THE TWO COMPETING EXPLANATIONS FOUND IN K-6 BOOKS: Here is the typical "Airfoil shape" or "Popular" explanation of airfoil lift which commonly appears in childrens' science books:

As air approaches a wing, it is divided into two parts, the part which flows above the wing, and the part which flows below. In order to create a lifting force, the upper surface of the wing must be longer and more curved than the lower surface. Because the air flowing above and below the wing must recombine at the trailing edge of the wing, and because the path along the upper surface is longer, the air on the upper surface must flow faster than the air below if both parts are to reach the trailing edge at the same time. The "Bernoulli Principle" says that the total energy contained in each part of the air is constant, and when air gains kinetic energy (speed) it must lose potential energy (pressure,) and so high-speed air has a lower pressure than low-speed air. Therefore, because the air flows faster on the top of the wing than below, the pressure above is lower than the pressure below the wing, and the wing driven upwards by the higher pressure below. In modern wings the low pressure above the wing creates most of the lifting force, so it isn't far from wrong to say that the wing is essentially 'sucked' upwards. (Note however that "suction" doesn't exist, because air molecules can only push upon a surface, and they never can pull.)

MY NOTES: (1996) Uh oh, wind tunnel photographs of lift-generating wings reveal a serious problem with the above description! They show that the divided parcels do not recombine at the trailing edge. Whenever an airfoil is adjusted to give lift, then the parcels of air above the wing move far faster than those below, and the lower parcels lag far behind. After the wing has passed by, the parcels remain forever divided. This has nothing to do with the wing's path lengths. This even applies to thin flat wings such as a "flying barn door." The wind tunnel experiments show that the "wing-shape" argument regarding difference in path-length is simply wrong.

Also, real-world aircraft demonstrate another fallacy. In order to create lift, must a wing have greater path length on the upper surface than on the lower? No. Thin cambered (curved) wings such as those on hang gliders and on rubberband-powered balsa gliders, have equal path length above and below, yet they generate lift. Still the air does flow faster above these wings than below. However, since there is no difference in path length, we cannot refer to path length to explain the difference in air speed above and below the thin wing. The typical "airfoil shape" explanation cannot tell us why a paper airplane can fly, because it does not tell us why the air above the paper wing moves faster.

It is also a fallacy that in order to create lift, a wing *must* be more curved on top. In fact, wings which are designed for high speed and aerobatics are symmetrical streamlined shapes, with equal curvature above and below. Some exotic airfoil shapes are even flat on top and more curved on the bottom! (NASA's "supercritical" wing designs, for example.)

If the typical "popular" or "airfoil-shape" explanation is correct, then how can symmetrical wings and thin cambered wings work at all? How can rubberband balsa gliders work? Those who support the "path length" explanation will sometimes suggest that some other method must be used to explain these particular wings. But if so, why then do so many books put forth only the above "popular" explanation as the single explanation of aerodynamic lift? Why do they avoid detailing or even mentioning any other important explanations of lifting force?

The cloth aircraft of old had single-layer wings having identical path length above and below. If the "Wing-shape" or "popular" explanation is correct and path-length is very important, how can the Wrights' flyer have worked at all? Conversely, we do find that thin airfoils such as the Wrights' have faster flow on the upper surface than the lower surface. Since the path lengths are identical, how can we explain this?

The above "path length" viewpoint would predict that the addition of a lump to the top of a wing should always increase the lift (since it increases the upper surface path length.) In fact, the addition of a lump does not increase lift. This suggests that there is a problem with the "airfoil shape" explanation of lift.

Forces on sailboat sails are explained using the typical "pathlength/wingshape" explanation above. But sailboat sails are thin cloth membranes with identical path-lengths on either side. Why should air on either side of a sail have different velocities if the path length is the same?

Children have experience with rubberband-powered balsa wood aircraft having wings composed of a single flat layer of very thin wood. Paper airplanes usually have flat thin wings. These aircraft cannot fly? How can the "path length" version explain their successful operation?

