10 February 2012

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?

02 February 2012

New publication in French Academy journal

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)