I have a special treat this evening. Colin Palmer has been kind enough to write a guest post on the relative performance advantages and dynamics of thin and thick wings, especially in the context of animal flyers. Colin is located at Bristol University. He is an accomplished engineer with an exceptional background in thin-sectioned lifting surfaces (particularly sails). Colin has turned his eye to pterosaurs in recent years, and he has quickly become among the world's best pterosaur flight dynamics workers. You can catch his excellent paper on the aerodynamics of pterosaur wings here. Press release on it can be found here.
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Thin And Thick Wings
Colin Palmer
In the early days of manned flight the designers took their inspiration from birds. One of the consequences was that they used thin, almost curved plate aerofoil sections. This seemed intuitively right and certainly resulted in aeroplanes that flew successfully. However towards the end of the First World War the latest German Fokker fighters suddenly started to outperform the Allied planes. Counterintuitively their wing sections were thicker-surely these sections would not cut the air so well so how could they possibly have enabled aeroplanes to fly faster and climb more quickly. But that was what was happening, the Germans had done their research and discovered that a combination of a cambered aerofoil with the correct thickness distribution gave superior aerodynamic performance. Subsequently all aircraft had similar teardrop shaped wing sections and soon there was a massive body of experimental and theoretical work available that enabled designers to select just the aerofoil they required.
Fast forward to the period after the Second World War and an explosion of interest in applying the latest aerospace science to the traditional arts of sailing. Many people looked to aircraft and logically assumed that sailboats would perform better if only they could be fitted with wing sails, like up-ended aircraft wings. Surely this had to be more efficient than the old-fashioned sails made of fabric and wire, just like the earliest aircraft. But the results were disappointing. Not only on a practical level where the wing sails proved unwieldy and unsuited to operating in a range of wind conditions, but perhaps more worrying they offered no obvious performance advantage and indeed in light winds they were significantly inferior, area for area. What was going on? Why didn't the massive investment in the development of aircraft wing sections have anything to offer to sailboats?
The answer lay in understanding the effect of Reynolds number. From the very earliest days of manned flight aircraft were operating at Reynolds number approaching 1 million and as speeds increased so did the Reynolds numbers, so it became customary for aerofoils to be developed for operation at Reynolds numbers of 2 to 3 million or more. But sailboats are much slower than even the slowest aircraft so the operational Reynolds numbers are lower than for aircraft, typically in the range from 200,000 to 500,000, right in the so-called transition region. It turns out that in this Reynolds number range the experience and intuition gained from studies at significantly higher values can be very misleading indeed. In the transition region a curved plate, (membrane) aerofoil can be more aerodynamically efficient than a conventional thick aerofoil.
This transition Reynolds number range is also where most birds and bats operate, and from what we know of pterosaurs it was also their domain. Consequently natural forms are not necessarily disadvantaged by having the membrane wings of bats or pterosaurs or the thin foils of the primary feathers in the distal regions of bird wings.
But there is a complication. A curved plate or, to an even greater extent, a membrane aerofoil has very little intrinsic strength and requires some form of structure to keep it in place and keep it in shape. On sailing yachts this structure is a thin tension wire that supports the headsail or the tubular mast in front of the mainsail. In order to tension the wire for the headsail, very large forces are required which places the mast in considerable compression, normally requiring a guyed structure that can have no direct analogue in nature. Natural forms are restricted to using a supporting structure which is loaded in bending and restrained by attached muscles and tendons. Generally speaking, the bending resistance of a structure depends upon the depth of the cross-section, so as bending load increases the diameters of the bones must increase otherwise the wing will become too flexible.
This is where the apparent superiority of the membrane wing may be compromised, because the presence of structural member severely degrades the aerodynamic performance. The structural member may be along the leading edge of the aerofoil as in the case of bats and pterosaurs, or close to the aerodynamic centre as in the case of the rachis of the primary feathers of birds. In all cases the loss of performance is less if the supporting structure is on the pressure side (the ventral side) of the aerofoil. It is therefore most likely no coincidence that this is the arrangement of the wing bones and membrane in bats and the rachis and vane in primary feathers. It was therefore also most likely that the wing membranes of pterosaurs were similarly attached to the upper side of the wing finger. Even in this configuration there is a substantial penalty in terms of drag, although it may result in some increase in the maximum lift capability of the section, due presumably to an effective increase in camber. (Palmer 2010).
This aerodynamic penalty arising from the presence of the supporting structure may perhaps be the reason why birds’ wings have thickness in the proximal regions, where the performance of such a thick aerofoil is superior to a thin membrane obstructed by the presence of the wing bones. More distally, where the wing bones become thinner or are not present, the wing section reverts to a thin cambered plate formed by the primary feathers. On the bird’s wing the proximal fairing of the bones into an aerofoil section is achieved by the contour feathers with very little weight penalty. This is not possible in bats (and presumably also in pterosaurs) where any fairing material would, at the very least, need to be pneumatised soft tissue, resulting in a considerable weight penalty as compared to feathers. In the absence of aerodynamic fairing around the supporting structure, aerodynamic efficiency can only be improved by reducing the cross-section depth of the bones - the general shape of the section having very little effect. But reducing the section depth results in a large increase in flexibility since the bending stiffness varies as the 4th power of section depth, so there are very marked limits to the effectiveness of this trade-off.
