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HOW A WING WORKS
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The main goal of a wing is to create a force that is equal to the weight of the aircraft but pointing in the opposite direction, so trust the wing can lift the aircraft's weight. A wing flying through air creates this force by causing lower air pressure above the wing and higher pressure under the wing.
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The wrong explanation of how a wing works
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The popular description of how a wing creates lift
Is often something like this: "Because the top of the wing is curved and the bottom is flat, the distance on top is longer and the air on top has to go faster. Fast air has less pressure, so the slower air on the bottom pushes the wing up, and this is the lift force that carries the weigh of the aircaft.
What is wrong about this explanation:
- Air doesn't have to go faster because of a longer path, there is no law in nature that states that air volumes have to reunite after being separated. In fact: the air doesn't reunite at the trailing edge of a wing.
- There is no such thing as fast air. Speed is a relative thing and not a property of mass. As fast air doesn't exists, there must be an other cause of the reduced pressure on top of a wing.
Some examples of airfoils that do or do not produce lifting thrust:
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Both sides have the same distance
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Both sides have the same distance
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A longer distance on top
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Both sides have the same distance
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A really longer distance on top
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A typical airfoil
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A typical airfoil, upside down
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A wing flying up side down can produce upward thrust to lift the craft!
(not very well, but it works) |
How (I think) a wing does work
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Imagine a flat & extremely thin wing, having a speed of say 200 km/h relative to the ambient air, and zero angle of attack. This "airfoil" obviously produces no upward thrust. There is something happening though: drag. The air and the surface interact where they touch; the boundary layer. The air and the surface kind of stick together on an atomic level. This causes both the airfoil and the air to accelerate, in opposite directions. The air becomes turbulent by this boundary layer acceleration, and the wing slowes down.
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Now, if the angle of attack is made positive, the air coming under the wing (from the left) runs into a conflict with the wing. The wing is solid and the air is a fluid. As these two push on each other, the air will become compressed but the solid surface will not (not noticeable). Air will flow away from a higher pressure area to any area of less pressure, which in this case is mainly downwards. The wing is pushed upwards and a bit backwards by the compressed air.
At the same time, the flat wing's solid body leaves a void behind, a vacuum space on top & behind it. Vacuum space has an air pressure of zero point nothing. The ambient air has a pressure of about 1 KG per square centimeters, that is about 10.000 KG per square meter! Air flows from a higher to a lower pressure area, so it will travel to this vacuum with great acceleration! The "information" about air pressure travels through the air with the speed of sound, about 340 M/S. Air weighs only about 1,2 KG per cubic meter, so the vacuum is filled really fast. As long as the wing moves through the air (or the air to the wing), a vacuum is created, and the pressure on top of the wing will be lower .
Both processes are continuously, so this wing with keep having higher pressure on the bottom and lower pressure on top.
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An extremely thin wing isn't realistic, because a wing's surface needs structural support, and there should be room in the wing for all kinds of stuff. Placing the support at the front bottom is the only good option, because it doesn't enlarge the frontal area. But the higher pressure area is now only in front of the wing, which is a major drag, a great force in the wrong direction.
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Air flows curved, not corner wise. It (like all mass) "doesn't want" to accelerate. The shape of the airfoil should be friendly to the air. If the airfoil is hard on the air, the air will resist, give "air resistance".
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The result of these few logic steps brought us to a typical airfoil shape.
One more step for now: the Coanda effect.
The air in front of the airfoil is being pushed away, thus of higher pressure. Air wants to flow from higher to lower pressure. The higher pressure air from the top-front can accelerate to the lower pressure area on the rear-top of the wing's airfoil. This can give a strong Coanda effect on the top-front of the wing. The Coanda effect will reduce the air pressure a lot. By lowering the front of the airfoil, more air will shoot over the top, increasing the Coanda effect. Read more about the Coanda effect here.
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An illustration of an airfoil, showing (imaginationary-) air parcels around it.
What a wing does to the ambient air
The air on top of a wing is accelerated backward and downward.
The air on the bottom of a wing is accelerated a bit downward and a bit forward.
In front of the wing is: higher pressure
On top of the wing is: lower pressure
On the bottom of the wing is: higher pressure
Looking at it in a Newtonian way: the wing accelerates air-mass mainly downwards, this action force on the inertia of mass gives a reaction force upwards: thrust (used for lift). (Inertia = the slowness of mass, not wanting to accelerate)
Looking at it from an air-pressure point of view: a wing creates a continues implosion on top (lower pressure), and with a higher than normal angle of attack it also creates a continues explosion on the bottom (higher pressure).
Extra information:
At the leading edge (the front of an airfoil), the air has to move away and is therefore of higher presence. This pressure "information" is "communicated" forwards by a pressure wave traveling through the air with the speed of sound (about 340 m/s). When flying with an airspeed of 330 m/s, the pressure wave from the leading edge is traveling forward with only 10 m/s (relative to the wing). This causes an enormous build-up of pressure; nearing the sound-barrier. The leading edge of most aircraft is round because the angle of attack must vary, and it's aerodynamic. A razor sharp leading edge prevents most pressure build-up, but can only be used with one angle of attack. A delta wing with a sharp leading edge can create vortex-lift, but that is an other story!.
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