Retreating Blade Stall
Nov 19, 2017
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Retreating Blade Stall
Today I want to talk about retreating blade stall, not to be confused with retreating brain stall, which is what a lot of students get when they're trying to study and learn all this information.
Retreating BLADE stall happens in the rotor system of the helicopter. Retreating BRAIN stall is what happens when people get confused trying to learn all this stuff. I'm Kenny Keller, the creator of Helicopter Online Ground School, and that's why we created Helicopter Online Ground School, is to help people with the retreating brain stall.
I want to hit some of the things that you're going to have to know minimum even for the private pilot level. At the commercial level and CFI level, maybe you'll be up there drawing things on a board and diving more into this. At the private level, you need to know what it is, what are you going to notice when it happens, and how are you going to get out of it.
First you need a definition for retreating blade stall. A simple definition is when the retreating blade exceeds the critical angle of attack, which is approximately 15 degrees. Something else important to know, the critical angle of attack in retreating blade stall is a limiting factor for the forward speed of a helicopter.
Retreating Blade Stall
One reason is structural, and the other reason is what we call VNE, or never exceed speed. You have to know that if you exceed the VNE, you can get into retreating blade stall, but you need to know what's going on in the rotor system.
As the helicopter starts traveling faster, the airflow over the advancing blade speeds up. The airflow slows down over the retreating blade as the forward air speed of the helicopter increases. The advancing blade is moving into the wind. Retreating blade is moving away from the wind. You're getting more lift on one side, less lift on the other.
The retreating blade angle of attack must be increased to help equalize the lift. We know if we go past the VNE we can get into retreating blade stall, but there are some other factors that lower that VNE.
The first two are high gross weight and high-density altitude. Those are always two factors when you're talking about a problem with a helicopter, is high gross weight and high-density altitude.
Retreating Blade Stall
Then there's three more that also contribute to the retreating blade stall. One of them is low RPM. The other is steep or abrupt turns, and then there's also turbulent air. You have to understand those five things, that they contribute to the retreating blade stall. You have VNE, and then these other five factors lower that VNE, and it can cause you to get into retreating blade stall at a slower speed than VNE.
Next you're going to have to know what happens at the onset of retreating blade stall. Three things are going to happen, and you have to commit these to memory. The first one is you're going to have abnormal vibrations. After the abnormal vibrations, you're going to have a pitch up of the nose due to gyroscopic presession, and then after that pitch up of the nose, you're going to have a roll to the retreating side. Depends on what aircraft you're flying whether it's counterclockwise or clockwise.
The aircraft is going to roll to the retreating side. Next you have to know the corrective measures for retreating blade stall. You get into it. You're aware of the conditions where it can happen. You feel it. You notice it start to happen. You need to be able to reduce collective pitch the first thing, because you're going to decrease the angle of attack and start helping that problem that you're getting yourself into.
Retreating Blade Stall
Next is increase RPM if your RPM is a little bit low. If the RPM is low, you're going to have to get back up into the green range. Reduce forward airspeed, and then minimize maneuvering. Basically what they're saying is slow down, lower the collective, check your RPM, don't do anything rash, and you will get out of the retreating blade stall.
These are the types of things that we go through in Helicopter Online Ground School with audio and visual cues to help you learn the information and help you get through the dreaded check ride. If you're working on a helicopter rating and you need help with the ground school, click here: Enroll
