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Once again, I'm Jacob and in this video I want to expand more on what I covered in my first video Dissymmetry of Lift. More specifically, I want to break down the 5 different areas of the rotor disk that have drastically different aerodynamic characteristics. By expanding more on this, it makes explaining Retreating Blade Stall a little easier.

If you remember from my earlier video, I explained how the advancing blade will produce more lift than the retreating blade if left uncompensated. This is due to differences in airflow and how they affect the blades. But to go into detail, is can be broken down even further. The part of the blade closest to the hub on the retreating side experiences reverse flow. This means that the airflow literally flows from trailing edge to leading edge and therefore does not produce lift. This is all based on the fact that at any given blade RPM the blade tip travels faster than the hub. In this reverse flow region, the forward airflow through the disk is more than the rotational velocity.
The next region is the Negative Stall region. Simply put, a blade stalls if the resultant airflow is outside the critical angle. It creates boundary layer separation, turbulent air, and loss of lift. What makes it negative is that the airflow is impacting the airfoil above the chord line. In essence, the induced flow causes the airflow to push down on the airfoil outside the critical angle where it isn't useful.

Moving more outboard the next region is the Negative Lift region. This region. Is also Negative because the airflow impacts the airfoil above the chord line. But at this point in the blade the rotational speed combined with the induced flow causes the Resultant to be within the critical angle. This means the airfoil is no longer stalling and now creates lift. But because it is negative lift, it pushes the airfoil down. It's aerodynamically the same as if you were to impart negative pitch in the blade. These 3 regions are known collectively as the "NoLift Areas" and they grow in size as airspeed increases. Outside of these regions is where positive lift occurs. Just like the name implies, this is the good stuff that we need to fly. The Resultant airflow is inside the critical angle and below the chord line which produces lift. For more info on how lift is created, check out my video here (   • How Lift is Created  ).

With enough forward airflow, a 5th region can develop. This is a Positive Stall condition that begins to form on the retreating side. It's positive because the airflow is below the chord line and is a stall because the airflow is outside the critical angle of the airfoil. Here the airflow separates from the airfoil and lift no longer increases with increases in Angle of Attack. My next video will expanded more on this subject (   • Retreating Blade Stall/VNE in Helicop...  ).

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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