Almost everyone says it's Bernoulli's theorem. Air moving over a surface reduces the pressure perpendicular to the surface. Wings with a camber (the top surface is curved more than the bottom surface) make the air go faster over the longer top surface. Voila! Higher air stream velocity and lower pressure on the top of the wing "sucks" the airplane up. When velocity over a surface increases, pressure on the surface decreases, and vice versa.
But couldn't it be the "kite" effect - air hitting the bottom surface of the wing and pushing it up - like the "lift" generated by kites and water skis? You know, Newton's third law of motion - for every action there's an equal and opposite reaction.
I've wondered about this for years, ever since I built a small wind tunnel. I couldn't get any measurable lift from a small wing section until I cranked the angle of attack up to perhaps five degrees. Since then, I've been noticing some other things about airplanes that might argue for the "kite" effect, or Newtonian lift:
Everyone knows, too, that with enough power you can make a barn door fly. If that isn't angle-of-attack lift from the pressure on the bottom surface, what is it?
Can it be that the kite-effect on the bottom of wings and flaps really gives an airplane its lift? Does the cambered airfoil simply make airflow behave better as the wing's angle of attack approaches the stall?
I had been passing on this wisdom for many years, until I met the Professor.
Dr. John Anderson Jr., one of the world's leading authorities on aerodynamics, was nearby at the National Air and Space Museum. He is the special assistant for aerodynamics there, and also professor of aerodynamics at the Maryland University's department of aerospace engineering.
Dr. Anderson explained patiently, with sketches and drawings, that Bernoulli's theorem is alive and well, and the effect keeps our airplanes flying. I must admit I was both disappointed and reassured.
Telegraphing Your Arrival
Two facts are important for you to keep in mind when thinking about lift, Professor Anderson said:
In the subsonic flight regime, here is what happens when you fly through air with a cambered-airfoil wing. Out in front of the wing, in undisturbed air, the pressure is ambient (about 15 pounds per square inch at sea level). Because of the wing's "bow wake," the pressure is a little higher right at the point where the air stream separates to go around your wing - maybe five percent more than atmospheric pressure. That's called the stagnation point.
Aerodynamicists studying subsonic wings commonly refer to ambient pressure as "zero." Their charts then show test pressure coefficients as minus over the top surface (compared with the ambient pressure) and positive over the bottom surface. As air molecules flow over the top of the wing, they accelerate. Those closest to the wing really speed up - to about twice the aircraft speed in a light aircraft like a Cessna 152.
"If you look at smoke-flow photographs, you can see the streamlines really squashed, indicating increased speed of the airflow," says Dr. Anderson. This means greatly increased velocity, and reduced pressure over the top surface of the wing. You can demonstrate the effect very easily. Just blow gently over the top surface of a piece of paper, and watch what happens. (It's not fair to blow over the bottom, too. Fold a "leading edge" on the paper to avoid this).
A typical diagram of pressures over a low-speed airfoil clearly shows reduced pressures over the top of the airfoil (above the line in illustration B) and increased pressures on the bottom of the wing (below the line). Notice that the higher pressures on the bottom are not as strong as the reduced pressures on the top. "For normal angles of attack," Dr. Anderson says, "the pressure decrease over the top of the wing is much more severe than the increase in pressure over the bottom surface."
At general aviation aircraft speeds of 150 to 200 mph, the drop in pressure on top may be only five percent. The increase on the bottom surface is a little more than one percent at cruise angles of attack.
The air does speed up, of course, flowing over a cambered wing's bottom surface, but most wings still have positive pressure on the underside. "I would say that under typical conditions 75 percent of the lift is due to reduced pressure on the top surface," Dr. Anderson says. "There's only a small pressure difference, really, but there's a lot of wing area. That's where lift comes from - all that wing area. Flow expands over the top surface, and pressure drops dramatically!"
So much for the "kite" effect, or Newtonian lift. Bernoulli wins.
