12. HELICOPTER AERODYNAMICS(All explanations is based on counter clockwise rotating rotors viewed from above)12.1 HELICOPTER TERMS‚· PLANE OF ROTATIONThe plane in which the rotor blades travel. The plane of rotation is perpendicular to axis ofrotation. In a stable hover flight the shaft axis and axis of rotation will coincide. (See axis ofrotation below)‚· AXIS OF ROTATIONThe line through the rotor head always at right angles (90 degrees) to the plane of rotation.The blades rotate around this axis.‚· ROTORSHAFT AXISThe line extended out of the rotor shaft (mast).
When the helicopter is in a stable hover theaxis of rotation coincide with the rotorshaft axis. (See picture above under Plane ofrotation)For a helicopter in forward flight or any direction for that matter the axis of rotation androtorshaft axis will not coincide.‚· TIP PATH PLANEPlane within which the tips of the rotor blades travel. It is always parallel to the plane ofrotation. Through movement of the cyclic this path can be altered.‚· ROTOR DISCThe circular area created by the spinning blades of a helicopter, when viewed from above itlooks like a disc.
The rotor disc is manipulated by the pilot to move the helicopter in whichever direction the disc is tilted.‚· ROTOR DISC AREAThis is the total area covered by all the blades while spinning around the axis of rotation.The disc area is not a constant in flight, since it is affected by the coning angle of theblades.‚· DISC LOADINGThe gross weight of the helicopter divided by the disc area, expressed as lb/sq inch orkg/m2 , disc area is not constant in flight, it changes with the coning angle, therefore discloading cannot be a constant.‚· BLADE LOADINGThe gross weight of the helicopter divided by the combined area of the helicopter blades,expressed as lb/sq inch or kg/m2. It is the amount of the total weight each square footof a helicopter rotor blade supports. Since blade area does not change, blade loading isa constant in flight.ROTOR DISC ROTORDISC AREAHelicopter gross weightDivided byDisc areaRotor disc area isreduced by upwardsconing of bladesRotor disc area isincreased by lessconing of blades12.2 FORCES DIAGRAM AND ASSOCIATED TERMINOLOGY‚· FORCES DIAGRAMForces acting upon a helicopter in a stable hover:In a stable hover there are 2 forces acting upon a helicopter. The first one is Total RotorThrust or Lift created by the blades of the helicopter which holds the helicopter in the air.The second one is the Weight of the helicopter which works in the opposite direction of lift,pulling the helicopter down. To maintain a stable hover these two forces will be equal toeach other.If the Lift force is more than Weight, the helicopter will climb and vice versa if the Weight ismore than the Lift force.Forces acting upon a helicopter in forward flightIn forward flight two more forces is added to the equation Thrust and Drag. To move thehelicopter forward a horizontal force named Thrust is introduced. The combined forceHelicopter gross weightDivided byCombined area ofhelicopter bladesStable hover Forward flightbetween the Lift vector and the Thrust vector is Total Rotor Thrust. The Total Rotor Thrustas you can see in the diagram above is now leaning forward of the mast and thus thehelicopter is moving in a forward direction.The second force is called Drag, this is a rearward, retarding force caused by disruption ofairflow by the rotor, fuselage and other protruding objects. Drag opposes thrust and actsrearward parallel to the relative wind. Drag consists of 3 parts, Profile, Induced andParasite drag.PROFILE DRAG: Develops from the frictional resistance of the blades passing through theair. It does not change significantly with the airfoil’s angle of attack, but increasesmoderately when airspeed increases. (Also see description under blade drag)INDUCED DRAG: Is generated by the airflow circulation around the rotor blade as itcreates lift. Thus the higher the Pitch angle (see description below) the higher the induceddrag.PARSITE DRAG: Parasite drag is present any time the helicopter is moving through the air.This type of drag increases with airspeed. All non lifting components of the helicopter, suchas the cabin, rotor mast, tail and landing gear, contribute to Parasite drag.If the helicopter’s Total rotor thrust is more than the Total Drag, the helicopter willaccelerate/climb. If the Total Drag Force is more than Total Rotor Thrust the helicopter willslow down/decent.‚· PITCH ANGLEPitch angle is also known as blade angle.This is the angular difference between the chord line of the blade and the Plane ofRotation.This angle can be altered by the pilot through the collective lever.(See diagram below)‚· INDUCED AIRFLOWDefined as the mass of air that is forced down through the rotors of the helicopter.Induced airflow is the resultant of lift that has deflected air downward.(See diagram below)‚· RELATIVE AIRFLOWA rotor blade in movement experiences two airflows. One from straight ahead caused byRotation and one from straight above caused by Induced flow.These two flows combine to create a resultant airflow called Relative airflow onto theblade. The relative airflow is no longer parallel to the Plane of rotation. (See diagrambelow)‚· ANGLE OF ATTACKThis is the angle that is formed between the Chordline and the Relative airflow. (Seediagram below)