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.