Hello again! First of the new fresh posts to this blog. Hopefully these past few days have been sufficient recovery time from that last monster of a post. Wings are funny like that, and we’re only covering the barest minimum of functional basics… pretend I didn’t just say that.

So last time, we discovered that the total lift generated by wings is focussed around a point. Well guess what? This point moves backwards and forwards!

I’m sorry, but it’s true. In fact, in certain situations, this point wander off to an infinite distance in front of the wing. Tilt the wing a bit, and it shoots off to infinity in the other direction.

Put your worries to rest, though, because it’s controlled by a very simple (but brilliant) concept called moments. No, not moments in time (it had me confused for a while at first). We’re talking levers here. You know how taking a wooden crate apart is way easier with a crowbar than with your hands? That works because moments make it so. And now, let’s go back to that blessed see-saw analogy. I know, it gets old, but see-saws are bloody brilliant at demonstrating this stuff.

Put simply: Moment = Force x Distance

Let’s set some rules to fix how they work.

• If we increase the weight on one side, we’ve got to move it closer to the reference point (in this case, the pivot) to keep the moment constant (make sure the value doesn’t change)
• If we reduce the weight on one side, we’ve got to move it further away from the pivot to keep the moment constant
• If the moment on one side = the moment on the other, then the see-saw will remain still.
• If the moment on the left = bigger than the moment on the right, then the see-saw will rotate to the left (in the direction of the greater moment – anti-clockwise, in this case)
• If the moment on the right = bigger than the moment on the left, then the see-saw will rotate to the right (clockwise, in this case)

One easy way of remembering the first three points is this:  for a constant moment, an increase in force means a decrease in distance from the reference point, and a decrease in force means an increase in distance from the reference point.

 

Got it? I hope so. If not, that BBC article I grabbed the illustration off seems a reasonable place to start. I’m afraid that to understand wings, one must first understand moments. To reiterate: THIS IS SOMETHING YOU CANNOT SKIP.

 

Okay, if you’re reading this, then you either understand moments, or you’ve given up and are cheating. Or you’re skim-reading, and… you know, my parents always told me not to judge.

So, dear reader (can I call you dear reader? I’m going to go with yes), we established that the centre of pressure (the point along the wing at which lift does its lifting thing) moves. But how do we tell where that little rascal has gotten off to? And how do we predict where it’s going to go? Well, for present purposes, it depends on the angle at which the wing meets the airflow. We call this the angle of attack.

Those lines represent the paths of the incoming air, and the Greek symbol alpha represents the angle between it and the wing. If the wing tilts upwards, it is said to have a positive angle of attack, and will be generating lift. If it’s tilting down, the angle of attack becomes negative, and the wing will start producing negative lift (pushing the aircraft down). If it’s pointing straight into the airflow, the angle of attack is 0.

And now that’s sorted, we’ll introduce the Aerodynamic Centre. This sweet spot is the point around which the centre of pressure moves. For most aerofoils (the 2D shape you see below), it resides more or less 25% of the chord length back from the leading edge.

Are you ready for some brain hurting? I’ll assume that’s a yes.

The Aerodynamic Centre is the point at which the lifting moment remains constant.

Don’t panic! Let’s go back to the moment ground rules we set out earlier. We know that a constant moment requires an increase in force to be matched with a decrease in distance from the reference point, and vice versa.

The wing’s aerodynamic centre is treated in the same way as the pivot – it is our reference point. If the moment around the aeodynamic centre must remain constant:

• If lift increases, then the centre of pressure must move closer to the aerodynamic centre.
• If lift decreases, then the centre of pressure must move further away from the aerodynamic centre.

So, taking this to two logical conclusions:

• The centre of pressure is closest to the aerodynamic centre when the wing is generating its maximum lift (positive or negative)
• The centre of pressure is furthest from the aerodynamic centre when the wing is generating zero lift. In fact, it’s infinitely far away. Crazy, right?

Now, we know that lift is connected to the wing’s angle of attack. Making some reasonable assumptions, and as long as the wing is flying normally:

• The further the angle of attack is from 0, the more lift is generated, and the closer the centre of pressure is to the aerodynamic centre. Remember, this applies whether the wing is pointing up or down!
• The closer the angle of attack is to 0, the less lift is generated, and the further the centre of pressure is from the aerodynamic centre*.

*Just a note  – many wings actually generate 0 lift at negative angles of attack, depending on their designs. We’ll go over this later.

 

Resorting temporarily to second person, you’re just going to have to bear with me on this one. This might seem like a pointlessly complex and unnecessary topic, but it’s absolutely crucial for keeping aircraft flying controllably. And anyway, that’s the worst of wings covered. Next time, we’ll get to look at some pretty graphs which will tell us all we need to know about how a wing behaves.