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. YES AND NO.
  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 with equal curvature top and bottom.  The lifting force
     does not vanish if an airplane flys upside-down.  Explanations for
     flight involve other things, and not airfoil asymmetry.

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 100% 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 100% of their lift because of the Bernoulli 
     Equation.  This is true because 100% of the lifting force comes from
     pressure differences on the wings' surfaces.

     In fact, we can explain the lifting force by "Newton," by ignoring 
     the pressure differences and instead measuring the dense deflected 
     air and calculating the change in momentum.

     And of course we can explain 100% of the lifting force 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.  YES AND NO.

     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."

     Static wings are allowed to have all sorts of crazy airfoil shapes, 
     but if they don't have a downwards-tilted trailing edge which is 
     sharp, they won't lift an airplane.

     Other features of wing-shape 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: angle of attack is critical, but wing shape
     has effects too.  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.

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

    Incorrect, since wind tunnel experiments and aerodynamic math will both
    show that the upper and lower air flows do not recombine.  See these 
    wind-tunnel photos which illustrate this lack of recombining.  Also 
    see the NASA Site which debunks this widespread fallacy.

6. 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.

9. 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.
     Both the upper and lower surfaces create the lifting force.

     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.  Also, the pressure below the wing APPEARS
     to provide less lift.  This happens because a thick wing must push 
     air out of its way, and as the flowing air splits up and down to make
     a space for the oncoming wing, air below the wing takes a straighter 
     path.  It takes a straighter path because the thickness of the wing
     bends air upwards at the same time as the tilt of the wing bends air
     downwards.  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 AS IT WAS FOR A THIN WING.  The thick wing is 
     making us confused.  The thick wing SEEMS to get more lift from the 
     curved streamlines above the wing than from the straight streamlines 
     below, but this is an illusion.  The thick wing distorts the stream-
     lines.  Examine the streamlines surrounding a thin wing to see the 
     truth.  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 lifting 

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" 

- 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 '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!)

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