Aerodynamics Helicopter Glossary

The actual distance an object is above the ground

The blade moving in the same direction as the helicopter or gyroplane In rotorcraft that have counterclockwise main rotor blade rotation as viewed from above the advancing blade is in the right half of the rotor disc area during forward movement

Any surface designed to obtain a useful reaction of lift or negative lift as it moves through the air

Aline along which there is no magnetic variation

The density of the air in terms of mass per unit volume Dense air has more molecules per unit volume than less dense air The density of air decreases with altitude above the surface of the earth and with increasing temperature

When referenced to an aircraft it is the movement about its lateral or pitch axis Movement of the cyclic forward or aft causes the nose of the helicopter or gyroplane to pitch up or down

Is the movement of the aircraft about its longitudinal axis Movement of the cyclic right or left causes the helicopter or gyroplane to tilt in that direction

When an unsafe condition exists with an aircraft the FAA issues an airworthiness directive to notify concerned parties of the condition and to describe the appropriate corrective action

An instrument that indicates flight altitude by sensing pressure changes and displaying altitude in feet or meters

The angle between the airfoil’s chord line and the relative wind

The pedal used to control the pitch of the tail rotor or air diffuser in a NOTAR system

A rotor system in which each of the blades is connected to the rotor hub in such a way that it is free to change its pitch angle and move up and down and fore and aft in its plane of rotation

Those units and components that furnish a means of automatically controlling the aircraft

The condition of flight during which the main rotor is driven only by aerodynamic forces with no power from the engine

The imaginary line about which the rotor rotates It is represented by a line drawn through the center of and perpendicular to the tip-path plane

The weight of the standard rotorcraft operational equipment unusable fuel and full operating fluids including full engine oil

An upward sweep of rotor blades as a result of lift and centrifugal force

A device attached to the drag hinge to restrain the fore and aft movement of the rotor blade

The rotation of the blade around the spanwise (pitch change) axis

The ability of the rotor blade to move in a vertical direction Blades may flap independently or in unison

The part of the hub assembly to which the rotor blades are attached sometimes referred to as blade forks

The fore and aft movement of the blade in the plane of rotation It is sometimes called hunting or dragging

The load imposed on rotor blades determined by dividing the total weight of the helicopter by the combined area of all the rotor blades

The part of the blade that attaches to the blade grip

The length of a blade from its tip to its root

The condition of the rotor blade when it is operating at an angle of attack greater than the maximum angle of lift

The further most part of the blade from the hub of the rotor

The relationship of the blade tips in the plane of rotation Blades that are in track will move through the same plane of rotation

The mechanical procedure used to bring the blades of the rotor into a satisfactory relationship with each other under dynamic conditions so that all blades rotate on a common plane

The variation in the angle of incidence of a blade between the root and the tip

The tendency of the rotor disc to tilt aft in forward flight as a result of flapping

Indicated airspeed of an aircraft corrected for installation and instrumentation errors

The theoretical point where the entire weight of the helicopter is considered to be concentrated

The point where the resultant of all the aerodynamic forces acting on an airfoil intersects the chord

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

An imaginary straight line between the leading and trailing edges of an airfoil section

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

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

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 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 driveshaft

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 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

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

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 vertical movement of a blade about a flapping hinge

A maneuver accomplished prior to landing to slow down a rotorcraft

A turboshaft engine with no physical connection between the compressor and power output shaft

A component of the transmission or power train that automatically disconnects the main rotor from the engine when the engine stops or slows below the equivalent rotor rpm

The sum of the basic empty weight and useful load

A usually beneficial influence on rotorcraft performance that occurs while flying close to the ground It results from a reduction in upwash downwash and bladetip vortices which provide a corresponding decrease in induced drag

Self excited vibration occurring whenever the frequency of oscillation of the blades about the lead-lag axis of an articulated rotor becomes the same as the natural frequency of the fuselage

Trademark applied to gyroplanes designed and produced by the Bensen Aircraft Company

An inherent quality of rotating bodies which causes an applied force to be manifested 90° in the direction of rotation from the point where the force is applied

The study of how people interact with their environment In the case of general aviation it is the study of how pilot performance is influenced by such issues as the design of cockpits the function of the organs of the body the effects of emotions and the interaction and communication with other participants in the aviation community such as other crew members and air traffic control personnel

Movement of a blade with respect to the other blades in the plane of rotation sometimes called leading or lagging

The property of matter by which it will remain at rest or in a state of uniform motion in the same direction unless acted upon by some external force

Hovering close to the surface (usually less than one rotor diameter distance above the surface) under the influence of ground effect

That part of the total drag that is created by the production of lift

The component of air flowing vertically through the rotor system resulting from the production of lift

Lines on charts that connect points of equal magnetic variation

A unit of speed equal to one nautical mile per hour

The maximum ratio between total lift (L) and total drag (D) This point provides the best glide speed Any deviation from the best glide speed increases drag and reduces the distance you can glide

A vibration in which the movement is in a lateral direction such as imbalance of the main rotor