It may therefore be no coincidence that where the cross section depth has to be greatest, in the proximal regions of the wing, both bats and pterosaurs have a propatagium, which means that the leading-edge of the wing section is more akin to the headsail of a yacht, stretched on a wire, than a membrane with the structural member along the leading-edge. Wind tunnel tests have shown that moving the structural member back from the leading-edge, while keeping it on the underside of the wing section, results in a significant increase in aerodynamic performance.
Showing posts with label Wings. Show all posts
Showing posts with label Wings. Show all posts
Thursday, July 5, 2012
Wednesday, June 20, 2012
Fun Facts
Been on a paper crunch recently, so haven't had the time or wherewithal to post much. I will try to get up some more real "articles" soon, but here are some fun flying/swimming facts for you guys in the meantime. Some of these may turn into full posts:
- Flight is impossible without viscosity. You can't generate lift in a superfluid.
- Advance ratio refers to the distance traveled relative to the number (or total arc) of foil/wing/tail strokes. The highest advance ratio for a swimmer belongs to the manta and cownose rays, which use their entire body as a wing while aquaflying.
- The main flight muscles in more basal winged insects, like dragonflies, pull directly on the wing base. In more derived taxa, the muscles typically pull primarily on the exoskeleton and beat the wings by flexing the body wall.
- The slots at the tip of bird wings reduced induced drag, but only are effective at low speeds for broad wings. Broad-winged species only open the slots when flying slowly, and species with high-aspect ratio wings don't have slots. Pelicans have the highest AR wings among those birds that use wingtip slots (AR 11-12).
- Flight is impossible without viscosity. You can't generate lift in a superfluid.
- Advance ratio refers to the distance traveled relative to the number (or total arc) of foil/wing/tail strokes. The highest advance ratio for a swimmer belongs to the manta and cownose rays, which use their entire body as a wing while aquaflying.
- The main flight muscles in more basal winged insects, like dragonflies, pull directly on the wing base. In more derived taxa, the muscles typically pull primarily on the exoskeleton and beat the wings by flexing the body wall.
- The slots at the tip of bird wings reduced induced drag, but only are effective at low speeds for broad wings. Broad-winged species only open the slots when flying slowly, and species with high-aspect ratio wings don't have slots. Pelicans have the highest AR wings among those birds that use wingtip slots (AR 11-12).
Friday, May 11, 2012
Winglets
A colleague of mine asked the other day what the upturned wing tips on many modern commercial aircraft are doing, and if they have any similarity to the split wingtips used by many birds. A full explanation would take pages, but here's a quick and dirty one:
There is a price on producing lift called 'induced drag'. Induced drag can be modeled a few different ways, but basically it involves some efficiency-reducing downwash at the wingtips involved in the formation of the wingtip vortices. A wing without a tip does not suffer from induced drag, as a result, but in most real situations wings obviously must have tips (in a wind tunnel you can build a wing that goes from wall to wall to eliminate induced drag effects).
An upturn of the wingtip can, in certain speed regimes, reduce induced drag. Splitting the wingtip into smaller, high aspect ratio wings can also reduce induced drag. Fixed wing aircraft can be designed with the first of these two tricks. Many birds use both: they spread the primary feathers in both the transverse and vertical planes - this means they get the multiple-tip bonus and the vertical displacement effect.
Induced drag is particularly problematic at low speeds, so these tricks are best for slow soaring, landing, and takeoff. Of course, in a fixed-wing aircraft, the winglets are set in place and cannot be differentially deployed. In animals, however, slotted wingtips are only deployed when it is most useful - that is, at low speeds.
There is a price on producing lift called 'induced drag'. Induced drag can be modeled a few different ways, but basically it involves some efficiency-reducing downwash at the wingtips involved in the formation of the wingtip vortices. A wing without a tip does not suffer from induced drag, as a result, but in most real situations wings obviously must have tips (in a wind tunnel you can build a wing that goes from wall to wall to eliminate induced drag effects).
An upturn of the wingtip can, in certain speed regimes, reduce induced drag. Splitting the wingtip into smaller, high aspect ratio wings can also reduce induced drag. Fixed wing aircraft can be designed with the first of these two tricks. Many birds use both: they spread the primary feathers in both the transverse and vertical planes - this means they get the multiple-tip bonus and the vertical displacement effect.
