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Another question for my comic: my friend has made a demand that I give one of her characters a flying mount, preferably griffin-like. I have some designs but I want them to be plausible:

  • How bulky and how big can I make this creature?
  • What air density and such can I change in the planet's atmosphere to facilitate this thing?

The planet so far is quite earth-like but I can change it if need be and the creature's anatomy, other than six limbs and a beak, is up for debate. I already know of things like lift, thrust, drag, high air density = easier flight and so on, so don't feel as if it needs explaining.

Update

The 'griffin' will have small forelimbs (front feet) that are more like arms than legs, with low muscle mass, sitting under the winged limbs, which have strong muscles anchored on a deep breastbone.

The wing membrane extends back from the wings, onto the hind legs and then onto the sides of the tail, which is a more reptilian tail, not the tiny lion tail of an earthly griffin.

The hind legs are low and short and mostly fold neatly against the body to reduce drag, but partially splayed to increase wing area.

The creature has a large chest cavity with large lungs and a light stature.

How big do the wings have to be if it were still the size of a big cat, but significantly lighter and with these other changes? (You can twist the creature's anatomy to suit your answer if you see fit, as long as it can be ridden).

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Flight, and the dimensions of the surfaces that allow it, roughly follow the square-cube law. Simply stated, if you double each dimension of an aircraft's fuselage, the fuselage will intuitively look four times as big when viewed from any one angle (2*2), but the volume the aircraft can now hold is actually eight times the original aircraft (2*2*2). The wings must therefore be able to provide eight times the lift of the smaller ones, and that almost always translates to a wing that is larger in proportion to the fuselage than on a smaller aircraft. So, the larger the object you have to get off the ground, the larger the wings, and the size of the wings doesn't scale linearly to any cross section of the object.

For example, the two families of vultures are among the larger flying birds. The Black Vulture, common in the Southern U.S. and Latin America, has a wingspan about 6 feet and a chord length about 2 feet, to lift a mass of about 6 pounds, giving the vulture a wing loading of about 0.5 lb/ft^2. To lift a Pegasus, mass about 1,200 lbs, with the same wing loading allowing similar flight characteristics, would require 2,400 sqft of wing area, comparable to the foundation footprint of a large ranch-style house, and if the wingspan to chord length were proportional to the vulture's we're talking a total wing about 90 feet by 25 feet, so the chord length is about 3 times that of the horse.

The three workarounds, two fairly real-world and one totally fantastic, are:

  • Make the animal lighter. A griffin might be closer to the mass of a big cat, about 400 pounds. That would only require a wing area of 800 sqft, or a wing about 50 by 16 feet. Still beyond practical but not quite so ludicrous.

  • Make the animal fly faster. Vultures and other large birds like raptors don't tend to fly all that fast, preferring to soar over their territory looking for prey or carrion (though they can reduce their wing area and increase their wing loading for less drag and greater speed; the '70s vogue of "swing wings" on military aircraft was based in part on this ability). Given a light breeze and a thermal off the ground, many of these birds can more or less hover. Humans usually use flying machines (or animals) as transports to get somewhere more quickly, so even airplanes prized for their docile handling behavior like the Cessna 152 have wing loadings closer to 10 lb/sqft, which reduces drag but increases stall speed (and thus takeoff/landing speeds). Even hang gliders and paragliders have at least double the wing loading of birds. If we increase wing loading by an order of magnitude to 5 lb/sqft, our 400lb griffin would only need 80 sqft wing area, or a wingspan about 16ft and chord about 6 feet. That's quite a bit more proportional to the size of the animal; if the wings tucked similarly to a vulture's they'd pack up into about a 3 foot span along each flank. However, in the real world the animal would need to be at a dead sprint to take off, while most depictions of these creatures allow for a bird-like takeoff from a standing start.

  • Magic. Many fantasy worlds have flying creatures with ridiculously undersized wings. The only explanation for how such a creature could fly is that its ability isn't completely natural.

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  • $\begingroup$ Of course, one could increase the wing surface area a touch in order to get a corresponding reduction in stall speed. Someone would have to work out the precise math, but I'm sure there's a balance that gets you a leisurely trot and reasonable wing size. Although, do factor in the fact that in order to carry stuff you're going to need to over-estimate your gryph's body weight. $\endgroup$ – Draco18s Dec 14 '15 at 15:10
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For any flying thing (living or mechanical), there are two primary concerns:

  • The weight of the object should be as small as possible.
  • The creature should be able to push as much air downward/backward as possible.

Birds and airplanes are best examples for that. Birds have really lightweight skeleton and their muscles are much more powerful (in terms of mass) than a mammal's. So for the same muscle mass, birds' muscles provide a more powerful thrust that is used to push air.

Now as for the largest flying creature in the history of Earth, I recommend reading in detail about Quetzelcoatlus and Hatzegopteryx. Both of them were pterosaurs (cousins of dinosaurs, living in their times) and both of them had wingspans more than 10 meters (33 ft) long! Their masses are a question mark, with lower limit being at ~40 kg while some scholars think they might have had masses approaching ~200 kg! Aerodynamic calculations predict that it is impossible to have a living creature with a larger size than that.

