TOP  |  
MISCONS  |  
GOOD STUFF  |  
SEARCH

Airfoil Lifting Force Misconception
Widespread in K-6 Textbooks

William Beaty 1996
º Misconceptions
º Articles Here
º GIF Diagrams
º Links
º Refs



Beat around in the underbrush of aerodynamics and you'll encounter an interesting question:

HOW DO AIRPLANE WINGS *REALLY* WORK?

Amazingly enough, this question is still argued in many places, from elementary school classrooms all the way up to major pilot schools, and even in the engineering departments of major aircraft companies. This is unexpected, since we would assume that aircraft physics was completely explored early this century. Obviously the answers must be spelled out in detail in numerous old dusty aerodynamics texts. However, this is not quite the case. Those old texts contain the details of the math, but it's the *interpretation* of the math that causes the controversy. There is an ongoing Religious War over both the way we should understand the functioning of wings, and over the way we should explain them in children's textbooks. The two sides of the controversy are:

  • The physics explanation, NEWTONIAN or ATTACK ANGLE: wings are forced upwards because they are tilted and they deflect air. A wing's trailing edge must be sharp, and it must be aimed diagonally downward if it's to create lift. Both the upper and lower surfaces of the wing act to deflect the air. The upper surface deflects air downwards because the airflow "sticks" to the wing surface and follows the tilted wing (this phenomena is called "Coanda effect" or "Flow Attachment.") After the wing has passed by, air remains flowing downwards. Airplanes fly because of Newton's 3rd law (action/reaction forces,) the law of Conservation of Momentum, and the Coanda effect.
  • The popular explanation, PATH-LENGTH or AIRFOIL-SHAPE: wings do not deflect air, instead they are sucked upwards because the "airfoil" shape has a longer surface on top. Airfoils are curved on top and flat below, and therefore the air follows a longer path above than below. Air that is divided at the leading edge of a wing must rejoin at the trailing edge. Since the upper surface of the wing is longer, it causes the air to flow faster over the upper surface, which (by Bernoulli's principle) creates lower pressure above. Because lift is caused by the shape of the wing, wings can create lift at zero attack angle. They can create lift simply from path length difference which leads to pressure difference, and no air needs to be deflected. After a wing has passed by, the air does not remain moving downwards. (THIS EXPLANATION IS SERIOUSLY FLAWED.)
This webpage is biased towards the ATTACK ANGLE view. See just about any K-6 science book for the PATH LENGTH argument. (Note that the controversy extends far beyond grade school, and even some pilot training manuals contain the discredited "path length" explanation.)

Also three other explanations of lift exist: the circulation explanation, the flow-turning or streamline-curvature explanation, and the vortex-based explanation. These three appear in advanced textbooks, where they form the basis of the mathematics used by aircraft designers. The misleading "popular" or "airfoil-shape" explanation commonly appears in children's science books, magazine articles, and in pilot's textbooks. On the other hand, the public rarely or never encounters explanations based upon circulation, upon curvature of streamlines, or upon Newton's Laws.

A possible solution to the controversy:

Billb's Airplane Flight Analogy
http://amasci.com/wing/rotbal.html
Note well: Newton and Bernoulli do not contradict each other. Explanations which are based on Newton's and on Bernoulli's principles are completely compatible. Each explanation describes 100% of the lifting force. For the most part they're just two different ways of simplifying a single complicated subject. Much of the controversy arises because one side or the other insists that only *THEIR* view is correct. They insist that only a *SINGLE* explanation is possible, and the opposing view is therefore wrong. In other words, which is the single best way to crack an egg? This is a war between the Big-endians and Little-endians from "Gulliver's Travels." They simply refuse to acknowledge that there are several valid yet independant approaches to solving the problem, and they insist that their version must be the single right answer.

However, there are also several serious mistakes usually associated with the "popular" explanation described above. People who believe the "popular" explanation wrongly insist that any parcels of air divided by the wing's leading edge must meet again at the trailing edge. This is incorrect. They also believe that wings don't deflect the oncoming air downwards. Also incorrect. These and several other mistakes commonly appear in elementary science texts, as well as in popular articles on aircraft physics. These mistakes change the popular "airfoil-shape" explanation into a system of misconceptions. I explore these below.

