Airflow around a wing

You may have read an article that appeared in Skywings last August about porosity. In that article I referred to the diagram in Ozone’s paper about SharkNose technology which showed how the difference between the internal wing pressure and the external pressure is greatest at the leading edge (Figure 1). The question I promised to return to was why would that be the case?

I guess most of us remember being told for our BHPA exams that the air flowing over the wing goes faster, and that below the wing goes slower. And that Bernoulli’s theorem tells us that faster moving air is lower in pressure than slower moving air. So there you are. But that explanation always niggled with me. It seemed a bit incomplete, and doesn’t explain why the pressure difference is greatest at the leading edge.

I also had an example that called into question whether it is simply a matter of air moving faster that results in a lower pressure. I remembered a demonstration from my physics class from long ago, which involved blowing next to a piece of paper. Done correctly, the paper seems drawn upwards towards the faster-moving, lower pressure air. However, this works only if you follow the instruction we were given at school, and doesn’t if you hold the paper differently (Figure 2). Try it.

Well, I did some reading around, and grappled with things like circulation, the Kutta-Zhukovsky theorem and the Coanda effect. I understood enough of it to realise that the full explanation of aerodynamic lift is really very complex, and not easy to summarise here. But I did identify a few simple ideas that gave me some insight, that are more straightforward to explain.

First off, I also remembered from my physics class was that air has mass. Of course it does, but it’s easy to overlook. As an aside, Fred Pieri at Ozone tells me that the mass of air inside a solo paraglider is around 10kg, and inside a tandem is 18kg. So around two-thirds of the inertia of our wing in flight or ground handling comes from the air contained within it, and only perhaps one-third from the weight of the glider itself.

Because air has mass, it requires a force, in a fluid usually exerted by a pressure, to cause it to change direction or speed. That’s Newton’s first law. In front of our wing there’s what’s called a stagnation point, where the air approaching the wing becomes stationary (Figure 3). Air on flowlines above or below this point go over or under the wing. The pressure required to bring the approaching air to a halt, the Pitot Pressure, increases in proportion to the square of the airspeed of the wing. The innovation of Ozone’s SharkNose was to ensure that the stagnation point, which moves around at different angles of attack and speeds, was always as close as possible to the wing’s air intake position, maximising the internal pressure in differing situations.

This higher pressure in front of the wing causes the approaching air to slow down and divert either over or under the wing. You can think of the effect of pressure as a bit like the effect of a hill on the speed of a bike: going up the hill (approaching higher pressure) slows the bike down. It’s also true that, just like going over the brow of the hill and speeding up, air does the same, as the higher pressure behind causes the air to accelerate.

The next thing I realised is that air will only follow a curved flowline if the pressure is lower on the inside of the flowline curve than on the outside. It’s this difference in pressure that makes air divert from a straight path. It’s a bit like in a tornado, or low-pressure system. I used to wonder why the air didn’t just blow across the isobars from high to low pressure, but now I realise that in part, the low pressure at the centre of a cyclone is the consequence of the huge mass of air swirling around in a circle. It’s like the pressure difference needed (higher outside, lower inside) is the equivalent of leaning over on a bicycle to go around a corner, and the pressure difference forces the air to describe a circle around the centre of the low pressure system: the faster I go on my bike and the tighter the corner, the more I have to lean – in the same way, the bigger the pressure difference has to be on either side of the flowline if the curve is tight, or the speed of the air is higher.

Looking at the shape of the leading edge and the curve of the flowline around it tells us that the pressure on the inside of the curve must be lower than on the outside, otherwise the air simply wouldn’t go around the curve – it would just continue in a straight line. (Figure 4). This also explains why the school demonstration works when the paper is curved, but not when it’s straight.

What’s more, as the air flows away from the higher pressure in front of the wing, it accelerates – just like the bike going back down the hill. In fact, as the air passes over the top surface it reaches a speed faster than it had in front of the wing, before slowing down as it arrives at the trailing edge. Even so, it arrives at the trailing edge quicker than the air going underneath the wing, as a screenshot from Professor Babinsky’s YouTube video demonstrates (Figure 5). But explaining that gets really complex.

However, the curve of the flowlines around the leading edge does help explain why the pressure difference is greatest at that point. And that’s one of the reasons why manufacturers use heavier weight cloth in this part of the wing. For example, on the Sigma 10, Advance specify Skytex 38 on the leading edge, but use Skytex 32 on the rest of the external surface of the wing.

Enjoy your flying!

If you want to chat anything through, we’re always happy to offer help and advice over the phone (01433 627195) or by email (info@aerofix.com).

 

2018-10-10T11:57:29+00:00Porosity|