Anyhow, just get something in the size range of these creatures and give it a mass ~150 kg (so that it is able to fly while carrying a mass of ~70 kg on its back).

One more thing, the thicker a planet's atmosphere, the easier it becomes for the creature to lift up/forwards (although it's not as simple as that). But do not go for an atmosphere more than 1.5 times as dense as Earth's.

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  • $\begingroup$ thank you for the answer, i know well of Quetzelcoatlus, i was crazy about dinos and such as a kid but its body shape doesnt quite fit the design and they were light weight which is what lead me to think it wasnt possible for my creature, though with enough twisting if limbs and flesh i think itll work $\endgroup$ – XenoDwarf Oct 15 '15 at 14:09
  • $\begingroup$ You have no options about wingspan. It has to be ~10 meters. Other than that, you are free to twist the body shape according to your needs, but you must keep it aerodynamic (birdlike or like an airplane). Also, the metabolism has to be warmblooded. Also, if you can give it a flap of skin membrane between its legs (just like rhamphoryncoid styled primitive pterosaurs), it would help it lift even more weight as there would be extra surface available for it for gliding sake. $\endgroup$ – Youstay Igo Oct 15 '15 at 15:28
  • $\begingroup$ thanks this is great information, its the first time ive actually put a lot of thought into a flying creature so all criticism is appreciated. $\endgroup$ – XenoDwarf Oct 15 '15 at 23:54
  • $\begingroup$ " It has to be ~10 meters." Well, sorta. What matters isn't the length of the wings, it's the surface area. You could have a creature with wider, shorter wings, at least in theory.They'd still have to be pretty freaking huge, though. $\endgroup$ – Saidoro Nov 5 '15 at 20:27
  • $\begingroup$ No. It is biologically impossible to support a wingspan greater than 10-11 meters. Maybe mother nature has other things in mind, but the scientists of today think that way. $\endgroup$ – Youstay Igo Nov 6 '15 at 6:53
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If you're open to messing with the gravity and atmosphere of the planet, heavy fliers become much more plausible (although at the cost of potentially making the planet less hospitable for visiting humans).

Relevant parameters:

  • Lower gravity
    • Less weight for the wings to lift
    • Less weight for the bones and muscles to need to support, so the bones themselves can be lighter
  • A denser atmosphere
    • Each stroke of the wings can provide more thrust, which should permit faster flight using less power
      • Then again, denser air means proportionally greater pressure drag- but I do not know how much that matters
    • Gliding will provide more lift, since, again, there's more air to push off of
      • Then again, less gravity probably means a slower glide speed, which may or may not cancel out the lift boost

While I am certainly not qualified to analyze all the possible effects of all those different factors, I can take a stab at some of the simpler-looking ones.

Say the ideal griffin has the body of a lion (250 kg and 3.0 meters long, call it), and the wings of a bald eagle scaled up to match the length and width of the lion. Typical bald eagles can have a length of 1.02 m, a 2.3 m wingspan, and 6.3 kg mass. Scaling that up by a factor of 3 gives a bird 3.06 m long, with a 6.9 m wingspan. If I assume that lift is probably proportional to the area of the bird's wings, which is proportional to the square of the wingspan, this eagle should be able to lift 56.7 kg (9 times its Earth mass) and then some. Which is less than the mass of the bird itself (170.1 kg = 6.3 kg * 3^3), and nowhere near the mass of our lion. That's the Square-Cube Law in action.

However, that's not the end of the story. If we want the griffin to be able to lift its 250-some kilograms as easily as our giant Earthbound eagle can lift 56.7 kg of its mass, we can reduce gravity and thicken the atmosphere to compensate.

To fly on the planet (call it P), we need $$F_{l_P} \ge F_{g_P}$$ (read that as "Force of lift on P $\ge$ force of gravity on P")

Since a denser atmosphere there will give more lift as compared to Earth $$F_{l_E} \cdot \frac{\rho_P}{\rho_E} \ge F_{g_P}$$ Substituting in mass * gravitational acceleration for those forces (from Newton's second law of motion, F = ma) $$m_{equiv} \cdot g_E \cdot \frac{\rho_P}{\rho_E} \ge m \cdot g_P$$ where $m_{equiv}$ is the 56.7 kg figure calculated above.

Rearranging that a bit gives $$\frac{m_{equiv} \cdot \rho_P}{\rho_E} \ge \frac{m \cdot g_P}{g_E}$$ After substituting in some known values $$\frac{56.7 \cdot \rho_P}{\rho_E} \ge \frac{250 \cdot g_P}{g_E}$$ And after rearranging again $$\frac{\rho_P}{\rho_E} \ge 4.41 \frac{g_P}{g_E}$$

So the atmosphere on this planet would need to be about 4.5 times as dense as Earth's, or gravity at its surface would need to be 4.5 times weaker, or somewhere in between. In theory.

