Basic Helicopter - Prelims

Accumulator

A device to aid in unfeathering a propeller.

Aerodynamic twisting moment

An operational force on a propeller which tends to increase the propeller blade angle.

Angle of attack

The angle between the chord line of a propeller blade section and the relative wind.

Anti-icing system

A system which prevents the formation of ice on propeller blades.

Automatic propeller

A propeller which changes blade angles in response to operational forces and is not controlled from the cockpit.

Back

The curved side of a propeller airfoil section that can be seen while standing in front of the airplane.

Blade

One arm of a propeller from the hub to the tip.

Blade angle

The angle between the blade section chord line and the plane of rotation of the propeller.

Blade station

A distance from the center of the propeller hub.

Boots

Ice elimination components which are attached to the leading edge of propeller blades.

Boss

The center portion of a fixed-pitch propeller.

Centrifugal force

The force on a propeller which tends to throw the blades out from the propeller center.

Centrifugal twisting moment

The force on a propeller which tends to decrease the propeller blade angle.

Chord line

The imaginary line which extends from the leading edge to the trailing edge of a blade airfoil section.

Comparison unit

The unit in a synchronization or synchrophasing system which compares the signals of the master engine and the slave engine and sends a signal to correct the slave engine speed or blade phase angle.

Constant-speed system

A system which uses a governor to adjust the propeller blade angle to maintain a selected RPM.

Controllable-pitch propeller

A propeller whose pitch can be changed in flight by the pilot's control lever or switch.

De-icing system

An ice elimination system which allows ice to form and then breaks it loose in cycles.

Effective pitch

The distance forward that an aircraft actually moves in one revolution of the propeller.

Face

The flat or thrust side of a propeller blade.

Feather

The rotation of the propeller blades to an angle of about 90 degrees which will eliminate the drag of a wind milling propeller.

Fixed pitch propeller

A propeller, used on light aircraft, whose blade angles cannot be changed.

Frequency generator

The engine speed signal generator for some synchronization systems.

Geometric pitch

The theoretical distance that an aircraft will move forward in one revolution of the propeller.

Governor

The propeller control device in a constant-speed system.

Ground adjustable propeller

A propeller which can be adjusted on the ground to change the blade angles.

Hub

The central portion of a propeller which is fitted to the engine crankshaft and carries the blades.

Leading edge

The forward edge of a propeller blade.

Pitch distribution

The twist in a propeller blade along its length.

Plane of rotation

The plane in which the propeller rotates, 90 degrees to the crankshaft centerline.

Propeller

A device for converting engine horsepower into usable thrust.

Propeller disc

The disc-shaped area in which the propeller rotates.

Pulse generator

The unit which generates a speed and blade position signal in a synchrophasing system.

Reversing

Rotation of the propeller blades to a negative angle to produce a braking or reversing thrust.

Safetying

The installation of a safety device such as safety wire or a cotter pin.

Selector valve

Propeller control unit in a two position propeller system.

Shank

The thickened portion of the blade near the center of the propeller.

Slip

The difference between geometric pitch and effective pitch.

Snap ring

A component of a splined or tapered shaft installation which is used to aid in removal of the propeller.

Spider

The central component on many controllable-pitch propellers which mounts on the crankshaft and has arms on which the blades are installed.

Synchronization system

A system which keeps all engines at the same speed.

Synchrophasing system

A refined synchronization system which allows the pilot to adjust the blade relative position as they rotate.

Tachometer

The speed sensing unit of some synchronization systems.

Thrust bending force

An operational force which tends to bend the propeller blades forward.

Tip

The portion of the blade farthest from the hub.

Torque bending force

An operational force which tends to bend the propeller blades in the direction opposite to the direction of rotation.

Two position propeller

A propeller which can be changed between two blade angles in flight.

Helicopters

Classified as rotary wing aircraft, and their rotary wing is commonly referred to as the main rotor or simply the rotor.

Relative wind

The direction of the airflow with respect to the blade.

Pitch angle

It is the acute angle between the rotor blade chord and a reference line.

Angle of attack

The acute angle between the chord line of the airfoil and the relative wind.

Lift

The force produced by the airfoil that is perpendicular to the relative wind and opposes gravity.

Drag

The force which tends to resist the airfoil's passage through the air.

Center of pressure

An imaginary point where the result of all aerodynamic forces of the airfoil are considered to be concentrated.

Blade Stall

The condition under which the streamline flow of air separates from the camber of the blade and reverse flow occurs, resulting in an almost complete lost of lift.

Torque

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

Blade twist

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

Dissymmetry of lift

The unequal lift across the rotor disc resulting from the difference in the velocity of air over the advancing and retreating blade half of the rotor disc area.

Monocoque construction

The construction of a metal or composite material tube or cone without internal structural members.

Bonded construction

Structure whose parts are joined together by chemical methods rather than mechanical.