Also, those who firmly adhere to the popular explanation have been successful in convincing many authors that there can only be a single best method for explaining aerodynamic lift, and that the "Airfoil-shape" method is far better than the "Attack-angle" method. I strongly disagree with this, and believe that the correct versions of both explanations should be in constant use. Since the Newton method gives a better intuitive grasp of the issues, that method is more appropriate for elementary explanations aimed at the public and for introductory material for science students and pilots. On the other hand, the "Airfoil Shape" explanation is less intuitive, yet it dovetails very well with lifting force calculations, so it is very useful in mathematical modeling, for physics students, for aircraft design, fluid flow simulation software, etc.


The Truth shall set your free... But first it will piss you off!
-Anon






















Bill B.'s "Newtonian" Articles & Misc.Controversy


The "Airfoil Mistake" in the news

       

Discussions, message threads

Relevant forums

BOOKS

Other websites



"Newtonian" Lifting Force References

Cliff Swartz, "Numbers Count", editorial in THE PHYSICS TEACHER, p536, Vol34, Dec 1996

Gale Craig, NEWTONIAN AERODYNAMICS FUNDAMENTALS, 1995, Regenerative Press, Anderson Indiana 46011, ISBN: 0964680602

Prof. Klaus Weltner, AERODYNAMIC LIFTING FORCE, The Physics Teacher (magazine), Feb 1990, pp78-82

K. Weltner, BERNOULLI'S LAW AND AERODYNAMIC LIFTING FORCE, The Physics Teacher, Feb 1990, pp84-86

K. Weltner, A COMPARISON OF EXPLANATIONS OF THE AERODYNAMIC LIFTING FORCE, Am. J. of Physics, 55 (1) Jan. 1987 pp50-54

Langewiesche, Wolfgang, STICK AND RUDDER, 1975 Tab Books, ISBN: 0070362408

N.H. Fletcher, MECHANICS OF FLIGHT, Physics Education, Wiley, NY 11975, pp385-389

HOW AIRPLANES FLY


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:

[GIF: add an off-center bump, then tilt the wing to

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, inertial effects will cause a thin, curved airfoil to deflect air downwards from its trailing edge more than it deflects air upwards at its leading edge, and the unequal deflection generates lift even at zero angle of attack. This type of wing may APPEAR to have zero attack angle, but 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 of zero.

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 I said before, 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 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, 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 the Bernoulli principle. And 100% of lift can be explained by 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 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 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 
	subjected to a huge downblast of air.  They are essentially feeling 
	a portion of the pressure which supports the plane.

	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 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. 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 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.

Parts of the Airfoil Misconception

  1. Wings create lift because they are curved on top and flat on the bottom. Incorrect.
  2. Part of the lifting force is due to Bernoulli effect, and part is due to Newton's 2nd law. Incorrect.
  3. To produce lift, the shape of the wing is critical. Incorrect.
  4. The Bernoulli effect pertains to the shape of the wing, while Newton's laws pertain to the angle of attack. Incorrect.
  5. Air which is divided by the leading edge must recombine at the trailing edge. Incorrect.
  6. The upper surface of an airfoil must be longer than the lower surface. Incorrect.
  7. The tilt of the wing produces part of the lift. The shape of the wing produces the rest. Incorrect.
  8. A wing is really just the lower half of a venturi tube. Incorrect.
  9. The upper surface of a wing will deflect air, but the lower surface is horizontal, so it has little effect. Incorrect.
  10. Airfoils need not deflect any air; pressure differences alone can produce lift. Incorrect.
  11. Ship propellors, rudders, rowboat oars, and helicopter blades all deflect water or air. But airplane wings are entirely different. NOPE.
  12. The "Coanda effect" only applies to thin liquid jets, not to airfoils and flow attachment. Incorrect.
  13. An airfoil can create lift even at zero attack angle. Misleading.
  14. Cambered airfoils create lift at zero AOA, which proves that the "Newtonian" theory of lift is wrong. Incorrect.
  15. The "Newtonian" theory of lift is wrong because downwash happens far behind the wing where it can have no effect. Incorrect.


