocity in its plane of rotation when the center of mass moves closer or further from the axis of rotation.

The increase or decrease in the velocity of a rotor blade as its effective radius of rotation lengthens or shortens – as through flapping. Drag and flappin hinges help relieve stresses and imbalances brought about by this force.

Rotors, as in a coaxial, tandem or intermeshing arrangement, which rotate in opposite directions to cancel torque.

The mechanical change of the angle of incidence, or pitch, of individual rotor blades independently of other blades in the system.

The cyclic control usually located between the pilot’s legs and is commonly called the cyclic stick is similar to an aeroplanes ailerons but not only controls rolling left and right to achieve turns, it also moves forward and back to obtain it’s directional control. The variation of the pitch of each rotor blade individually during each revolution; controls the horizontal flight direction of a helicopter.

Specific Helicopter Terms: The control for changing the pitch of each rotor blade individually as it rotates through one cycle to govern the tilt of the rotor disc and. consequently, the direction and velocity of horizontal movement.

A flapping hinge with a skewed axis so that the flapping motion introduces a component of feathering that would result in a restoring force in the flap-wise direction.

Pressure altitude corrected for nonstandard temperature variations.

A compass error caused by magnetic disturbances from the electrical and metal components in the aircraft. The correction for this error is displayed on a compass correction card place near the magnetic compass of the aircraft.

The upwardly inclined angle from true horizontal at which semi-rigid rotor blades are generally mounted.

The ability to maneuver a rotorcraft by tilting the rotor disc and changing the pitch of the rotor blades.

A shaft turbine engine in which the compressor and power section are mounted on a common dnveshaft.

The circular area swept by the rotor in one complete revolution.

The area swept by the blades of the rotor. It is a circle with its center at the hub and has a radius of one blade length.

The total helicopter weight divided by the rotor disc area.

The equal life between the advancing and retreating halves of the rotor disc as relative wind alters the effective velocity of each blade. – The unequal lift across the rotor disc resulting from the difference in the velocity of air over the advancing blade half and retreating blade half of the rotor disc area.

The downward rush of air produced by the powered rotor.

Resistance of the rotor blades to movement as a result of inertia and wind resistance; the opposite of thrust.

The difference between the deflected position of the blade, due to drag, of an engine-driven rotor, and the straight position. – An aerodynamic force on a body acting parallel and opposite to relative wind.

A rotor system utilizing two main rotors.

The tendency of a helicopter to continue rolling when the critical angle is exceeded, if one gear is on the ground, and the helicopter is pivoting around that point.

Periodic variations in the angle of incidence of an airfoil during forward flight to equalize lift. – The action that changes the pitch angle of the rotor blades by rotating them around their feathering (spanwise) axis.

The axis about which the pitch angle of a rotor blade is varied. Sometimes referred to as the spanwise axis.

The transmittal of forces, which are initiated by aerodynamic action on rotor blades, to the cockpit controls.

The hinge that permits the rotor blade to flap and thus balance the lift generated by the advancing and retreating blades.

The up and down movement of a rotor blade in response to the dissymmetry of lift. – The vertical movement of a blade about a flapping hinge.

A maneuver accomplished prior to landing to slow down a rotorcraft.

Feet per minute describing speed of ascent or descent.

Unrestricted spinning of the rotor when not under power, permitting autorotation.

A universal coupling, permitting the swash plate to tilt at any angle relative to the mast.

An electrical or mechanical device that automatically maintains engine RPM with varying flight loads.

A cushion of air created by the rotor’s downwash, enhancing lift and generally in effect at a distance of approximately one rotor blade from the ground.

Severe fuselage oscillations, often leading to capsize or structural breakdown, initiaded when shock waves transmitted from the landing gear to the rotor create an imbalance. This condition is rare and generally confined to three-bladed, atriculated rotor.

The principle in which a force applied to a spinning rotor becomes manifest (such as through tilting of the disc) after a 90° lag from the point where thee forcewas applied.

Specific Helicopter Terms: Hover in ground effect relates to the helicopters ability to hover at a certain altitude where the downwash of air from the main rotor is able to react with a hard surface (the ground), and give a useful reaction to the helicopter in the form of more lift force available with less engine power required.

Specific Helicopter Terms: Hover out of ground effect relates to the helicopters ability to hover at a certain altitude where there are no hard surfaces for the downwash to react against. For example a helicopter hovering 150ft above the ocean surface will be in an OGE condition and will require more power to maintain a constant altitude than if it was hovering at 15ft. Therefore a helicopter will always have a lower OGE ceiling than IGE due to the amount of engine power available.

Another relatively helicopter specific term is hovering. A flight maneuver in which the helicopter is maintained in a fixed position above the ground, both vertically and horizontally.

Speed, Kilometers (1000 meters) per hour – the amount of distance expressed as kilometers travelled within an hour.