Induced drag is particularly problematic at low speeds, so these tricks are best for slow soaring, landing, and takeoff. Of course, in a fixed-wing aircraft, the winglets are set in place and cannot be differentially deployed. In animals, however, slotted wingtips are only deployed when it is most useful - that is, at low speeds.
Wednesday, May 9, 2012
The Bumblebee Myth
I had an interesting quick discussion on Twitter earlier today where the issue of the old "bumblebees can't fly myth" came up. This is an old story that has turned into something of an urban legend. There are all sorts of versions running around the internet (brief overview here), but they typically suggest that an aerodynamicist of some note showed that it was mathematically impossible for a bumblebee to fly. Obviously, bumblebees do fly, and the story is often used in a derogatory sense to
put down scientists and engineers or being intelligent yet (in the views of the story-tellers) unable to grasp something obvious to everyone else. It is, as it were, another version of the Straw Vulcan argument to deride logical approaches to problems.
I could go on for a while about how ridiculous this sort of thing is, but I imagine it is self-evident to anyone that reads this blog, so I'll hold my typing on that front. Needless to say, the story is based upon a flawed understanding of the situation.
What was shown at some point (though the details are foggy when this was done the first time) is that a standard steady-state, oscillating foil model using a rigid wing does not accurately describe bumblebee flight. If bees had to fly like airplanes, they would indeed by grounded. But, of course, bees don't fly like airplanes. For one thing, insects have flexible wings. The shape changes in the wings are critically to their lift production. Second, insect wings carry a significant virtual mass of air on them as they move, owing to the "stickiness" of the air at that size scale. This means the effective profile of the wing is not the same as the anatomical profile. Thirdly, bees and many other insects with high wing beat frequencies seem to rely heavily on unsteady effects. This means that the air never reaches equilibrium on their wings. The flow is kept in a constant state of imbalance; this works best at very high wing beat frequencies and small size scales and can produce very high lift coefficients (well above steady state maxima). Small birds and bats can use some of these tricks, too, it would seem, because the measured maximum lift coefficients are often quite large in small animal flyers (about 5 for flycatchers, for example, compared to the 1.6 theoretical maximum for a thick feathered wing with slight camber).
So, in short: yes, bumblebees can fly. We understand how they fly and have for a long time, though there is plenty left to learn about insect flight overall. The story that someone proved bumblebees can't fly on paper is a myth (or, at best, a major misinterpretation). Finally, suggesting that logic is a bad way of solving problems is silly.
I could go on for a while about how ridiculous this sort of thing is, but I imagine it is self-evident to anyone that reads this blog, so I'll hold my typing on that front. Needless to say, the story is based upon a flawed understanding of the situation.
What was shown at some point (though the details are foggy when this was done the first time) is that a standard steady-state, oscillating foil model using a rigid wing does not accurately describe bumblebee flight. If bees had to fly like airplanes, they would indeed by grounded. But, of course, bees don't fly like airplanes. For one thing, insects have flexible wings. The shape changes in the wings are critically to their lift production. Second, insect wings carry a significant virtual mass of air on them as they move, owing to the "stickiness" of the air at that size scale. This means the effective profile of the wing is not the same as the anatomical profile. Thirdly, bees and many other insects with high wing beat frequencies seem to rely heavily on unsteady effects. This means that the air never reaches equilibrium on their wings. The flow is kept in a constant state of imbalance; this works best at very high wing beat frequencies and small size scales and can produce very high lift coefficients (well above steady state maxima). Small birds and bats can use some of these tricks, too, it would seem, because the measured maximum lift coefficients are often quite large in small animal flyers (about 5 for flycatchers, for example, compared to the 1.6 theoretical maximum for a thick feathered wing with slight camber).
So, in short: yes, bumblebees can fly. We understand how they fly and have for a long time, though there is plenty left to learn about insect flight overall. The story that someone proved bumblebees can't fly on paper is a myth (or, at best, a major misinterpretation). Finally, suggesting that logic is a bad way of solving problems is silly.
Thursday, April 19, 2012
Microraptor - Brief Note
I am putting together a paper for PNAS with three coauthors on flight dynamics in Microraptor gui. One thing that is becoming increasingly apparent as I combine my various calculations is that Microraptor was probably pretty darn good in the air, overall. It is not known if it had powered flight ability, but even if Microraptor was an unpowered gliding animal, it has an awful sophisticated set of control surfaces (including some that you don't see in modern birds). This was a highly maneuverable little beast.
I will be posting more about Microraptor and other paravians once our paper is out (assuming that all goes well in review). I may also be giving a talk on the subject at SVP, and Justin Hall (who is also on the paper in question) has recently given some talks on the subject at conferences on the West Coast.
I know that this blog is quite new, and readership is obviously still limited (though growing!), but I would like hear what your most pressing questions are regarding early avian (and near-avian) flight. I am hoping to do a relatively sophisticated series on the topic here once the semester wraps up and my teaching obligations are complete for the summer.
