Wing Loading

Regarding wing loading, although the overall correlation shown in Eq. (1-5) seems reasonable, Greenewalt [55] found that, in many cases, the relation between wing loading and mass increases more slowly than that indicated in Eq. (1-5). For exam­ple, the three families of birds (i. e., the Passeriforms, the Shorebirds, and the Ducks) do not follow the one-third law. As indicated in Table 1.3, for hummingbirds, wing loading is almost independent of body mass; hence different species can have the same wing loading. Tennekes [29] used the data collected by Greenewalt [55] and summarized the various scaling relations for seabirds, shown in Table 1.4. All gulls and their relatives have long, slender wings and streamlined bodies, so it was rea­sonable to assume geometric similarity. From Table 1.4 it is obvious that the wing loading and cruising speed generally increase with weight.

1.2.4 Aspect Ratio

As for aircraft, the aspect ratio (AR) can give an indication of the flight characteristics for flapping animals. The AR is a relation between the wingspan b and the wing area S:

b2

AR = j. (1-7)

[4] When the AoA is increased (from 4.03° to 7.82°), the adverse pressure gra­dient on the upper surface grows, which intensifies the Tollmien-Schlichting (TS) wave, resulting in an expedited laminar-turbulent transition process. A shorter LSB leads to more airfoil surface being covered by the attached tur­bulent boundary-layer flow, resulting in a lower drag. This corresponds to the lift-drag polar’s left turn.

[5] At a lower AoA, for example, 2.75°, there is a long bubble on the airfoil surface, which leads to a large drag.

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