Updates

Well, it's been months since I ran off to ASB and promised more blog posts, so it seems I should give an update.

I have had to focus on two major tasks since moving to Los Angeles (other than making my apartment livable and such): teaching human gross anatomy and starting work on a new team project in collective behavior and swarming.

I was hired by USC largely for my expertise in teaching anatomy, and so I was well aware of that schedule coming in.  However, I have also taken on a second course, called Clinical Perspectives on Human Anatomy (MEDS 320) which is brand new (I'm building it from scratch).  MEDS 320 is part of a new Minor in Health Care Studies at USC, which is basically an intensive pre-med supplement set of courses designed to get medical school bound undergraduates an edge for applications and their two years in an MD program.  I am having a blast with it, but needless to say building the course has had to take priority over other things (such as blogging).  If you're curious about the program, you can look here: http://dornsife.usc.edu/minor-in-health-care-studies/

The Collective Behavior bit may be a bit of a surprise to those that are used to me being "that pterosaur guy".  To make a long story a bit shorter, I have an eclectic background in biomechanics, zoology, and anatomy (including 8 years of professional animal husbandry work), and those all ended up playing a role in my joining a three-person team to study swarm motion for the Navy.  Our project, entitled "Biologically Inspired Human Supervision and Control of Agent Teams" was approved for funding over the summer, and the grant kicked in September 1st.  This means I have rapidly retooled my schedule and research focus to some extent.  I will be working with live animals (fish), and that requires a full on aquatic motion lab (thanks Office of Naval Research for funding that entire set-up; it's pretty darn awesome).  In addition, I am working to get a hold of whatever existing data I can.  This has left less time for flying things.

All that said, I will be getting back to working on Aero Evo as soon as I can.  The dragon post(s) will be first, and then probably some reviews of recent cool papers regarding animal flight.  In the longer term, this blog may end up being integrated with my new lab website (not active yet, so stay tuned on that front), and the material may begin to include some discussion of decision-making and swarm behavior for fairly obvious reasons.  Don't be surprised if the visual scheme ends up evolving, as well.

I am off to the annual meeting of the Society of Vertebrate Paleontology next week.  I will be presenting on some animal flight related research (tail function in Microraptor gui), so that will also show up here at some point.  I hope to have some material up before I leave, but it's hard to promise as much at this time.

Cheers everyone!

ASB

I will be resuming the Dragon series on Aero Evo soon.  I am currently at the annual American Society of Biomechanics conference (ASB 2012).  Some good talks this morning; I was particularly intrigued by the experiments of T. Gross from Washington University showing that the primary trigger of bone loss following muscle paralysis is not the loss of mechanical loading.  An endocrine or paracrine effect seems to be the primary influence.  Very cool stuff.

Tomorrow I will give my presentation in the Comparative Session, entitled "Mesozoic Speed Demons: Flight Performance of Anurognathid Pterosaurs".  If you happen to be at ASB, come on by.

 

Here There be Dragons

As my previous teaser post suggested, I am going to spend some time on dragons over the next week.  This is mostly because dragons are fun, but also because the shapes of things humans have imagined to fly highlight some myths about animal motion and anatomy. To start off with, though, I would like to look at some real dragons.

Draco volans: Flying Dragon
There are actually 20-30 species in the genus Draco (depending on your preferred taxonomy).  Most members of the group have elongate ribs that they can extend, forming airfoils that allow gliding to varying degrees.  The most famous of the group is Draco volans.  This small lizard can extend gliding surfaces on both the trunk and the neck, and while the aspect ratio of the wings is pretty low (so the glide angle isn't great), these animals can manage glides of tens of meters or more.

The wings, when extended, have an elliptical shape, and while this is not the only way to get an elliptical lift distribution (which is typically desirable) it is one way this can happen.  The relatively short span also keeps the inertia low during turns; anecdotal accounts indicate that Draco are actually quite maneuverable. 

The photograph at left  (taken from here) gives an idea of the scale of the lizards and the structure of the wing. Note that the ribs seem to be relatively compliant; part of each spar is actually cartilage. We might suppose the the planform in Draco is just the result of constraint - in other words, we might suspect that there just aren't many shapes that are viable for a lizard (or other lepidosaur) using ribs to make wings. 

However, the fossil record shows that other wing shapes are viable.  Icarosaurus is pictured at left (image by Julius Csotonyi; note the obvious copyright notice and watermark - respect the copyright, please.  You can find it on his website here).  Icarosaurus hails from the Triassic.  Note that the wings have a high aspect ratio shape and that the overall span is relatively much greater than in Draco.  Interestingly, Icarosaurus was also a substantially larger animal than the largest Draco.  As a result, even with a greater relative span, Icarosaurus sported a higher wing loading.  With the greater AR, it would have had a smaller glide angle (gone further for a given amount of lost height), and with the greater wing loading, Icarosaurus would have glided faster than Draco.  Both of these likely came at the expense of lower maneuverability.  The shape of the spars (i.e. the ribs) supporting the wings in Icarosaurus curved posteriorly near the tips, particularly the ribs the near the mid-section of the wing.  This gave a broad, backswept tip shape to the wing of Icarosaurus.  To the best of my knowledge, the specific aerodynamics of this tip shape have not be investigated in the literature for Icarosaurus.

If you are interested in reading more about Draco gliding, and the evolution of gliding in other, similar, fossil forms, then I recommend reading this paper by McGuire and Dudley.  Sadly, a subscription is required.


Chrysopelea: Gliding Tree Snakes
Yup, that's right, gliding snakes.  They may not have the dragon namesake, but many of the historical reconstructions of dragons show flying, serpentine animals (see text from my most recent blog post before this one).  The closest thing to such an animal among real species are the snakes of the genus Chrysopelea.  Jake Socha and his group have done most of the leg work on understanding gliding in these animals.  You can check out his lab page here.  I have already blogged about these critters here, so I won't go into it at length, but there image below (by Tim Laman, from here - again, respect the copyright please) gives a great shot of how these snakes flatten their bodies during gliding. 

 Vortices are spun off either side of the body in these snakes during glides, and this produces a decent lift profile that allows glides averaging 10 meters of horizontal distance (Socha et al., 2005: available here).  This is particularly impressive because the snakes must use a rather unusual launch method (hanging and dropping from the tail) to take off; all other gliders are able to leap to begin flight, and that helps a great deal.


Next time: a look at some pterosaurs, then we begin building a fantasy dragon and consider the limits of size in vertebrate flyers.

Dragons

I have a soft spot for old texts, and Google Books, BHL, and others are a real boon in that they make some older texts available for web browsing. I particularly enjoy old bestiaries and natural history books, and I was pointed to this little gem earlier today: http://biodiversitylibrary.org/page/38946323#page/105/mode/1up

I am not sure how the aerodynamics would work out on those creatures, but I might have to give it a whirl just for fun.

Stay tuned for a post on real aerial dragons.

Why Turkeys are Like Rockets

The photograph at left was taken by David Hone at the Pittsburgh Zoo.  They are actually quite common as wild individuals in that surrounding area, so it's a bit amusing that the shot ended up coming from the zoo. In any case, I give you this turkey to highlight two brief myths.  

