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Generation of lift on a wing
How do planes fly you might ask? Planes (and even birds) fly through the generation of lift. Lift is defined as the force that occurs due to an imbalance of pressures (or pressure gradient). How do these imbalances of pressures occur? Keep on reading to find out! Pressure in air is dependent on several things such as the air density (rho) and temperature (T, usually expressed in Kelvins). Also, the pressure can be viewed as the summation of two types of pressure, static and dynamic. Static pressure being the force per square area exerted by the air onto any material whereas the dynamic pressure is the pressure associated with the flow. Flow here means the flow of air, which includes but is not limited to the velocity of the flow. As the flow speed increases so does the dynamic pressure. This dynamic pressure (often referred to as Q) is related as .5*rho*S*V² (where S is the wing surface area and V is the velocity of the flow, or of the wing). An important rule is that the sum of the dynamic pressure and static pressure is relatively constant within the flow. Immediately one can see that as the dynamic pressure increases, the static pressure must decrease! Now that we understand that the dynamic pressure increases with the speed of the flow we can look into what this means for a plane. Planes utilize airfoils which are shaped fatter in the front and usually curve upward on both top and bottom sides. It is this shape that forces air above the wing section. When more air is forced over the wing, the air speeds up, and as we already learned, as the air speeds up the dynamic pressure increases causes a decrease in static pressure. So what happens on the underside of the wing? Basically the exact opposite happens! On the underside of the wing the flow speed decreases as it expands. So we have a decrease in dynamic pressure and an increase in static pressure. If we recall from earlier that the static pressure is what causes that lovely pressure gradient that it is safe to say that it is higher on the bottom and lower on top, causing an upwards force (known as lift) on the surface of the wing. And that's how planes fly! |
Here you can see the pressure differences between top and bottom.
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