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I know that for rockets they often use the circular cross-section S_reference for the drag prediction: drag = rho/2 v^2 * c_D * S_reference

For aircrafts they use the chord length of a wing as S_reference.

So when I want to compare those to for their aerodynamic performance, can I just use S_reference{rocket} * C_D{rocket}

and S_reference{aircraft} * c_D{aircraft}

So to my mind: I can always use any reference area I want for determining c_D , but when I want to compare two bodys for their aerodynamic performance, I have to use: c_D * S_reference

Thanks for your opinion,

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The reference area is chosen according to the flow profile:

  • For long slender bodies, the wetted area is chosen because the main drag source is the shear forces between the boundary layer and the free flow.
  • For relatively short bodies, the cross-sectional area is most relevant because of the much larger contribution of pressure differential between front and aft of the stream.

Directly comparing $C_D$ of rockets and aeroplanes would not be very useful, because the reference area for aeroplanes is the wing area, which rockets do not have. Indeed $C_D$ * A is a much more useful comparison, if one knows what reference area A was chosen.

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