Model tests of gliding with different hindwing configurations in the four-winged dromaeosaurid Microraptor gui

Abstract

Fossils of the remarkable dromaeosaurid Microraptor gui and relatives clearly show well-developed flight feathers on the hind limbs as well as the front limbs. No modern vertebrate has hind limbs functioning as independent, fully developed wings; so, lacking a living example, little agreement exists on the functional morphology or likely flight configuration of the hindwing. Using a detailed reconstruction based on the actual skeleton of one individual, cast in the round, we developed light-weight, three-dimensional physical models and performed glide tests with anatomically reasonable hindwing configurations. Models were tested with hindwings abducted and extended laterally, as well as with a previously described biplane configuration. Although the hip joint requires the hindwing to have at least 20°of negative dihedral (anhedral), all configurations were quite stable gliders. Glide angles ranged from 3°to 21°with a mean estimated equilibrium angle of 13.7°, giving a lift to drag ratio of 4.1:1 and a lift coefficient of 0.64. The abducted hindwing model's equilibrium glide speed corresponds to a glide speed in the living animal of 10.6 m·s −1 . Although the biplane model glided almost as well as the other models, it was structurally deficient and required an unlikely weight distribution (very heavy head) for stable gliding. Our model with laterally abducted hindwings represents a biologically and aerodynamically reasonable configuration for this four-winged gliding animal. M. gui's feathered hindwings, although effective for gliding, would have seriously hampered terrestrial locomotion.E vidence now exists that should settle the long-running debate over a ground-up origin of avian flight vs. the evolution of avian flight from a trees-down glider. This evidence shows that the protoavian was arboreal (1) rather than a terrestrial cursor as many have suggested (2-4). The leading protagonist in this controversy is presently a dromaeosaur, Microraptor gui, with a fully formed hind wing that is closely similar to its completely avian forewing (5), having elongate, aerodynamically advanced "primary feathers" coming off the metatarsi (5). There seems little reason to doubt the aerodynamic function of the hindwing, but there has been controversy over exactly how it was arranged and used for flight. We decided to address this problem by creating and testing models that closely replicate the anatomical features preserved in the now numerous fossils of Microraptor and its close relatives (discussed in SI Text).Primitively, early archosaurs are sprawling, with the legs set laterally and elevated at around 75°(6), a preadapted posture for gliding. Modern birds normally have the thigh elevated and sprawled to the side in different degrees; for example, it is nearly perpendicular to the midline in loons and grebes (7). This variation shows that the degree of splaying needed to use the hindlegs in gliding is not unusual when compared with that in modern birds. The absence of an antitrochanter and a supraacetabular shelf (SAC) i...

Aerodynamic performance of the feathered dinosaur Microraptor and the evolution of feathered flight

“…2b,c; see Methods for this calculation), the legs-down configuration has the higher aerodynamic efficiency (glide ratio 4.7 (:1)) compared with the sprawled-legs configuration (glide ratio 2.8 (:1)). Equilibrium glide velocities were calculated from the corresponding SCR with an estimated Microraptor mass of 0.5 kg (consistent with previous work) [15][16][17]22 ; this shows that lesser aerodynamic performance goes hand-in-hand with a lower glide velocity (Fig. 2d).…”

“…Virtual models used for this work were based on measurements of fossil casts and digital reconstructions of wing planform. Chatterjee and Templin 15 analysed flight performance using two pieces of stream tube theory software 33 : ANFLTPWR (animal flight power) and ANFLTSIM (animal flight simulation) 33 (we have been unable, despite repeated attempts, to obtain copies of these codes from Chatterjee & Templin 15 , or to replicate their methods) and, in contrast to subsequent modelling work 16 , assumed that because of the biplane configuration the feathers on the femur and tibia only served to streamline and thus reduce the arodynamic drag of these leg bones and did not contribute to the production of lift. This study 15 concluded that likely flight speeds for Microraptor would have been between 9 and 15 m s À 1 and that the dinosaur would have been a moderate glider, traveling from tree to tree in a phugoid motion, achieving at least 40 m of horizontal distance with a terminal flight speed of ca.…”

“…Alexander et al 16 tested their models as free-flying gliders and concluded that the mean steady-state glide ratio for each of their three configurations was 4.1 (ref. 16).…”

“…Such estimates are highly dependent on the assumed L/D; therefore, if the much lower values reported by other workers were applied, the range would be greatly reduced. Alexander et al 16 subsequently studied the aerodynamic properties of several different Microraptor configurations by producing free-flying models with wing profiles based on those of a pigeon. They based the overall shape on the Microraptor gui holotype (Institute of Vertebrate Paleontology and Paleoanthropology, Beijing) V13352 and investigated the following three configurations they considered plausible: the forewing and tail attached in the same frontal plane with varying incidence; the hind wings mounted with 20°of anhedral and 75°of sweep (similar to the previous configuration but with the tips of the hind wings horizontal); and a biplane configuration where the feet forming the hind wings are located beneath the main wing-a configuration supported by the earlier geometric study of Chatterjee and Templin 15 .…”

“…Previous interpretations have included the following: static interpretation of fossils 14 , computer-based flight performance modelling of fossils 15 , analysis of free-flying models 16 and scale model wind tunnel experiments 17 . No consensus on likely wing configurations (thus degree of stability and glide performance) has been attained as reflected in a factor of six range of predicted lift-to-drag (L/D) ratios in previous work (that is, in Chatterjee and Templin 15 : 15; Alexander et al 16 : 4.1; Koehl et al 17 : 3). L/D is one fundamental measure of flight performance determining minimum glide angle and thus maximum flight range under steady conditions.…”