1. Wings create lift because they are curved on top and flat on the
   bottom.   INCORRECT.

     Incorrect because only some wings look like that, while other wings are
     symmetrical (they're the same on top and bottom,) while still others 
     are flat on top  ...and curved on the bottom!   And don't forget the 
     hang-gliders and the Wright Brothers' flyer, both of which used thin 
     fabric wings.



2. Part of the lifting force is due to Bernoulli effect, and part is due 
   to Newton.   INCORRECT

     Incorrect because ALL wings, regardless of shape or degree of tilt,
     must create ALL of their lift because of Newton.  To say otherwise 
     would mean that a wing could violate Newton's Laws!  Yet at the same 
     time, ALL wings create ALL of their lift because of the Bernoulli 
     Equation.  This is true because all of the lifting force comes from
     pressure differences on the wings' surfaces.

     In fact, one hundred percent of the lifting force can be explained
     by "Newton," by ignoring the pressure differences and instead 
     measuring the deflected air and calculating the change in momentum.

     And of course 100% of the lifting force can also be explained by
     "Bernoulli", by looking at air speeds and then calculating the air 
     pressure on every part of the wing surface.  See the NASA site.



3. To produce lift, the shape of the wing is critical.  INCORRECT.

     Incorrect because aerodynamic scientists have found that there are
     two critical features of all airfoils:  the trailing edge of the wing
     must be fairly sharp, and the trailing edge of the wing must be 
     angled downwards.  This is discussed in advanced textbooks in the
     chapters on circulatory flow, in the section on "Kutta Condition."

     Wings are allowed to have all sorts of crazy airfoil, but if they 
     don't have a downwards-tilted trailing edge which is sharp, they 
     won't produce much lift.

     Other featues of the wing are important but not critical.  For
     example, in order to prevent "stall" the leading edge of the wing
     must be fairly bulbous and the wing's upper surface must lack
     sharp curves as well as being fairly smooth (no bumpy screws or
     rivets allowed.)  If the wing's leading edge is too sharp, or if
     its upper surface is made wrong, then the flow of air above the wing 
     will break loose or "detach," and it will no longer be guided 
     downwards by the upper surface.  This problem is called a "stall," 
     and during a stall the amount of lifting force contributed by the 
     upper wing surface becomes very small.



4. The Bernoulli effect pertains to the shape of the wing, while 
   Newton's laws pertain to the angle of attack.  INCORRECT.

     Incorrect because Newton's laws pertain to all features of the wing; 
     both to wing shape and attack angle.   Exactly the same thing is true 
     of Bernoulli's equation.  Wings don't violate Newton's laws, and
     wings in conventional flight (slower than the speed of sound) don't
     violate Bernoulli's equation.  See #2 above.




     Incorrect because aerodynamic scientists have found that there are


  There are even some wings which
     

3. Flat thin wings generate lift entirely because of Newton; because they
   are tilted, while thick curved wings generate lift exclusively because 
   of "Bernoulli Effect?"  INCORRECT.

Think a moment: if a wing
  when a flat thin wing is given a positive angle of attack,
    the air above the wing speeds up, and the air below the wing slows
    down.  100 percent of the lifting force can be explained using 
    either the "Bernoulli effect" or the Newton/Coanda principles.
    These two simply are a pair of alternate viewpoints on the same
    situation, and it's wrong to try to break the lifting force 
    into a separate percentage of "Bernoulli" force and an "attack angle" 
    force.


longer path
http://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.html



- In order to generate lift, the upper surface of an airfoil must be more
  strongly curved than the lower surface?  INCORRECT

   Incorrect, since lift can be generated by symmetrical airfoil such as 
   those used on acrobatic aircraft.  Lift can also be generated by
   thin fabric airfoils, by sheets of paper (paper airplanes), by tilted
   pieces of flat plywood, or by "supercritical" airfoils which are more
   curved on the BOTTOM than the top.



5.Air which is divided by the leading edge must recombine at the 
trailing edge.  INCORRECT.

