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In aeronautics, the aspect ratio of a wing is the ratio of its span to its aerodynamic breadth or chord. A long, narrow wing has a high aspect ratio, whereas a short, stubby wing has a low aspect ratio.〔Kermode, A.C. (1972), ''Mechanics of Flight'', Chapter 3, (p.103, eighth edition), Pitman Publishing Limited, London ISBN 0-273-31623-0〕 For a constant-chord wing of chord ''c'' and span ''b'', the aspect ratio is given by: : If the wing is swept, ''c'' is measured parallel to the direction of forward flight. For most wings the length of the chord is not a constant but varies along the wing, so the aspect ratio ''AR'' is defined as the square of the wingspan ''b'' divided by the wing area ''S''.〔Anderson, John D. Jr, ''Introduction to Flight'', Equation 5.26〕〔Clancy, L.J., ''Aerodynamics'', sub-section 5.13(f)〕 In symbols, :. For such a wing with varying chord, the standard mean chord ''SMC'' is defined as : ==Aspect ratio of aircraft wings== Aspect ratio and other features of the planform can be used to predict the aerodynamic performance of a wing. For a given wing area, the aspect ratio, which is proportional to the square of the wingspan, is of particular significance in determining the performance. Roughly, an airplane in flight can be imagined to affect a circular cylinder of air with a diameter equal to the wingspan.〔Clancy, L.J., ''Aerodynamics'', section 5.15〕 A large wingspan is working on a ''large'' cylinder of air, and a small wingspan is working on a ''small'' cylinder of air. For two aircraft of the same weight but different wingspans the small cylinder of air must be pushed downward by a greater amount than the large cylinder in order to produce an equal upward force. The aft-leaning component of this change in velocity is proportional to the induced drag''. Therefore the larger downward velocity produces a larger aft-leaning component and this leads to larger induced drag on the aircraft with the smaller wingspan and lower aspect ratio. The interaction between undisturbed air outside the circular cylinder of air, and the downward-moving cylinder of air occurs at the wingtips and can be seen as wingtip vortices. This property of aspect ratio AR is illustrated in the formula used to calculate the drag coefficient of an aircraft 〔Anderson, John D. Jr, ''Introduction to Flight'', section 5.14〕〔Clancy, L.J., ''Aerodynamics'', sub-equation 5.8〕〔Anderson, John D. Jr, ''Fundamentals of Aerodynamics'', Equation 5.63 (4th edition)〕 : where : There are several reasons why not ''all'' aircraft have high aspect wings: * Structural: A long wing has higher bending stress for a given load than a short one and therefore requires higher structural-design (architectural and/or material) specifications. Also, longer wings may have some torsion for a given load, and in some applications this torsion is undesirable (e.g. if the warped wing interferes with aileron effect). * Maneuverability: a low aspect-ratio wing will have a higher roll angular acceleration than one of high aspect ratio, because a high-aspect-ratio wing has a higher moment of inertia to overcome. In a steady roll, the longer wing gives a higher roll moment because of the longer moment arm of the aileron. Low aspect ratio wings are usually used on fighter aircraft, not only for the higher roll rates, but especially for longer chord and thinner airfoils involved in supersonic flight. * Parasitic drag: While high aspect wings create less induced drag, they have greater parasitic drag, (drag due to shape, frontal area, and surface friction). This is because, for an equal wing ''area'', the average chord (length in the direction of wind travel over the wing) is smaller. Due to the effects of Reynolds number, the value of the section drag coefficient is an inverse logarithmic function of the characteristic length of the surface, which means that, even if two wings of the same area are flying at equal speeds and equal angles of attack, the section drag coefficient is slightly higher on the wing with the smaller chord. However, this variation is very small when compared to the variation in induced drag with changing wingspan. For example,〔Dommasch, D.O., Sherby, S.S., and Connolly, T.F. (1961), ''Airplane Aerodynamics'', page 128, Pitman Publishing Corp. New York〕 the section drag coefficient of a NACA 23012 airfoil (at typical lift coefficients) is inversely proportional to chord length to the power 0.129: A 20 percent increase in chord length would decrease the section drag coefficient by 2.38 percent. * Practicality: low aspect ratios have a greater useful internal volume, since the maximum thickness is greater, which can be used to house the fuel tanks, retractable landing gear and other systems. * Airfield Size: Airfields, hangars and other ground equipment define a maximum wingspan, which cannot be exceeded, and to generate enough lift at the given wingspan, the aircraft designer has to lower the aspect-ratio and increase the total wing area. This limits the Airbus A380 to 80m wide with an aspect ratio of 7.8, while some Boeing 777 have an aspect ratio of 9.0, influencing flight economy.〔Hamilton, Scott. "(Updating the A380: the prospect of a neo version and what’s involved )" Leehamnews.com, 3 February 2014. Accessed: 21 June 2014. (Archived ) on 8 April 2014.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Aspect ratio (aeronautics)」の詳細全文を読む スポンサード リンク
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