Friday, June 29, 2012

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.


Tuesday, June 26, 2012

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. Lagerst├Ątten 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

 

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).


Thursday, June 14, 2012

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.  

Wednesday, June 13, 2012

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.

Sunday, June 10, 2012

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).


Friday, June 8, 2012

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!