I think that one of the most under-appreciated wonders of the modern world – besides maybe the smartphone and no-stick cooking spray – is the airplane. Nowadays, people travel from continent to continent in a matter of hours, crossing oceans and distances that at one point took months to travel. Sure, we like to complain about the discomfort of flying – the inconvenience of security, the bag fees, the lack of legroom – but at one point, in history, the very idea of flying was a fantastical concept.
At the dawn of the last century, many people thought that heavier-than-air flight was simply impossible. It was absurd. Absolute lunacy.
However, in 1903, two brothers from Ohio set off to Kitty Hawk armed with nothing more than a canvas flying machine and the good old American spirit for invention. There, on the sandy dunes of North Carolina, the first flight was made.
While only just shy of a minute long, that flight changed the world.
Nowadays, no one ever really stops to think about how flight happens; or, if they do, they’re exposed to many misconceptions. Lift generation boils down to something simple: a difference in pressure between the bottom and top of the wing. This causes an upwards force that we call lift. While most theories touch on this, they don’t accurately explain what causes the pressure difference.
EQUAL TRANSIT THEORY:
The most common wrong explanation of lift is called the ‘longer path’ or ‘equal transit time’ theory. This theory states that it is the speed of the airflow due to the shape of the wing that causes lift. The top is curved up, and is therefore longer than the bottom. This theory relies on the idea that the air has to travel a longer distance (and for some reason) has to do it in the same amount of time as the air at the bottom (this is where this theory falls flat). This means that the air on the top is moving faster, which results in a lower pressure than the air at the bottom, which in turn causes an upwards force on the wing.
This theory relies on something called Bernoulli’s principle to explain why the faster air is at a lower pressure. In it’s simplest form, Bernoulli’s equation states that the sum of the energy for a given volume will be the same before and after. Energy is conserved. In Bernoulli’s equation, this statement takes the form of a pressure term and kinetic energy term (Let’s just neglect gravity for now. Air is light.) and can be written as follows:
While Bernoulli’s equation explains why faster airflow results in lower pressure, there is no reason that the air on the top of the wing should have to move faster to have equal transit time as air passing underneath the wing. In fact, it is actually the lower pressure that causes the air to speed up in the first place, not the other way around – and once this air goes over the top of the airfoil, it is usually moving at a speed much faster than would be needed to read the tail at the same time as the bottom air molecules.
So what is the actual reason behind this pressure difference? Strap in, because you’re going to get a lesson in streamline curvature.
THE REAL REASON? STREAMLINE CURVATURE:
Let’s start with this handy-dandy diagram. On the top, we have some spooky looking equations and on the bottom, there is what we call a streamline. A streamline is just a term for the path that air molecules take; it’s a way of visualizing the flow. Bernoulli’s equation only applies to air molecules along a streamline; i.e. if the pressure changes for air somewhere along that pathway, the molecules will speed up or slow down. “Streamline curvature” is the analysis of how these pathways curve and what that we can gather about the pressure forces acting on the air.
That equation up there? Basically it relates the gradient of pressure along the radii of streamlines. If your radius is infinity (you have a straight line) your change of pressure in the normal direction (direction of radius) is zero, so there is no pressure force acting. However, if you have a small radius (which implies a tight curve, maybe around the tip of a wing) you will have a large pressure gradient. Think of it like the string of a tetherball, keeping the ball swinging around in a circle. If you shorten the string, the ball goes around in a faster circle. Streamline curvature behaves in a similar way; if you have a tighter circle, the flow will need a large pressure difference to keep it following that path, and it will speed.
As a result, when the airflow curves up around the tip of a wing, the pressure drops. At this part of the wing, there is something that aerodynamicists like to call the “suction peak”, where the pressure is incredibly low due to the air having to fly up or down and around the tip of the airfoil. The air then follows the shape of the airfoil, and follows the behavior described in the curvature analysis. On the bottom of the airfoil, the flow curves up and then down again. In order for the flow to be “pulled” back down, there is a higher pressure along the bottom surface of the wing. By the same logic, there is lower pressure on the top surface, with a higher pressure far beyond the top of the wing (let’s call it our baseline atmospheric pressure) that causes the air to bend and follow the wing, and then straighten out once more at the tail.
The other key point to how wings generate lift is angle of attack – the angle from horizontal that the wing is flying at. If you were trying to fly on symmetrical wings at zero angle of attack, you wouldn’t generate any lift because the streamlines – and therefore the forces – would be symmetrical on the top and bottom of the wing.
To generate any lift, you will have to make sure that the top of your wings have a lower pressure than the bottom by mounting them at a slight angle. (or, conversely, by always flying slightly upwards, but this is problematic)
You may also be asking how planes fly upside down – and to that, I say “with extreme difficulty.”