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 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)

The Dickinson lab website in new dress

Browsing around I recently found that the Dickinson lab, where AFL memeber Florian Muijres now is a postdoc, has a new look. The old classic version can still be accessed, while the new lab website "FlyRanch" demarcates the move by this lab from CalTech, Los Angeles, to University of Washington, Seattle. The webiste shows the great range of research done by this lab. Have a look and get impressed. The lab members are assigned different characters and characteristics of typical fantasy literature creatures. The leader of the clan, overlord Michael himself is "ruthless and fearless"! Now we look forward to see which character Florian will assume.

A new paper in Current Biology from FlyRanch deals with polarized light perception in fruit flies, and could be consulted as a typical contribution.

Climate change affects flight speed in wandering albatrosses

A new study published in the current issue of Science reports about climate change related effects in flight behavior in wandering albatrosses, breeding on Crozet Islands in the southern Indian Ocean. Due to climate change related phenomena the mean wind speed has increased in the southern oceans, between 50 and 60 S. Albatrosses use dynamic soaring, which is a way of extracting energy by soaring in the wind gradient in the boundary layer of the sea. The stronger the wind, the faster the albatrosses can fly. A french team, lead by Henri Weimerskirch, has studied wandering albatrosses on the Crozet Islands during 20 years. During this period the winds have increased, and the scientists have been able to measure related properties in the albatrosses. The albatrosses now forage further to the south, their flight speed has increased from 10 to 12 m/s, and their daily travel rate during foraging journeys has increased from 500 to 700 km/day. Better foraging success has led to improved breeding success, and the albatrosses have increased by about 1 kg in body mass. It seems as if we here have a positive effect related to the ongoing climate change, but the scientists mention that the predicted scenario of wind change will come to a deterioration further down the trail, so the observed effects may be temporary. It is very nice, though, to see a study reporting an association between climate change and flight speed.

Span efficiency of desert locusts

In a new paper, AFL-postdoc Per Henningsson, now working in the flight group at Oxford University, and Richard Bomphry have published a paper about "Time-varying span efficiency through the wingbeat of desert locusts" in the journal Interface. They use time-resolved PIV to work out the span efficiency, which is something that we have tried also in Lund. In a previous paper Richard Bomphrey had estimated that span-efficiency is 0.89 at mid-downstroke in the locust. The Oxford Group uses a slightly different calculation method that is customary in Lund, and so we will read this paper for our next lab meeting. The question is how much the difference in method matters to the final result, if it matters at all. It is nice to see the spread of PIV in the animal flight research community, and there are certainly still lots of research needed to be done. The answer, by the way, was 0.79.

New paper about aerodynamics in slow-flying flycatcher

Just of the press from the Journal of the Royal Society Interface is a paper about pied flycatcher aerodynamics, by aouthors Florian Muijres, Melissa Bowlin, Christoffer Johansson and Anders Hedenström. Stereo flow-visualization was used to study the wake vortices shed off the wings of flycatchers as they flew slowly in the wind tunnel. The results sow that the aerodynamic force is mainly from the downstroke, resulting in a closed vortex loop. The results also show that the tail is involved in deflecting the downwash, resulting in an aerodynamic efficiency comparable to that of cruising flight. During the upstroke the wings generate no significant forces, but the body-tail configuration does to some degree, and so the upstroke phase of slow flight is not completely uninteresting from an aerodynamics viewpoint.

Old and New Activities within Animal Flight Lab

First of all, a happy new year to AFL members and followers. At the transition between years one usually contemplates what has happened during the past year, and what will happen in the year to come, in the present case 2012. Last year, 2011, was a really productive year in the AFL, with Florian Muijres dissertation in April being a local peak. For the occasion Tom Daniel visited as opponent and we all had a great time. Now Florian and family has already arrived in Seattle, where Florian will make a postdoc with Michael Dickinson. We wish them the best of luck and look forward to get regular progress reports. During the autumn, postdoc Sophia Engel, Germany, left after a 2-year period here. Master student Gide Koekkoek, The Netherlands, finished his project on the bat flapper before the summer, and just after Christmas the corrected proofs of a paper was sent back to the journal. Another master student was Naid Mubalegh, France, who did a project on zebra finch flight during the spring. Here in Lund, we welcome Jonas Håkansson as a new PhD student. Last year we received an infrastructure grant that we are currently trying to spend, and hopefully the new equipment will serve us well in the wind tunnel. Finally, a reading tip in the form of a JEB paper about immune function decrease immediately after endurance flights in starlings. The picture is from our pre-christmas lab celebrations, with the hope we get reasons to celebrate many more times during 2012!