Airfoils, Stalls and Critical Angle of Attack – Video Transcript
Today we’re going to talk a little bit about airfoil stalls. I want to make sure that we’re clear as to what that means. Most people, when they hear the term stall, they typically infer their car. Specifically, their car engine stalling out and then therefore not producing any power. In the case of airplanes, this is a little bit different. We’re referring to an airfoil stall, which means an airfoil, such as the wing, is going to stop producing lift. Maybe not stop but produce significantly less lift. So, we’re going to talk a little bit about the aerodynamics involved in how a wing produces lift to start with, and ultimately then, how it would stall and dramatically reduce the amount of lift that it produces.
To get us started, I want to go over some basic terminology so that we can all be on the same page with some of the vocabulary that I’m going to use. First, we’ll take a look at one of our archer wings and we see from the wingtip here, we can create a cross-section of what the wing would look like. If we draw an imaginary line from the leading edge to the trailing edge of the wing, this line is referred to as the chord line. Additionally, if we imagine the airplane flying forward, the equal and opposite wind we call the relative wind. Now, there’s going to be an angle then formed between this chord line and this relative wind. That angle, we refer to as our angle of attack.
So up here on the board, I have this cutaway of an airfoil. We have the chord line drawn in red, we have this relative wind that we can see would continue like this, and that angle that’s formed between the chord line and this relative wind is our angle of attack (AOA). So that angle of attack, it really drives one thing. We can associate the angle of attack with the pressure difference between the top and the bottom of the wing. What that means, is if this angle continues to grow, what’s going to change is how air flows over and under the wing.
In this example, we notice that the airflow will end up coming over the wing like so, and then it basically remains fairly flat across the bottom. What that means then, is this air creates a venturi effect between the top camber of this wing and the free-flowing air above it. That venturi effect that’s made causes this air to accelerate and would result then in a decrease in static pressure. This is where we get this concept of a relatively lower pressure above the wing and a relatively higher pressure below the wing. As I increase my angle of attack, like if we go over to this image here as we increase our angle of attack, we see that the chord line and this relative wind the angle is greater. That larger angle of attack means then that now, this air flow is going to create an even tighter venturi effect across the upper camber of the wing, which means that ultimately, we’re going to produce more total lift. As you’re probably noticing where that venturi effect is, is where the majority of that lift is produced. So, we call that point, the center of pressure or the center of lift.
Another key element to this that’s going to be important in understanding the concept of a stall is what’s happening as this air continues to flow around the airfoil. In this example, where we were at a relatively low angle of attack, the airflow would basically stay connected. There’s this sort of boundary layer where the air is relatively connected to the surface of the wing, however the greater our angle of attack becomes, what happens then is kind of just like any fluid. So, air is a fluid very similar to that of something like water. If I were to put a drop of water on my finger, the drop of water would not run straight off like this. Instead, it would run down my finger and sort of wrap around and then drop off.
Well, why does it stay connected? The answer is the same sort of deal here. We would refer to this as sort of like a boundary layer. So, the idea of this air flow, is that it should stay connected to the surface of the wing. But as this angle of attack increases, what happens is the air is not able to stay connected because that angular difference becomes too significant. The result is that at some point it starts to separate which means it kind of creates its own free stream of air. Now that the air flow is not connected to the trailing edge of this wing, it means that this portion of the wing is not really doing much for us. As we increase the angle of attack more and more and more, this separation point is just going to continue to work its way forward.
To retrace our steps here, we understand that the angle of attack is pretty much associated with the pressure difference between the top and the bottom of the wing. Therefore, if I increase my angle of attack, I’ll produce more lift, but I’m also creating more separation of the boundary layer from the upper surface of the wing. This means that at some point that separation could get to a place where this separation is occurring all the way near the leading edge of the wing. Once that separation reaches a point where it reaches this center of pressure, once that separation works its way all the way up to reach that point, now where the majority of our lift is being produced is sort of stalled. There’s no more airflow at that point, this is what we’d identify as being in a stalled condition or at least the early onset of that stalled condition.
This then can be referenced to a specific angle of attack and that angle of attack where that stall occurs is referred to as the critical angle of attack. This basically remains predominantly the same for an aircraft and obviously it’s pretty exaggerated in the case of this image, but I think it serves its function. So, where a stall occurs is contingent upon the angle of attack reaching this critical angle of attack. This would then apply to various configurations of the aircraft. Meaning, I can stall the airplane at different air speeds, at different attitudes, and different power settings, different weights. None of that really applies as much to the stall as does our angle of attack. This is the only contributor. Those other factors may affect where the relative wind is coming from or where our chord line is, but the reality is the only thing driving this stalled condition is when we reach our critical angle of attack.
Now let’s transition to stall recovery. Most airplanes have some kind of stall warning indicator which is some kind of a device that’s found on the leading edge of the wing that ultimately will sense the relative wind and then in some way, inform the pilot through a horn or siren or some kind of something that identifies that you are nearing the critical angle of attack. That would then prompt the pilot to make some kind of an adjustment, either to reduce the angle of attack by pitching down, or maybe adding power or sometimes a combination of both. In either way, this is how we can ensure that we prevent ourselves from entering a stalled condition. If we did find ourselves in a stalled condition, obviously the airplane would be producing significantly less lift and it would be very difficult to maintain altitude. I hope that this video has been helpful, and you’ve learned a little bit more about the airfoil stall and some of the vocabulary associated with it.