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Optimum airplane design plays a vital role in the development of efficient and effective aircraft because it primarily relies upon the best possible combination of excellent aerodynamic qualities, lightweight structure, economy of operation, etc., which satisfy required flight characteristics. Optimum design procedures are intended to reduce the dependence upon a designer's individual skill, experience, and intuition. In the present work, an attempt has been made to use a multivariate optimization technique in the preliminary aerodynamic design of a civil subsonic jet aircraft. The design problem is formulated as a constrained nonlinear optimization problem. The solution is obtained by using the complex method of M.J. Box. The necessary relations used for weight estimation, aerodynamic characteristics, engine properties, etc., are either simplified or empricial in nature. However, the program developed can easily be modified to include more accurate formulas and additional conditions if desired. The numerical exercise carried out here is mainly for illustrating the approach suggested. H A fi a b C, g H L M M cr n Po Re S Nomenclature = wing aspect ratio = ratio of length to characteristic diameter for fuselage or engine = speed of sound = wing span = ratio of fuselage diameter to square root of wing planform area =coefficient of lift of an airplane = coefficient of drag of an airplane = coefficient of airplane drag at zero lift = coefficient of drag for fuselage or engine nacelle =coefficient of profile drag for wing =coefficient of profile drag for tail = specific fuel consumption = coefficient of thrust (= 770.5 p V 2 S) -airplane drag = fuselage diameter = objective function = gravitational acceleration (= 9.81 m/s 2 ) = altitude of flight = lift acting on airplane = lapse rate of temperature =design range = cruising Mach number (= Via) = critical Mach number =load factor = design wing loading (= W 0 /S) = Reynold's number = wing planform area = summation of the frontal areas of cross section for fuselage and external engines 5, = planform area for tail plane TE = temperature, K t 0 = thickness ratio of wing V = cruising speed W t W 0 = weight of airplane at any condition and its design value W = weight of power plant W str = weight of fuel W p =payload a = throttle coefficient T? = taper ratio of wing (= root chord/tip chord) A = density ratio of air P,P 0 = density of air at any altitude and at sea level, respectively /* = coefficient of dynamic viscosity of air \l/ = coefficient representing Mach number effect on fuel consumption \>X t = wing sweep at quarter chord line and trailing edge, respectively

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