Pet peeve time. I'd like to focus on something so trivial in the great scheme of things that it may cause your brain to sludge up like molasses at absolute zero. It has nothing to do with politics, terrorism, economics, psychology, political correctness, or why an allegedly merciful God allows the Kardashians to exist. My petty peeve is the enduring myth, famed of high school physics classes, that airplanes fly because a wing is curved on top.
What prompts this pettish jeremiad is recently seeing this myth promulgated on PBS, of all places. I do not recall if they dragged Bernoulli in as an avatar of scientific authority on the subject, but my H.S. science teacher did, and so did, shudder, the teacher of the general physics class I took in college. The college prof was not pleased when I called him on his explanation. I had just finished reading the veriest and ancient bible of flight by Wolfgang Langewiesche titled Stick and Rudder: An Explanation of the Art of Flying. It's still available today, and is beloved by primary flight instructors the world over. At any rate, I airsplained to the prof why aircraft do not fly by Bernoulli alone, and mirabile dictu he bought it. As well he should.
By far the simplest explanation of why an aircraft flies is Newton's Third Law of Motion. To wit: For every action there is an equal and opposite reaction. The action with which we are concerned is the wind flowing past the wing of an aircraft as it is propelled through the sky by either its engine, or by gravity in the case of a "glider". Imagine a sheet of thin plywood, an air plane, being hauled through the air exactly parallel to the wind. It will produce no lift and a small amount of drag. Tilt that woody plane up at the front and now we're talking lift. This lift is a reaction to the wind impinging on its lower surface---Newton's third law in airborne action.
The high school explanation is that the upper curvature of a wing causes the air to speed up and reduce the pressure thereby lifting the plane into the air as shown by Mr. Bernoulli's equations. Now here's where it gets a little tricky. The usual demonstration of pure Bernoulli lift is a high school teacher blowing over the top of a sheet of paper, with the class acknowledging that the paper really is lifting up just because of the higher speed air blowing over the "top". So indeed pure Bernoulli lift can "fly" a sheet of paper. By extension, it is averred, the curved upper surface of a wing can reduce the pressure enough to lift a plane into the air.
Sadly, an actual aircraft can not fly solely because of the pressure reduction caused by the upper curvature of its wing. Got that? To actually fly, an aircraft must tilt its wing up in front by varying degrees (referred to as "angle of attack") so that the "relative wind" impinging on the bottom can push the plane up. The consequent lowered air pressure over the top of a real wing is an effect of lift, not its cause. The high school explanation turns this on its head by putting the effect before the cause.
This is not just semantics. As an illustration stick your hand out your car window at speed with your palm flat to the wind. Naturally you will feel a great deal of pressure. The air pressure is certainly reduced behind your palm, but it is obvious that this reduced pressure is not a proximate cause of the pressure you feel on your palm.
The specific curvature of a wing surface, the "airfoil", is there for two main reasons. The first is to reduce drag and thus make the wing more efficient. The second and more important function is to control the flow of air around the wing so that the desired operating parameters of a given aircraft are met. A thickly airfoiled flat-bottomed wing suits the needs of an aircraft intended to take off slowly, fly slowly, land slowly (think Piper Cub), and to give the aircraft gentle handling and more easily controlled stall (loss of lift) characteristics. The thin and relatively small wing of a jet fighter sports an airfoil configured for high speed combat and may appear to have very little curvature at all.
In fact many jet fighters have symmetrical (same curves top and bottom) airfoils so that their flight characteristics are similar whether upright or inverted. Symmetrical airfoils are also preferred on aircraft dedicated to aerobatics, and for the same reason. Countless millions of hours in wind tunnels, computer simulations, and contemplation by febrile noggins have been devoted to optimizing airfoils in the past century. This has resulted in tens, if not hundreds, of thousands of different airfoil designs that optimize a wing for duty stretching all the way from gossamer indoor free-flight models traveling at one or two miles per hour, all the way to the Space Shuttle which is the fastest winged beastie ever to fly.
Airfoils trick the wind, hence the "foil", into proceeding smoothly around and across a wing in the desired fashion, but they are not the primary cause of the lift a wing generates. Mr. Bernoulli's equations do indeed describe what happens when a wing generates lift, but they do not appear to properly order cause and effect when applied to an airfoil. Pure Bernoulli pressure reduction is indeed seen in very many hydro-dynamic situations. Carburetors spring to mind most readily, but pure Bernoulli "lift" can not, will not, ever cause a real full-sized aircraft to rise into the sky.
Pass it on, and if you see a copy of Stick and Rudder, pick it up. It's an entertaining read even for a non-pilot.
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