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, 104 (5), 1576-80

Biplane Wing Planform and Flight Performance of the Feathered Dinosaur Microraptor Gui

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Biplane Wing Planform and Flight Performance of the Feathered Dinosaur Microraptor Gui

Sankar Chatterjee et al. Proc Natl Acad Sci U S A.

Abstract

Microraptor gui, a four-winged dromaeosaur from the Early Cretaceous of China, provides strong evidence for an arboreal-gliding origin of avian flight. It possessed asymmetric flight feathers not only on the manus but also on the pes. A previously published reconstruction shows that the hindwing of Microraptor supported by a laterally extended leg would have formed a second pair of wings in tetrapteryx fashion. However, this wing design conflicts with known theropod limb joints that entail a parasagittal posture of the hindlimb. Here, we offer an alternative planform of the hindwing of Microraptor that is concordant with its feather orientation for producing lift and normal theropod hindlimb posture. In this reconstruction, the wings of Microraptor could have resembled a staggered biplane configuration during flight, where the forewing formed the dorsal wing and the metatarsal wing formed the ventral one. The contour feathers on the tibia were positioned posteriorly, oriented in a vertical plane for streamlining that would reduce the drag considerably. Leg feathers are present in many fossil dromaeosaurs, early birds, and living raptors, and they play an important role in flight during catching and carrying prey. A computer simulation of the flight performance of Microraptor suggests that its biplane wings were adapted for undulatory "phugoid" gliding between trees, where the horizontal feathered tail offered additional lift and stability and controlled pitch. Like the Wright 1903 Flyer, Microraptor, a gliding relative of early birds, took to the air with two sets of wings.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
Feathers of M. gui. (A) Holotype of M. gui [Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) V13352] as preserved [modified from Xu et al. (12)]. (Scale bar, 5 cm.) (B) The long feathers on the hand and metatarsal sections had evolved for flight; they were asymmetric with interlocking barbules. (C) In the rest of the wing and hindleg, the feathers are symmetric (10).
Fig. 2.
Fig. 2.
Wing planform of Microraptor. (A–D) Different possible hindlimb postures during flight. (A) Hindlimb backwardly directed as in modern birds. (B–D) Biplane configuration. (B) Hindlimb backwardly sloping position. (C) Hindlimb forwardly sloping in predatory strike position. (D) Hindlimb in z-fashion with a body silhouette showing the animal in lateral view with an upwardly tilted tail for pitch control. (E) Cross-section of the tibia–fibula showing a streamlining and stretching effect of the cylindrical tibia by adding feathers caudally. (F) Cylindrical structure offers maximum resistance to the airstream as the airflow behind it becomes broken up into eddies, creating turbulence. (G) Filling the spaces in a cylindrical structure in front and behind improves streamlining, as in the case of the feathered tibia of raptors. (H) Pouncing posture of a raptor, Falco. (I) A typical staggered biplane (Stearman 75) for comparison with Microraptor; in biplane aircraft of the 1920s, there was a large additional drag of wires, struts, etc. between the two wings, which eventually made the biplane obsolete except for a niche application; such drag-induced structures were absent in Microraptor. (J) Life reconstruction of M. gui (IVPP V13352) in dorsal view showing the morphology and distribution of hindlimb feathers (Left) and orientation of the hindlimb (Right) during gliding, based on Fig. 1 A; proximal feathers on the humerus and femur are inferred (data are from ref. 12). (Scale bar, 5 cm.)
Fig. 3.
Fig. 3.
Flight performance of Microraptor. (A) Power curves (steady level flight) for Microraptor. The horizontal line represents the estimated maximum continuous power available. Two curves are shown for the level flight power required. Curve 1 is from streamtube theory (20), and curve 2 is based on the simpler aircraft theory (22). They converge for speeds of greater than ≈6 m/s. (B) Glide polars for Microraptor, compared with a seabird (frigatebird, M = 1.5 kg) and a pterosaur (Nyctosaurus, M = 1.85 kg) (ref. ; see Table 1 for aerodynamic data). (C) Glide paths of Microraptor from a perch. Curve 1 shows phugoid gliding. Curve 2 shows a final rapid pitchup with high drag. Curve 3 shows gliding path with pitch damper on. Curve 4 shows a parachuting trajectory.
Fig. 4.
Fig. 4.
A simple cladogram of eumaniraptoran theropods showing the distribution of leg feathers in selected taxa (modified from refs. , , and 26). In Microraptor, the contour feathers are present on the femur, tibia, and metatarsus, but only the metatarsal feathers are asymmetric and form the ventral wing of the biplane design; the feathers on the femur and tibia are symmetric (12). In Pedopenna, long metatarsal feathers are present to form the ventral wing of the biplane layout, but they appear to be symmetrical (25). In Archaeopteryx, long contour feathers are present on the femur and tibia, but they appear to be lost on the metatarsus (15). At this stage, the evolution of monoplane design probably took place. In an unnamed enantiornithine bird, long contour feathers are present on the femur and tibia but absent in the metatarsal region (26). In modern raptors such as the falcon Falco, similar contour leg feathers persist on the femur and tibia for streamlining, but metatarsal feathers are generally reduced or absent.

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