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This video's topic covers the Forces Acting on an Airfoil. In doing this it also covers quite a lot of terminology associated with airfoils as well.

Starting with the basics we have a basic airfoil shape. This can come in either symmetric or asymmetric shapes, each with their own benefits. From the leading edge to the trailing edge of the airfoil we have what's known as the Chord Line. This line gives us a baseline for how to measure forces in relation to the blade. As the rotor turns about the mast it creates a Rotational Relative Wind (also known as the Tip Path Plane). This is generally perpendicular to the axis of rotation when the aircraft is level. Due to the fact that the rotor blades follow each other along the same path around the mast with pitch applied, a downwards velocity of air is present known as Induced Flow or Downwash. What's leftover is now the Resultant Relative Wind, or the wind that has been modified by induced flow.

It is from here that we can find the Angle of Attack. This angle is the angle between the Chord Line and the Resultant Relative Wind. This angle depicts the amount of Lift the airfoil is producing. Angle of Attack should not be confused purely with pitch angle in the blades. The difference between the Chord Line and Rotational Relative Wind is the Angle of Incidence. As pilots we control the Angle of Incidence with cyclic and collective inputs because this is a mechanical angle. On the other hand, Angle of Attack is an aerodynamic angle which is a product of our Angle of Incidence and Induced Flow.

From here it's necessary to note that Lift acts perpendicular to the Resultant Relative Wind. It's from this point that we can compare Lift to Drag in order to find our Total Aerodynamic Force on the blade. This is sometimes referred to as the Resultant Force and is generally inclined up and to the rear. This is the basic structure of the forces acting on an airfoil. From here you can see how changes in flight profiles can affect Lift. For example, if you were to have an increase in Induced Flow with no change to Angle of Incidence, the Angle of Attack would be reduced and therefore Lift would be reduced. This is exactly what would happen if you made an approach to an OGE (Out of Ground Effect) hover with without an increase in collective.

While I wouldn't expect every pilot to actively think about this diagram during every flight maneuver, it is good to understand this diagram in order to have a better understanding of how Lift can change depending on the flight profiles you find yourself in.

That wraps up Forces Acting on an Airfoil! Thanks for watching! Don't forget to hit like and subscribe below. Be sure to like us on Facebook as well. As always, safe flying!


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