New wake study of a bat from Brown university

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.

Sized by the wingbeat

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?

New PhD thesis from Animal Flight Lab

Florian Muijres has received his thesis from the printer and is busy distributing them. The thesis contains seven different studies about aerodynamic performance in bats and birds. Two papers describe a high lift mechanism - Leading Edge Vortex - used by bats and pied flycatchers when flying slowly. This vortex enhances lift by 40% or more in these vertebrates, and before Florians work it was thought to be a typical insect mechanism. In other papers time-resolved PIV is used to work out aerodynamic efficiency of bats and birds, and to evaluate an adaptation of the classic actuator concept of flapping flight. Florian is to be congratulated to marvelous thesis, which is not only interesting on the inside but also beautiful on the outside. The public defense will take place on 28 April at 10:00 am, in the Blue Lecture Hall, Ecology Building, Lund. As faculty opponent we are very glad to welcome Professor Tom Daniel, University of Washington, Seattle.

Rhea gets PhD

On last Friday our group member Rhea von Busse passed her oral examination for a PhD at the Humboldt University, Berlin. Dr Busses thesis is about "The trinity of energy conservation: kinematics, aerodynamics and energetics of the lesser long-nosed bat (Leptonycteris yerbabuenae)", including chapters on kinematcs, aerodynamics (wakes), a comparison between kinematics and wakes, and an investigation about the flight metabolic rate. This thesis is the result of the collaboration between AFL and York Winter. The discussions leading up to this collaboration were initiated some 10 years ago, when AH visited the Max Planck Institute at Seewiesen to give a talk about the wind tunnel studies on birds that were underway. Then Rhea came with the first batch of Glossophaga soricinas used for our first studies (and Rheas master thesis). We congratulate Rhea to this magnificent achievement and wish her the best of luck with her new studies on bats in the Brown University bat biology group.

Operation snow storm: lab meeting

During the lab meeting today (29 November 2010) the AFL-members that made it to the Ecology Building, in spite of the snow-storm, discussed the recent paper about “The effect of body size on the wing movements of pteropodid bats, with insights into thrust and lift production”. This paper, from the Brown University Group, reports on kinematic parameters studied in six species of bats. The paper concludes that “the ways that bats modulate their wing kinematics to produce thrust and lift over the course of a wing beat cycle are independent of body size”. How convenient if this proves to be true! The authors however thinks that small bats, such as Phyllostomids, may differ from the medium to large sized bats studied here. One reason put forward could be that small bats, using a leading edge vortex, are more insect-like in their use of aerodynamic mechanism, and therefore different from other bats. It will be interesting to find out of smaller species of bats differ in flight.

A new paper on bat flight has now been published by the Brown University group in the Journal of Experimental Biology (Hubel et al. 2010), where they report on the wake dynamics in a medium sized bat, Cynopterus brachyotis, when it is flying at intermediate speeds (5 and 6.7 m/s). The wake is very similar to that found for other bat species studied in Lund, which lend support for the notion of the generality of these results. Hence, it is very nice to see results of bat aerodynamics being confirmed by anther lab. Since papers of bat flight are still quite rare, this paper will be the focus of our next lab meeting, which will be Monday 11 October, 2010.