I will be posting more about Microraptor and other paravians once our paper is out (assuming that all goes well in review). I may also be giving a talk on the subject at SVP, and Justin Hall (who is also on the paper in question) has recently given some talks on the subject at conferences on the West Coast.
I know that this blog is quite new, and readership is obviously still limited (though growing!), but I would like hear what your most pressing questions are regarding early avian (and near-avian) flight. I am hoping to do a relatively sophisticated series on the topic here once the semester wraps up and my teaching obligations are complete for the summer.
Monday, April 16, 2012
Wait, penguins can fly!?
In short, no, penguins cannot fly in the traditional sense. But they can aquafly, which is today's topic.
There are relatively small numbers of flying animals that can also swim with the wings - most of these are birds, though some insects can perform this trick, as well. When the wings are used for lift-based motion through the water this is termed "aquaflight". Most aquaflying birds do also fly in the aerial sense, but penguins have abandoned aerial locomotion and use the wings exclusively for aquatic locomotion. This evolutionary transition is among the most interesting functional morphology problems in ornithology. Understanding how penguin morphology has changed over time in response to their aquaflying habits not only sheds light on aquatic locomotion, but it also gives us information regarding the constraints of aerial locomotion (which is primarily why I figured it was worth a discussion on Aero Evo).
The image at left is a reconstruction (by Chris Gaskin) of Kairuku, as described by Daniel Ksepka and colleagues. The full paper is available here for free. Kairuku hails from the Oligocene. Penguins have quite a good fossil record, and we know from the record that early penguins were often quite large (Kairuku stood nearly five feet tall). All of the known fossil penguins are also from clearly flightless animals - even early penguins like Kairuku, or the even earlier Waimanu, possessed wings clearly dedicated to swimming at the expense of aerial locomotion. However, we can deduce from phylogeny that penguin ancestors were flying animals, probably amphibious flyers (those animals that use the wings to both fly and swim). As mentioned above, there are some modern examples of amphibious flyers, and we can use them in functional comparisons with penguins (both living and fossil) to gain a better understanding of what mechanical changes have occurred in penguins.
It might be expected that using the wings underwater would dominate the physical forces acting on the wings of amphibious flyers. If this were true, then the wing geometry and bending resistance of the wing bones in amphibious flyers (like puffins) should be quite similar to that of penguins. However, this turns out not to be the case. In 2010 I published a paper showing that the wings of penguins are tremendously "overbuilt", with regards to their bending strength, when compared to other birds - including other living birds that swim with their wings. One of the figures from that paper is shown at left. You will note that the residuals in humeral strength are much greater for penguins (Sphenisciformes) than for any other birds in the sample. Puffins and auks, which all aquafly as well, are in the Alcidae. The Procellariiformes include a sample of aquaflyers, as well. As such, penguins are unique, even among aquaflyers.
In fact, this trend is not even limited to comparisons with aquaflying birds that still engage in aerial locomotion (i.e. those that "fly" proper). Last November I published a paper with Gareth Dyke and Xia Wang (available from PLoS ONE here) that described new material of plotopterids and compared the functional characteristics of plotopterid structure to penguin morphology. Plotopterids were flightless, aquaflying birds of the middle Cenozoic that were similar to penguins in many anatomical respects and which also reached large sizes (particularly in the Miocene). Some of the material we described in that manuscript is pictured at left.
As it turns out, the wings of plotopterids appear to be more similar (in terms of geometric variables known to correlate with mechanical properties) to living auks than to living penguins. This implies that the swimming stroke of plotopterids was more like that of auks than penguins, even though plotopterids were flightless.
There are more groups that need comparing in this way to generate a firm conclusion (Mancalline auks and Great auks, for example), but the emerging pattern is that the mechanics and evolutionary history of penguins is quite unique. Their morphology, it seems, is not just about having lost flight - it is presumably the specific trajectory by which they left the aerial realm, as well their specific phylogenetic history, that has placed penguins in a unique morphological space. In the 2010 Biological Journal paper I suggested that at least one major factor of importance may be the mirrored aquaflying stroke of penguins - unlike other aquaflying birds, living penguins produce nearly the same magnitude of lift with the wings on the upstroke and downstroke (albeit with opposite signs, of course). This mode of swimming greatly increases efficiency by reducing surge accelerations. Where this feature entered penguin evolutionary history has not yet been examined, but may be a fruitful topic of future research.