Regardless of the angle of attack, if a wing does not deflect air downwards, it creates no lift at all. To say otherwise would go against the law of Conservation of Momentum. Yet those who believe in the "airfoil-shape" explanation commonly state that wings operate only by pressure, and Newton's laws are unimportant. This is a direct violation of basic physics principles. Bernoulli's equation incorporates basic physics, and anyone who depart from Newton must automatically depart from Bernoulli as well. Besides buoyancy and helium balloons, the only way to remain aloft is to take some matter and accelerate it downwards. The downward force applied to the matter is equal to the upward force applied by the matter against the craft. Rockets work like this, as do ship propellers, jet engines, helicopters, ...and wings!

Some people argue that the "path length" explanation must be right, since some wings generate lift even at zero angle of attack. However, Attack-angle is determined geometrically, by drawing a line between the tip of the leading and trailing edge. This geometrically-determined attack angle can be misleading:



Small bumps on the leading edge of a blunt-nosed wing have a large effect on where the line is drawn. These bumps strongly affect the determination of "attack angle, yet these bumps may have little if any effect on the lifting forces being generated. Also, once the "zero AOA" geometry has convinced us to tilt the trailing edge downwards, inertial effects will cause the airfoils to deflect air downwards from its trailing edge more than it deflects air upwards at its leading edge. The downward tilt of the trailing edge generates significant lift even when the wing as a whole is lift even angle of attack. This type of wing may APPEAR at zero attack angle. The inertia of air causes the air to flow straight from the trailing edge of the airfoil. Because of inertia, the trailing edge of a cambered airfoil itself behaves as a tilted plane, and therefore the airfoil effectively has a positive angle which causes air to be deflected. Other cambered wings are similar; they still have a positive "effective" attack angle even when their geometrical attack angle is zero. The trailing edge, not the airfoil itself, determines lift.

Some people argue that flat wings, symmetrical aerobatic wings, Supercritical wings, and thin cloth wings do not employ the Bernoulli Effect, and these wings must instead be explained by Newton and attack angle. But as mentioned above, if jet fighters and the Wright Flyer use Attack Angle rather than Bernoulli Effect, why do the books teach only Bernoulli Effect? At the very least, these books are ignoring an entire class of aircraft by never mentioning Attack Angle. However, even these thin wings and symmetrical wings exhibit the full-blown Bernoulli principle! There is a large difference in speed between the upper and lower air streams along flat wings. If a flat sheet of plywood is tilted into the air stream, the air flows faster above the sheet than below, the divided parcels never rejoin, and lift is generated by the pressure difference. But the flat sheet also deflects the air, and just as much lift is generated by deflection of air. In fact, 100% of aerodynamic lift can be explained by pressure forces and the Bernoulli principle. And 100% of lift can be explained by F=mA and Newton's third law. They are two different ways of explaining a single event. However, any appeals to differences in path length are simply wrong, and any book which uses that explanation is acting to spread science misconceptions. An alternate explanation of lift: "ATTACK ANGLE"

As air flows over a wing, the flow adheres to the surfaces of the wing. This is called flow-attachment, also the "Coanda effect." Because the wing is tilted, the air is deflected downwards as it moves over the wing's surfaces. Air which flows below the wing is pushed downwards by the wing surface, and because the wing pushes down on the air, the air must push upwards on the wing, creating a lifting force. Air which flows over the upper surface of the wing is adhering to the surface also. The wing "pulls downwards" on the air as it flows over the tilted wing and off the trailing edge, and so the air pulls upwards on the wing, creating more lifting force. (Actually the air follows the wing because of reduced pressure, the "pull" is not really an attraction.) The lifting force is created by Newton's Third Law and by conservation of momentum, as the flowing air which has mass is deflected downward as the wing moves forward. Because of Coanda Effect, the upper surface of the wing actually deflects more air than does the lower surface.