    Incorrect, since mathematical models and wind tunnel experiments both
    show that the upper and lower air flows do not recombine.  See these 
    wind-tunnel photos which illustrate this 
    lack of recombining.








- Asymmetrical airfoils produce lift because of their special shape, while 
  symmetrical airfoils produce lift because they are tilted?  INCORRECT.

- A symmetrical airfoil cannot create lift? INCORRECT


- Aircraft cannot fly upside down?  INCORRECT


- The decreased pressure above an airfoil creates much more lifting force
  than the increased pressure below the airfoil.  Since the decreased
  pressure above is supposedly caused by the Bernoulli effect, while the
  increased pressure below is supposedly caused by collision of air with
  the tilted wing, the "Bernoulli effect" supplies the lift.  Therefore 
  the "angle of attack" effects are of less importance and can be ignored 
  in order to simplify the explanation?  INCORRECT.

    Incorrect, because both the increased pressure below the airfoil and
    the decreased pressure above are created entirely by the Bernoulli
    effect.  ALSO, both are caused by the angle of attack and the forces
    resulting from the deflection of massive air.  100% of the lifting
    force can be explained by appeals to the Bernoulli effect.  But also
    100% of the lifting force can be explain by the process of deflection
    of air by the wing.  However, explaining the difference in air speed
    above and below the wing is not straightforward.


- The low pressure above an airfoil produces suction.  The lifting force
  is an upwards suction force. INCORRECT.

    Incorrect.  Air molecules produce pressure upon a surface by colliding
    with that surface.  They do not attract that surface.  In other words,
    SUCTION DOES NOT EXIST.  When you suck air through a straw,
    you are lowering the pressure within the straw.  There is no suction.
    Instead, the outside atmosphere PUSHES the air into the straw.
    So, while it is true that the pressure above the wing is low, it is
    not true that the lifting force is caused by suction.  Instead, the
    lifting force is caused by the pressure-difference.  If the pressure
    above the wing should fall, then the ambient pressure below the wing
    will force the airplane to move upwards.


- The air in front of the leading edge of an airfoil and the air behind
  the trailing edge are moving at zero degrees deflection?  INCORRECT.

    Incorrect, since with a real aircraft, the air moves slightly upwards
    to meet the leading edge of the wing, but then it is projected greatly
    downwards from the trailing edge, creating a "downwash" flow.
    Although the "upwash" equals the "downwash" in a 2-dimensional wind 
    tunnel experiment, this is not true in practice with real airplanes. 
    (2D wind tunnels depict ground-effect flight, not normal flight.)
    With a real airplane flying high above the earth, if the "upwash" and
    the "downwash" flows were equal, yet the lifting force was non-
    zero, then this would totally violate the law of conservation of
    momentum.  Unfortunately for the "airfoil-shape" camp, fundamental
    physics principles must be satisfied, and Newton's laws are not
    selectively violated by airfoils. In order to create an upwards
    lifting force, there must be a net downward acceleration of parcels of
    air.  Planes fly by pushing air downwards, which creates a pressure
    difference across a wing.  Air-deflection and pressure are linked.  
    You cannot have one without the other.


- Airplane propellors, rudders, jet turbine blades, and helicopters all 
  function by deflecting air to create force.  They throw the air one way,
  and the air pushes them the other way.  But airplane wings are 
  different? Wings operate by a separate kind of physics, and are "sucked 
  upwards" by the Bernoulli effect?  INCORRECT.

    Incorrect, because the real world cannot tell the difference between 
    an airplane wing and a helicopter blade.  It does not know that a
    ship's rudder and an airplane wing are different.  Wings, rudders, 
    propellors, oars;  all these devices work by identical principles: 
    they throw massive fluid one way, and are thrown the other way by 
    action/reaction forces.  Bernoulli's equation does have bearing, since 
    the action/reaction forces express themselves as a pressure difference 
    across the surfaces of the object which deflects the fluid.


- An airfoil can generate lift without deflecting air downward? INCORRECT.

