The Square-Cube Law and Giant Fliers: Part Two

In the last post, we learned a little bit about the square-cube law and how it influences flight mechanics. To recap, the bigger animals get, the harder it is to fly because the effect of weight increases faster than the effect of lift. This makes things tricky for “giant fliers.” Airplanes can overcome the issue through massive amounts of thrust, or forward motion. This is because more thrust makes air move more quickly around the wings, which increases lift.

But animals don’t have jet engines. They have muscles, and guess what? Those are stuck on the “square” end of the square-cube law too! This is because the amount of force a muscle can move depends on the area of its cross-section, rather than the muscle’s overall volume.

So how do big birds get more thrust?

The simple answer is, with thick chest muscles. Flying birds have a specialized ribcage with a “keel” that juts out of the front. This gives their flying muscles more area to connect to and lets the muscles themselves grow thicker relative to the bird’s size.

Pterosaurs had keels too, by the way.

As a side note, see how long and thin-looking that parrot’s neck is and how short the wings look? That goes back to how birds keep themselves as lightweight as possible. A lot of what we see in birds is just their feathers, especially in the wings. This helps keep the wings big and flexible without adding too much extra weight!

But all that can only go so far. The bigger a flying animal gets, the more it has to compensate for the square-cube law. And the bigger the wings get, the trickier it is to take off. So how did the giant fliers of the ancient world, and the slightly-less-giant fliers of today, manage it?

The bigger they are, the harder they soar

I had to do some digging here. It’s harder to tell how extinct animals worked when you can’t, you know, watch them do it. Looking at modern day giants like the condor and the albatross, you might start to notice that these birds do a lot of soaring. This lets them take advantage of things like wind and thermals to help with lift. That way, they spend less energy on flapping and takeoffs. They also tend to take advantage of wind and sloping lands or cliffs to help get into the air.

It’s believed that teratorns, extinct relatives of the condor and the largest known flying birds to exist, were similar. These giants, some of which were comparable in size to small planes, had wings that were too large to take off just by flapping. Scientists believe they used strong winds and slopes to get going, and then soared through the sky in search of food. Also like their modern relatives, the food they were most interested in was probably carrion.

Quetzalcoatlus, the biggest of the pterosaurs and so huge that scientists have debated over whether they could fly at all, are now thought to have also been soarers. Their takeoff strategy is still under debate, since they moved on the ground differently from birds. But it looks like they did their actual hunting on the ground like modern-day storks.

So, in short: giant fliers have really big wings, relatively light bodies for their size, and really, really like strong winds.

Okay, but can I ride one?

Unfortunately, the idea of flying mounts is where the whole concept of giant fliers starts to fall apart. Notice how all of the animals we just discussed are or were either carrion eaters or ground- or water-hunters? Yeah, being built for soaring isn’t really conducive to carrying off your meal. And putting all that effort into making yourself relatively light and then carrying your own weight makes taking heavy loads (like, say, a grown human) into the air tricky. It’s hard to say for sure, since we’re still working out the details on animals like Quetzalcoatlus… But it would be difficult at least to make a flying animal mount work on hard science alone.

Fortunately, this is the point where we remember we’re talking about animals in a fantasy setting.

Now, I’m not saying we should ignore everything we’ve just discussed and focus on just making cool stories. That’s perfectly acceptable, but we wouldn’t be here if we weren’t interested in getting scientific. What I’m saying is that, once you understand the science, you can start letting your magic system work with it.

Let’s say your setting has elemental magic. Maybe critters like dragons and gryphons have a natural ability to manipulate the way the air flows over their wings. This could increase lift and even thrust enough to compensate for both their weights and whatever they might be carrying. It would even make an engaging description, if you take the time to talk about how the unnatural rush of wind feels as it flows around the rider.

Or maybe there’s a band of rocs that are literal living teratorns nesting near your kingdom, and the local wizard has just figured out a way to manipulate gravity. As the spell for lightening oneself becomes more available, people could start taming some of the rocs as flying mounts or pack animals.

And that’s the great thing about fantasy: the world is yours to tinker with and expand beyond what we already know. Bringing real science into the mix just gives us some new foundations to work with.

Sources: Earthlife, Functional morphology of Quetzalcoatlus