Myth 1: Galliform birds (chickens and relatives) are "poor" flyers.  

This shows up in the literature consistently, especially in paleontological studies seeking to create nice categories among living flyers to compare their plots of fossil attributes to.  Now, I agree that domestic chickens are pretty poor at flying by most estimates, but their derived from a group that is, on the whole, not so much "bad" at flying as quite specialized.  Galliform birds are, on the whole, adapted for burst launching - that is, they spend most of their time on the ground, and when startled, can take off with very high accelerations at a steep angle.  This requires large muscles (including a large pectoralis minor; Galliformes includes species with some of the largest relative pec. minor fractions among birds) and stiff forelimb elements.  In short, because takeoff is energetically and mechanically rigorous, being particularly good at takeoff means being "overbuilt" compared to more typical flyers.  So even though galliform birds, such as turkeys, cannot stay in the air very long (their fast twitch flight muscles get tired quickly) they have more extreme flight adaptations than many other birds.  Note that these avian fast twitch muscles generate huge amounts of power: about 390 W/kg (compared with roughly 175 W/kg or less for aerobic muscle in birds).

Myth 2: Big birds have to run to take off.

This one comes up pretty often in general texts (see Vogel, 2003) and paleontological discussions of flight performance in fossil taxa.  The turkeys apparently didn't get the memo, though, as they are among the heaviest living flying birds and (as discussed above) are not only able to launch without a run, but are actually burst launchers, so they are taking off at a steeper angle than many smaller birds.

As it turns out (and I'll write more on this some other time) running launch in birds has very little association with size, assuming you correct for habitat differences.  You see, water birds are, on average, a bit bigger than land birds, and water birds often run to take off - but that's because of the dynamics of water launching, not size.

Aquaflyers Again: Skates and Rays

Previously I wrote a bit about the wonders of aquaflying in penguins.  This time, I thought it would be fun to write briefly on some of the interesting details of aquaflying in skates and rays.

Not all rays are aquaflyers in the sense I am using here.  Many rays propel themselves by moving a series of waves down either pectoral complex like this.  I'd like to talk more about that in the future, but for now, I am talking about those rays that propel themselves by flapping underwater flight - that is, reciprocating the entire pectoral complex on either side of the body as wings, like this.

Aquaflying rays, such as cownose rays, often move in large groups (see photograph above by Chris Hobaugh.  She retains all copyright; do not use without permission).  There are some quite interesting potential dynamics there in terms of motion in neighbor wakes and combined tip vortex effects, but so far as I am aware there are no data on those aspects for rays, so there isn't much that can be said on it for now (though there might be something on it down the road, hint hint).

One thing that is known, however, is that aquaflying rays move with almost absurdly large advance ratios.  Ridiculous, even.  To understand what this means, we need to examine the idea of advance ratios.

For an airplane with a propeller, advance ratio is simple: it is the ratio of the forward speed over the product of the revolution rate of the propeller and the diameter of the circle made by the propeller blades.  So, we have:

Advance Ratio = v/(f * d)

Where v is forward speed, f is the rate of propeller spin, and d is the diameter of the swept disc.

For a flapping animal, we have to take into account the reciprocating wings/fins, and this can be done using amplitude as an added variable (see Ellington, 1984; Vogel, 2003).  So, this gives us:

Advance Ratio = v/(2*r*f*l)

Where v is forward speed, r is the amplitude of the stroke (in radians), f is the flapping frequency, and l is the wing length.  To get a number you can compare to an airplane or other machine using a propeller, multiple by π.

Now, flapping swimmers often do quite well.  Penguins, for example, manage advance ratios around 0.5, which is quite good for motion in water (Hui, 1988).  However, cownose rays exceed an advance ratio of 2 (Heine, 1992).  This is an extraordinary amount of forward motion for each wing cycle.  The trick is that they use their entire bodies as aquafoils, and therefore get lift (mostly as thrust) not just from motion of the "wings", but also from motion of their bodies.

Now, one thing that's interesting about this in rays is that, theoretically, they should be able to get a highly mirrored stroke.  I mentioned the issue of mirrored strokes in the penguin post, and if you want a more technical discussion check out Habib (2010).  The upshot is that if both the upstroke and downstroke produce similar amounts of thrust, then the animal will proceed at a relatively constant speed, rather than lunging forward on each downstroke.  That "lunging" is called a surge acceleration.  The orthogonal motion (up and down for an aquaflyer) is called a heave acceleration.

Aquaflying animals can waste a lot of energy in surge accelerations if they don't have equal phases to their swimming strokes (example: puffins).  Rays probably have very small surge accelerations, because their stroke cycle is close to a true sine wave and their bodies (which are the aquafoil) are relatively symmetrical in the dorsal and ventral aspects. However, to my knowledge this has not been examined in detail despite the fact that accelerometer data do exist for rays.  If this prediction is accurate, however, rays are getting the best of two worlds of aquatic efficiency: high advance ratios from using the entire body for thrust, and low surge accelerations through stroke mirroring.  Presumably this comes at the cost of some additional heave acceleration, but it's still an awfully good bargin, and some of the rays can get a good head of speed going, too.  So much so, that they can do things like leap multiple body lengths out of the water.  See photo at left (taken from here).  More photos of leaping mobula rays by Barcroft here.













References

Ellington CP. 1984. The aerodynamics of hovering insect flight. Philosophical Transacations of the Royal Society of London, Series B. 305: 1-181

Habib M. 2010. The structural mechanics and evolution of aquaflying birds. Biological Journal of the Linnean Society. 99(4): 687-698

Heine C. 1992. Mechanics of flapping fin locomotion in the cownose ray, Rhinoptera bonasus (Elasmobranchii: Myliobatidae). Ph.D. dissertation, Duke University, Durham NC

Hui  CA. 1988. Penguin swimming. I. Hydrodynamics. Physiological and Zoology. 61: 333-343

Vogel S. 2003. Comparative Biomechanics. Princeton University Press. 580 pp

Guest Post: Thin vs Thick Wings

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.

------------------------
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.

Mosquitoes in the Rain

A recent paper by Dickerson et al. in PNAS explains how mosquitoes are able to fly effectively in rainy conditions (remember: many of them hail from humid tropics), even though a single raindrop by weigh 50x what a mosquito weighs.  If you cannot access the full paper, feel free to read this summary on BBC (complete with video).

Essentially the answer comes down to poor momentum transfer by water droplets to the flying mosquitoes.  The insects have a hydrophobic surface, and most rain drops only score glancing blows, so the water slides off quickly before it can affect the flight path a great deal.  Even direct hits only drop the mosquitoes a short distance, because very little of the momentum actually transfers to the ultralight mosquito - the water basically briefly engulfs them and then continues on its way.  The expanded surface area for wetting on the wings produced by the fringed hair margin mosquitoes possess further improves their ability to shrug off water strikes.

This manuscript answers one intriguing question, but raises some new interesting questions about aerial stability in small insects and body shape effects during flight in adverse conditions.

It even inspired a comic strip.

How Many Mesozoic Birds are We Missing?

Very cool new paper out in PLoS ONE, by Brocklehurst et al. (2012), entitled "The Completeness of the Fossil Record of Mesozoic Birds: Implications for Early Avian Evolution". 

