Airflow Through a Rotor System

Introduction to aeronautical science |

Airflow Through a Rotor System |

In Ground Effect vs Out of Ground Effect |

|

William Fish |

1/9/2013 |

This paper is a brief explanation of the aerodynamics of the main rotor system of a helicopter while hovering in and out-of-ground effect.

Since Igor Sikorsky transformed ink and paper into metal and wood the rotor system of a helicopter has changed and advanced many times. However the basic principles of the aerodynamics effecting flight remains constant. My intent with this paper is to give the reader a glimpse into the life of air on a zero wind day as it enters and leaves the main rotor system while hovering at different altitudes.

When a helicopter is at 100% rotor RPM, the air moving through the rotor system is down and away. This is called induced flow. The blades are at or “flat pitch” and they are not creating lift. To start producing lift the pilot has to increase the pitch of the blades with the collective control in his left hand, which increases the mechanical angle of the airfoil (the angle of incidence). This control input collectively changes the pitch of all the blades of the main rotor at the same time through each revolution. The change in blade pitch increases drag which requires an increase in power to maintain the same rotor RPM.

When lift overcomes weight the aircraft leaves the ground to a hover until lift and weight are balanced again. At this point the helicopter is in-ground effect (IGE). IGE is most effective over smooth hard surfaces. The majority of rotary wing aircraft will remain IGE as long as its altitude is within approximately 1 rotor diameter above ground level (AGL) when measured from the surface to the rotor disk. This is the most efficient hover position for a helicopter to be in because of the interference of the airflow near the ground. Maintaining a stationary hover creates vortices (swirling air at the tip of the blade) to build to some extent and coupled with the...