Busting Bernoulli's Lift

Today I'm sick with a cold, and the weather's been crap for almost a week now, so no flight training at the moment. Tomorrow is looking up however.

In the meantime I've been lazing around the house, playing computer games and browsing some aviation blogs. I came across one blog by an American flight instructor, which in one post was explaining the theory behind lift - incorrectly. That same incorrect explanation has been a persistant myth among laypeople and pilots alike for a long time now, and every time I hear it repeated it just raises my pet peeveness of it one more notch. So today we'll set the record straight, and I'll do my best to provide proof for any persistent skeptics out there.

The myth is that lift is produced by the curvature, or camber, of the top surface of a wing. The air being divided by the wing at the leading edge must meet up again at the trailing edge, and because the air has further to go over the top (due to the camber), its velocity increases and pressure decreases. The decrease in pressure causes a pressure differential between the top and bottom of the wing, which sucks the wing upwards, thus creating lift. This decrease in pressure is in reference to Bernoulli's Principle, which states that as the velocity of a fluid increases, its pressure decreases.

I am not disputing the validity of Bernoulli's Principle, because that part is true. The error however comes in the assumption that the pressure differential can create enough lift to counteract the weight of the airplane. If this explanation were true, it immediately raises some questions if one thinks about it:

For one, how does an aerobatic airplane fly upside-down? The quick answer is that aerobatic aircraft are equipped with a symetrical wing, which is a wing with an equal camber (curvature) on both the top and bottom. But that explanation creates yet another problem. How does a symetrical wing fly at all?! According to Bernoulli's Principle, it can't, because due to the symetry of both the top and bottom the air wouldn't create a pressure differential. Similarily, in theory, a flat wing, like what you would find on a balsa wood model, or many fabric wings like on hang-gliders and ultralights, would not be able to produce any lift at all either, but we've all seen balsa models fly.

Another problem we come upon is that our current explanation completely ignores what we know about angle of attack. Every pilot knows that term. Angle of attack is the angle that the wing meets the oncoming air. The illustration above would be a wing at an angle of attack of zero degrees. The myth would seem to indicate that a wing could produce lift at an angle of attack of zero. The myth also seems to say that it would be possible to maintain that zero angle of attack through the entire speed range of an aircraft, since lift is created by the Bernoulli Principle, and not angle of attack. This is at odds with any pilot's observations of how an airplane reacts as it is decelerated. It only takes a single demonstration of slow flight to show that as an airplane slows down, the angle of attack increases, which can be observed by the fact that the nose raises higher and higher into the air to the point of an aerodynamic stall. It is this observation of angle of attack that leads us forward to the true explanation of how a wing creates lift.

The majority of lift is in actual fact produced as a result of the angle of attack, which creates both a deflection force underneath the wing (resulting in higher pressure), and a vacuum effect on top of the wing (lower pressure). The vacuum effect above is not however a result of a wing's camber, it is a result of angle of attack. With a wing at a positive angle of attack, the air flows over the top of the wing and is forced to turn a corner down towards the wing. It is at this corner the low pressure zone develops. Note the illustration below of a flat wing moving through the air at a certain positive angle of attack. You'll have to forgive the pathetic visual, I made it in paint, but it gets the point across. I chose to illustrate a flat wing because its easy to visualize the "corner" that the air has to turn as it passes over the top.


So if all a wing needs is angle of attack to fly, why the heck do they give it a camber?! Well the primary reason is for smooth airflow, at a range of attack angles. A flat wing will stall at a very low angle of attack because it creates a very sharp corner over the top of the wing for the air to have to travel. At some point the air won't be able to make that corner, and will seperate from the wing, producing a turbulent airflow. We know this as a stall. When the top surface of a wing is cambered, it allows the air to make that corner over the top of the wing at much higher angles of attack without seperating and causing a stall. The camber makes for a much smoother corner. This, in effect allows the wing to produce lift at slower speeds, which is generally a good thing. This also explains effectively why airplanes designed to fly slow and carry big loads have a wing with a high camber. Its not because the camber itself produces the lift, but because the camber allows the wing to fly at greater angles of attack without stalling, and the greater the angle of attack, the more lift a wing will produce.

This explanation also neatly answers all of the questions that came up with the popular mythical explanation. Aerobatic airplanes can fly either right-side up or upside-down, regardless of whether they have symetrical wings, because they just have to maintaint a positive angle of attack. Flat wings like balsa gliders and hang-gliders are also explainable. Finally it also fits better in with a piot's observations during slow flight, because as the aircraft decelerates, the angle of attack must increase, which is observable in the form of a raised nose.

While the mythical explanation of lift may not be ENTIRELY false (the Bernoulli Principle), it may contribute to the total lift of an airfoil a little bit, but the effect is negligable for practical purposes. One thing in that explanation that is completely false is the idea that air being split at the leading edge must meet back up at the trailing edge. There's no law anywhere that says air particles have a "memory" to them and must return to the particles they began with. They are carried to wherever the forces acting take them. Wind tunnel tests with time-pulsing coloured smoke actually show that the above air accelerates far past where the lower air ends up at the back, and also are forced downwards. The downward flow is known as downwash (which is also an unexplainable phenominon with our mythical understanding of lift).

So, now you know the rest of the story...