My notes on "attack angle":

If you understand the "attack angle" explanation, then the causes of other aircraft phenomena such as wingtip vortex will suddenly become clear. The air at the trailing edge of the wing is streaming downwards into the surrounding still air. The edge of this mass of air curls up as the air moves downwards, creating the "wingtip vortex." A similar effect can be seen when a drop of dye falls into clear water: the edge of the mass of dye curls up as the dye forces itself downwards into the water, resulting in a ring vortex which moves downwards.

There is one major error associated with the "attack angle" explanation. This is the idea that only the LOWER surface of the wing can generate a lifting force. Some people imagine that air bounces off the bottom of the tilted wing, and they come to the mistaken belief that this is the main source of the lifting force. Even Newton himself apparantly made this mistake, and so overestimated the necessary size of man-lifting craft. In reality, air is deflected by both the upper and the lower surfaces of the wing, with the major part being deflected by the upper surface.

Because a large, heavy aircraft must deflect an enormous amount of air downwards, people standing under a low-flying aircraft are, after a short delay, subjected to a huge downblast of air. They are essentially feeling a portion of the pressure which supports the plane. Imagine standing below a helicopter that hovers a few tens of yards above the ground. Enormous downwash? Now imagine that helicopter flying along at 150mph, or imagine the blades detaching and flying away perpendicular to travel, like wings, and you end up with the usual physics of fixed-wing aircraft. All aircraft wings are essentially sucking in air from all directions and flinging it downwards. This fact gets lost when the aircraft moves horizontally much faster than its downwash moves vertically. Some people even come to believe that wings don't deflect air at all, or leave air moving downwards after the aircraft has passed by.

The downwash can be useful: when a cropduster flies low over a field, the spray is injected into the airflow coming from the wings. Rather than trailing straight back behind the craft, the spray is sent downwards by the downwash from the wings. Also, during takeoff the downwash interacts with the ground and causes lift to greatly increase. Pilots often use this effect to gain a large airspeed just after takeoff. Because of downwash "ground effect," their engine needs to do much less work in keeping their aircraft aloft, therefore the extra power available can be used to speed up the plane.

To create adequate lift at extremely low speeds, an airfoil must be operated at a large angle of attack, and this leads to airflow detachment from wing's the upper surface (stall.) To prevent this, the airfoil must be carefully shaped. A good low- speed airfoil is much more curved on the top, since lift can be created only if the wing surface carefully deflects air downwards by adhesion. Thus one origin of the misconception involving "more curved upper surface." The surface must be curved to prevent stall, not to create lift but to avoid losing lift. The situation with the lower surface is different, since the lower surface can deflect the air by collision. Even so, it makes sense to have the lower surface be somewhat concave, so that the air is slowly deflected as it proceeds along, and so the upwards pressure is distributed uniformly over the lower surface.

Why does flowing air adhere to the upper surface of the wing? This is called flow-attachment, also "the Coanda effect." Apparently Dr. Bernoulli has a better PR department than Dr. Coanda, (grin!), since everyone has heard of Bernoulli, while Coanda is rarely mentioned in textbooks.

The only correct part of the "wingshape/pathlength" explanation of lift is the description of the Bernoulli effect itself. But the "Bernoulli Effect" can also be interpreted thus: because the wing is tilted, it creates a pocket of reduced pressure behind its upper surface. Air must rush into this pocket. And at the tilted lower surface, air collides with the surface and creates a region of increased pressure. Any air which approaches the high pressure region is slowed down. Therefore, the pressure is the cause of the air velocity, not vice-versa as in the "airfoil-shape" explanation above. Also, it is wrong to imagine that the low pressure above the wing is caused by the "Bernoulli effect" while the high pressure below the wings is not. Both pressure variations have similar origin, but opposite values.

The "airfoil shape" explanation could be very useful in calculating the lifting force of an airfoil. Knowing the fluid velocity at all points on the airfoil surface, the pressure may be calculated via Bernoulli's equation at all points, and if the pressure at each point is vector summed, the total lifting force upon the wing will be obtained. The trick then is knowing how to obtain the fluid velocities. Appeals to differences in pathlength do not work, so other methods (circulation and Kutta condition) must be used.

