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NOTE: The two extremities of the airfoil profile also differ in appearance. The end, which faces forward in flight, is called
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igure 2-6. Airfoil designs.Double wedge airfoil(Supersonic)Circular arc airfoil(Supersonic)Laminar flow airfoil(Subsonic)Clark 'Y' airfoil(Subsonic)Later airfoilEarly airfoil
Figure 3-7. Airfoil designs.
C
amber of upper surfaceCamber of lower surfaceTrailing edgeLeading edgeMean camber lineChord line
Figure 3-6. Typical airfoil section.
the leading edge, and is rounded; the other end, the trailing edge, is quite narrow and tapered.
A reference line often used in discussing the airfoil is the chord line, a straight line drawn through the profile connecting the extremities of the leading and trailing edges. The distance from this chord line to the upper and lower surfaces of the wing denotes the magnitude of the upper and lower camber at any point. Another reference line, drawn from the leading edge to the trailing edge, is the mean camber line. This mean line is equidistant at all points from the upper and lower surfaces.
An airfoil is constructed in such a way that its shape takes advantage of the air’s response to certain physical laws. This develops two actions from the air mass: a positive pressure lifting action from the air mass below the wing, and a negative pressure lifting action from lowered pressure above the wing.
As the air stream strikes the relatively flat lower surface of a wing or rotor blade when inclined at a small angle to its direction of motion, the air is forced to rebound downward, causing an upward reaction in positive lift. At the same time, the air stream striking the upper curved section of the leading edge is deflected upward. An airfoil is shaped to cause an action on the air, and forces air downward, which provides an equal reaction from the air, forcing the airfoil upward. If a wing is constructed in such form that it causes a lift force greater than the weight of the aircraft, the aircraft will fly.
If all the lift required were obtained merely from the deflection of air by the lower surface of the wing, an aircraft would only need a flat wing like a kite. However, the balance of the lift needed to support the aircraft comes from the flow of air above the wing. Herein lies the key to flight.
It is neither accurate nor useful to assign specific values to the percentage of lift generated by the upper surface of an airfoil versus that generated by the lower surface. These are not constant values and vary, not only with flight conditions, but also with different wing designs.
Different airfoils have different flight characteristics. Many thousands of airfoils have been tested in wind tunnels and in actual flight, but no one airfoil has been found that satisfies every flight requirement. The weight, speed, and purpose of each aircraft dictate the shape of its airfoil. The most efficient airfoil for producing the greatest lift is one that has a concave, or “scooped out” lower surface. As a fixed design, this type of airfoil sacrifices too much speed while producing lift and is not suitable for high-speed flight. Advancements in engineering have made it possible for today’s high-speed jets to take advantage of the concave airfoil’s high lift characteristics. Leading edge (Kreuger) flaps and trailing edge (Fowler) flaps, when extended from the basic wing structure, literally change the airfoil shape into the classic concave form, thereby generating much greater lift during slow flight conditions.
On the other hand, an airfoil that is perfectly streamlined and offers little wind resistance sometimes does not have enough lifting power to take the airplane off the ground. Thus, modern airplanes have airfoils that strike a medium between extremes in design. The shape varies according to the needs of the airplane for which it is designed. Figure 3-7 shows some of the more common airfoil sections.
Low Pressure Above
In a wind tunnel or in flight, an airfoil is simply a streamlined object inserted into a moving stream of air. If the airfoil profile were in the shape of a teardrop, the speed and the pressure changes of the air passing over the top and bottom would be the same on both sides. But if the teardrop shaped airfoil were cut in half lengthwise, a form resembling the basic airfoil
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igure 2-8. Pressure distribution on an airfoil & CP changes with an angle of attack.High angle of attackNormal angle of attackLow angle of attack attackofAngle -8°CP attackAngle of +10°CP attackofAngle +4°CP
Figure 3-8. Pressure distribution on an airfoil and CP changes with AOA.travel is very important, since it affects the position of the air loads imposed on the wing structure in both low and high AOA conditions. An airplane’s aerodynamic balance and controllability are governed by changes in the CP.
(wing) section would result. If the airfoil were then inclined so the airflow strikes it at an angle (angle of attack (AOA)), the air moving over the upper surface would be forced to move faster than the air moving along the bottom of the airfoil. This increased velocity reduces the pressure above the airfoil.
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Pilot's Handbook of Aeronautical Knowledge飞行员航空知识手册(40)