Here's the abstract:
"Many palaeobiological analyses have concluded that modern birds (Neornithes) radiated no earlier than the Maastrichtian, whereas molecular clock studies have argued for a much earlier origination. Here, we assess the quality of the fossil record of Mesozoic avian species, using a recently proposed character completeness metric which calculates the percentage of phylogenetic characters that can be scored for each taxon. Estimates of fossil record quality are plotted against geological time and compared to estimates of species level diversity, sea level, and depositional environment. Geographical controls on the avian fossil record are investigated by comparing the completeness scores of species in different continental regions and latitudinal bins. Avian fossil record quality varies greatly with peaks during the Tithonian-early Berriasian, Aptian, and Coniacian–Santonian, and troughs during the Albian-Turonian and the Maastrichtian. The completeness metric correlates more strongly with a ‘sampling corrected’ residual diversity curve of avian species than with the raw taxic diversity curve, suggesting that the abundance and diversity of birds might influence the probability of high quality specimens being preserved. There is no correlation between avian completeness and sea level, the number of fluviolacustrine localities or a recently constructed character completeness metric of sauropodomorph dinosaurs. Comparisons between the completeness of Mesozoic birds and sauropodomorphs suggest that small delicate vertebrate skeletons are more easily destroyed by taphonomic processes, but more easily preserved whole. Lagerstätten deposits might therefore have a stronger impact on reconstructions of diversity of smaller organisms relative to more robust forms. The relatively poor quality of the avian fossil record in the Late Cretaceous combined with very patchy regional sampling means that it is possible neornithine lineages were present throughout this interval but have not yet been sampled or are difficult to identify because of the fragmentary nature of the specimens."

It's an extensive paper with quite a bit of information regarding discovery bias.  If you're interested in fossil birds and the origins of modern avian diversity, this is a must-read (and open access!)

The manuscript does not discuss flight much (as that's not really the topic at hand), but there is one mention that I thought might be worth discussing here.  The authors note that: "Avian species today, and in the past, are typically small-bodied and lightly built because of the constraints imposed by powered flight."

Overall, this is almost certainly true: birds are (both historically and today) overwhelmingly represented by small species, and flight certainly adds constraints to body size and build.  I am curious, though, whether birds are actually more skewed in their body size distribution than other, non-flying animals.  Most mammals are small, for example (about half of all the mammal species are rodents, and these are mostly quite small).  Squamates and amphibians are also overwhelmingly represented by small forms.  Now, that said, these groups also include some giant forms, and most of the large birds have historically been flightless.  However, some of the larger flying birds (the largest pseudodontorns and teratorns, for example) were reasonably large, all considered. Argentavis may have tipped the scales at 75-80 kg, and while that's not huge, it's well within the body size range of larger mammalian predators alive today (it's more massive than a leopard by a fair margin, for example).

This is not to say that the body size distribution of birds is not skewed by their volancy, but rather than I'm not sure this has been rigorously demonstrated.  Many supposedly "obvious" facts go untested because they seem to intuitive.  Perhaps this is another one worth a serious look.

References

Brocklehurst N, Upchurch P, Mannion PD, O'Connor J (2012) The Completeness of the Fossil Record of Mesozoic Birds: Implications for Early Avian Evolution. PLoS ONE 7(6): e39056. doi:10.1371/journal.pone.0039056

 

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).

Producing Lift

Excepting very tiny animals, all flying species produce more lift than drag (usually by many times), and use lift for weight support and thrust.  To produce substantial lift, a wing must be held at some effective angle of attack to the oncoming flow.  Angle of attack is the angle between the chord and the direction of travel.  Note that effective angle of attack is different from the raw angle of attack – the effective angle of attack also includes the effect of camber, which is curvature in the wing along the chord.  A cambered wing has a positive effective angle of attack even if the raw angle of attack is zero (Pennycuick, 1989; 2008): camber adds to the effective angle of attack.

There are multiple methods for modeling the production of lift, but most engineers now favor the use of a vortex model.  A vortex model works on the observation that a lift-producing foil has two mathematical components to the flow about the foil: a translational component and a circulation component (See image at left).  The circulation is a component only; no fluid actually travels around the wing in a full loop, but there is a component of the overall flow that can be represented as a “bound vortex”: fluid rotating on the wing itself. 

The image at left is a quick schematic I put together that shows flow components of a wing. The translational flow is indicated by A and A’ (above and below the wing, respectively).  The label B indicates the circulation component.  When the wing is at a positive angle of attack, circulation is present on the wing.  The sum of B and A is then greater than the sum of B and A’ (note that the direction of B and A’ are opposite), such that flow above the wing is faster than that below it.

This results in shed vortices: rotational elements of fluid pushed along behind the wings that balance the angular momentum of the vortices on the wings.  It is the circulation that producing asymmetrical flow: the circulation adds to the velocity of the air above the wing while it simultaneously reduces the net velocity of the flow below the wing (Alexander, 2002; Vogel 2003; Pennycuick, 2008).  This produces a differential pressure that pushes upwards on the wing.  The same process can be viewed in terms of momentum: the circulation about the wing means that air coming off of the wing is deflected (generally downwards and backwards, for a horizontally flying animal), and this added momentum means that force is being exerted on the air, which pushes back on the foil (in accordance with classic mechanics, specifically the Third Law of Motion).  The rate of momentum transfer is equal to the total fluid force (Vogel, 2003).

The lift produced by a wing can therefore be examined in terms of vorticity: the strength of the circulation on the wing and the shape and strength of the vortices that swirl behind a flying animal (or machine).  These shed vortices are collectively called a “vortex wake”.  One method of distinguishing modes of flapping flight is through the differences in the trailing vortices, which indicate differences in how momentum is added to the incoming flow.  

Record-Breaker

Among flying birds, some of the strongest wings (structurally speaking) belong to peregrine falcons (that measurement comes from my own work).

Here's why: http://www.airspacemag.com/flight-today/falcon.html

Frightful, the world record holder, exceeds 242 mph in a stoop, and can pull out of such dives carrying a lure equal in mass to herself.  The best comparison I can think of is this: drop down the middle of a spiral stairwell, and catch yourself on the railings at the bottom with your arms.  With a compact car attached to your back.

Clap and Fling

One really cool mechanism used by some flying animals to quick-start lift on the wings is called a "clap and fling": the wings are clapped together above the animal on the upstroke, and then peeled apart.  This forces the vorticity to start on the wings almost immediately, and produces a counter vortex above the animal that results in a handy low-pressure zone above their body.  Insects used this mechanism the most, but some birds do, too.  The photo at left shows a pigeon using a clap and flight during launch.  This is why pigeon takeoff often produces a clapping sound. 

(In the photo, if you look closely, you'll see that the pigeon is just finishing toe-off.  As usual, the legs produce most of the launch power, then the wings will engage immediately - thanks to the clap and fling, the wings will hit max lift almost immediately, and that allows a very steep climb-out for the pigeon after it leaves the ground).

The photograph was taken by Joe Hancuff.  You can check out his work here and here.  He's also on twitter (@joehancuff).  He has a particularly extensive gallery of dancers.

