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In the new Supergirl trailer for the CBS TV Show there is a quick line about how she needs a cape because it would help with the aerodynamics. As far as I can tell its just a piece of material that flaps about in the breeze with no rigid structure (like bones in a birds wing) to stabilize the cape to make a viable wing.

Is there any physics that back the claims of a cape aiding aerodynamics?

closed as off-topic by Rogue Jedi, Jason Baker, Ward - Reinstate Monica, Meat Trademark, Cherubel Apr 28 '16 at 5:55

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    Hmm.. I'm tempted to VTC with "Questions seeking scientific solutions or explanations are off-topic unless they relate directly to a cited work of fiction." as this is not asking In-Universe, but IRL. The fact that people can't actually FLY IRL (unaided, anyway), makes me hesitate... – K-H-W May 27 '15 at 1:12
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    "No capes!" -Edna, Incredibles – Jim2B May 27 '15 at 1:26
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    No Capes is a lesson we should learn from Watchmen... – MC_Hambone May 27 '15 at 1:26
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    @MC_Hambone My mistake. We have clarified that questions such as these are ok for this site. Air works the same in the Superman/Girl universe, so it is comparable to real-world physics. Good luck! – Möoz May 27 '15 at 1:50
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    I WISH super girl wore her underwear on the outside... wait... – user16696 May 27 '15 at 17:34
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Since the speaker didn't expound at all on what he meant by aerodynamics, an explanation has to come from two places:

  • What happened on-screen just before he says this
  • How a cape would affect flying in a world where birds and airplanes and hanggliders work.

Just before that line is uttered, Kara is in a car chase, and completely misses a hard left turn. She ends up plowing into a fence on the side of the road. Thus, we can assume that having the cape, in-universe, helps her navigate while flying.

This, of course, just begs the question. It doesn't explain why it helps her navigate. For that, we get no help from Supergirl, but we can can picture what a cape should do if attached to a flying thing.

Mostly, it changes the way drag builds up behind her. The cape, being separate and elevated slightly from her body, might be expected to behave in a way similar to a spoiler on a car. We know that spoilers are used in high-performance cars partly to help them make turns at high speed. There are two problems here, though. First, the rear spoilers work by pushing the rear of the car down, increasing traction on the road and decreasing drift. For Kara, though, she's not in contact with the ground, so the effect of a spoiler would be to shove her into the ground — rather counterproductive.

More importantly, a spoiler is a fixed addition to a car. Since the cape moves with air flow, however, the dynamics are much more complex (and I couldn't find a good analysis of them online.) It would create some effect on the drag; it's possible that it may help slow Kara down when she banks, or disrupts the air flow around her, in such a way that it increases her maneuverability.

Or, at least, it would if it wasn't so small and thin. Given how flimsy her cape is, the actual amount of air displaced by it, and the change in drag relative to the rest of her body, probably isn't enough to make any difference at all.


For reference: at no point do I think any Superman story explained the cape as aiding in flight: it was almost always there for sentimental and/or decorative reasons. Out-of-universe, it was meant to mimic the look of circus strong-men.

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    A flapping cape is far worse that a spoiler. At speed, it will flap but will not flow uniformly. This causes turbulence. At faster speeds, this turbulence with oscillate with more energy and cause instability in the supporting structure. As an example, if you are ever around a sailing vessel on a F4-F5 day, observe the sails when they are being hoisted. The whole vessel will shake. This is not fast. You will not see a net downward force from the cape. – Gusdor May 27 '15 at 7:14
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    Plus one just for for using “begs the question” correctly. – Paul D. Waite May 27 '15 at 10:14
  • @Gusdor that's true, but a sail is much bigger, and generally anchored at both ends, so when it catches the wind it causes the rest of the boat to move. A cape - small, thin, and only anchored at one end - will behave more like those flags people hang off their car windows. Not at all like a spoiler, but not really that much like a sail either. – KutuluMike May 27 '15 at 10:43
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    @MichaelEdenfield I'm afraid you misunderstood me. I was referring to a sail that is partially rigged. A flag is a better analogy. The turbulance still applies – Gusdor May 27 '15 at 11:44
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    @Aron If you disapprove of the oversimplifications in my answer then I heartily encourage you to write your own with a complete analysis of the fluid dynamics of automotive spoiler in comparison to blonde alien women flying around in a skin-tight outfit with bulletproof cape attached. – KutuluMike May 28 '15 at 3:17
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Absolutely none, the cape would be ridiculously detrimental to any aerodynamics. There is a reason why professional cyclists and other speed athletes wear skintight suits with no pockets or extra material whatsoever (except back pocket for cyclists). Any flapping material will act like a little parachute.

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    oddly enough, one of the early reasons given for why Superman wore a cape was because his suit had no pockets, and his cape is where he kept his Clark Kent clothes :) But I don't think the cape would act much like a parachute, either. The image we see of Batman gliding to Earth on his cape is also rather unrealistic -- the cape doesn't catch and resist air, it just flaps around in it a bit. – KutuluMike May 27 '15 at 11:54
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    Isn't Batmans cape some high tech material that goes rigid when he puts some electricity through it? – Davor May 27 '15 at 12:56
  • What if the whole point of the cape is to act like a parachute therefore to stabilize flight? In addition, what if the "aura effect" (which protects Superman's clothes from being destroyed) renders the cape somewhat more firm like a fin or wing? – mg30rg May 27 '15 at 14:23
  • Batman either makes it rigid like in the movies, or grabs the ends to make it into a parachute or hang glider. – user16696 May 27 '15 at 17:39
  • @Davor And how about my assumption with the aura effect frequently mentioned in the TV adaptations of Superman? (I've never seen superman's cape knoting itself mid-air.) – mg30rg May 28 '15 at 7:43
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I don't think there is. I would actually think the turbulence they produce would slow down the hero, not that some of them would even care. As they say in the comments, the sole purpose of the cape is aesthetics.

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    To elaborate on that, I think they were introduced so early life action filming against a stationary backdrop would appear to be moving by having the cape flutter. – Jim2B May 27 '15 at 2:40
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    I'm pretty sure Superman's cape pre-dates Superman on film by ~20 years. – KutuluMike May 27 '15 at 3:37
  • should read this: scifi.stackexchange.com/questions/25123/… – hjhjhj57 May 27 '15 at 3:39
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The closest explanation (and proof against at the same time) is similar to how a parachute works. From io9:

It's easier to understand why parachutes have holes when observing laundry hung out on a line on a windy day. A large sheet will be pushed upwards by the wind. Gentle breezes make it billow at the center, looking a little like a parachute. When winds pick up, the sheet will start to move more erratically. First the one corner, then the other, will flip outwards in the wind. This will make the sheet start twisting one way, then another. The more violent the wind, the more the sheet will twist, as each corner in turn flaps out to relieve the pressure from the wind.

This looks peaceful enough if watched from one side. If watched from below, it's more ominous. If air can't easily move in a way that won't disturb the billowing shape parachute, it will rush out one side, then another. This will make the parachute twist unpredictably, and make it hard for the parachute operator to control the jump. If the twisting gets bad enough, the parachute will collapse. The sides will flip up and then central bulge will snap shut like a fan, leaving the jumper in free fall with some string and loose fabric attached. Building in a weak point that can be controlled is better than insisting on invulnerability that can't. http://io9.com/5612765/why-do-parachutes-have-holes

Super girl missed a hard turn because she could not stop or redirect enough. The turbulence of the Cape slows her down and breaks up the air flow enough to make that turn. It turns a typical weak point into solution.

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Just to create a pro-cape argument [please note that this is fully speculation and is not backed with neither scientific- nor in universe facts - which I guess is ok in case of a fictional character].


Let's just assume, thet the vector supergirl creates to be able to fly originates at her center of gravity! Let's also assume that this vector has nothing to do with her actual position. This would make her levitation/flight a balancing game. (I guess levitating upside-down or slowly rotating is not very impressive in case of a superheroine.)

In this case aerodynamics at real huge speeds were a quite huge deal since once she takes a slight assimetric movement (like turning her head or moving one of her hands) enornous turbulences would occur making her mindlessly rotating in the air. Now, if she wears a cape which undoubtedly acts like an airbreak it will somewhat stabilize her mid-air position.

If you also add the - so called - aura effect which protects the dress of superman - and I assume supergirl's too - from being burnt, broken or hurt in other manners, it might also stabilize the cape enough to act like an actual fin or wing.

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    this is fully speculation and is not backed with neither scientific- nor in universe facts -- that means this isn't an answer. Anyone can make up anything they like, doesn't mean it should be posted. – Matthew Read May 27 '15 at 14:48
  • @MatthewRead Well, more like mostly than fully speculation with quite acceptable - even if speculative - explanation why would a cape help a flying superhero. Since AFAIK no canonical source backs the statement in the question I think my answer is just as justified as the question above. – mg30rg May 28 '15 at 7:22

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