The fore (lead) and aft (lag) movement of the rotor blade in the plane of rotation

Basic empty weight not including full engine oil just undrainable oil

One of the four main forces acting on a rotorcraft It acts perpendicular to the relative wind

The ratio of a specified load to the total weight of the aircraft

A term used when two hands of an instrument are superimposed over each other as on the engine/rotor tachometer

The component that supports the main rotor

Action of the rotor head striking the mast occurring on underslung rotors only

Any visual or electronic device airborne or on the surface that provides point-to-point guidance information or position data to aircraft in flight

The time between the end of evening civil twilight and the beginning of morning civil twilight as published in the American Air Almanac

An engine that does not compensate for decreases in atmospheric pressure through turbocharging or other means

A low frequency vibration having one beat per revolution of the rotor This vibration can be either lateral vertical or horizontal

Hovering greater than one diameter distance above the surface Because induced drag is greater while hovering out of ground effect it takes more power to achieve a hover out of ground effect

The part of total drag created by the form or shape of helicopter parts

The term used for passengers baggage and cargo

The lateral or longitudinal oscillation of the fuselage due to it being suspended from the rotor system

The angle between the chord line of the rotor blade and the reference plane of the main rotor hub or the rotor plane of rotation

In a gyroplane it is the spinning of the rotor to a sufficient rpm prior to flight

The height above the standard pressure level of 29.92 in Hg It is obtained by setting 29.92 in the barometric pressure window and reading the altimeter

Drag incurred from frictional or parasitic resistance of the blades passing through the air It does not change significantly with the angle of attack of the airfoil section but it increases moderately as airspeed increases

Airflow from rotation that is modified by induced flow

Any blade located in a semicircular part of the rotor disc where the blade direction is opposite to the direction of flight

A stall that begins at or near the tip of a blade in a helicopter because of the high angles of attack required to compensate for dissymmetry of lift In a gyroplane the stall occurs at 20 to 40 percent outboard from the hub

A rotor system permitting blades to feather but not flap or hunt

The component of relative wind produced by the rotation of the rotor blades

A complete system of rotating airfoils creating lift for a helicopter or gyroplane

A device used to stop the rotor blades during shutdown

The force produced by the rotor in a gyroplane It is comprised of rotor lift and rotor drag

A rotor system in which the blades are fixed to the hub but are free to flap and feather

A turbine engine used to drive an output shaft commonly used in helicopters

A flight condition in which the rate of turn is too great for the angle of bank

Plates attached to the bottom of skid landing gear protecting the skid

A flight condition in which the rate of turn is too slow for the angle of bank

The ratio of the total rotor blade area to total rotor disc area

The dimension of a rotor blade or airfoil from root to tip

A term used to describe the position of the two needles on the engine/rotor tachometer when the two needles are not superimposed

A hypothetical atmosphere based on averages in which the surface temperature is 59°F (15°C) the surface pressure is 29.92 in Hg (10132 Mb) at sea level and the temperature lapse rate is approximately 35°F (2°C) per 1000 feet

A device used to limit the blade flap or rotor flap at low rpm or when the rotor is stopped

A condition when a rotorcraft is in straightand-level unaccelerated flight and all forces are in balance

An airfoil having the same shape on the top and bottom

A rotor turning in a plane perpendicular to that of the main rotor and parallel to the longitudinal axis of the fuselage It is used to control the torque of the main rotor and to provide movement about the yaw axis of the helicopter

A hinge that permits the rotor blades of a semirigid rotor system to flap as a unit

The force developed by the rotor blades acting parallel to the relative wind and opposing the forces of drag and weight

The imaginary circular plane outlined by the rotor blade tips as they make a cycle of rotation

In helicopters with a single main rotor system the tendency of the helicopter to turn in the opposite direction of the main rotor rotation

The rearmost edge of an airfoil

The tendency of the single-rotor helicopter to move laterally during hovering flight Also called tail rotor drift

The additional lift obtained when entering forward flight due to the increased efficiency of the rotor system

A condition of increased drag and decreased lift in the aft portion of the rotor disc caused by the air having a greater induced velocity and angle in the aft portion of the disc

The actual height of an object above mean sea level

A turbine engine transmitting power through a shaft as would be found in a turbine helicopter

The power control on the end of the collective control

A rotor hub that rotates below the top of the mast as on semirigid rotor systems

The state of a rotor when rotor force has been removed or when the rotor is operating under a low or negative G condition

The difference between the gross weight and the basic empty weight It includes the flight crew usable fuel drainable oil if applicable and payload

The angular difference between true north and magnetic north; indicated on charts by isogonic lines

A vibration in which the movement is up and down or vertical as in an out-of-track condition

A transient condition of downward flight (descending through air after just previously being accelerated downward by the rotor) during which an appreciable portion of the main rotor system is being forced to operate at angles of attack above maximum Blade stall starts near the hub and progresses outward as the rate of descent increases

One of the four main forces acting on a rotorcraft Equivalent to the actual weight of the rotorcraft It acts downward toward the center of the earth

The movement of a rotorcraft about its vertical axis