Feathers vs Membranes

A recent discussion arose on the Dinosaur Mailing List that included some questions regarding the relative merits of membrane wings and feathered wings, mostly in the context of pterosaurs vs birds. In that spirit, I thought I'd give a little rundown of the relative advantages/costs of each type of vertebrate wing.

Avian Wings
Birds are the only flying vertebrates to use keratinized, dermal projections (i.e. feathers) to form their wings.  Feathers have the distinct advantage of being potentially separate vortex-generating surfaces, meaning that a bird can split its wing up into separate airfoils, thereby greatly changing its lift and drag profile as required (Videler, 2005).  Tip slots are the most obvious example of this mechanism, whereby the tip of the wing is split into several separate wingtips by spreading the primary feathers of the distal wing.  The alula, which lies along the leading edge of a bird’s wing, and is controlled by digit I, is another example of a semi-independent foil unit (Pennycuick, 1989; Videler, 2005).  The splayed primaries of a slotted avian wingtip passively twist nose-down at high angles of attack (and therefore at high lift coefficients), and this feather twist reduces the local angle of attack at the distal end of slotted avian wings, preventing them from stalling (Pennycuick, 2008).  Slotted avian wingtips may therefore be nearly "unstallable", though this does not prevent the overall wing from stalling (Pennycuick, pers comm.).  Feathered wings can also be reduced in span without an accompanying problem of slack and flutter – the feathers that form the contour of the wing simply slide over one another to accommodate the change in surface area.  Despite these advantages, feathers have some costs as wing components, as compared to membranous wings.  Feathered wings are relatively heavy (Prange et al., 1979) and cannot be tensed and stretched like a membrane wing (which has ramifications for cambering). Theoretically, avian wings should not be able to produce maximum lift coefficients as high as an optimized membrane wing (Cunningham, pers comm.), but experimental data to determine if transient, maximum lift coefficients actually differ significantly between bats and birds are not yet available (Hedenstrom et al., 2009).

Chiropteran Wings
Bats have a wing surface formed primarily by a membrane stretched across the hand, antebrachium, brachium, and body down to the ankle.  Unlike birds, which have a limited number of muscles that produce the flapping stroke (two, primarily: m. pectoralis minor and m. pectoralis major), bats have as many as 17 muscles involved in the flight stroke (Hermanson and Altenbach, 1983; Neuweiler, 2000; Hedenstrom et al., 2009).  The membranous wings of bats are expected to have a steeper lift slope than the stiffer, less compliant wings of birds (Song et al., 2008).  This results from the passive cambering under aerodynamic load that occurs in a compliant wing: as lift force increases, the wing passively stretches and bows upwards, producing more camber, and thereby further increasing the lift coefficient and total lift.  While there are some advantages for a flying animal in having such a passive system, bats presumably must mediate this effect with the many small muscles (and fingers) in their wings – tensing the wings actively while under fluid load will mediate the amount of camber that develops.  This would be important to mediate drag and stall, though no empirical data currently exist to indicate exactly how bats respond to passive cambering.  The work by Song et al. (2008) also indicates that compliant, membrane wings achieve greater maximum lift coefficients than rigid wings, but data have yet to be collected demonstrating that this holds in vivo for bats and birds.  Compared to birds, the distal wing spar in bats is quite compliant (Swartz and Middleton, 2008).

Pterosaur Wings
The structure and efficiency of pterosaur wings is obviously not known in as much detail as those of birds or bats, for the simple reason that no living representatives of pterosaurs are available for study.  However, soft tissue preservation in pterosaurs does give some critical information about their wing morphology, and the overall shape and structure of the wing can be used (along with first principles from aerodynamics) to estimate efficiency and performance.

It is known from specimens preserving soft tissue impressions that pterosaur wings were soft tissue structures, apparently composed of skin, muscle, and stiffening fibers called actinofibrils, though the exact nature and structure of actinofibrils has been the topic of much debate (Wellnhofer 1987; Pennycuick 1988; Padian and Rayner 1993; Bennett 2000; Peters 2002; Tischlinger and Frey 2002).  Associated vasculature is also visible in some specimens, especially with UV illumination (Tischlinger and Frey, 2002).  Recent work on the holotype of Jeholopterus ningchengensis (IVPPV12705) seems to confirm that the actinofibrils were stiffening fibers, imbedded within the wing, with multiple layers (Kellner et al., 2009).  The actinofibrils were longer and more organized in the distal part of pterosaur wings than in the proximal portion of the wing, which may have implications for the compliance of the wing going from distal to more proximal sections.  The inboard portion of the wing (proximal to the elbow) is called the mesopatatgium, and was typified by a small number of actinofibrils with lower organization, which would have made this part of the wing more compliant than the outboard wing.

The outer portion of the wing, which was likely less compliant the mesopatagium, is termed the actinopatagium (Kellner et al., 2009).   Because pterosaurs had membrane wings, they could presumably generate high lift coefficients, but exactly how high depends on certain assumptions regarding their material properties and morphology (pteroid mobility and membrane shape being two of these factors).

Now, for some punchlines...
Based on the structural information above, we might expect the following regarding pterosaurs and birds:

- Pterosaurs would have a base advantage in terms of maneuverability and slow flight competency.

- Pterosaurs would also have had an advantage in terms of soaring capability and efficiency

- Pterosaurs would have been better suited to the evolution of large sizes (though this was affected more by differences in takeoff - see earlier posts about pterosaur launch).

- Birds will perform a bit better as mid-sized, broad-winged morphs (because they can use slotted wing tips and span reduction).

- Birds would have an advantage in steep climb-out after takeoff at small body sizes (because they can work with shorter wings and engage them earlier).  This might pre-dispose them to burst launch morphologies/ecologies.

Interestingly enough, the fossil record as we currently know it seems to back up all of these expectations.  For example, the only vertebrates that seem to have been adapted to dedicated sustained aerial hawking in the Mesozoic were the anurognathid pterosaurs.  Large soaring morphs in the Mesozoic were dominated by pterosaurs, also.  On the other hand, mid-sized arboreal forms in the Cretaceous were largely avian.

Full references for all of the above literature is available upon request.  I'll post the full refs here as soon as I have a chance, but just email me in the meantime if need be (currently traveling in Boston).

Back in Civilization

I am back from the field! A quite successful bit of work locating Late Cretaceous vertebrate fossils in New Mexico. More on that over at H2VP soon. In the meantime, the flight posts shall commence here again shortly. Cheers!

Off to the field

I'm headed to New Mexico for a bit of field work. Won't be posting again until June 5th as a result, but here's a post at H2VP about the field work (Justin is our expedition leader):

http://h2vp.blogspot.com/2012/05/field-work.html?m=1

I also have a guest post at Archosaur Musings:

http://archosaurmusings.wordpress.com/2012/05/30/academics-on-archosaurs-mike-habib/

See everyone when I return!

Beijing Pterosaur Meeting

This was one of four abstracts I was on for the Beijing Pterosaur Meeting a couple of years back.  The next pterosaur meeting is schedule for Rio - more on that in the not-to-distant future.

