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Effect of wing loading on the drag curve
If we change the wing area of the aircraft while keeping the weight constant we change its wing loading (aircraft weight/wing area). The effect of this is shown in Fig. 7.5 where it can be seen that the result of an increase in wing loading is to move the drag curve to the right of the picture without altering the drag values.
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Fig. 7.5 Effect of wing loading and altitude on drag Increase in altitude or wing loading moves drag curve to the right |
The explanation of this is quite simple. At any point, A, in Fig. 7.5 the lift (equal to the weight of the aircraft) is given (Chapter 1) by the lift coefficient multiplied by the wing area and the dynamic pressure (-pV2). Assume that we reduce the wing area. If only the size of the wing is changed but its geometrical shape and angle of attack remain the same, the lift coefficient will be unaltered. We can then obtain the same lift force by increasing the speed to raise the dynamic pressure, so compensating for the area reduction.
The drag coefficient will also be unchanged for our smaller wing. Therefore, since the product of dynamic pressure and wing area is unaltered, the drag force will also be unchanged. Point A is therefore simply moved horizontally to A’ (Fig. 7.5) and the entire curve is shifted as shown and somewhat spread out, the minimum drag retaining the same value as before.
At this point it is worth pointing out that our argument is somewhat approximate. Unless the fuselage and tail assembly are scaled in the same way as the wing, the drag coefficient will be changed as we go from point A to point A’ (Fig. 7.5). Thus some change in the minimum drag value will be obtained.
A further factor we have ignored is that a change in the size of the wing will also require a change in the weight of the structure, so our assumption of constant weight is questionable, particularly if we consider large changes in wing area.
However, it remains generally true that increasing the wing loading means that the aircraft can fly faster with little penalty in terms of increased drag. This has led to an increase in wing loadings for many aircraft types, and this is further described in Chapter 9. It must be remembered that any such increase in wing loading will mean a higher minimum flying speed, and so a compromise must be reached between requirements for cruise and landing and take-off performance.
