Let's say that there is a wind going 45 mph and the flight direction of the humanoid creature is in the same direction as the wind. As the flight is in the same direction as the wind, air resistance is minimum, so small it is basically 0. And let's say that this humanoid creature has a wingspan of 5 feet(2 feet per wing + 1 foot for the body) at the arms and a wingspan of 7 feet at the legs(3 feet per leg + 1 foot for the body). And let's say that the number of arm flaps per minute is 120 flaps(so 2 flaps per second) and the number of leg flaps per minute is 60 flaps(so 1 flap per second). And let's assume that it is synchronized so that the 2 arms flap at the same time, the 2 legs flap at the same time and for every 2 arm flaps completed, 1 leg flap is completed. Let's say that every arm flap moves you 3 feet and every leg flap moves you 5 feet in no wind. This makes it easier to calculate the speed.

Mass is going to be important here as is height so lets say those measures are 120 lbs and 5 feet.


  • Arm length: 2 feet
  • Leg length: 3 feet
  • Wind speed: 45 mph
  • Arm wingspan: 5 feet
  • Leg wingspan: 7 feet
  • Arm flapping speed: 2 flaps per second
  • Leg flapping speed: 1 flap per second
  • Height of humanoid: 5 feet
  • Mass of humanoid: 120 lbs

Now here are my questions:

1) Can the humanoid creature fly at all assuming his/her arms and legs don't get sore after 1 minute of flapping?

2) If the humanoid creature can fly, how fast can it fly assuming it follows the wind the whole way?


3) Is the maximum speed assuming 70 mph wind speed max for no storms anywhere close to the speed of sound at 767 mph?

You notice I am using 100% imperial measurements. That is because my Kepler Bb people use a system very similar to the imperial system but with different numbers of units equaling any given unit. They do however share some base units like inches and seconds and ounces. Once I know the answers in the imperial system, it will be easy for me to convert into the Kepler measurement system(a lot of multiplication and division but that is easy(so like I would convert mph into inches per second and then use the Kepler conversion factors to convert it into Kepler miles per Kepler hour)).

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    $\begingroup$ First thing to bear in mind is that mach number is measured from airspeed, not groundspeed, so head/tail winds are completely irrelevant for this question. $\endgroup$ Dec 13, 2016 at 16:11
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    $\begingroup$ As an additional comment, air resistance would be minimal when traveling at the same speed and in the same direction as the wind. Once you are exceeding the wind-speed, you will be encountering air resistance. $\endgroup$ Dec 13, 2016 at 16:57
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    $\begingroup$ Is he African or European? $\endgroup$ Dec 13, 2016 at 20:36
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    $\begingroup$ Are you unfamiliar with the lore of laden vs. unladen air speed velocity of flying beings? Without knowledge of history, we are doomed to repeat it. $\endgroup$ Dec 13, 2016 at 20:46
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    $\begingroup$ So I try to help people find inspiration, so I usually try not to debunk the questions too thoroughly, but the physics you describe is so far from the real world physics of flight that it's basically impossible to reconcile them. Would you find an answer coming from that direction useful, or would it just be unhelpful? $\endgroup$
    – Cort Ammon
    Dec 13, 2016 at 21:03

4 Answers 4


I'm going to take a stab at a very simple no, on the basis that the world's fastest bird can't even hit mach 0.15 in level flight White-throated needletail - 105mph.

In a dive you might have a bit more of a chance, birds can fly to nearly 40,000 feet. I'd have to let someone with a much better knowledge of terminal velocities to work out whether supersonic falling would be possible at that altitude. (I suspect not - Joseph Kittinger jumped from over 100,000 feet and didn't break the sound barrier, so if your humanoid has a similar drag co-efficient then he definitely won't).

Your person can definitely only fall though, as with a weight of 120lbs and an effective wingspan of about 10-12 feet he certainly isn't going to fly. Look at the wingspans of large birds - 8-12 feet - and their weight - about 15-25lbs.

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    $\begingroup$ Terminal velocity by definition cannot exceed the speed of sound in a universe that has laws of physics like ours. $\endgroup$
    – tillmas
    Dec 13, 2016 at 16:25
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    $\begingroup$ Tell that to Felix Baumgartner ... $\endgroup$ Dec 13, 2016 at 16:26
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    $\begingroup$ Valid point, I should have included a proviso about atmospheric density as well. I would argue that he achieved a velocity higher than terminal velocity exoatmospherically, the same way that a re-entry craft does. $\endgroup$
    – tillmas
    Dec 13, 2016 at 16:41
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    $\begingroup$ Peregrine falcon can dive at 389 km/h. Still far from sound barrier. $\endgroup$
    – njzk2
    Dec 13, 2016 at 20:20
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    $\begingroup$ That's not really true - he accelerated to terminal velocity (for his altitude) in exactly the same way that any falling object will. Of course, as altitude decreases so does terminal velocity - so he would have slowed down as he fell - but ultimately he started off perfectly stationary and accelerated to TV in a perfectly normal way. $\endgroup$ Dec 13, 2016 at 23:23

No, your humanoid won't fly

The lift to weight ratio for your humanoid isn't good enough for the speeds you're talking about. Assuming an earth like atmosphere, your humaniod is about 6 times too heavy to lift itself. Note that the albatross has the widest wingspan of living birds at about 9 feet across but it weighs just 19 pounds. Your humanoid could keep the same wing size but go much faster to generate the required lift but that's problematic because drag increases quadratically $v^2$ as velocity increases. Going fast is hard.