Soaring efficiency and long distance travel in giant pterosaurs
Authors: Michael Habib and Mark Witton

Azhdarchid pterosaurs include the largest known flying animals, with the largest species reaching a potential mass of over 250 kg.  Prior work suggests that several features of azhdarchid anatomy could be associated with a soaring-dominated lifestyle, including large size, burst-flapping adapted pectoral girdle and proximal forelimb, moderate to high wing aspect ratio, and exceptional pneumaticity.  However, long-range flight ability of azhdarchid pterosaurs has not been quantified in the literature. Furthermore, while the flight of giant pterosaurs has been modeled for a range of large species (Hankin and Watson 1914; Bramwell and Whitfield 1974; Brower 1983; Chatterjee and Templin 2004) and researchers have invariably concluded that they were capable of flight, some recent studies have called into the question the flight abilities of pterosaurs at large body masses (Chatterjee and Templin, 2004; Sato et al. 2009), especially the relatively ‘heavy’ masses in the recent literature (Paul 1991, 2002; Witton 2008). Here we present the results from a quantitative analysis of long-distance travel efficiency in azhdarchid pterosaurs, demonstrating that the largest pterosaurs should not only have been effective flyers, but had the potential to be the furthest-traveling animals known to science.

Power analysis indicates that the largest pterosaurs needed to reach external sources of lift, following launch, before they exhausted anaerobic muscle endurance.  Following climb out, even large azhdarchids should have been capable of staying aloft by using external sources of lift.  A quantitative framework already exists for estimating maximum migration range in soaring birds using thermal lift.  We have extended this framework to pterosaurs by altering existing models to accommodate the membrane wings of pterosaurs and uncertainty in potential muscle physiologies.  Maximum fuel capacity (stored as fat and additional muscle) was estimated by taking the difference between body masses scaled from skeletal strength (maximum) versus mass for maximum wing efficiency (maximizing lift coefficient according to reconstructed aspect ratio).  This new migration model indicates that the largest azhdarchid pterosaurs had the capacity for non-stop flights exceeding 10,000 miles.

The ability of large pterosaurs, especially azhdarchids, to effectively reach external sources of lift was great augmented by 1) adaptations for a powerful launch (Habib, 2008) that would allow them to exceed stall speed without utilizing excessive amounts of valuable anaerobic capacity, and 2) adaptations for rapid generation of full circulation on the wing, which would have substantially reduced the time and energy expenditure of climb out. Approximately 2.5 chord lengths are usually required before a wing develops full steady state circulation, known in the literature as the “Wagner Effect” (Wagner, 1925). Analysis of the tensile support in azdarchid wings suggests a potential for rapid translation and twisting of the outboard wing, which would be promoted by the T-shaped cross section of the wing phalanges.  Such rapid translation can develop full circulation up to five times faster than otherwise possible and greatly reduce the flapping cycles needed to reach maximum circulation during climb out, an observation previously made by at least one other pterosaur worker (Cunningham, pers comm.) but previously unmentioned in the formal pterosaur literature. These improvements to the efficiency of the initial climb out from launch would have extended the required proximity to external lift sources, and broadened the potential habitat range of giant pterosaurs. 


Literature Cited

Bramwell CD, Whitfield GR (1974). Biomechanics of Pteranodon. Philosophical Transactions of the Royal Society of London 267: 503-581.

Brower, JC (1983). The aerodynamics of Pteranodon and Nyctosaurus, two large Pterosaurs from the Upper Cretaceous of Kansas.  Journal of Vertebrate Paleontology. 3: 84-124

Chatterjee S. and Templin RJ (2004).  Posture, Locomotion and Palaeoecology of Pterosaurs. Geological Society of America Special Publication, 376, 1-64.

Habib, M.B. 2008. Comparative evidence for quadrupedal launch in pterosaurs. Zitteliana, B28, 161-168.

Hankin, EH and Watson DMS (1914). On the flight of pterodactyls. Aeronautical Journal, 18, 324-335.

Paul. G. S. 1991. The many myths, some old, some new, of dinosaurology. Modern Geology, 16, 69-99.

Paul GS (2002) Dinosaurs of the Air: The Evolution and Loss of Flight in Dinosaurs and Birds. John Hopkins University Press, Baltimore. 472 p.

V is for vulture

On a bus headed towards Washington, DC and watching some turkey vultures soar in the distance. They are easy to pick out because the fly with their wings in a vertical "v" shape - this is called a dihedral. A dihedral tends to stabilize a flyer in roll (though this depends on the sweep of the wings). In strong gusts, vultures will roll back and forth passively, which we call canting. Just a fun aero fact for the day.

KLI

Another in the series of abstracts.  This was my abstract for the think-tank conference at the Konrad Lorenz Institute in Vienna, Austria in September 2010.  These are invite-only sessions on various hot topics related to evolutionary biology.  Ours was on the "Constraints and Evolution of Form" - basically an Evo Devo related gig.  I was the resident biomechanist for this one.

Emergence of convergent forms under fluid load in plants and animals
Very few biomechanists examine both plants and animals in parallel, apparently under a tacit assumption that the rules of shape determination must differ substantially between such distantly related groups.  However, convergent structures suggest that the rules of shape governing these groups are largely the same.  Such similarities suggest that environmental constraints are important in determining shape, and/or that genomes are more plastic and prone to morphological convergence than often accepted.  I suggest that reference to physical first principles should be made whenever shape is examined in multi-cellular organisms, regardless of their phylogenetic position.  As a case example, I report on the presence of highly convergent structures related to resistance and passive yield under aerodynamic fluid load in plants and animals.  I utilize examples from both living and fossil forms, including broad-leafed trees, neornithine birds, and azhdarchid pterosaurs.

Walker Symposium

This was my abstract (co-authored with David B. Weishampel) for the Walker Symposium at the SVP 2009 Meeting.  It got a bit complicated as my Romer session was concurrent with this one...

Flight morphology and launch dynamics of basal birds, and the potential for competition with pterosaurs
Birds inherited a bipedal gait and feathered airfoils from their theropod ancestry. These features produce specific tradeoffs with regards to launch, maximum size, lift coefficient, and limb disparity. There are subtle effects related to the use of feathered wings, such as the ability to utilize separated wingtip slots and extensive span reduction, which have also influenced avian flight evolution. Combining information from structural mechanics, aerodynamics, and phylogeny, we conclude that the basal state for avian takeoff was a leaping launch, not a running launch. We find that several morphological features of early birds, inherited from theropod ancestry, predisposed them to radiation in inland habitats. We find that Archaeopteryx could sustain substantial loads on both its forelimbs and hindlimbs, but structural ratios between the forelimb and hindlimb of Archaeopteryx are indicative of limited volancy. Limb strength in Confuciusornis was modest, suggesting an emphasis on cruising flight and limited launch power.  We find little evidence to support extensive competition between birds and pterosaurs in the Mesozoic. Prior literature has suggested that pterosaurs competed with early birds for resources and may have helped shape the early evolution of birds. There is some evidence of partitioning between pterosaurs and birds in ecological space. Evidence from the Jehol fauna suggests that pterosaurs dominated near coastlines during the Early Cretaceous, while birds were more diverse and important inland. However, flight is not a single, compact character. Flight mechanics vary considerably across volant animals. Some flyers experience only limited competition for resources with other flying species, and might compete most intensely with non-flying taxa. As a baseline for understanding the interactions between Cretaceous birds and pterosaurs, the flight dynamics of the two groups need to be compared in a quantifiable framework. Birds and pterosaurs inherited different morphologies, and this impacted their flight regimes. Comparing the two systems provides a basis for hypotheses related to competition in the Cretaceous, and the influences on early avian evolution.

