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. 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. 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. Gross weight.  The sum of the basic empty weight and  useful load. Gravity.  See weight. Fully articulated rotor system.  See articulated rotor system. 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. Free turbine.  A turboshaft engine with no physical  connection between the compressor and power output shaft. Flare.  A maneuver accomplished prior to landing to slow  a helicopter. Flapping hinge.  The hinge that permits the rotor blade to  flap and thus balance the lift generated by the advancing and  retreating blades. 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. 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. 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. 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. 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. G-3