The total drag on an airfoil can be estimated bywhere D = drag, σ = ratio of air density b...

The total drag on an airfoil can be estimated by

where D = drag, σ = ratio of air density between the flight altitude and sea level, W = weight, and V = velocity. As seen in Fig. P7.35, the two factors contributing to drag are affected differently as velocity increases. Whereas friction drag increases with velocity, the drag due to lift decreases. The combination of the two factors leads to a minimum drag.

(a) If σ = 0.6 and W = 16,000, determine the minimum drag and the velocity at which it occurs.

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(b) In addition, develop a sensitivity analysis to determine how this optimum varies in response to a range of W = 12,000 to 20,000 with σ = 0.6.

FIGURE P7.35: Plot of drag versus velocity for an airfoil.