Automatic aeroelastic devices in the wings of a steppe eagleAquila nipalensis

Abstract

SUMMARY Here we analyse aeroelastic devices in the wings of a steppe eagle Aquila nipalensis during manoeuvres. Chaotic deflections of the upperwing coverts observed using video cameras carried by the bird (50 frames s–1) indicate trailing-edge separation but attached flow near the leading edge during flapping and gust response, and completely stalled flows upon landing. The underwing coverts deflect automatically along the leading edge at high angle of attack. We use high-speed digital video (500 frames s–1) to analyse these deflections in greater detail during perching sequences indoors and outdoors. Outdoor perching sequences usually follow a stereotyped three-phase sequence comprising a glide, pitch-up manoeuvre and deep stall. During deep stall, the spread-eagled bird has aerodynamics reminiscent of a cross-parachute. Deployment of the underwing coverts is closely phased with wing sweeping during the pitch-up manoeuvre,and is accompanied by alula protraction. Surprisingly, active alula protraction is preceded by passive peeling from its tip. Indoor flights follow a stereotyped flapping perching sequence, with deployment of the underwing coverts closely phased with alula protraction and the end of the downstroke. We propose that the underwing coverts operate as an automatic high-lift device, analogous to a Kruger flap. We suggest that the alula operates as a strake, promoting formation of a leading-edge vortex on the swept hand-wing when the arm-wing is completely stalled, and hypothesise that its active protraction is stimulated by its initial passive deflection. These aeroelastic devices appear to be used for flow control to enhance unsteady manoeuvres, and may also provide sensory feedback.

Unsteady dynamics of rapid perching manoeuvres

“…A landing bird may thus generate greater time-averaged lift than drag during a pitch-up manoeuvre. This may help explain why the eagle observed by Carruthers et al (2007) would consistently approach its perch from below, and exhibited a rapid increase in altitude during its pitch-up phase. As the authors of that study suggested, this may be a means to aid deceleration by transferring kinetic energy to potential energy.…”

“…But this would also increase the total added mass in time, making it difficult to stop the body late in the manoeuvre by producing a net thrust through −m aU . The sudden change in the direction of forces would tend to yield an uncontrolled perching manoeuvre, which is not observed (Green & Cheng 1998;Berg & Biewener 2010;Carruthers et al 2007). In the next two sections we extend the simple 1D force model equation (2.3) by discussing the effects of rotation on the added-mass force, and by modeling the circulation forces induced by vortex shedding.…”

“…We next study equation (2.4) through a prescribed set of kinematics representing simultaneous deceleration and pitch-up to large α, relevant to perching manoeuvres in birds (Carruthers et al 2007):…”

“…Though the exact kinematic motion used by birds in perching manoeuvres varies between species (Berg & Biewener 2010;Provini et al 2014), one relatively simple motion is for a bird to pitch its wings continuously from near-horizontal to approximately 90 • angle of attack. Carruthers et al (2007) observed this motion in a Steppe Eagle and noted that the eagle gained altitude quickly during the landing phase, implying that large lift was produced. However, steady-state analysis predicted that the bird would lose lift at high angles of attack due to stall effects and reduced airspeed (Carruthers et al 2010).…”

Arbitrary Lagrangian-Eulerian approach in reduced order modeling of a flow with a moving boundary

“…The examples include dangerous phenomena like §utter and bu¨eting of the wings and fuselage [4 6], vibrations in turbine engines [7] and helicopter blades [8], as well as applications in the design of bio-inspired air vehicles [9,10]. Furthermore, recent research on the growth of the lift force and drag reduction by active electromorphing [11] and aeroelastic boundary actuators [12,13] requires the analysis of FSI.…”