Wing Area

Norberg [4] reports that there are greater variations between groups of animals in wing area than in wingspan. The departure from the geometrical relation is obvious, which is shown in Table 1.3. As for wingspan, hummingbirds have the largest deviation from the geometrical relation. They seem to have a larger wing area for a given body mass than do birds in general. Based on the varia­tion of the wing area for different groups of birds, Greenewalt [55] subdivides birds into different classes or “models.” His model offers the following correla­tions:

In general, for both human-made and natural flyers, the agility and maneuverability improve with a smaller AR. High AR wing flyers, such as an albatross and the U-2, are excellent flyers for steady-state forward flight, but are not suitable for fast course changes.

Another consideration is that the induced drag, which is caused by the lift, tends to decrease with higher AR. Obviously, the minimum induced drag is obtained with an infinitely long wing. Similarly, for steady forward flight, with a large AR, the lift-to-drag ratio (L/D) or the so-called glide ratio increases with an increasing AR. The largest AR for birds is found among species that typically spend a substantial portion of their time in soaring instead of flapping flight. A typical example is the wandering albatross (Diomeda exulans), which has an aspect ratio of about 15. As noted by Tennekes [29], the glide ratio for the wandering albatross is around 20. Human engineering can now offer a sailplane with a glide ratio of around 60.