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In this lesson I'll be covering Airflow at a Hover. For the purposes of explanation, these diagrams refer to helicopters operating at a stationary hover and in a no wind condition. This is because motion or wind from any direction or velocity will change the lift characteristics due to dissymmetry of lift, transverse flow, and translational lift. In another video I will explain how all of these factors come together to affect the helicopter. But for now I'll keep it simple and cover one subject at a time.

Airflow at a hover can be broken down into two types: In Ground Effect (IGE) and Out of Ground Effect (OGE). This leads to the question, "what is ground effect?" Ground effect is the increase in lift that occurs when a hovering helicopter is within one rotor diameter above a surface. When a helicopter hovers, airflow through the rotor system comes down vertically and vortices develop around the rotor tips. While in ground effect, this down flow of air is restricted due to ground friction as the airflow is redirected out words and away from the rotor disk. Because of this, the high velocity of a downwards flow of air (induce flow) is reduced as well as the rotor tip vortices are pushed away from the helicopter. While hovering out of ground effect, there is nothing to restrict or redirect the downward flow of air. Therefore there is a high velocity of induced flow through the rotor system and the wing tip vortices are allowed to develop without any restriction.

Because there is more induced flow while operating out of ground effect, the pilot must increase the Angle of Incidence in the blades in order to maintain the same Angle of Attack. Keep in mind that Angle of Incidence is a mechanical angle that a pilot can directly control by manipulating the collective. However, the Angle of Attack is in aerodynamic angle that is the product of pitch and induced flow. Due to the fact that the pilot has to increase pitch in order to maintain Angle of Attack while out of ground effect, there is more drag on the rotor system and therefore requires an increase in power to maintain lift. This is why it requires more power to hover OGE than IGE.

One last factor to consider while operating at a hover ground effect is the surface condition. Tall grasses, rough terrain, ground obstructions, water, etc. can all affect airflow as it passes through the rotor disc. This can cause uneven air flow and therefore uneven lift.

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If you're just getting started and want more information, pictures, and more explanations, I'd recommend reading the Rotorcraft Flying Handbook http://amzn.to/2ifPlnZ

If you've already got a basic understanding, and want to further your professional helicopter education with advanced helicopter concepts, I'd recommend reading Cyclic and Collective, by Shawn Coyle http://amzn.to/2ifQGLx