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Given a constant load factor, which of these statements about the variation of induced drag with changes of speed and mass are correct or incorrect?

1. When speed decreases, induced drag increases.
2. When mass decreases, induced drag increases.

  • A
    1 is correct, 2 is incorrect.
  • B
    1 is incorrect, 2 is correct.
  • C
    1 is incorrect, 2 is incorrect.
  • D
    1 is correct, 2 is correct.

Induced drag is created as a byproduct of lift. The pressure differential between the top and bottom of the wing creates a upwash around the tip of the wing that turns into a downwash behind the wing. This is known as a vortex, which creates a rearward force known as induced drag.

This question is asking for the effect of:

  • Speed: The induced drag is inversely proportional to the speed. Therefore, induced drag increases as the speed decreases, and, induced drag decreases and the speed increases.
This is because, to maintain lift constant, as the velocity decreases, the coefficient of lift will have to be increased. At higher coefficients of lift, the pressure differential between the top and bottom of the wing will be greater and therefore produce stronger vortices = stronger induced drag. (and vice versa)
  • Changing mass: Induced drag is proportional to the mass. Therefore, induced drag increases with increasing aeroplane mass, and, induced drag decreases with decreasing aeroplane mass.

This is because, as the aircraft get heavier, the lift that the wings will have to produce increases. To achieve that, the coefficient of lift will have to increase, meaning a higher AoA, which creates stronger vortices. (and vice versa)

Summary:

  • Higher AR = Lower induced drag
  • Higher AoA = higher induced drag
  • Higher weight = higher AoA = higher induced drag
  • Higher speed = lower AoA = lower induced drag
  • Winglets = lower induced drag.

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