    Incorrect. If it did so, it would be staying in the air without
    ejecting mass downwards, and this would violate the Conservation
    of Momentum law.  Yes, balloons remain aloft without ejecting mass,
    but balloons function via bouyancy forces, and an airplane wing
    obviously does not.  Think about it: a helicopter hovers because it
    throws air downwards.  Yet a 'copter blade is simply a moving wing!
    If wings did not fling air downwards, if wings remained aloft only
    through pressure differences, then helicopter blades would do the
    same, and there would be no downblast below a helicopter.


- An airfoil can generate a lifting force without causing a reaction 
  force against the air?  INCORRECT.

    Incorrect.  If it did so, it would violate Newton's Third Law of
    Motion, the law of equal action and reaction forces.


- The majority of textbooks use the popular 'path length' or 'airfoil
  shape' explanation of lift, and it is inconceivable that this many books
  could be wrong.  Therefore, the "path length" explanation is the 
  correct one?  INCORRECT.

    Incorrect, this argument from authority is simply wrong.  It is also
    dangerous, since it convinces us to never question authority and to 
    close our eyes to authors' errors.  If we trust the concensus 
    agreements of others, then we become sheep which follow a leaderless 
    herd.  Beware of this habit!  As the NASA space shuttle managers who 
    closed their eyes to the Challanger booster seal problem found out, 
    the real world is all too real.  Nature ignores politics, and 
    scientific facts are determined by evidence, not by majority votes.




?. The upper surface of a wing will deflect air, but the lower surface is 
   horizontal, so it has little effect. INCORRECT.

     Incorrect, but for an interesting reason.  If a thin flat wing
     deflects air downwards, diagrams show that the air above the wing and 
     the air below the wing are equally deflected.  If we then make this 
     wing thicker and streamlined, the total amount of deflected air and 
     the lifting force remain the same...  but the air below the wing
     appears less deflected, and the air above the wing appears more
     deflected.  This happens because a thick wing must push air out
     of its way, and as the flowing air curves away from the oncoming
     wing, it takes a straighter path in the region below the wing.
     This has no effect on the lifting force, since the air above the
     wing takes a more curved path, so the pressure difference remains
     the same.  The thick wing SEEMS to get more lift from the curved
     streamlines above than from the straight streamlines below, but 
     this is an illusion.  The lift comes from the DIFFERENCE between
     the two flows, and changing the thickness of the wing will alter
     the appearance of the air flows without changing the difference or
     changing the lifthing force.




- The 'Coanda effect' only involves narrow jets of air, and has little to
  do with airfoil operation, so its exclusion from explanations of lift is
  understandable and justified?  INCORRECT.

    Incorrect, the Coanda effect involves the adhesion of a flow to a
    surface.  It applies to ANY flowing fluid, not just to narrow jets. 
    If the airflow across a wing did not adhere to the wing, the wing
    would be permanently in the 'stall' regiem of operation.  During
    "stall", it would not deflect air across its upper surface, and it
    would produce a greatly diminished lifting force.



- There are two explanations of airfoil lifting force: angle of attack, and
  pressure differential.  The 'pressure differential' explanation is correct,
  and the 'angle of attack' is misleading and can be ignored?  INCORRECT.

    Incorrect.  Both explanations are useful once the incorrect parts of
    the "path length" explanation have been removed.  They are two
    different "mental models," they are two different ways of looking at
    one complicated situation.  Paraphrasing the physicist R. Feynman:
    "Unless you have several different ways of looking at something, you
    don't really understand it."  A complete understanding requires that
    we easily shift between alternate viewpoints.  Wings really do
    produce lift when velocity differences create a vertically-
    directed pressure differential across their surface area.  But also,
    they really do produce lift by reacting against air and driving it
    downwards.  Unfortunately the airfoil-shape-based explanation has 
    become connected with several incorrect add-on explanations; the
    "path-length" fallacy for example.



- An airfoil can generate lift at zero angle of attack?  MISLEADING

    Not entirely wrong: depending on how we define 'angle of 
    attack', a wing may be at zero angle of attack even though it 
    obviously *acts* tilted and deflects the oncoming air downwards.
    This is a fight between semantics and reality.  If the rear portion of
    a wing is tilted downwards and deflects the air downwards, shouldn't
    it by definition have a positive angle of attack?
    No, not if 'angle of attack' is measured by drawing a line between the
    tips of the leading and trailing edges of the wing crossection.  If
    the leading edge is bulbous, then small details on the leading edge
    can radically change the location of the drawn line without radically
    changing the interaction of the wing with the air.  If such a wing is
    then rotated to force it to take a "zero" angle, that rotation in
    reality tilts the wing to a positive attack angle and generates lift.



