| 摘要 |
<p>An airplane is a fixed-wing aircraft that is propelled forward by thrust from a jet or propeller engine. Modern airliners have two wings, typically with an engine mounted below each wing. A key measurement of the wing's aerodynamic quality is its lift-to-drag (L/D) ratio: the amount of lift generated by a wing divided by the drag it creates by moving through the air. A higher ratio is one of the major goals in aircraft design; since a particular aircraft's required lift is set by its weight, delivering that lift with lower drag leads directly to better fuel economy, climb performance, and glide ratio. The wings are subject to the stress and requirement of having to lift the aircraft frame while burdened with the weight of the jet or propeller engine. Some heavy aircraft have more than two engines, and some smaller commercial aircraft place their engines on the rear fuselage (rather than on the wings). The wing's angle-of-attack impacts the drag it places on the fuselage. By varying the angle, the pilot can affect an airplane's cruising, turning, ascent, descent, and braking parameters. The the angle of the wing to the oncoming air that gives zero lift is defined to be zero degrees. If one then changes the angle of attack both up and down one finds that the lift is proportional to the angle. Furthermore, the lift from wings is proportional to their area, so the heavier aircraft the bigger that wing area must be. The area is the product of the tip-to-tip wingspan times the width (mean chord) of the wing. For efficient steady flight the ratio of span to chord (aspect ratio) should be as high as possible. For more lift the wing can divert more air (mass) or increase its vertical velocity or a combination of the two. This vertical velocity behind the wing is the "downwash". The greater the angle of attack of the wing, the greater the vertical velocity of the air and the greater the speed of the wing the greater the vertical velocity of the air. The increase in the speed and the increase of the angle of attack give the wing lift. The LD and aspect ratios intertwine to determine a particular airplane's flight characteristics. A higher lift-to-drag ratio and optimum aspect ratio suggests that a significantly smaller thrust is required to propel the wings through the air at sufficient lift. Wings also have spoilers or flaps that assist in braking. Typically, these flaps are deployed by the pilot to increase drag on runway braking. There are many types of wings, but common to all is that they are directly affixed to the fuselage and have as their primary intent to assist the aircraft frame through a number of flight and braking manoeuvers: a) single wings; b) one-and-a-half wings in what is sometimes called a sesquiplane where one wing is significantly smaller than the other either in span, chord or both; c) biplane wings (two set of wings) connected to the fuselage either directly above and below each other, or in some cases staggered; and, d) multi-wing of 3 or more and like a biplane, connected to the fuselage either directly above and below each other, or in some cases, staggered. This design was part of the aircraft evolution in the early 1900's, with the German tri-fokker being a well-known model. Each of 'b', 'c' and 'd' above would apply to smaller, and not large commercial, aircraft. Wings can be of various shapes; a generic photo is below. It is contemplated that the min-wing would be targeted to the commercial swept-back wings, and the mini-wing could be swept-back or straight depending on the particular aerodynamics of the wing it is affixed to.</p> |