P.S. If you love penguins then you need to check out the definitive fossil penguin blog by Daniel Ksepka: http://fossilpenguins.wordpress.com/
References
Ksepka, DT, Fordyce RE, Ando T, and Jones CM. 2012. New fossil penguins (Aves, Sphenisciformes) from the Oligocene of New Zealand reveal the skeletal plan of stem penguins. Journal of Vertebrate Paleontology 32: 235-254
Habib M. 2010. The structural mechanics and evolution of aquaflying birds. Biological Journal of the Linnean Society. 99(4): 687-698
Dyke GJ, Wang X, Habib M. 2011. Fossil plotopterid seabirds from the Eo-Oligocene of the Olympic Peninsula (Washington State: USA): descriptions and functional morphology. PLoS ONE 6(10): e25672. doi:10.1371/journal.pone.0025672
There are relatively small numbers of flying animals that can also swim with the wings - most of these are birds, though some insects can perform this trick, as well. When the wings are used for lift-based motion through the water this is termed "aquaflight". Most aquaflying birds do also fly in the aerial sense, but penguins have abandoned aerial locomotion and use the wings exclusively for aquatic locomotion. This evolutionary transition is among the most interesting functional morphology problems in ornithology. Understanding how penguin morphology has changed over time in response to their aquaflying habits not only sheds light on aquatic locomotion, but it also gives us information regarding the constraints of aerial locomotion (which is primarily why I figured it was worth a discussion on Aero Evo).
As it turns out, the wings of plotopterids appear to be more similar (in terms of geometric variables known to correlate with mechanical properties) to living auks than to living penguins. This implies that the swimming stroke of plotopterids was more like that of auks than penguins, even though plotopterids were flightless.
There are more groups that need comparing in this way to generate a firm conclusion (Mancalline auks and Great auks, for example), but the emerging pattern is that the mechanics and evolutionary history of penguins is quite unique. Their morphology, it seems, is not just about having lost flight - it is presumably the specific trajectory by which they left the aerial realm, as well their specific phylogenetic history, that has placed penguins in a unique morphological space. In the 2010 Biological Journal paper I suggested that at least one major factor of importance may be the mirrored aquaflying stroke of penguins - unlike other aquaflying birds, living penguins produce nearly the same magnitude of lift with the wings on the upstroke and downstroke (albeit with opposite signs, of course). This mode of swimming greatly increases efficiency by reducing surge accelerations. Where this feature entered penguin evolutionary history has not yet been examined, but may be a fruitful topic of future research.
P.S. If you love penguins then you need to check out the definitive fossil penguin blog by Daniel Ksepka: http://fossilpenguins.wordpress.com/
References
Ksepka, DT, Fordyce RE, Ando T, and Jones CM. 2012. New fossil penguins (Aves, Sphenisciformes) from the Oligocene of New Zealand reveal the skeletal plan of stem penguins. Journal of Vertebrate Paleontology 32: 235-254
Habib M. 2010. The structural mechanics and evolution of aquaflying birds. Biological Journal of the Linnean Society. 99(4): 687-698
Dyke GJ, Wang X, Habib M. 2011. Fossil plotopterid seabirds from the Eo-Oligocene of the Olympic Peninsula (Washington State: USA): descriptions and functional morphology. PLoS ONE 6(10): e25672. doi:10.1371/journal.pone.0025672
Friday, April 13, 2012
Bennettazhia humerus
Monday, April 9, 2012
Insect Flight and Human Health
Many of the practical applications of understanding animal flight relate to areas like robotics and aeronautics. However, there are implications for human health, as well. Since flying animals experience intense skeletal loads and have rather strict muscle output requirements, many of them (particularly vertebrate flyers) make good models for understanding the limits of biological tissues, which in turn can play a role in future breakthroughs for biomaterials, tissue engineering, and the like.
However, there is a more direct link with human health, as well: many of the world's most serious diseases are carried by flying insects. Parasite life cycles can be quite complex, and many of them include more than one host. It is common for one of these hosts to be a smaller, more mobile organism, and that is often an insect. The more mobile host is typically called the vector, though in truth the definition of a disease vector is more specific than that.
The conditions under which vectors are the most mobile will therefore often be the conditions under which disease spreads most quickly. As it turns out, insect flight is rather sensitive to atmospheric conditions, on account of insect muscle and respiratory physiology, as well as their typically size range. Many insects, such as mosquitos, sit in a size range where changes in atmospheric density or temperature can change flight performance quite abruptly. At left is a wonderful photograph of an Aedes triseriatus eastern treehole mosquito taking a blood meal, by Alexander Wild. To see more of Alex's spectacular photography (and trust me, he has stuff you've never seen before, guaranteed) go to http://www.alexanderwild.com/
The limits on vector performance may getting more relevant in some locations. For example, in Hawaii, avian malaria has decimated many of the endemic bird populations. Many (if not most) of the lowland species present before European habitation of the islands are now extinct, or nearly so. However, the highlands have been protected from this effect in the past because they were cool enough to keep the malarial loads inside the mosquitoes low, and also because the cooler air and lower oxygen density inhibited the malaria-carrying mosquitos (different mosquito species take over at high altitudes). Unfortunately, Freed et al. discovered a about six years back that more mild high altitude temperatures were allowing malaria to spread up the mountains. They attribute this mostly to malarial loads within mosquitos, but it is likely that flight performance of the mosquitoes themselves is also improving at higher altitudes. The Freed et al. study can be found here.
