The aerodynamic center of any airfoil will be immediately aft of the point of maximum thickness on a cambered wing, this will be on the top side, usually well forward of the center point. This means they will produce exactly zero lift at zero AOA, and require some angle to produce lift.įinally, to respond to the true-or-false questions about your two statements, it would depend on each airfoil's curvature. On the other hand, symmetrical wings (airfoils) have no aerodynamic camber, but rather have equal distances for the air to travel over both the top and bottom surfaces. If you inverted the airfoil, so the curved surface was on the bottom, there would be negative lift (downward pressure) at zero degrees angle of attack. The result of all this head-spinning aerodynamics is that the pressure directly on the bottom (at right angles to) surface of the airfoil is higher than that on the top surface, resulting in aerodynamic lift on that wing (airfoil), even at zero degrees angle of attack. Since Total air pressure = Static (directly onto the airfoil) Pressure plus Dynamic Pressure (speed of the air), and the Dynamic pressure (speed) on the top is higher, that means to balance the total pressure, the static pressure on the top must be lower. This means air on the top surface flows at a higher relative speed. The result of this is that air passing over the top surface of the airfoil has a longer distance to travel than air passing over the bottom surface. Adding slots between segments will make those work even better than a solid airfoil and allow to use more camber.A cambered, or "airfoil-shaped" wing cross section will have a significant curve (bulge) on the top surface, usually with the thickest part nearer the leading edge, while the bottom surface will have no or minimum curve. Again, as soon as a lot of lift at low speed is needed, thin, highly cambered airfoils are the best choice. Note that aircraft with a high wing loading use extensive and extensible high lift devices which turn their wings into thin, highly cambered structures for landing. This is similar to the use on propellers: A wider operating range requires to move away from the narrow optimum offered by those highly cambered airfoils. As soon as the aircraft needs to cover a wider speed range, however, a lower camber is needed to keep drag low at high speed. Those with low maximum speed like human powered or electric propulsion aircraft prefer those airfoils because they create the needed lift at the lowest possible speed, so the aircraft can fly with the limited installed power. Use on wings: Some aircraft do indeed use highly cambered airfoils. Those are more stubby and enjoy much narrower variations in flow conditions, so the highly cambered, thin airfoil is indeed the best choice here. Note that indeed thin, highly cambered airfoils are used on compressors and turbines in jet engines. In off-design points (i.e., most of the time) the propeller would have poor performance when compared with one which tolerates more diverse conditions. Of course you can pre-twist the blade so it will assume the correct shape in the desired operating point, but a propeller needs to work over a wide range of operating points, from take-off roll to high speed flight at altitude. First propeller use: A highly cambered airfoil would cause high pitching moments and twist the propeller blade.
0 Comments
Leave a Reply. |
Details
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |