Are Capes Aerodynamic?

By Daniel Kolitz on at

Not all superheroes wear capes, but many do, and it’s a long-established fact that these capes are crucial when it comes to flying around town to fight crime and brood and whatnot. And yet the people still don’t know, with anything approaching certainty, how exactly capes facilitate this process (at least, the capes that aren’t themselves imbued with the power of flight).

To clear up the confusion, for this week’s Giz Asks we reached out to a number of experts in engineering to figure out whether capes are actually aerodynamic. The consensus seems to be that capes are not really that aerodynamic at all, when you get right down to it. Just the opposite, really. But the mechanics here are interesting, and for those of you still interested in cape-assisted flight, a number of plausible alterations are proposed below.

Jamey Jacob

Professor, Mechanical and Aerospace Engineering, Oklahoma State University

Capes by themselves are not aerodynamic in the sense of producing lift or reducing drag. Thus, Superman’s cape serves no real purpose other than complimenting the Man of Steel’s costume colour scheme, or providing some moderate warmth from upper level air currents. However, they do have an aerodynamic impact. Much like a flag flapping in the breeze, they would generate a drag force and flutter under certain conditions. Thus, from a purely aerodynamic perspective, an unsupported cape is detrimental—not beneficial—to flight.

That said, it is possible to make a cape provide useful aerodynamic forces if you can design it to retain a specific shape. Hang gliders or Rogallo wings do this for example, by stretching a thin fabric between a supporting frame, while parafoils do this by using ram air to inflate the fabric. In these cases, the flexible fabric forms an aerofoil shape to produce a lift force. Thus a cape with this structure could be configured as a wing to produce aerodynamic lift and provide limited flight control for a superhero. Wingsuits do this by enhancing the a skydiver’s ability to produce limited lift by providing additional surface area and allowing the skydiver to control the distribution of aerodynamic forces using their limbs. However, this is moving away from what we normally consider to be termed a cape.

As Edna Mode said, “No capes!”

Geoffrey Spedding

Professor, Aerospace and Mechanical Engineering, USC

In some way [yes, they are], since anything that travels through the air experiences aerodynamic forces, then of course they must be. But the real answer is no. The question really means can they generate useful aerodynamic forces, and as usually configured, they will instead flutter uselessly like flags.

You could get them to act like wings, if you could have a leading edge spar that stiffened, and then some trailing spars and/or some way of maintaining tension in the cape. Perhaps the trailing edge would be elastic and then fixed tightly to the hips in a belt that could tighten the sheet up. What you have now ended up with is something that resembles a bat wing, or a hang-glider. Bats and hang-gliders use flexible membranes and they work quite well.

Is the human body aerodynamic?

We might look at expert parachutists as they free-fall, who show that all kinds of tricks can be performed by small shape changes at speed. We also have wing suits, where a small triangle goes from arm to body on each side. When the speed is sufficiently high, the wings don’t have to be very big, or efficient, to generate enough lift to support the weight. I understand one of the tricky parts is figuring out how to slow down in controlled fashion.

Bottom line—I can imagine an aerodynamic Batman and his cape, provided there is something to provide thrust. The cape would be used both to provide thrust, and control in death-defying twists and turns, and to confuse smart missiles, like a modern day aerial bullfighter.

Carl Ollivier-Gooch

Professor, Mechanical Engineering, University of British Columbia

Capes definitely aren’t aerodynamic. The cape is going to add drag, no matter whether they themselves are flying, like Superman, or are being propelled by various gadgets like Batman. Roughly speaking, I’d expect the drag for a cape to be about the same as for a flag flapping in the breeze. So let’s say Superman is flying at only 300 km/hour (considerably slower than a speeding bullet!), and that his cape is about 60 cm by 150 cm. The drag on his cape is going to be about 375 N (around 40 kg); that’s about half what I’d estimate the drag for his body would be. So the cape looks cool, but it’s terrible aerodynamically.

Mark Drela

Professor, Aeronautics and Astronautics, MIT Aeroastro

It depends on what you mean by “aerodynamic.”

Common definitions are: Close to lowest possible drag; or close to highest possible lift; or close to highest possible lift/drag ratio.

A freely flapping cape has relatively high drag, and cannot generate lift, so it fails miserably on all counts. The human body is also a very poor aerodynamic object on all counts.

If the rear edge of the cape is restrained, i.e. attached to the feet, then it could generate lift like a wing-suit does, but it’s still very poor compared to a good wing on all three counts.

A freely-flapping cape attached at the neck is also very destabilising, because its drag will want to flip the wearer around so that he’s flying feet-first. So you won’t see a skydiver wearing a cape because it would be pointless at best, and possibly dangerous at worst.

Michael Dickinson

Professor, Bioengineering and Aeronautics, Caltech

The basic answer is, it depends on how the cape is attached.

A typical Superman cape, which attaches to the neck but is otherwise free, is not aerodynamic at all. It is basically a flag, which like a flag would simply flap in the wind.

In the case of Superman, the wind is generated by Superman’s own motion. Such a cape would do nothing but generate drag, and thus slow Superman down.

A tight cape that attaches along the arms or legs, as is sometimes depicted on Batman, could serve an aerodynamic function in gliding. This is basically how base jumping works.

As Batman jumps, his so-called “glide angle” would be extended by lift created by his cape if he held it taut along his body. This is basically what fly squirrels (like Rocky of Rocky and Bullwinkle fame) can do. However, compared to high performance wings of albatrosses and other birds, such short cape-wings are not very impressive from an aerodynamic perspective. In the jargon of the aeronautics, they have a “low aspect ratio,” which means that their length to width ratio is very small; long slender wings (like those of an albatross or sailplane) which have a high aspect ratio are much better.

The best humans could do is glide; they do not have sufficient muscle mass to generate lift via flapping like a bird, bat, or insect. They do not even come close—ergo all the nutballs who jumped off of medieval castles to their deaths.

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