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An aeroplane accelerates from 80 kt to 160 kt at a load factor equal to 1. By what factors will the induced drag coefficient (i) and the induced drag (ii) change?
  • A
    (i) 1/4 (ii) 2.
  • B
    (i) 1/16 (ii) 1/4.
  • C
    (i) 1/2 (ii) 1/16.
  • D
    (i) 4 (ii) 1/2.
  • Lift formula: L = CL x ½ rho V2 x S
Rho – does not change in level flight
If V doubles => CL must decrease by a factor of 4 (V is squared), in order to keep lift constant.
  1. CDI varies with CL2. Therefore, CDI must decrease by a factor of 16 => 42 when speed doubles.
  • Induced drag formula: Induced drag = CDI x ½ rho V2 x S
  1. Unlike lift, it varies with speed in level flight. If V is doubled => induced drag decreases by a factor of 4.
However, as we have seen previously Cdi has decreased by a factor of 16.
Overall, induced drag decreases by a factor of 4/16 = 1/4
Note:
Parasite drag changes by a factor of V2
Induced drag changes by a factor of 1/V2
Induced drag coefficient changes by a factor of 1/V4

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