What Camber Does
The truth is that the advantages of curved, or cambered, airfoils were known before the Wright brothers. Things like the center of lift on a cambered wing were not understood for sure until the Wright's wind-tunnel and flight research. But as far back as the late 1700s, aircraft designer George Cayley understood that a cambered shape produces more lift than a flat plate.
Toward the end of the 18th century, Horatio D. Phillips patented many cambered airfoil shapes, developed through wind-tunnel testing. Dr. Anderson calls Phillips the "Grandparent of the modern airfoil."
The Wrights, puzzled by the conflicting information available to them about air pressures over wings, threw away much of the early data and, more or less, started over. They spent about a year with their wind tunnel, testing various airfoils, and decided on a five-percent-thick airfoil for their Wright Flyer. That's a thickness ratio of 20 to 1, where the chord (wing width) is 20 times the wing's thickness.
And so airfoils remained, even though they were cambered, until late in World War I. Remember the Fokker triplane immortalized by the Red Baron? One reason the little triplane climbed and fought so well is that it flew with the first thick airfoil in the modern history of aviation.
In his Gottingen laboratory, aeronautical pioneer Ludwig Prandtl discovered the substantial advantage of a thick airfoil. Anthony Fokker immediately picked the Gottingen 298 airfoil for his triplane.
Surprisingly, a thicker airfoil develops less drag at slow speeds, for the lift produced, and a higher rate of climb than an equivalent thin wing, like that on the SPAD XIII (similar to the French Eiffel 14 airfoil). Airflow separates over the top of a thin wing at smaller angles of attack, creating drag, and producing a near-stall condition. Thicker airfoils generally stall at a greater angle of attack, providing improved maneuverability.
In addition to the lift bonus provided by the 13-percent-thick airfoil, it provided room inside the wing for bracing that hung outside in other World War I fighters. No wires on the triplane.
The fatter wing was so inherently strong, in fact, that Fokker's later D-VII biplane really needed no struts between the upper and lower wings. Fokker added them only to make pilots and purchasers more comfortable, in that era of bird-cage wiring and bracing.
Tried and True Airfoils
The airfoils that you fly today on general aviation aircraft haven't changed a great deal since the early 1930s. The National Advisory Committee on Aeronautics (NASA's predecessor, before the days of space flight) gave us several famous series of airfoil designs in the 1930's.
The Cessna 150 flies with a 1933 NACA 2412 airfoil. (Maximum camber {curvature} of 2/100ths, or two percent, of chord; maximum camber location along the chord at 4/100ths chord, or four percent, and a thickness of 12/100ths of chord, or 12 percent). The Cessna Citation and the Beech King Air fly with a late 1930s airfoil, the NACA 23012, which goes to show that - if you do your research right - it lasts.
A modern airfoil series, developed by NASA since 1965 for light general aviation airplanes, provides substantially more lift. A revived wing tunnel test program at NASA Langley, with the power of computers to calculate airflow, delivers airfoils with almost 80 percent more lifting power per square foot than conventional airfoils.
Flat Plates
So how about the proverbial barn door - an absolutely flat plate - with little thickness and no curve or camber at all? You know you can get it to fly - with enough power. Some early gliders (using altitude for power) flew with no camber at all.
Surprisingly, much of the lift on a barn door also derives from a greater reduction in pressure on the top surface. This occurs because the airflow just underneath the leading edge of the barn door, at low angles of attack, stagnates, flows backwards toward the leading edge, and wraps around it. "And that's your restriction again," Dr. Anderson says. "The air stream necks down to a very small area as it goes around that leading edge and gives you a very low pressure."
Bernoulli again.
With a flat plate, obviously, you get a higher percentage of the total lift from the positive pressure on the underside than you would from a Cessna 150 wing. As the angle of attack for a flat plate increases, you eventually reach a point where more lift comes from the underside than the upper surface, but the angle of attack will be so great - and the drag so tremendous - that it questions the definition of "flying."
