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| I'm sure that everyone has seen illustration 1 at some time or other. These are the four main forces that must be equal to sustain flight. If thrust is equal to drag and lift is equal to the weight of the plane, then you are in straight and level flight. If any of the forces becomes unequal then you will either be ascending or descending. |
| Airplanes use two kinds of lift. Pressure differential and deflection. Figure 2a depicts pressure differential lift. The air moving over the top of the wing has to go farther creating a lower pressure than is under the wing thus creating lift. In figure 2b the angle of attack is greater which causes a downward deflection of air. This causes a compression of air under the wing and adds to the pressure differential |
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A symmetrical airfoil wing as in figure 3 always has a positive angle of attack as it creates lift. This angle of attack is so small that it is not noticeable in flight. A symmetrical airfoil has an advantage over the flat bottom airfoil in aerobatic flight as it will perform just as well inverted as it will right side up. looking at the illustration you will see that the top portion of the airfoil looks much like that of a flat bottom airfoil and the air is moving farther across it. The bottom rear area is more aligned to the flight path and the air here doesn't have to move as far. The bottom front area is creating a compression zone because of it's higher angle of attack than the top front area. One important thing to remember is that when a wing creates lift, it also creates a proportional amount of drag. This makes it imperative that an airplane be as light as possible. If you have a 10 pound plane with a .60 in the nose, it has to fly fast and won't slow down very much. If the same plane weighs only 5 pounds, it will fly just as fast but will slow down much better.
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