Avian malaria has been used as a model for understanding human malaria for decades (in fact, two of the Nobel Prizes for malaria research were given for discoveries in avian malaria). It is reasonable to suspect that human health will also be influenced by the impacts of climatic changes on vector mobility. In the coming weeks, I will post more examples of how insect flight performance is critical to human health and economies - both for better and for worse.
References
Freed LA, Cann RL, Goff ML, Kuntz WA, Bodner GR. 2005. Increase in avian malaria at upper evelvation in Hawai'i. The Condor 107: 753–764
However, there is a more direct link with human health, as well: many of the world's most serious diseases are carried by flying insects. Parasite life cycles can be quite complex, and many of them include more than one host. It is common for one of these hosts to be a smaller, more mobile organism, and that is often an insect. The more mobile host is typically called the vector, though in truth the definition of a disease vector is more specific than that.The conditions under which vectors are the most mobile will therefore often be the conditions under which disease spreads most quickly. As it turns out, insect flight is rather sensitive to atmospheric conditions, on account of insect muscle and respiratory physiology, as well as their typically size range. Many insects, such as mosquitos, sit in a size range where changes in atmospheric density or temperature can change flight performance quite abruptly. At left is a wonderful photograph of an Aedes triseriatus eastern treehole mosquito taking a blood meal, by Alexander Wild. To see more of Alex's spectacular photography (and trust me, he has stuff you've never seen before, guaranteed) go to http://www.alexanderwild.com/
The limits on vector performance may getting more relevant in some locations. For example, in Hawaii, avian malaria has decimated many of the endemic bird populations. Many (if not most) of the lowland species present before European habitation of the islands are now extinct, or nearly so. However, the highlands have been protected from this effect in the past because they were cool enough to keep the malarial loads inside the mosquitoes low, and also because the cooler air and lower oxygen density inhibited the malaria-carrying mosquitos (different mosquito species take over at high altitudes). Unfortunately, Freed et al. discovered a about six years back that more mild high altitude temperatures were allowing malaria to spread up the mountains. They attribute this mostly to malarial loads within mosquitos, but it is likely that flight performance of the mosquitoes themselves is also improving at higher altitudes. The Freed et al. study can be found here.
Avian malaria has been used as a model for understanding human malaria for decades (in fact, two of the Nobel Prizes for malaria research were given for discoveries in avian malaria). It is reasonable to suspect that human health will also be influenced by the impacts of climatic changes on vector mobility. In the coming weeks, I will post more examples of how insect flight performance is critical to human health and economies - both for better and for worse.
References
Freed LA, Cann RL, Goff ML, Kuntz WA, Bodner GR. 2005. Increase in avian malaria at upper evelvation in Hawai'i. The Condor 107: 753–764
Saturday, April 7, 2012
Friday, April 6, 2012
Pterosaur Water Launch: Preliminary Results
Back again for more water launching goodness. These results were presented at SVP 2011; with luck they will be finalized and appear in a formal journal (PLoS ONE) this summer.
Here's what I get for Anhanguera, using the technique from yesterday's post:
The initial escape phase could be accomplished with a net remaining acceleration of 17.6 m/s2, and an acceleration of up to 39.2 m/s2 on the subsequent, unhindered propulsive bound. Sufficient contact area to provide a useable propulsion phase would require that the wing finger be opened 15-25 degrees. This contact area was greatly augmented by the exceptionally broad MCIV-PHIV joint. The escape phase would require exceptional shoulder adduction musculature, and I note that Anhanguera appears to have had such expanded musculature: the orientation and enlargement of m. subscapularis appears to be of particular importance, along with the reinforcement of the joint between the scapula and notarium (Bennett, 2009). My model predicts that Anhanguera would have used a series of repeated propulsions when launching from water (unlike terrestrial launch), which would have occurred as a series of “hops” across the water surface.
This repeated propulsion is required because the animal loses energy to the initial escape from surface tension, and because no lock and release is available on the wing finger joint during water launch, which somewhat reduces power output. The quadrupedal water launch still greatly outperforms any kind of bipedal launch, however. In fact, bipedal launch from the water was almost certainly impossible for pterosaurs of practically any size (bipedal terrestrial launch was likely impossible, as well for most species - not to mention inferior in performance in just about every way). This is particularly true given the recent work on floating position done by Dave Hone and Don Henderson.
One important note is that this water launch model makes predictions about morphological features one should expect to find in pterosaurs adapted to water launching. In this way, it makes testable predictions from the theoretical model. This is important, as we will presumably never get water launch trackways.