Speed, Nautical miles – a measurement of distance or speed travelled.

An upward force created as the rotation of the rotor blades overcomes the pull of gravity.

The vertical shaft to which the rotor blades are attached.

Speed, Miles per hour – the amount of imperial miles travelled within an hour.

In a helicopter they are more correctly referred to as "anti-torque pedals". The anti-torque pedals are located in the same position as the rudder pedals in an airplane, and serve a similar purpose, namely to control the direction in which the nose of the aircraft is pointed.

Application of the pedal in a given direction changes the pitch of the tail rotor blades, increasing or reducing the thrust produced by the tail rotor and causing the nose to yaw in the direction of the applied pedal. The pedals mechanically change the pitch of the tail rotor altering the amount of thrust produced.

The acute angle between the chord of an airfoil and the plane of rotation.

The longitudinal shape or outline of a rotor blade as viewed from above.

A plane described by, or parallel to, the spinning rotor disc.

A condition which can occur during a low hover in ground effect. Imagine the airflow which was directed to the ground to create the air cushion in a ground effect is now rebounding off the ground and going back up into the top of the rotor system. When it passes back through the rotor again it gets accelerated. This process may continue with the air velocity increasing each time it passes through the rotor.

Eventually the velocity is so great that the air going into the rotor from above causes a loss of lift and the helicopter will sink toward the ground unless the pilot increases power. This means that if recirculation is occuring, the helicopter will need more power to hold a constant height.

Recirculation will not always happen but will be aggravated by the type of ground or nearby obstacles causing the air which is trying to escape out to the sides of the helicopter to be directed back up toward the rotor system. The result is a “recirculation” of downwash air.

Credit: Helis.com

The direction and speed of the flow of air or wind in relation to the rotor blades.

A single force derived from the combination of several component forces.

A rotor system in which the blades are mounted to the hub without flappin or drag hinges.

Rate of Climb – the speed of which the rotorcraft ascends.

An assembly comprised of one (with counterweight) or more airfoils and designed to provide a helicopter’s lift and thrust.

The rotating "wing" of the helicopter. (Also applies to the helicopter tail rotor)

The cenral structure to which the rotor blades are secured.

A device used for indicating the main rotor’s revolutions per minute.

Revolutions Per Minute in relation to any spinning component of the rotorcraft.

A rotor system in which the blades are free to flap, or teeter, on a central hinge pin.

Also known as the over-run clutch, is basically a one way clutching device that allows the rotor blades to continue to turn, automatically disengaging the engine in the event of an engine failure. Mandatory in any helicopter design.

The distance between the root and tip of a rotor blade.

The tendency of a mass to remain in balance or equilibrium despite disruptive forces.

Very specific to helicopter terms is the swashplate. A circular structure generally mounted on the gimbal ring and able to tilt in any direction relative to the mast through cyclic control; rods connecting the swash plate to the pitch-control arms tilt the rotor by cam action. Swash plates can be mounted on, inside or below the main rotor mast.

Same as the antitorque rotor on a piston engine driven helicopter.

The see-saw like flapping action of a semi rigid rotor.

Driving force; the opposite of drag.

The path traced by the rotor-blade tips in their rotation.

The rotational speed of the rotor at the blades tips.

The tendency of a helicopter’s body to turn in a direction opposite to that of the rotor when the engine is fuselage mounted.

Alignment of the tip paths and plane of rotation of blades to minimize vibration.

The extra lift gained when going from hovering to forward flight, or when hovering in a wind of 15 mph or more.

A small, adjustable rectangular plate fastened to the trailing edge of a rotor blade near its tip; used as an aid to alter tracking.

Referring to the more traditional helicopter landing gear comprising of two parallel metal tubes connected to the airframe by impact absorbing cross tubes or similar.

Helicopter rotors are designed to operate at a specific rotational speed. The throttle controls the power produced by the engine, which is connected to the rotor by a transmission. The purpose of the throttle is to maintain enough engine power to keep the rotor speed within allowable limits in order to keep the rotor producing enough lift for flight.

In many helicopters, the throttle control is a single or dual motorcycle-style twist grip mounted on the collective control (rotation is opposite of a motorcycle throttle), while some multi-engine helicopters have power levers.

In many piston engine-powered helicopters, the pilot manipulates the throttle to maintain rotor speed. Turbine engine helicopters, and some piston helicopters, use governors or other electro-mechanical control systems to maintain rotor speed and relieve the pilot of routine responsibility for that task. (There is normally also a manual reversion available in the event of a governor failure.)

Very High Frequency relating to the AM band aircraft radio transceiver use for communication.

The vortex ring state, also known as settling with power, is a dangerous condition that may arise in helicopter flight, when a vortex ring system engulfs the rotor causing severe loss of lift. – Read more here: Copters.com