Hovering flight in birds
The research on hovering flight in birds has focused on nectar feeding hummingbirds, which use a horizontal figure eight motion of the outstretched wings, similar to insects, resulting in lift support during both the downstroke and the upstroke. However, other birds also hover, but then use a different wing motion, where the wings are beaten downwards and forwards during the downstroke and are folded and moved close to the body during the upstroke.
This latter form of hovering, known as asymmetric or avian hovering, is performed by for example flycatchers. The flycatcher is a small passerine that hunts insects on the wing when hovering or flying slowly. To shed light on the aerodynamics of avian hovering we performed the first study of the wake of slow flying (close to hovering) pied flycatchers. The results show that the birds produce all the lift during the downstroke and make their wings aerodynamically inactive during the upstroke by retracting the wings and spreading the wing feathers. The resulting wake dynamics is much more similar to that of passerines flying at medium flight speeds than it is to the wake of hovering hummingbirds.
As in all hovering, generating enough force when the speed of the air across the wing depends only on the flapping motion is difficult. In addition, compared to the hummingbirds that generate some lift during the upstroke, the flycatcher needs to generate all the required lift during the downstroke. Our wake measurements show that even at speeds as high as 2-3 m/s, the flycatcher uses force coefficients above what is the maximum attainable during steady state conditions. The high force coefficient suggests the use of unsteady aerodynamic mechanisms.
In order to determine the mechanism behind this high force production we visualized the flow directly above the wing of slow flying pied flycatchers. We were able to demonstrate that the main mechanism is the use of leading edge vortices (LEVs), vortices in close proximity to the wing surface, which boost the lift production in the flycatchers. The LEVs are relatively strong compared to hummingbirds, which is in agreement with the higher demands on the flycatcher. Despite the strength of the LEV, it does not burst or shed from the wing during the downstroke and seem to be controlled by dynamic twisting of the wing and aeroelastic bending of the primary feathers.