Based on this water launch model, we can expect that the following features should be better represented in marine taxa than terrestrial taxa:
expanded scapula
reinforced scapular-notarial joint
expanded deltopectoral crest
Warped deltopectoral crest or expanded tip of dp crest
extra-broad MCIV-PHIV wing finger joint
limb length disparity
expanded posterior brachial musculature
So far, this pattern appears to hold. While azhdarchids do have expanded dp crests, and somewhat expanded triceps they lack most of the other features in the list (at least comparatively speaking). So far, only marine pterosaurs exemplify all of the above simultaneously.
Thursday, April 5, 2012
Swimming Eagle
This video has been making the rounds, so many of you have probably seen it: Swimming Eagle of Baton Rouge.
What I find particularly rewarding about this little clip is that the quantitative model I built over the last year to estimate water launch in pterosaurs also predicts that eagles (and some other birds) should be able to do this, as unusual it is. Always validating to see expectations met.
I will post more about water launch in pterosaurs later, but the basic gist is this: the folded wing pivot of a bird (wrist) or pterosaur (base of fourth finger) can produce quite a bit of flat plate drag in the water, if the wing is still mostly folded. Combined with the powerful flight muscles, this provides a mechanism for generating substantial forces in the water without compromising flight anatomy.
What I find particularly rewarding about this little clip is that the quantitative model I built over the last year to estimate water launch in pterosaurs also predicts that eagles (and some other birds) should be able to do this, as unusual it is. Always validating to see expectations met.
I will post more about water launch in pterosaurs later, but the basic gist is this: the folded wing pivot of a bird (wrist) or pterosaur (base of fourth finger) can produce quite a bit of flat plate drag in the water, if the wing is still mostly folded. Combined with the powerful flight muscles, this provides a mechanism for generating substantial forces in the water without compromising flight anatomy.
Wednesday, April 4, 2012
Reconstructing the past
One of the things I hope to do with Aero Evo is to give some insights into how biomechanists like myself manage to make estimates of performance for fossil animals. In particular, of course, I will focus on how we know things about extinct flying animals.
Three of the parameters of typical interest are the speed at which a flying animal flew, the rate at which it flapped its wings, and how much room it needed to flap them: velocity, flapping frequency, and flapping amplitude, respectively.
These three factors are all correlated, and can be summarized by the Strouhal Number. The Strouhal Number is equal to fA/U, where f is frequency, A is amplitude, and U is velocity. As it turns out, the range of strouhal numbers at which a flapping wing or undulating fin can operate efficiently is pretty darn narrow. For creatures as seemingly different as dolphins, birds, fish, and dragonflies, Str in cruising locomotion only varies from about 0.2-0.4. This pattern occurs because of the narrow range of motion in which vortex shedding is efficient.
The classic paper on the topic is by Taylor et al., and can be found here. That link will take you to the abstract; those on campuses should be able to get the full paper, though it is behind a paywall. Fortunately, Nudds, Taylor, and Thomas wrote a followup paper with much of the same information (specifically on birds), and it is open access here.
Now, it should be noted that the limits on Str mentioned above only apply to cruising - that is, the animal is moving steadily at its efficient, long-distance gait. Burst performance is different, and on average, the Str will be higher during things like rapid climbs (say, just after takeoff) or bursts after prey. Nonetheless, we can get a good idea of how extinct animals worked by using Str. I'll post more on how to use Str, and how to combine it with other equations to solve for multiple variables, soon.
Three of the parameters of typical interest are the speed at which a flying animal flew, the rate at which it flapped its wings, and how much room it needed to flap them: velocity, flapping frequency, and flapping amplitude, respectively.
These three factors are all correlated, and can be summarized by the Strouhal Number. The Strouhal Number is equal to fA/U, where f is frequency, A is amplitude, and U is velocity. As it turns out, the range of strouhal numbers at which a flapping wing or undulating fin can operate efficiently is pretty darn narrow. For creatures as seemingly different as dolphins, birds, fish, and dragonflies, Str in cruising locomotion only varies from about 0.2-0.4. This pattern occurs because of the narrow range of motion in which vortex shedding is efficient.
The classic paper on the topic is by Taylor et al., and can be found here. That link will take you to the abstract; those on campuses should be able to get the full paper, though it is behind a paywall. Fortunately, Nudds, Taylor, and Thomas wrote a followup paper with much of the same information (specifically on birds), and it is open access here.
Now, it should be noted that the limits on Str mentioned above only apply to cruising - that is, the animal is moving steadily at its efficient, long-distance gait. Burst performance is different, and on average, the Str will be higher during things like rapid climbs (say, just after takeoff) or bursts after prey. Nonetheless, we can get a good idea of how extinct animals worked by using Str. I'll post more on how to use Str, and how to combine it with other equations to solve for multiple variables, soon.
