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

Density altitude.  Pressure altitude corrected for nonstandard temperature variations. 

Deviation.  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 placed near the magnetic compass of the aircraft. 

Direct control.  The ability to maneuver a helicopter by tilting the rotor disk and changing the pitch of the rotor blades. 

Direct shaft turbine.  A single-shaft turbine engine in which the compressor and power section are mounted on a common driveshaft. 

Disk area.  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. 

Disk loading.  The total helicopter weight divided by the rotor disk area. 

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

Drag.  An aerodynamic force on a body acting parallel and opposite to relative wind. 

Dual rotor.  A rotor system utilizing two main rotors. 

Dynamic rollover.  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. 

Emergency Position Indicator Radio Beacon (ERIPB).  A device used to alert search and rescue services in the event of an emergency by transmitting a coded message on the 406 MHz distress frequency, which is relayed by the Cospas- Sarsat global satellite system. 

Feathering.  The action that changes the pitch angle of the rotor blades by rotating them around their feathering (spanwise) axis. 

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

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

Flapping.  The vertical movement of a blade about a flapping hinge. 

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

Flare.  A maneuver accomplished prior to landing to slow a helicopter. 

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

Freewheeling unit.  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. 

Fully articulated rotor system.  See articulated rotor system. 

Gravity.  See weight. 

Gross weight.  The sum of the basic empty weight and useful load. 

Ground effect.  A usually beneficial influence on helicopter 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. 

Ground resonance.  Selfexcited 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. 

Gyroscopic procession.  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. 

Human factors.  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. 