Flaps
It's tempting to think that much of the added lift from flaps comes from the air's impact on their bottom surface, and not from the well-advertised "increase in camber" of the wing. How can a split flap, on the rear of the wing's underside, give it more camber? On this point, too, I have to admit defeat.
In effect, the flaps do increase camber, because the air molecules "see" the wing coming with flaps down, and change their ideas about flow around the wing! The airflow is "being faked out," as Dr. Anderson puts it. "It is seeing an airfoil section that looks more cambered than it really is. Even with split flaps." (And, believe it or not, split flaps provide a greater boost in lift than simple hinged flaps.) Airflow moves even faster over the top of a wing with flaps down. The point of greatest lift on the wing moves rearward, requiring a "nose up" trim change.
Flaps also increase drag, which is evident when you drop the flaps on approach. In fact, Dr. Anderson says, you should think about flaps as a lift/drag control device. On takeoff, you want to increase lift, without paying a high drag penalty. On landing, you also want high lift, but you don't worry much about drag. The drag of full flaps even helps to increase your glide angle and landing accuracy. However, you can see why it's essential, when you make a go-around, to milk the flaps up as recommended, to win the lift/drag contest.
A garden variety airfoil, the NACA 23012, has a design landing speed of 100 mph without flaps. A simple hinged flap reduces landing speed to 81 mph. A split flap reduces it to 78 mph. A slotted trailing-edge flap brings the landing speed down to 75 mph, and a double-slotted flap to 70 mph. The Beech Bonanza uses the 23012 airfoil on its wingtip, and the NACA 23016.5 airfoil at its wing root.
Symmetrical Airfoils
Bernoulli is still winning, even with symmetrical airfoils, where the curve is the same over the top and bottom surface. Symmetrical airfoils are found on aerobatic aircraft, designed intentionally to fly upside down as well as right side up. The truth is that these airfoils also develop most of their lift from an increase in the velocity of air moving over the "top" surface, whichever that is.
Symmetrical airfoils develop somewhat less lift, other things being equal, than a conventional airfoil, and they don't start developing any lift at all until they move to a positive angle of attack. Most normal airfoils, by comparison, develop lift even at slightly negative angles of attack.
Drooping Fighter Stores
You've probably noticed the way external stores hang nose-down on aircraft designed for supersonic flight, when they're at rest on the apron. This may have suggested to you that the high-performance aircraft's lift is derived more from the "kite" effect, than it is from the Bernoulli effect. Wrong again. Two factors are at work here:
Relatively straight supersonic wings don't do very well at subsonic speeds. Airplane design is always a compromise, says Dr. Anderson. "Look at the F-15 and F-16. They don't have wings as thin as the F-104." Current fighter design philosophy accepts the fact that dog-fighting is a subsonic activity, and only rarely do fighters fly at supersonic speed.
Therefore, when supersonic aircraft with thin, sharp wings fly at subsonic speed, their wing performance is poor. They fly with their wings at a greater angle of attack than subsonic aircraft. More lift comes from the impact pressure of the air on the bottom surface - but most of the lift still comes from the Bernoulli effect.
With all wings, thick or thin, the lift force continues to build as the angle of attack increases, up to the stall point (around 15 degrees for a conventional subsonic light aircraft airfoil). Thin supersonic wings have better landing and takeoff performance if they are delta-shaped. The delta wing stalls at a much higher angle of attack than straight or swept wings.
I now know why my wind tunnel couldn't prove Bernoulli right until I increased the wing's angle of attack to five degrees or more. "The forces were just too small," Anderson says. The airflow, generated by an old vacuum cleaner motor with an electric fan blade, probably didn't generate a wind of much more than 20 miles an hour. And a wing section six inches long and six inches wide doesn't provide much area for the air stream to work on.
I've got to accept the evidence: lift comes from the increased velocity of air movement over the top of a surface. Even barn doors do it.
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