Distance Champs
For those that haven't seen it, there is a press release for the description of Guidraco here. This is yet another example of a large pterosaur whose closest known relative was half a world away (other examples include the anhanguerids/ornithocheirids of Brazil and Europe, giant azhdarchids of Europe and North America, etc). This is rather exciting for me, personally, as I have been suggesting for the last couple of years that large pterosaurs might have had cross-continental travel abilities. I even got to babble about it on NPR: you can check it out here.
Do the Twist
Arguably the most adept fliers in the world are those animals who carry flight as their namesake: the dipteran insects, i.e. the flies.
Alexander Wild (whose photography you should check out if you are not already familiar) recently posted some new shots of fruit flies approach a fungal feeding patch. I am particularly fond of this shot.
Note how the plane of the wings are almost perpendicular to the direction of travel at the moment the shot was taken. The degree of wing rotation used by insects, particularly during landing and takeoff, can be quite extraordinary. Sadly, the precise effects and roles of wing rotation in animal flight are poorly understood. Some good work has been done with bees and flies, but even there we are still quite naive. According to Sharon Swartz of Brown University, next to nothing is known about the role of wing twisting in bats. The knowledge base situation is only marginally better in birds.
Those looking for a great experimental project on animal flight: think about working on wing twist. Theoreticians [which I suppose includes myself, though my work is about a 50/50 split] have some good ideas of what should happen, but we need experimentalists to play it out and get the real nuts and bolts.
Alexander Wild (whose photography you should check out if you are not already familiar) recently posted some new shots of fruit flies approach a fungal feeding patch. I am particularly fond of this shot.
Note how the plane of the wings are almost perpendicular to the direction of travel at the moment the shot was taken. The degree of wing rotation used by insects, particularly during landing and takeoff, can be quite extraordinary. Sadly, the precise effects and roles of wing rotation in animal flight are poorly understood. Some good work has been done with bees and flies, but even there we are still quite naive. According to Sharon Swartz of Brown University, next to nothing is known about the role of wing twisting in bats. The knowledge base situation is only marginally better in birds.
Those looking for a great experimental project on animal flight: think about working on wing twist. Theoreticians [which I suppose includes myself, though my work is about a 50/50 split] have some good ideas of what should happen, but we need experimentalists to play it out and get the real nuts and bolts.
Pterosaur Acrobatics
I received a fun message from David Hone of Archosaur Musings fame yesterday. Here's the relevant bit:
"A biology teacher in Ireland tweeted me to ask if I thought a pterosaur could pull off a loop-the-loop or victory roll or similar. My guess was 'probably' especially something like and ornithocherid, but I thought you'd love it as a thought problem..." -- David Hone
It's a fun idea to ponder. The precise maneuvers available to fossil animals are can be difficult to work out with much confidence, but there are a few things that can be said with confidence:
- The most maneuverable pterosaurs were probably anurognathids, and they could certainly pull off a loop-the-loop or just about anything else you could want from a flying animal. In fact, anurognathids were probably among the most agile flying animals of all time, right up there with living vesper bats, swifts, and the like. I talked a bit about their abilities at Pterosaur.net here. Mark Witton dealt with them more while debunking the vampire anurognathid concept here.
- For larger pterosaurs, things get a bit trickier, but the inertial problems would still be pretty minor. The real question becomes whether the animals could handle multiple extra body weights of force on the wings. Based on work I've done on pterosaur wing strength, as well as work by Colin Palmer, it seems that at least pterosaurs as large as Anhanguera (4-5 meter wingspan) could have handled major rolls or loops. I have yet to check the numbers for something bigger, like Quetzalcoatlus northropi, but I might have to give it a shot. I may post a more extensive conversation on it the idea at H2VP soon.
"A biology teacher in Ireland tweeted me to ask if I thought a pterosaur could pull off a loop-the-loop or victory roll or similar. My guess was 'probably' especially something like and ornithocherid, but I thought you'd love it as a thought problem..." -- David Hone
It's a fun idea to ponder. The precise maneuvers available to fossil animals are can be difficult to work out with much confidence, but there are a few things that can be said with confidence:
- The most maneuverable pterosaurs were probably anurognathids, and they could certainly pull off a loop-the-loop or just about anything else you could want from a flying animal. In fact, anurognathids were probably among the most agile flying animals of all time, right up there with living vesper bats, swifts, and the like. I talked a bit about their abilities at Pterosaur.net here. Mark Witton dealt with them more while debunking the vampire anurognathid concept here.
- For larger pterosaurs, things get a bit trickier, but the inertial problems would still be pretty minor. The real question becomes whether the animals could handle multiple extra body weights of force on the wings. Based on work I've done on pterosaur wing strength, as well as work by Colin Palmer, it seems that at least pterosaurs as large as Anhanguera (4-5 meter wingspan) could have handled major rolls or loops. I have yet to check the numbers for something bigger, like Quetzalcoatlus northropi, but I might have to give it a shot. I may post a more extensive conversation on it the idea at H2VP soon.
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