Because lift is a force, it is a vector quantity , having both a magnitude and a direction associated with it.
Lift acts through the center of pressure of the object and is directed perpendicular to the flow direction. There are several factors which affect the magnitude of lift. There are many explanations for the generation of lift found in encyclopedias, in basic physics textbooks, and on Web sites. Unfortunately, many of the explanations are misleading and incorrect. Theories on the generation of lift have become a source of great controversy and a topic for heated arguments.
To help you understand lift and its origins, a series of pages will describe the various theories and how some of the popular theories fail. Lift occurs when a moving flow of gas is turned by a solid object. When extended, the flaps increase the deflection of the air and provide greater lift for takeoff and landing.
As it flies, a plane is in the center of four forces. Lift upward force and thrust forward push, provided by a propeller get a plane into the air. Gravity and drag air resistance, which is friction caused by air rubbing against the plane try to pull the plane down and slow its speed. A plane must be built so that lift and thrust are stronger than the pull of gravity and drag by just the right amount. This approach exists not on the level of numbers and equations but rather on the level of concepts and principles that are familiar and intelligible to nonspecialists.
It is on this second, nontechnical level where the controversies lie. Two different theories are commonly proposed to explain lift, and advocates on both sides argue their viewpoints in articles, in books and online.
The problem is that each of these two nontechnical theories is correct in itself. But neither produces a complete explanation of lift, one that provides a full accounting of all the basic forces, factors and physical conditions governing aerodynamic lift, with no issues left dangling, unexplained or unknown.
Does such a theory even exist? Bernoulli came from a family of mathematicians. In other words, the theorem does not say how the higher velocity above the wing came about to begin with. There are plenty of bad explanations for the higher velocity. Because the top parcel travels farther than the lower parcel in a given amount of time, it must go faster. The fallacy here is that there is no physical reason that the two parcels must reach the trailing edge simultaneously.
And indeed, they do not: the empirical fact is that the air atop moves much faster than the equal transit time theory could account for. It involves holding a sheet of paper horizontally at your mouth and blowing across the curved top of it. The page rises, supposedly illustrating the Bernoulli effect.
The opposite result ought to occur when you blow across the bottom of the sheet: the velocity of the moving air below it should pull the page downward. Instead, paradoxically, the page rises. On a highway, when two or more lanes of traffic merge into one, the cars involved do not go faster; there is instead a mass slowdown and possibly even a traffic jam.
That lower pressure, added to the force of gravity, should have the overall effect of pulling the plane downward rather than holding it up. Moreover, aircraft with symmetrical airfoils, with equal curvature on the top and bottom—or even with flat top and bottom surfaces—are also capable of flying inverted, so long as the airfoil meets the oncoming wind at an appropriate angle of attack.
The theory states that a wing keeps an airplane up by pushing the air down. The Newtonian account applies to wings of any shape, curved or flat, symmetrical or not. It holds for aircraft flying inverted or right-side up. The forces at work are also familiar from ordinary experience—for example, when you stick your hand out of a moving car and tilt it upward, the air is deflected downward, and your hand rises.
But taken by itself, the principle of action and reaction also fails to explain the lower pressure atop the wing, which exists in that region irrespective of whether the airfoil is cambered. It is only when an airplane lands and comes to a halt that the region of lower pressure atop the wing disappears, returns to ambient pressure, and becomes the same at both top and bottom.
But as long as a plane is flying, that region of lower pressure is an inescapable element of aerodynamic lift, and it must be explained. Neither Bernoulli nor Newton was consciously trying to explain what holds aircraft up, of course, because they lived long before the actual development of mechanical flight. Their respective laws and theories were merely repurposed once the Wright brothers flew, making it a serious and pressing business for scientists to understand aerodynamic lift.
Most of these theoretical accounts came from Europe. All things that fly need air. Air has power to push and pull on the birds, balloons, kites and planes. In , Evagelista Torricelli discovered that air has weight. When experimenting with measuring mercury, he discovered that air put pressure on the mercury.
Francesco Lana used this discovery to begin to plan for an airship in the late s. He drew an airship on paper that used the idea that air has weight. The ship was a hollow sphere which would have the air taken out of it. Once the air was removed, the sphere would have less weight and would be able to float up into the air.
Each of four spheres would be attached to a boat-like structure and then the whole machine would float. The actual design was never tried. Hot air expands and spreads out and it becomes lighter than cool air. When a balloon is full of hot air it rises up because the hot air expands inside the balloon. When the hot air cools and is let out of the balloon the balloon comes back down. Airplane wings are shaped to make air move faster over the top of the wing. When air moves faster, the pressure of the air decreases.
So the pressure on the top of the wing is less than the pressure on the bottom of the wing. The difference in pressure creates a force on the wing that lifts the wing up into the air. Here is a simple computer simulation that you can use to explore how wings make lift. Laws of Motion Sir Isaac Newton proposed three laws of motion in These Laws of Motion help to explain how a planes flies. If an object is not moving, it will not start moving by itself.
If an object is moving, it will not stop or change direction unless something pushes it. When an object is pushed in one direction, there is always a resistance of the same size in the opposite direction. How does a plane fly? Let's pretend that our arms are wings.
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