To Quad or Not to Quad

I received a really good question about the relative advantages of bipedal and quadrupedal launch at the Royal Tyrrell Museum.  Essentially, they asked what the relative performance tradeoffs of each one might be, and why pterosaurs might have ended up locked into a quadrupedal launch style.

As it turns out, there are really two parts to the answer.  In terms of which launch mode ends up as the dominant method of takeoff within a clade, it is likely that phylogenetic inertia plays an important role: birds inherited an obligate bipedal stance from their ancestors, and so every bird (so far as we know) has been a biped and launches bipedally, at least from the ground (more on this later, but some birds are quad launchers from the water, which is pretty neat stuff).

Bats seem to have inherited an obligate quadrupedal lifestyle.  Pterosaur origins are more fuzzy, but they probably arose from one of a few different groups where bipedal to quadrupedal transitions were more common, and so their early evolution may have been more phylogenetically plastic with regards to stance.  They eventually ended up as obligate quadrupeds, with most species placing most of their weight on the forelimbs (this is apparent because manus tracks from pterosaurs are typically deeper than the pes prints). 

In terms of the relative performance advantages, it turns out that we can solve the question algebraically:

1) Both forms of launch will start as a leap (or a run ending in a leap).  This means that the immediate post-launch cycle is ballistic.  That's handy - ballistic math is easy.

2) Quad launch adds an upstroke immediately after push-off.  Bipeds can (and do) raise the wings as they toe-off, so they can engage the first downstroke as soon as the wings have clearance.

3) Quad launch adds more power for initial push-off, so this goes into the ballistic equation.  This will mean more height and speed, but at the cost of the added upstroke (the extra upstroke is accomplished with folded wings, so it's quite quick).


So, all we have to do is have an idea of max acceleration and unload time for takeoff, which gives launch speed, combined with the launch angle.  We can vary these a bit to get a range of plausible values, and these give us the ballistic trajectory.  So, for example, the maximum height gain can be calculated from the launch velocity squared x sin(launch angle) squared, divided by 2 x gravitation acceleration.

It turns out that for just about any flying animal, quadrupedal launch does better in nearly every way.  They get a lot more power (because the flight muscles are so strong and can add to the launch in a quad takeoff, whereas in a biped takeoff they add very little).  This means more clearance, ballistic time, and speed.  It also allows for a greater range of starting wing attack angles, and is essentially "safer" because of the much greater clearance for the wings and body (larger margin of error, as it were).  The extra time in the air from the greater push more than makes up for the extra upstroke time.  For example, even in a giant like Quetzalcoatlus northropi, the initial upstroke would only take about a tenth of a second.  It would have almost a third of second to reach the top of the ballistic leap, however, giving plenty of time to spare.

So, on the whole, quad launch is just "better" - with one exception.  A bipedal launcher with short wings and a very short flapping time can switch from ballistic phase to flapping phase a bit earlier.  This is not as efficient as the quad option, but it can mean a steeper and more immediate climb-out.  This is only useful for a burst-launching specialist at moderate or small sizes (at giant sizes quad launch is king), but it is perhaps noticeable that this exact set of morphological features and takeoff strategy is extremely common among living birds - it is highly typical of galliform birds (pheasants, grouse, etc), pigeons and doves, and many of the passerines.  It is also perhaps telling that there are no particularly short-winged pterosaurs.  For a quadrupedal launching animal, very short wings don't do nearly as much good.  The closest example might be anurognathids, and as noted in my GSA abstract, they are quite unique among pterosaurs.  More on that later...

Frogmouth Pterosaurs

I have (finally) some new material to post.  In addition to the new stuff, I've decided to post some of my past abstracts that might not have been easily accessible to everyone.  Here is my abstract from the GSA Northeast Conference in 2011.  I gave a platform talk on the biomechanics of anurognathids (some of you will already know that Mark Witton and I have a manuscript nearing completion on the topic, as well).

Functional Morphology of Anurognathid Pterosaurs
Anurognathid fossils include several exceptionally well-preserved specimens, some of which include extensive soft tissue preservation.  This exceptional amount of morphological information makes anurognathids prime candidates for functional biomechanical analysis.  Furthermore, anurognathids displayed a suite of unusual characteristics that make them of particular interest for functional study.  These traits included extensive pycnofiber coverings, fringed wing margins, shortened distal wings, shortened faces, and enlarged orbits. Prior authors have suggested that anurognathids were adapted to catching small insects on the wing.  I present a quantitative analysis that supports this general behavioral inference, and provides details regarding probable anurognathid locomotion. Results indicate that anurognathids were exceptionally maneuverable animals.

Bone strength analysis in Anurognathus ammoni reveals that each proximal wing was capable of supporting nearly 22 body weights of force.  The wing spar of A. ammoni was substantially stronger in bending than that of an average bird of the same size (residual of 0.72).  The calculated relative bone strength overlaps significantly with that of living birds that capture prey on the wing (p>0.92) but differs significantly from all other avian morphogroups (p<0.04).  Overall humeral robustness is similar between A. ammoni and megadermatid bats.

Anurognathid launch appears to have been particularly rapid and steep. Once airborne, anurognathid pterosaurs could likely generate high lift coefficients.  Leading edge structure in Jeholopterus suggests that anurognathids were capable of generating a leading edge vortex (LEV) as observed in some living bats and swifts.  Analysis of flapping efficiency suggests that the expansion of the proximal wing, coupled with reduction of the distal wing elements, would have increased flapping power at the cost of increased drag.  The proportions of the wing and details of the shoulder may be indicative of the ability to hover for brief intervals; power analysis also supports this conclusion. These results are consistent with reconstructions of anurognathids as highly maneuverable flyers, preferentially foraging in cluttered habitats on small aerial prey.
 

Gettin' Gigantic

I wrote up a short post about the potential selective advantage of giant size in pterosaurs over at the pterosaur.net blog.  You can check it out here.  I'm finishing the Chatham Maymester right now, so that's all for the moment.  Perhaps a bit more later today.

Thrust, lift, drag

I have had a smattering of questions lately about the roles of lift and drag in animal flight. An extensive review would require a book, but here are some basics:

1) Lift is the component of fluid force that is directed perpendicular to flow. This need not always mean that lift is directed upwards. For example, thrust in animal flyers is actually a component of the generated lift. By angling the flight stroke such that the power stroke sweeps down and forward, flying animals point some of their lift forward as thrust. The distal part of the wing produces more thrust and proportionately less weight support. As you move further inboard (proximal) on the wing, weight support becomes more important and thrust contribution diminishes.

2) Drag is the fluid force component parallel to the fluid flow. Most flying animals fly at a lift:drag ratio above one. This will typically mean that they use lift as the primary source of weight support and propulsion, and that minimizing drag improves propulsive efficiency. However, the situation can be complicated. Drag can contribute to weight support, and very tiny insects fly at L:D ratios less than one - as such, they paddle through the air rather than use true, lift-based flight.

