Chapter 1. GENERAL AERODYNAMICS

Unless otherwise indicated, this handbook is based on a helicopter that has the following characteristics:

1 - An unsupercharged reciprocating engine.
2 - A single main rotor rotating in a counterclockwise direction (looking downward on the rotor).
3 - An antitorque (tail) rotor.
4 - Skid-type landing gear.

Information is intended to be general in nature and should apply to most helicopters having these characteristics.

Before launching into a detailed discussion of the various forces acting on a helicopter in flight, it is first necessary that you understand the meaning of a few basic aerodynamic terms, how the force of lift is created, and the effect that certain factors have on lift.

Airfoil - An airfoil is any surface designed to produce lift or thrust when air passes over it. Propellers and wings of airplanes are airfoils. Rotor blades on helicopters are airfoils. The wing of an airplane is normally an unsymmetrical airfoil, that is, the top surface has more curvature than the lower surface.

The main rotor blades of most helicopters are symmetrical airfoils; that is, having the same curvature on both upper and lower surfaces (figure 1). Much research, however, is being conducted in the use of unsymmetrical airfoils for main rotor blades, and at least one currently manufactured make of helicopter is equipped with main rotor blades that are not considered true symmetrical airfoils.

On an unsymmetrical airfoil, the center of pressure is variable - as the angle of attack increases, the center of pressure moves forward along the airfoil surface; as the angle of attack decreases, the center of pressure moves rearward. On a symmetrical airfoil, center of pressure movement is very limited. A symmetrical airfoil is preferred for rotor blades so that a relatively stable center of pressure is maintained. Improvements in control systems may allow more latitude in blade designs in the future.

Chord line - The chord line of an airfoil is an imaginary straight line from the leading edge to the trailing edge of the airfoil (figure 2).  Figure 2 (above) - Chord line of an airfoil is the imaginary line joining the leading and trailing edges of the airfoil.

Relative wind - Relative wind is the direction of the airflow with respect to an airfoil. If an airfoil moves forward horizontally, the relative wind moves backward horizontally (figure 3). If an airfoil moves backward horizontally, the relative wind moves forward horizontally. If an airfoil moves forward and upward, the relative wind moves backward and downward. If an airfoil moves backward and downward, the relative wind moves forward and upward. Thus, the flightpath and relative wind are parallel but travel in opposite directions. (Forward and backward as used here are relative to the fore and aft axis of the helicopter - forward meaning in the direction the nose of the helicopter points, and backward meaning the direction the tail points.)  Figure 3 - Relationship between the flight path of an airfoil and relative wind. Relative wind is parallel and in the opposite direction to the flight path.

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 windspeed and direction.

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. For a helicopter, the relative wind is the flow of air with respect to the rotor blades. When the rotor is stopped, wind blowing over the blades creates a relative wind; when the helicopter is hovering in a no-wind condition, relative wind is created by the motion of the rotor blades through the air; when 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; and when the helicopter is in horizontal flight, the relative wind is a combination of the rotation of the rotor blades and the movement of the helicopter.

Pitch angle - The rotor blade pitch angle is the acute angle between the blade chord line and a reference plane determined by the main rotor hub. Since the rotor plane of rotation is parallel to the plane containing the main rotor hub, the rotor blade pitch angle could also be described as the acute angle between the blade chord line and the rotor plane of rotation (figure 4). The pitch angle can be varied by the pilot through the use of cockpit controls (collective and cyclic pitch controls) provided for this purpose.

Figure 4 - The pitch angle of a rotor blade is the angle between the chord line and a reference plane determined by the rotor hub or the plane of rotation.