For more general information on designing a flying humanoid, NASA has a great introduction section (Specifically the Aircraft Forces section) and that will tell you a lot of what you need to get started.

  • $\begingroup$ But I don't want the humanoids to be much bigger than that because then I encounter the Square-Cube law and will have to either change their bone material or make them have bigger bones which would in either case mean an increase in muscle mass is required. And I don't want them much smaller because then they might as well not be humanoids. So I have to stay within a reasonable size range(and the most reasonable size range for humanoids on a world where gravity is the same as earth's is the size range of humans). $\endgroup$
    – Caters
    Dec 13, 2016 at 20:38
  • $\begingroup$ Drag increase quadratically, not exponentially. $\endgroup$
    – Rob Watts
    Dec 13, 2016 at 20:55
  • $\begingroup$ @RobWatts You're totally right. Fixed it. $\endgroup$
    – Green
    Dec 13, 2016 at 20:58
  • $\begingroup$ @Caters I would spend a little time working through your lift/weight and thrust/drag ratios. That will help you a lot with how much weight you can carry and how big the wings will need to be. Maybe you can work out a lifting body humanoid. $\endgroup$
    – Green
    Dec 13, 2016 at 21:01

Mach number depends on air density which in turn depends on altitude. Concorde could only do Mach 2 at high altitudes.

There are two more serious problems. The first is the reason there are no supersonic propellor-driven aircraft: having a shockwave across the blades severely impairs their operation. The same would apply to flapping, even if you could flap wings at the desired speed there would be a point where the leading edge would be breaking the sound barrier while the rest of the flyer is subsonic. This would cause loss of lift.

The second is that the impact of the air on a supersonic object causes it to heat up. This would compensate for the freezing air at high altitudes, but again would have to be managed in order to avoid cooking the flyer.

(A sub-problem would be energy consumption; I've no easy way of working this out, but supersonic flight is usually achieved only with the aid of afterburners and extremely high fuel consumption. Only some aircraft are capable of "supercruise" without afterburners, and even they need them in order to accelerate to that speed.)

  • $\begingroup$ This. The other answers are accurate as well, but this is the critical physical constraint for supersonic flight. I'm adding my own answer as well to extrapolate a tiny bit but this is ultimately the right answer. $\endgroup$
    – thanby
    Dec 13, 2016 at 23:47

@pjc50 has the most basic physical constraint detailed very well but I'd like to expand on that answer a little bit.

In short, what you're asking is fundamentally impossible in our world of physics (but there's hope! Read to the end). Let's explore why with some simple examples.

Let's start with a definition. You want to break the sound barrier. The sound barrier is how fast sound travels through air. Thus, the direction of air flow is irrelevant, because if the air is moving 70mph west, the sound will have to move an additional 70mph west to compensate. However you would need 70mph (relative to the ground... this is why air speed is measured in knots, which are relative to the wind speed) less if you were going the opposite direction. This ends up being irrelevant to your question, however, because even if you're moving east against that current you still need to overcome the force of it. If it weren't for that effect, birds wouldn't be able to hover in the air like they do.

Basically, there's a reason we have no supersonic propeller-powered aircraft. It takes epic amounts of compression to pump air backwards fast enough to reach supersonic speeds. A propeller is, simply put, a very efficient wing-flap. Propeller blades are essentially just wings, they carve a bit of air and push it back, using the leverage to push the aircraft forward. The reason they are more efficient than wings is because they have no down-time. A bird wing has a moment of zero-thrust while the wing is returned to its forward position, while a propeller takes advantage of the ability to freely rotate to be constantly pushing on the air.

Yet despite the efficiency of a propeller and its superior efficiency over an animal wing, it still can't break the sound barrier by spinning in open air. It would eventually reach a speed of rotation where it would push more air out of the way than it would effectively push behind it, so you get diminishing returns per your energy investment. Think about splashing your hand in a pool of water, trying to hit a target with it. The harder and faster you splash, the less water you throw in the direction you're trying to splash it. It just spreads out in different directions and becomes less accurate.

That's were turbofans come in. They keep the air from moving out of the way by sucking it into a confined space, allowing the propellers to get better leverage over it. This is a step in the right direction, though I'm not sure whether or not any basic turbofan engines can break the sound barrier.

Jets are basically super-high-performance turbofans, and they definitely have the potential to break the sound barrier, because they tend to dump lots of extra energy into that compressed air (by burning fuel) to make it exit the engine even faster.

So... Unless your creatures have enormous amounts of energy to spend (which you did hand-wave a bit) AND have the ability to internally compress air (IE: not basic animal wings like you're describing), they won't be able to accomplish supersonic speeds. You could possibly build this feature into them, but it will require a bit more hand-waving. See questions like this for inspiration in that department. That one in particular might have an idea for you: Symbiotic creatures (the "turbofan" part would be a symbiotic creature, not directly part of the main body).


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