In the aquatic realm, both drag-based and lift-based propulsion is common. The former is particularly used in fast starts. For flyers, the same principle applies during their equivalent of a "fast start": namely, takeoff. I'll be writing about that basic derivation later.

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.

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.

The Strength of Color

Had a great extended dinner meeting with a friend and collaborator this evening to get a project rolling on the micromechanics of feathers.  One of the key features will be sorting out how pigments and structural colors affect the mechanical properties of feathers.  It's well documented that some pigments (melanins, particularly) strengthen feathers - but we don't know yet how much, by exactly what mechanism, and the relative effects on performance.  Other pigments probably also have an impact, but that's even more mysterious. 

Why do we care how feathers work?  Well, as a biologist with a strong interest in the evolution of flight in birds, I obviously have a personal stake in knowing more about feather mechanics.  But here are some traits of feathers that might make them interesting models for those with a more applied interest:

- Feathers have a high strength to mass ratio (particularly with regards to bending)
- Feathers are abrasion resistant
- Feathers are good thermal insulators
- Feathers are fast to replace - they are manufactured quickly with precision
- Feathers absorb impacts well
- Feathers are water resistant
- Feathers have notable aerodynamic properties (duh)

That's a rather solid set of attributes for a single biological structure.  With growing interest in biomaterials, we expect that feathers might hold some very intriguing clues about efficient material use and pigment effects.  Here's hoping!

Soaring is Good

Qualitatively, soaring flight is typically associated with large size in living flyers.  Only relatively large bats have been recorded soaring often, and soaring flight over long distances is well documented for many large birds (vultures, gannets, albatrosses, eagles, etc).  Soaring flight is less well documented for small birds, and it has typically been presumed that this is because gliding and soaring is less energetically useful for small birds than big ones.  But, as it turns out, migrating with long gliding phases is an efficient way to go even for relatively small birds - or at least, for one species of small bird.

Sapir et al., in a neat paper in PLoS ONE showed that bee-eaters run much lower heart rates when gliding and utilizing soaring flight than in continuous flapping flight (see figure from their paper at left).  If heart rate measures metabolic expenditure they way they suggest, then this means bee-eaters still get quite a good deal using unpowered flight mechanisms over long trips.

Now, this does not mean that soaring flight is not still more important to the biology of large flyers.  One reason that soaring might still be more critical to the evolution and ecology of giant flyers compared to average-sized ones is that long bouts of continuous flapping flight simply aren't available to large flying animals.  As size increases, flying animals start to face problems with mass-specific power scaling.  This is solved by laying down large fractions of anaerobic (i.e. "fast twitch") muscle.  Those high-powered muscle fibers have low endurance, however, so large flyers necessarily can only flap for short bursts - then they have to switch to unpowered phases of flight, and therefore use external sources of lift.  It is not surprising then, that large flyers also, more often than not, have more adaptations related to soaring than small sized flyers, and this means that large flying animals are probably better at soaring that small ones most of the time. 

Still, papers like Sapir et al. (2010) are important in dispelling our myths about the effects of size in animal flyers.  It is a greatly misunderstood area of biology, and one fraught with centuries of engrained concepts based on human intuition instead of careful measurement and analysis.  It's good that modern researchers are taking a second look at the biology of size in animal flyers.


References

Sapir N, Wikelski M, McCue MD, Pinshow B, Nathan R (2010) Flight Modes in Migrating European Bee-Eaters: Heart Rate May Indicate Low Metabolic Rate during Soaring and Gliding. PLoS ONE 5(11): e13956. doi:10.1371/journal.pone.0013956

Soap Box Moment

One of the quirks of being an animal flight specialist is that there are not many of us.  If you consider those that focus on fossil taxa, there are even fewer.  If I focused my publications to communicate only to other members of my field, I'd be talking to about 12 people, tops.  I might as well just send postcards.

But, as it turns out, there are thousands (if not millions) of people out there that find information on animal flight fascinating or even practical (see: robotics and aeronautics engineers).  So, I have a large, but dispersed and eclectic audience out there to reach.  How on Earth can I get to them all?

Easy: Open Access Publishing. 

Many of you already know that I'm rather firmly in the OA camp.  I admit that I have not done my part to promote OA in the same way as individuals like Andy Farke, Matt Wedel, and Mike Taylor (hats off to you guys!) but I at least favor OA journals like PLoS ONE for my publishing needs and give a nudge here or there when I can.

On that note, there is a great article out by Michael Eisen here that gets right to the core of the wimpish way that universities have dealt with a growing problem.  I would point out, as well, that while the academic institutions are the largest offenders in this case, other businesses have also fueled the fire (see: biomed engineering companies, etc).

Read and enjoy.

Viscosity

Had to work all night last night, so a quick post today. Figured I'd do a quick technical note.

One of the terms you'll see me use quite often here is 'viscosity'. The formal definition is that viscosity is the resistance of a fluid to changes in rate of shear. In colloquial terms, this means how thick and sticky the fluid feels (high viscosity means 'thicker' fluid). What you might not know is that the effects of viscosity aren't constant across body sizes: little things experience a stickier world. As it turns out, in biology, it is the relative viscosity that matters. For a tiny insect, air feels thick and heavy - the smallest flying insects basically paddle through the air, rather than fly in the normal sense.

I'll be talking more about this soon, but it's a great item to keep in mind as it explains a lot of biology with only a few simple rules.

Microraptor: Odds and Ends

The top image on the left is from Hone et al. (2010) and shows the holotype of Microraptor gui under UV light.  The image below, by Mick Ellison, shows a life restoration of Microraptor, and was taken from here (note: the hindlimbs could not actually get into the position shown in the image; that was done to show off all of the airfoils at once for comparative purposes).   One of the key questions regarding flight in Microraptor is whether it evolved flight independently of avialans, or if it represents a morphology that was a more direct precursor to flight in birds proper.

One thing I noticed a few years back is that it seems that Microraptor had a different set of "solutions" to the problem of aero control, as compared to living birds.  I have since put some math to it, and the calculus bears out the intuition.  Myself, Justin Hall, David Hone, and Luis Chiappe are writing this up now (see earlier cryptic blog post), but Justin has given a couple of talks on the hindwing use recently and some of you out there know that that I have been murmuring about the tail being used in aero control.  All will be revealed in the full manuscript (WFTP moment) but I do think it is quite interesting that the aero control surfaces in Microraptor took advantage of pre-existing maniraptoran anatomy.  In other words, you don't have to do much to your average dromaeosaurid to get it into the air.

This is a potentially critical observation.  For one, it suggests that the origin of flight in dinosaurs may have been more simple than previously supposed.  It also suggests that flight control may have had more to do with the gains and losses of aerodynamically active morphology we see near the origin of birds than simple weight support.  I am sad to say that most paleontologists don't seem to have a particularly good grip on what lift actually is, how it is used, and how it is generated.  Many of my colleagues also seem to struggle with how drag fits into the whole scheme.  Of course, I have lots of gaps in my knowledge, too, so I can't go pointing fingers.  Nonetheless, I suspect that we are going to see a major overhaul of the models for dinosaur flight evolution in the year or two.

