BLADE PITCH ANGLE RELATIVE WIND ANGLE OF ATTACK The pitch angle of a rotor blade is the angle between its chord line and the reference plane containing the rotor hub. [Figure 2-4] You control the pitch angle of the blades with the flight controls. The collective pitch changes each rotor blade an equal amount of pitch no matter where it is located in the plane of rotation (rotor disc) and is used to change rotor thrust. The cyclic pitch control changes the pitch of each blade as a function of where it is in the plane of rotation. This allows for trimming the helicopter in pitch and  roll during forward flight and for maneuvering in all flight conditions. Twisting a rotor blade causes it to produce a more even amount of lift along its span. This is necessary because rotational velocity increases toward the blade tip. The leading edge is the first part of the airfoil to meet the oncoming air. [Figure 2-3] The trailing edge is the aft portion where the airflow over the upper surface joins the airflow under the lower surface. The chord line is an imaginary straight line drawn from the leading to the trailing edge. The camber is the curvature of the air- foil’s upper and lower surfaces. The relative wind is the wind moving past the airfoil. The direction of this wind is relative to the attitude, or position, of the airfoil and is always parallel, equal, and opposite in direction to the flight path of the airfoil. The angle of attack is the angle between the blade chord line and the direction of the relative wind. When the angle of attack is increased, air flowing over the airfoil is diverted over a greater distance, resulting in an increase of air velocity and more lift. As angle of attack is increased further, it becomes more difficult for air to flow smoothly across the top of the airfoil. At this point the airflow begins to separate from the airfoil and enters a burbling or turbulent pattern. The turbulence results in a large increase in drag and loss of lift in the area where it is taking place. Increasing the angle of attack increases lift until the critical angle of attack is reached. Any increase in the angle of attack beyond this point produces a stall and a rapid decrease in lift. [Figure 2-5] Angle of attack should not be confused with pitch angle. Pitch angle is determined by the direction of the relative wind. You can, however, change the angle of attack by changing the pitch angle through the use of the flight controls. If the pitch angle is increased, the angle of attack is increased, if the pitch angle is reduced, the angle of attack is reduced. [Figure 2-6] Relative wind is created by the motion of an airfoil through the air, by the motion of air past an airfoil, or by a combination of the two. Relative wind may be affected by several factors, including the rotation of the rotor blades, horizontal movement of the helicopter, flapping of the rotor blades, and wind speed and direction.  For a helicopter, the relative wind is the flow of air with respect to the rotor blades. If the rotor is stopped, wind blowing over the blades creates a relative wind. When the helicopter is hovering in a no-wind condition, rela- tive wind is created by the motion of the rotor blades through the air. If the helicopter is hovering in a wind, the relative wind is a combination of the wind and the motion of the rotor blades through the air. When the helicopter is in forward flight, the relative wind is a combination of the rotation of the rotor blades and the forward speed of the helicopter. Axis-of-Rotation—The imaginary line about which the rotor rotates. It is represented by a line drawn through the center of, and perpen- dicular to, the tip-path plane. Tip-Path Plane—The imaginary circular plane outlined by the rotor blade tips as they make a cycle of rotation. Aircraft Pitch—When referenced to a helicopter, is the movement of the helicopter about its lateral, or side to side axis. Movement of the cyclic forward or aft causes the nose of the helicopter to move up or down. Aircraft Roll—Is the movement of the helicopter about its longitudi- nal, or nose to tail axis. Movement of the cyclic right or left causes the helicopter to tilt in that direction. 2-2 Figure 2-4. Do not confuse the axis of rotation with the rotor mast. The only time they coincide is when the tip-path plane is perpendicular to the rotor mast. Figure 2-3. Aerodynamic terms of an airfoil. TrailingGLYPH<.notdef> Edge C h o r d L i n e Angle of Attack F L I G H T P A T H R E L A T I V E W I N D UpperGLYPH<.notdef> Camber LowerGLYPH<.notdef> Camber LeadingGLYPH<.notdef> Edge Axis of Rotation Reference Plane PitchGLYPH<.notdef> Angle C h o r d L i n e