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Homework Help: Speed of Minimum Drag

  1. Sep 13, 2010 #1
    1. The problem statement, all variables and given/known data

    An aircraft is flying straight and level at 300 kilometers per hour
    at an altitude where the density is 0.9kg/m^3. The ratio of lift to drag
    coefficients for this aircraft at this flight condition is 15. Its weight at the
    moment under consideration is 98,000 Newtons. The aspect ratio of the
    wing is 8. Wing span is 15m. Assuming spanwise efficiency of 1, what is
    the induced drag coefficient? Thus, what is the profile drag coefficient?
    What is the speed for minimum drag for the aircraft at the same altitude and weight?

    2. Relevant equations
    V = 300 km/h = 83.3 m/s
    W = 98,000 N
    L/D = 15
    AR = b^2/S so S = 28.13 m^2
    e = 1
    rho = 0.9

    3. The attempt at a solution
    Lift coefficient = W/(q*S) = 2*W/((rho)*V^2*S) = 1.12
    Induced drag = Lift coefficient^2/pi*AR*e = 0.0500
    L/D = 15 = 0.114/Drag; Drag = 0.0747
    profile drag = 0.00759-0.000515 = 0.025

    Now, I know profile drag equals induced drag for minimum drag but how do I go about finding the velocity where this occurs?
    Last edited: Sep 13, 2010
  2. jcsd
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