The Ellison image is associated with a recent paper by Li et al. (2012) in Science.  The authors favor display characteristics for some of the feathered morphology, particularly the tail fan.  I don't discount this function at all, but it should be noted that it doesn't take much to provide a decent stabilizer or control surface for a mid-sized flying animal, and display surfaces don't have to be aerodynamically useless or costly.  (Just to shore a common myth, that is not the same as saying that tail fans, crests, flaps, etc would act as rudders on flying animals.  As a general rule, rudder use does not work well for a non-fixed wing flyer.  Even fixed-wing aircraft do not initiate turns by using rudders; the rudder systems are for stabilization).

References
Hone DWE, Tischlinger H, Xu X, Zhang F (2010) The Extent of the Preserved Feathers on the Four-Winged Dinosaur Microraptor gui under Ultraviolet Light. PLoS ONE 5(2): e9223. doi:10.1371/journal.pone.0009223


Li Q, Gao KQ, Meng Q, Clarke JA, Shawkey MD, D'Alba L, Pei R, Ellison M, Norell MA, Vinther J. 2012. Reconstruction of Microraptor and the Evolution of Iridescent Plumage. Science 335 (6073): 1215-1219

Spiders Incoming

Vadas Gintautas and I have done some more data collection on the spider aero-control work at this point, and the results are proving to be pretty darn exciting.

To bring everyone up to speed, here's the gist. We are working on the issue of aerial righting in spiders.  The way in which spiders right themselves after a fall from a ledge, branch, etc. has never been reported in the literature.  It is an interesting problem, because:

1) Spiders can't twist around in the air like cats and other mammals can.

2) Spiders that pursue prey over uneven terrain, rather than using burrows or webs for capturing food, are bound to fall sometimes.

3) Most spiders are small enough that they are going to fall in an intermediate Reynolds number regime - i.e. the air will be pretty sticky for them, but they won't just be "sinking" like a tiny gnat would.

4) Spider body plans are already of interest in robotics designs for movement over land.  Now imagine that the spider-bot can turn itself in the air using wake capture so that it always lands on its feet.  Yup, it's awesome.

What's particularly interesting here, though, is that we are not using the spiders we would expect to be particularly good at aero control.  We are using Black Hole Spiders, Kukulcania hibernalis, which hunt and move mostly over the ground.  They can climb, though, so we figure they might still have some kind of aerial righting response.  If they do, then it would suggest that aero control appears in even those species with limited arboreal habits.  A photo of K. hibernalis is shown at left.  It comes from the site of Ken the Bug Guy here, which is where we purchased our spiders.

What's cool is that we have found that our little spider stuntmen can, indeed, right themselves in the air, and they do it using a nice, simple trick: when dropped, the spiders immediately assume a leg position that makes them passively unstable in roll while inverted, but stable in roll when upright - this means that the spiders automatically flip right-side-up if they happened to be falling upside down, and then they stay that way for the rest of the trip.  All the spiders have to do is hit the critical position and hold it.  We are working on the algebraic solutions that demonstrate why the righting position works, but our simulation study already predicts their motion quite well (i.e. it matches our imaging studies of the real spiders).

The aerial righting response we have captured is somewhat similar to that seen in other arthropods, such as ants and stick insects.  Since we have found it in a mostly terrestrial animal, it is looking ever more likely that this is a method of aerial righting that is extremely widespread in terrestrial arthropods, and that's very neat stuff for understanding arthropod evolution (which is handy because arthropods represent most of the animal species on Earth).

We are not the only group working on falling spiders.  As I mentioned recently in an earlier post, Robert Dudley of Berkeley has a student working on arboreal spiders from the neotropics that, not surprisingly, are even better at aero control (I won't get into details because they have a paper pending, but it's awesome).

Vadas and I are setting up to work on some additional species of spiders, in order to take a more comparative approach.  We will include some arboreal spiders that we expect will be able to actually glide.  Not kidding; gliding spiders.  There are all sorts of other aerial behaviors we can find in spiders, as well, that take into account the use of silk for producing drag lines and parachuting.  There is an entire realm of aerial acrobatics in spiders that has been rather understudied (ballooning being the only one of the bunch to get much play in the literature).  So stay tuned for spinning, dropping, gliding, righting, leaping, parachuting spiders.

Back in Action

After over a week away (EB2012 and writing a grant proposal for the Office of Naval Research) Aero Evo is back in action. Multiple posts planned for later today - stay tuned.

One cool note of relevance to animal flight from EB2012: apparently the arborescent ganglia of honeybees become considerably more robust once they begin flying regularly.  The onset of flight is associated with a metabolic boost, and that might be related to the expansion of the neural architecture.  However, it also possible that the visual stimulus is important (flying animals need to keep track of rapidly changing horizon lines, etc).  More on this in a bit.

EB2012

Greetings all! I am en route to Experimental Biology 2012 in San Diego (actually started today but I had to teach yesterday). I will not be able to post much substantial until I get back.

I would like to take this moment to suggest that my fellow paleontologists also make a point of attending conferences with a neontological theme. I know that a handful of other paleontologists will be there at EB2012, but it would be great to see more of you at such gatherings. I also suggest that those who spend a significant amount of time working as theoreticians (myself included) take opportunities to exchange notes with experimentalists as much as possible.

Sure, some of us do a bit of experimental work, but I'm talking about the real down-and-dirty experimentalists. The ones that know their model organisms better than their own house. That's where most of our raw data ultimately arise (the other chunk comes from hardcore field biologists, who deserve similar respect).

Cross those discipline boundaries folks! See some of you in San Diego.

Giant Flyers: to the limit (or not)

I am preparing for my departure to the Experimental Biology Conference in San Diego (starts today), so another brief post today.

One of my pet projects over the last couple of years has been to make a go at estimating the maximum mechanically allowed size for each of the groups of  powered vertebrate flyers: birds, bats, and pterosaurs.  This is a rather difficult task for many reasons, not the least being that working out the "weak link" in any given scaling problem is time consuming.  Pterosaurs provide a special challenge since they provide no living representatives to work on.

I have a preliminary model now, however, and I am rather excited about it.  Because it's still rough, I am not going to get into too much detail, but here are some of the punchlines if I turn out to be right with my approach (emphasis on "if"):

- None of the flying clades ever produced an animal at the mechanical flight limit for the group, which means that ecological limits or other constraints have historically set the maximum size.

- Of the three clades, birds came the closest to reaching their mechanical limit (Argentavis)

- Giant pterosaurs, even though they were the largest flying animals, were further from their mechanical limit than the largest flying birds.

- Bats have the lowest absolute limit, but they also have the greatest gap between observed max size and potential max size.  As a result, ecological constraints on size might be particularly strong for bats.  A 3 meter wingspan bat does not seem impossible with the numbers I have right now.  That would more than double the maximum span of the largest bat on record.

All of these limits are estimated using the most giant-friendly morphology in each clade: teratorns for birds, azhdarchids for pterosaurs, and megachiropterans for bats.  There could be altogether novel morphologies out there for each group that would push the limit higher, though for various reasons I can get into later, I think the limits I am calculating are somewhat generalizable.