Flight in slow motion: aerodynamics of the pterosaur wing

Abstract

The flight of pterosaurs and the extreme sizes of some taxa have long perplexed evolutionary biologists. Past reconstructions of flight capability were handicapped by the available aerodynamic data, which was unrepresentative of possible pterosaur wing profiles. I report wind tunnel tests on a range of possible pterosaur wing sections and quantify the likely performance for the first time. These sections have substantially higher profile drag and maximum lift coefficients than those assumed before, suggesting that large pterosaurs were aerodynamically less efficient and could fly more slowly than previously estimated. In order to achieve higher efficiency, the wing bones must be faired, which implies extensive regions of pneumatized tissue. Whether faired or not, the pterosaur wings were adapted to low-speed flight, unsuited to marine style dynamic soaring but adapted for thermal/slope soaring and controlled, low-speed landing. Because their thin-walled bones were susceptible to impact damage, slow flight would have helped to avoid injury and may have contributed to their attaining much larger sizes than fossil or extant birds. The trade-off would have been an extreme vulnerability to strong or turbulent winds both in flight and on the ground, akin to modern-day paragliders.

Figure 1.

Generalized shape of the wing of a large ornithocheirid pterosaur. Redrawn from recent reconstructions [16,17,46], and showing the locations of the wing sections that were tested and the extent of the wing membrane assumed (cross-hatched area).

Figure 2.

Wing sections that were tested. (a) High-camber (14.6%) proximal region with ulna faired and unfaired, 20% and 40% of chord from anterior margin. (b) Medium-camber (11.5%) section representing central region of the wing fitted with WP1 and WP2 phalanx sections of two different sizes (nominally with depth of 7.5% and 10% of section chord) fitted on ventral side of the wing section. The WP1 section was also tested on the dorsal side and with a small aerodynamic fairing [8]. (c) Low-camber section (8.5%) fitted with WP1 and WP2 sections. WP1 section faired according to Padian & Rayner [8] and WP2 section faired to the least extent required to minimize separation, as predicted from XFOIL analysis [47]. (d) Flexible section with three different degrees of slackness (in the unloaded condition—the actual camber increased with aerodynamic load).

Figure 3.

Typical wind tunnel test results at Re = 200 000. (a) Variation of section lift coefficient (c_l) with angle of attack for WP2 wing bone section. Open circles: results with increasing angle of attack. Filled circles: results for decreasing angle of attack. The closeness of these results demonstrates good test repeatability and absence of hysteresis effects. (b) Effect of section fairing on c_l∶c_d for the WP2 wing section. The extensive fairing significantly reduces the minimum drag coefficient but has no effect on maximum lift coefficient.

Figure 4.

Polar glide curves. (a) Comparison with results from previous studies ((i) [10]; (ii) [12]), with results from present tests (WP1 medium camber section) at two extremes of mass ((iii) and (iv)). The WP1 wing has only half the aerodynamic efficiency (L/D ratio) and almost twice the sink rate of the earlier estimates. (b) Effect of different wing bone sizes and shapes: (i) section with no bone; (ii) small WP1; (iii) large WP1; (iv) small WP2; and (v) large WP2. The WP1 phalanges move the polar curve downwards with increasing size, resulting in little change in the optimum flight speed but a large increase in the sink rate. The WP2 phalanges have similar effects, but because they increase both drag and the maximum lift coefficient, the glide polar moves a little to the left, extending the low-speed flight envelope. (c) Potential performance of an optimized wing section: (i) 417a section used in earlier reconstructions; (ii) optimized modern S1223 laminar flow aerofoil section [48]; (iii) WP2 faired with XFOIL designed fairing; and (iv) small WP2 only. The two aerofoil sections give similar peak efficiencies of 20 : 1, but the S1223 section maintains good performance to higher values of lift coefficient, shifting the polar curve to the left. (d) Effect of flexibility: (i) envelope of rigid sections with same bone depth as flexible section, and (ii)–(iv) the flexible section with increasing membrane slackness.