Helicopter Glossary & Helicopter Terms

Advancing Blade:

the rotor blade that is moving through 180 degrees in the direction of flight and opposite to the relative wind.

Airfoil:

a rotor blade; specifically, a rotor blader defined by its surface curvature, as viewed in cross-section, and designed to provide lift.

Angle Of Attack:

the angle formed between the chord of a rotor blade and the relative wind.

Angle Of Incidence:

same as angle of attack; sometimes defined as pitch.

Antitorque Rotor:

a small rotor mounted on one side of the tail boom of helicopters having a single main rotor. Its sideward push or pull counteracts torque. Varying the pitch of the rotors blades provides directional control.

Articulated Rotor:

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

Aspect Ratio:

the ratio of a rotor blade's span to its chord.

Autorotation:

spinning of the rotor by airflow over the blades during conditions of power off or power failure (engine-out). Sufficient life is generally provided to allow a controlled descent and safe landing defined by the individual helicopters performance parameters.

Axis:

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

Blade:

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

Blade Loading:

a helicopter's gross weight divided by the total area of its rotor blades, expressing the load supported by the blades.

Camber:

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

Center Of Gravity:

the point in a helicopter where its total weight is centered for a given position.

Center Of Pressure:

the point of the chord where the resultant force of all aerodynamic forces is centered.

Centrifugal Force:

a hypothetical force exerting an outward pull on the rotor blades away from the axis of rotation.

Chord:

a straight, imaginary line from the leading edge to the trailing edge of an airfoil.

Collective Pitch:

the equal and simultaneous altering of the pitch of all rotor blades; controls the vertical flight of a helicopter.

Coning:

the result of the combination of tcentrifugal force pulling outward on the rotor blades and the downward pull of the helicopter's weight at the center of the rotor; this forms a shallow cone, or "V" shape in the plane of rotation.

Convertiplane:

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

Coriolis Force:

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 flapping hinges help relieve stresses and imbalances brought about by this force.

Counterrotating Rotors:

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

Cyclic Pitch:

variation of the pitch of each rotor blade individually during each revolution; controls the horizontal flight of the helicopter.

Dihedral:

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

Disc:

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

Dissymmetry Of Lift:

the unequal lift between the advancing and retreating halves of the rotor disc as the relative wind alters the effective velocity of each blade.

Downwash:

the downward rush of air produced by the powered rotor.

Drag (airfoil):

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

Drag Angle:

the difference between the deflected position of the blade, due to drag, of an engine-driven rotor, and the straight position.

Feathering:

periodic variations in the angle of incidence of an airfoil during forward flight to equalize life.

Flapping:

the up and down movement of a rotor blade in response to the dissymmetry of lift.

Freewheeling:

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

Gimbal Ring:

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

Ground Effect:

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

Ground Resonance:

severe fuselage oscillations, often leading to capsizing or structural breakdown, initiated when shock waves transmitted from the landing gear to the rotor create imbalance. This condition is rare and generally confined to three-bladed, articulated rotors.

Gyroscopic Precission:

the principle in which a force applied to a spinning rotor becomes manifest (such as though tilting of the disc) after a 90 degrees lag from the point where the force was applied.

Hovering:

a flight maneuver in which the helicopter is maintained in a fixed position above the ground, both vertically and horizontally.

Lift:

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

Mast:

the vertical shaft to which the rotor blades are attached.

Pitch:

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

Planform:

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

Plan Of Rotation:

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

Relative Wind:

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

Resultant Force:

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

Rigid Rotor:

a rotor system in which the blades are mounted to the hub without flapping or drag hinges.

Rotor (main):

an assemble (typically) comprised of two or more airfoils designed to provide a helicopter's life and thrust.

Rotor Hub:

the central structure to which the rotor blades are secured.

Semi-Rigid Rotor:

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

Span:

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

Stability:

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

Swash Plate:

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.

Tail Rotor:

same as antitorque rotor.

Teetering:

the see-saw like flapping of a semi-rigid rotor.

Thrust:

driving force; the opposite of drag.

Tip Path:

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

Tip Speed:

the rotational speed of the rotor at the blade tips.

Torque:

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

Tracking:

alignment of the tip paths and plan of rotation of blades to minimize vibration.

Translational Lift:

the extra lift gained when going from hovering to forward flight, or when hovering in a wind 15 m.p.h (24km/h) or more.

Trim Tab:

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

Aviation Terms

Most of the above terms are specific to rotary wing aircraft though many translate over to general aviation. By understanding these terms you will find it easier to learn about helicopter flight and helicopter construction. When discussing the many aspects of the helicopter with professionals and amateurs alike, it is imperitive that you embrace the basic aviation terminology as per above, or you may fail to have your interest in aviation be taken seriously.