|MLA Citation:||Bloomfield, Louis A. "Airplanes" How Everything Works 19 Jan 2018. Page 3 of 3. 19 Jan 2018 <http://www.howeverythingworks.org/prints.php?topic=airplanes&page=3>.|
In the other view, the blades "fly" through the fluid like the wings of an airplane. The fluid flow around each blade is such that the pressure is higher on one side of the blade than the other and the blade experiences a net force toward the lower pressure side. The blades move in the direction of this force, so the passing fluid does work on them—energy is transferred from the fluid to the blades.
These two views are completely equivalent. The fluid leaves the turbine blades traveling more slowly or at lower pressure, and it acquires a rotation in the direction opposite the turbine's rotation.
Another important view of flight involves air pressure in the streams of air flowing over and under the plane. When the air passing under the wing curves downward, it actually does so because the pressure just under the wing is higher than the pressure far from the wing—the air stream is experiencing an overall downward force due to this pressure imbalance and this downward force is deflecting the air stream downward. When the air passing over the wing arcs up, over, and down, it is also doing so because the pressure just above the wing is different from that far from the wing. In this case, the pressure just over the wing's leading edge is quite high—enough to deflect the air stream upward initially. But the pressure over the rest of the wing's upper surface is very low and the air stream curves inward toward the wing; arcing downward so that it leaves the wing's trailing edge with a small downward component to its motion. Overall, there is a low average pressure above the wing and a high average pressure below it. This pressure imbalance produces an overall upward force on the wing and supports the plane's weight.
These two views of flight—one involving deflection of the air stream and the other involving pressure imbalances—are intimately related to one another and really only two descriptions of the same process. Incidentally, the low pressure just over most the wing causes the air flowing over that wing to speed up. That's Bernoulli's equation in action—when air following a streamline experiences a drop in pressure, it accelerates in the forward direction.
The Newtonian perspective is the most intuitive and where we will start. The airstream arriving at the forward or "leading" edge of the airplane wing splits into two separate flows that travel over and under the wing, respectively. The wing is shaped and tilted so that these two flows experience very different accelerations as they travel around the wing. The flow that goes under the wing encounters a downward sloping surface that pushes it downward and it accelerates downward. In response to this downward push, the air pushes upward on the bottom of the wing and provides part of the force that supports the plane.
The air that flows over the wing follows a more complicated route. At first, this flow encounters an upward sloping surface that pushes it upward and it accelerates upward. In response to this upward force, the air pushes downward on the leading portion of the wing's top surface. But the wing's top surface is curved so that it soon begins to slope downward rather than upward. When this happens, the airflow must accelerate downward to stay in contact with it. A suction effect appears, in which the rear or "trailing" portion of the wing's top surface sucks downward on the air and the air sucks upward on it in response. This upward suction force more than balances the downward force at the leading edge of the wing so that the air flowing over the wing provides an overall upward force on the wing.
Since both of these air flows produce upward forces on the wing, they act together to support the airplane's weight. The air passing both under and over the wings is deflected downward and the plane remains suspended.
In the Bernoullian view, air flowing around a wing's sloping surfaces experiences changes in speed and pressure that lead to an overall upward force on the wing. The fact that each speed change is accompanied by a pressure change is the result of a conservation of energy in air passing a stationary surface—when the air's speed and motional energy increase, the air's pressure and pressure energy must decrease to compensate. In short, when air flowing around the wing speeds up, its pressure drops and when it slows down, its pressure rises.
When air going under the wing encounters the downward sloping bottom surface, it slows down. As a result, the air's pressure rises and it exerts a strong upward force on the wing. But when air going over the wing encounters the up and down sloping top surface, it slows down and then speeds up. As a result, the air's pressure first rises and then drops dramatically, and it exerts a very weak overall downward force on the wing. Because the upward force on the bottom of the wing is much stronger than the downward force on the top of the wing, there is an upward overall pressure force on the wing. This upward force can be strong enough to support the weight of the airplane.
But despite the apparent differences between these two descriptions of airplane flight, they are completely equivalent. The upward pressure force of the Bernoullian perspective is exactly the same as the upward reaction force of the Newtonian perspective. They are simply two ways of looking at the force produced by deflecting an airstream, a force known as lift.
But before fuel is injected into the engine's compressed air, there is a side duct that allows some of that compressed air to flow toward the passenger cabin. So the engine is providing the air you breathe during a flight.
There is one last interesting point about this compressed air: It is initially too hot to breathe. Even though air at 30,000 feet is extremely cold, the act of compressing it causes its temperature to rise substantially. This happens because compressing air takes energy and that energy must go somewhere in the end. It goes into the thermal energy of the air and raises the air's temperature. Thus the compressed air from the engines must be cooled by air conditioners before it goes into the passenger cabin.
But the airstream that goes over the wing has a complicated trip. First it encounters the leading edge of the wing and is pushed upward and forward. This air slows somewhat and its pressure increases. So far, this upper airstream isn't helpful to the plane because it pushes the plane backward. But the airstream then follows the curving upper surface of the wing because of a phenomenon known as the Coanda effect. The Coanda effect is a common behavior in fluids—viscosity and friction keep them flowing along surfaces as long as they don't have to turn too quickly. (The next time your coffee dribbles down the side of the pitcher when you poured too slowly, blame it on the Coanda effect.)
Because of the Coanda effect, the upper airstream now has to bend inward to follow the wing's upper surface. This inward bending involves an inward acceleration that requires an inward force. That force appears as the result of a pressure imbalance between the ambient pressure far above the wing and a reduced pressure at the top surface of the wing. The Coanda effect is the result (i.e. air follows the wing's top surface) but air pressure is the means to achieve that result (i.e. a low pressure region must form above the wing in order for the airstream to arc inward and follow the plane's top surface).
The low pressure region above the wing helps to support the plane because it allows air pressure below the wing to be more effective at lifting the wing. But this low pressure also causes the upper airstream to accelerate. With more pressure behind it than in front of it, the airstream accelerates—it's pushed forward by the pressure imbalance. Of course, the low pressure region doesn't last forever and the upper airstream has to decelerate as it approaches the wing's trailing edge—a complicated process that produces a small amount of turbulence on even the most carefully designed wing.
In short, the curvature of the upper airstream gives rise to a drop in air pressure above the wing and the drop in air pressure above the wing causes a temporary increase in the speed of the upper airstream as it passes over much of the wing.
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