- Cambered airfoils produce lift at zero AOA, which proves that the
  "Newton" explanation is wrong?  INCORRECT

    Incorrect.  Air has mass, and this means that it has inertia.  Because
    of inertia, an exhaust port can produce a narrow jet of air, yet an
    intake port cannot pull a narrow jet inwards from a distance.  This
    concept applies to wings.  When a cambered airfoil moves forwards at
    zero AOA (Angle of Attack,) air moves up towards the leading edge, and
    air also flows downwards off of the trailing edge.  The air which
    flows downwards behind the wing keeps moving downwards, and so the
    rear half of the wing controls the angle of the downwash, while the
    leading edge has little effect.  (In aerodynamics, this is called the 
    "Kutta Condition.")  In a cambered wing at zero AOA, the rear half of 
    the wing behaves as an airfoil with positive AOA.  On the whole, the 
    cambered airfoil BEHAVES as if it has a positive AOA, even though the 
    geometrical angle of attack is zero.



- A properly shaped airfoil gives increased lift because the air on the 
  upper surface moves faster than the air on the lower?  MISLEADING

    Not entirely wrong.  This is only half the story.  A properly
    shaped airfoil gives increased lift because the airflow does not
    easily "detach" from the upper surface, so the upper airflow can 
    generate lift even at large angles of attack and at low aircraft 
    speeds.  A sheet of plywood makes a poor wing because the airflow will 
    "detach" from the upper surface of the wood when the sheet is tilted 
    more than a tiny bit. This is called "stall", and it causes the upper 
    surface of the wing to stop contributing a lifting force.  A properly 
    designed wing must spread the net deflection of air widely across its 
    upper leading surface rather than concentrating all the deflection at 
    its leading edge.  Hence, the upper surfaces of most wings are 
    designed with the curvature which avoids immediate flow-detachment and 
    stall.  The shape of wings does not create lift, instead it only
    avoids stall.



- The "Newton" explanation is wrong because downwash occurs BEHIND the 
  wing, where it can have no effects?  Downwash can't generate a lifting
  force?   INCORRECT.

    Wrong, and silly as well!  The above statement caught fire on the
    sci.physics newsgroup.  Think for a moment: the exhaust from a rocket
    or a jet engine occurs BEHIND the engine.  Does this mean that
    action/reaction does not apply to jets and rockets?  Of course not.
    It's true that the exhaust stream doesn't directly push on the inner
    surface of a rocket engine.  The lifting force in rockets is caused
    by acceleration of mass, and within the exhaust plume the mass
    is no longer accelerating.  In rocket engines, the lifting force 
    appears in the same place that the exhaust is given high velocity:
    where gases interact inside the engine.

    And with aircraft, the lifting force appears in the same place that
    the exhaust (the downwash) is given high downwards velocity.  If a 
    wing encounters some unmoving air, and the wing then throws the air 
    downwards, the velocity of the air has been changed, and the wing will 
    experience an upwards reaction force.  At the same time, a downwash- 
    flow is created.  To calculate the lifting force of a rocket engine, 
    we can look exclusively at the exhaust velocity and mass, but this 
    doesn't mean that the rocket exhaust creates lift.  It just means that
    the rocket exhaust is directly proportional to lift (since the exhaust
    velocity and the lifting force have a common origin.)  The same is 
    true with airplane wings and downwash.   To have lift at high 
    altitudes, we MUST have downwash, and if we double the downwash, we 
    double the lifting force.  But downwash doesn't cause lift, instead 
    the wing's interaction with the air both creates a lifting force and 
    gives the air a downwards velocity (by F=MA, don't you know!)





http://amasci.com/wing/airfoil.html
Created and maintained by Bill Beaty. Mail me at: billb@eskimo.com.
If you are using Lynx, type "c" to email.