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.
Parts of the Airfoil Misconception
DESTROYED BY ISP CRASH IN MAY
-
Wings create lift because they are curved on top and flat on the
bottom. Incorrect.
-
Part of the lifting force is due to Bernoulli effect, and part is due
to Newton's 2nd law. Incorrect.
-
To produce lift, the shape of the wing is critical. YES AND NO.
-
The Bernoulli effect pertains to the shape of the wing, while
Newton's laws pertain to the angle of attack. Incorrect.
-
Air which is divided by the leading edge must recombine at the
trailing edge. Incorrect.
-
The upper surface of an airfoil must be longer than the lower surface. Incorrect.
-
The tilt of the wing produces part of the lift. The shape of the wing
produces the rest. Incorrect.
-
A wing is really just the lower half of a venturi tube. Incorrect.
-
The upper surface of a wing will deflect air, but the lower surface is
horizontal, so it has little effect. Incorrect.
-
Airfoils need not deflect any air; pressure differences
alone can produce lift. Incorrect.
-
Ship propellors, rudders, rowboat oars, and helicopter blades all deflect water or
air. But airplane wings are entirely different. NOPE.
-
The "Coanda effect" only applies to thin liquid jets, not to airfoils
and flow attachment. Incorrect.
-
An airfoil can create lift even at zero attack angle. Misleading.
-
Cambered airfoils create lift at zero AOA, which proves that the
"Newtonian" theory of lift is wrong. Incorrect.
-
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
force.
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.
- 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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