Retreating Blade StallVNE in Helicopters

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Welcome back to Helicopter Lessons in 10 Minutes or Less! • Check out my ebook covering this and more! Get your copy on iBooks: http://itunes.apple.com/us/book/id144... • or on Kobo for Android, Windows and Desktop users: https://www.kobo.com/us/en/ebook/heli... • Once again, I'm Jacob and in this video we're talking about Retreating Blade Stall/VNE. Before watching this video I'd recommend checking out my previous video Dissymmetry of Lift - Expanded ( • Dissymmetry of Lift - Expanded ) where I explained the different aerodynamic regions of the rotor disk. That is because this video builds off of that one quite a bit. • Just like how an airplane's wing can stall at lower speeds, the same can happen to the retreating blade of a helicopter. However, unlike planes, in helicopters this occurs in high-speed flight. In my last video I introduced the 5 aerodynamic regions of the rotor disk which included the positive stall region. This region is experienced on the retreating side only and is due to greater angles of attack at high forward airspeeds. The other regions include reverse flow, negative stall, negative lift, and positive lift. If a pilot does not understand the effects of flying into this condition, he or she could soon experience violent changes to aircraft pitch and roll as well as a temporary loss of control of the aircraft's flight path. • From here we will talk about some causes of retreating blade stall conditions. The first is excessively high forward airspeed. By this I mean operating at or above the speed labeled as VNE in your operators manual. VNE stands for velocity never exceed. This is the speed that engineers have determined that either retreating blade stall or structural damage on the aircraft can occur. Another calls is low rotor RPM. As you may remember from my previous videos on the lift equation, velocity has an exponential affect on lift and drag. Because of this, if NR (rotor speed) is too low, a stall condition can form on the retreating side of the rotor disk even easier. Too much forward cyclic can also contribute to retreating blade stall. This is usually a byproduct of too much for cyclic inputs to compensate for center of gravity being too far aft. This causes excessively high pitch angles at the 9 o'clock position which can aggravate a stall. Lastly, too high of a collective angle can cause retreating blade stall. Some manuals list high gross weight, high G loading, and high density altitude's as causes but these are major contributors. Helicopters can fly in all of these conditions without experiencing retreating blade stall. But when a helicopter attempts to fly at too high of a gross weight or too high of an altitude or with too much G loading in relation to collective angle applied, retreating blade stall can occur. • That said, there are a few symptoms as you approach retreating blade stall. The first being vibrations, potentially a vertical bounce as the blades stall at the 9 o'clock position. Depending on the control rigging you may feel and effectiveness or stiffening of the flight controls as well. These should be your first indication or hint to calm down whatever maneuver you are doing. Because the next will be the uncommanded pitching up of the nose and rolling in the direction of the stall side of the rotor. This could be potentially violent and resultant temporary loss of aircraft control. • It's best to avoid entering retreating blade stall to begin with. But if you find yourself in this condition the first thing you should do is lower the collective. This does three things: first it reduces angle of attack, next it reduces airspeed of the helicopter, and lastly it increases blade speed (NR). Next you should reduce the severity of the maneuver and adjust flight controls for normal flight. In some aircraft, more specifically those with semi rigid teetering rotor systems, the helicopter may attempt to self correct and you might not experience all of these conditions. But, if you continue to aggravate the retreating blade stall condition mast bumping can occur. • That wraps up retreating blade stall. It occurs when the retreating blade can no longer compensate for dissymmetry of lift and begins to stall. It is the most limiting factor for high-speed helicopter flight. 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

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