In a new study, published online today (14 March 2012) in Biology Letters, our lab shows that the high-lift mechanism Leading Edge Vortex (LEV) appear to be common to most animal flyers, as it appears in slow flying pied flycatchers. These birds hover and fly slowly when foraging on aerial insects, and they have a powerful downstroke when the LEV boost the lift by 100%. This is much stronger than found in for example hummingbirds, but that could be explained by the fact that the flycatcher has a feathered (inactive) upstroke. As flycatchers catch insects they need to be equally good at maneuvering in the air, and the LEV helps them to achieve the required turn radii. This mechanism was thought to be restricted to insects, as it was key to explaining why e.g. a bumblebee can fly, and subsequently our group has found this in slowly flying bats. Now, we extend the set of animas using LEV in slow flight to include also normal hoverers (i.e. animals having an inclined stroke plane and inactive back-/upstroke) in this spectacular study.
In a new study, published on-line in the Journal of the Royal Society Interface, the Brown University group report on wake measurements from the Brazilian free-tailed bat (Tadarida brasiliensis), using the PIV technique in a wind tunnel (Hubel et al.). This species differs in morphology and ecology from previously studied bat species, having higher aspect ratio wings and mainly flying in he open airspace when feeding on insects. So, does that make it's wake and aerodynamic properties different from other bats? The answer is "no"! Even if the authors argue that the Brazilian free-tailed bat has a wake very similar to that of the swift (Apus apus), it shows all of the characteristic wake features previously observed in bats. These include wing-root vortices and revers-vortices shed at the end of the upstroke. According to the authors when comparing the new data with previously studied bats: "the structure of their wakes is remarkably similar". Hence, it seems as if the notion of a typical "bat wake" receives support from this new study. Further studies will hopefully answer what features of the bat design make bat wakes different from those of birds.
The heaviest bat has a body mass of about 1.5 kg, which is about 10 times lower than the largest living bird species. Why this is so has puzzled scientists working on flight mechanics, since the power requirements increase approximately equally much for bats and birds. The solution lies in the muscular capacity in generating forces that beat the wings in active flight. While birds have one major depressor muscle responsible for a forceful downstroke, bats have several smaller muscles doing that same job. But the total muscle mass is smaller in bats, resulting in a lower maximum wingbeat frequency. When plotting scaling relationships for expected power required to fly, and power available from the flight muscles, it turns out that the power available curve (calculated on the basis of wingbeat frequency) does not increase as steeply as that of power required for flight. Where the two curves cross when plotted against body mass, you have the point of maximum mass for sustainable flight. In bats, this is about 1.5 kg, as shown in a new paper by Ulla Lindhe Norberg and Åke Norberg, of Gothenburg university, published in Journal of Experimental Biology. The same analysis was made earlier by Colin Pennycuick, which fixed the upper size sustained bird flight at about 12 kg. It seems as if birds have more muscle power allowing bigger size than bats. A question that follows is whether the basic bat design, having many flight muscles, prevent evolution of large size than about 1.5 kg, or if there are some additional factors limiting size?
In a new paper published in the French Academy of Science journal, Comptes Rendus Mechanique, members of the Lund Animal Flight Lab has published a paper about a bat-inspired flapper (see figure). The paper is entitled "Stroke plane angle controls leading edge vortex in a bat-inspired flapper", This is an attempt to mimic the flight of a true bat, and how a wing composed of a compliant membranous surface function. The advantage of working with a flapper is that it can be programmed to move its wings in all possible ways, i.e. also in ways that live bats usually don't do, and so the whole kinematic parameter space can be explored. The drawback is of course ,as with all models, to know how much the model depart in performance from the real system and if it matters. The current paper describes the development of a leading edge vortex (LEV) during the downstroke, which is an important aerodynamic ingredient of slow bat flight. The LEV dynamics was partially controlled by the stroke plane angle. A copy of the paper can be obtain my mailing (anders.hedenstrom@biol.lu.se)