I'd recommend reducing gravity by at least a factor of 3, if not 4. This goes back to the square-cube law: Even though the cross-sectional area (and thus the strength) of the eagle's wing bones increased by a factor of 9, the griffin weighs much more than 9 times what an eagle weighs. So the griffin's wings might just break whenever it tries to take off in Earth gravity, denser atmosphere or no.

The question then becomes: Could a planet with much less gravity than Earth actually hold onto such a thick atmosphere long enough for griffins to evolve? Sure. Venus and Titan both have much thicker atmospheres than Earth does; and Titan's gravity is much weaker as well. I do not know why these conditions exist on those worlds, but clearly they do.

Finally, would such a creature actually evolve in such an environment? I doubt it; not as I described. Lions are big and bulky; the classical griffin would surely be outcompeted by something with a smaller, lighter, more aerodynamic body- something more birdlike, in short. As for the more reptilian design in your question? Sure. Maybe. I don't know. I'm no biologist.

As for riding them: No idea, but you'll probably have better luck with birdlike wings that just attach to the griffin's shoulders than with batlike wings extending all the way to the hind legs and tail. If people are going to saddle these things, they'll need somewhere to put their feet, and bat wings would get in the way. Unless the rider was perched more on the griffin's shoulders, in which case the weight of the rider would be seriously unbalancing. Dragonriding has similar issues that have already been discussed here on Worldbuilding... somewhere.

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  • $\begingroup$ Not a bad answer at all here, but I'm pretty sure you meant multiplying a length of 1.02 metres to result in 3.06 metres (not 1.06). However, there are muscular and weight-bearing limits to wingspan; more than about 8 metres will likely make it impossible to flap the wings without bones breaking or muscles tearing, etc. As for your proposed tinkering with the planet, it could work. Lowering gravity too far will result in losing oxygen to atmospheric escape, though; anything below about 0.7G isn't safe, so most of the tinkering will be in atmospheric pressure rather than gravity. $\endgroup$ – Palarran Apr 2 '17 at 1:11
  • $\begingroup$ @Palarran You are quite correct about scaling up to 1.06 meters; that was a typo. It's fixed now. $\endgroup$ – Someone Else 37 Apr 2 '17 at 1:51
  • $\begingroup$ For less gravity causing the atmosphere to escape: Do you have a source for that? Titan's atmosphere has a pressure of 1.45 atmospheres at its surface. Furthermore, its surface gravity is only 0.14 g (source: same page, in the sidebar). So I'd say a planet with a thicker atmosphere than Earth and much less gravity is certainly plausible. But Titan is much farther from the Sun than Earth, you say, and experiences less solar wind. Well, I point to Venus: 92 atm of pressure. $\endgroup$ – Someone Else 37 Apr 2 '17 at 2:10
  • $\begingroup$ Atmospheric escape is particle-specific, @SomeoneElse37 . Titan is still retaining a dense atmosphere with that low gravity, yes, but it's made up of heavier gases only; last I checked, Titan has at best trace amounts of oxygen. Oxygen and nitrogen will escape at an unacceptable rate somewhere around 0.5-0.7G (I can't recall the exact figure offhand, but that's a lot more than 0.14G), and I'm assuming that the OP wants a generally Earth-like world. This is also why only the gas giants have any hydrogen in their atmospheres: 1G isn't nearly enough to retain hydrogen for very long. $\endgroup$ – Palarran Apr 2 '17 at 14:21
  • $\begingroup$ @Palarran I agree with your point about lighter gases escaping faster, but Titan's atmosphere is 98.4% nitrogen. No one's quite sure why it hasn't all blown off yet. Maybe Saturn's magnetic field shields it most of the time. Maybe it replenishes the nitrogen from comets or underground ammonia. Or maybe its atmosphere has been blowing off, but it had so much to begin with that it's only now down to 1.45 atm. We don't know. But there's certainly enough wiggle room for the asker to come up with something plausible. $\endgroup$ – Someone Else 37 Apr 3 '17 at 1:31
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In flight there are 4 forces acting in pairs:

  • Weight vs Lift
  • Drag vs Thrust

In order to fly, lift must equal or exceed weight and thrust must equal or exceed drag.

Ease of flight scales directly proportional to air density and inversely proportional to the value of gravity.

So if you increase the air density (proportionally related to pressure) and decrease gravitational acceleration you can get most anything to fly.

At the Moon's gravitational acceleration with terrestrial atmospheric pressures, humans could fly with wings that slip over their arms.

If you're interested in a more technical treatment, I ran through the calculations in another answer and I could link that in.

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  • $\begingroup$ send all the links you want i want as much as i can get, though i think ill modify the answer a bit so that people know i understand basic aerodynamics $\endgroup$ – XenoDwarf Oct 15 '15 at 14:46
  • $\begingroup$ i still want a better answer for this $\endgroup$ – XenoDwarf Oct 21 '15 at 9:56

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