Basics of Wings: Part 2

And the last of the Sett-imported posts rolls in. New content soon!

Subject! Wings! Yes! You’ve got to love wings. They allow us to fly and look pretty cool (in my opinion). You can even hang things like engines on them. Marvellous! We know they generate lift, and that’s great. But alas, it’s not that simple. We need to go deeper. I’m about to introduce you to the centre of pressure. Remember, this hurts me more than it hurts you.

Let’s imagine a wing flying through the air. We know it generates an area of lower pressure on its upper surface. We know this pushes the wing up. But it’s not like someone plonks their hand on the lower surface and just hauls the thing up. The pressure isn’t evenly distributed; it varies in magnitude along the chord (front-to-back length, as opposed to span, which is root to tip) of the wing. Let’s see how:

(Wikipedia: http://en.wikipedia.org/wiki/Lift_%28force%29)

Look to the wing’s upper surface, towards the leading edge (the front). See how the air particles are moving fastest here? As we learned from the last post, that faster speed means there must be a corresponding pressure drop, which means: lots of lift! But look! It starts to slow down again past a certain point. As the air slows down, pressure increases – you get less lift as you move further back. The further back you go, the higher the pressure gets, until it matches the pressure on the lower surface – this section of the wing isn’t producing any lift.

This is where the important steps happen. Pay attention. You… nouns.

• We know some sections of the wing will experience a stronger lifting force than others.

(VERY rough representation of lift components. Symmetrical sections wouldn’t behave like this!)

• Combine these force components together, and you have a resultant (overall) lifting force. This is the wing’s lift.

(VERY rough representation of total lift magnitude and point of action)

• This lift force is focussed at a certain point along the wing chord.
• This point is known as the centre of pressure.

Whew! That took a hell of a long time to figure out how to explain.

Right. So we know the wing is basically being pushed up through a specific point. It is REALLY REALLY IMPORTANT to know where this point is. Here’s why: you have to balance your aircraft. All solid objects balance around a certain point – this is known to engineers as the centre of gravity. This is where an object’s weight is focussed, in the same way as the mid-point of a see-saw. To keep the see-saw level, you’ve got to have the force pushing it up directly on top of that mid-point. Move in either direction, and the see-saw will start to rotate. In the same way, if an aircraft’s overall lifting force isn’t sitting on the centre of gravity, it will stop flying straight and level and start revolving around it, which can have perilous implications.

In the next post, we’ll learn how to calculate where the centre of pressure lies.