Is this Wingspan Enough/Necessary for My World's Parameters?

Question

I have a species of sapient humanoids in my world capable of flight using 24 foot wide wings. The world's being used for a science-fantasy world, so I'd like some realism, but some can be handwaved. Not all of it, though.

However, I don't know if it'd be necessary/enough for my world.

Here's my world's parameters:

1. 75% Earth gravity
2. 2x Earth's Diameter (Don't worry about the difference in diameter vs. gravity)
3. 3 times Earth's atmospheric pressure (Venus has 92 times Earth's gravity and likely isn't much lower in gravity, I think 0.75g and 3x atmospheric pressure is fine).
4. 36% atmospheric oxygen.

My species:

1. Is mammalian/avian(Both mixed) in nature, with some dinosaurian traits thrown in; They have feathers and hair and give birth to live young, which they breastfeed; Are warm-blooded; But have long tails like a reptile's.
2. Is about 7'6" on average as an adult, going down to 6'0" to 9'0".
3. Weigh about 180-240 pounds, up to 360 (Taller individuals) and down to 160 pounds (Shorter individuals)(These are using Earth Gravity, it's more like 120 lb to 270 lb).
4. Have 24 foot wingspans,
5. Their wings are bony, with two of their wing's fingers holding up the wing's end, and the other two are bound with the wing membrane (Different in structure to their body skin) to form an alula-like structure of the two fingers. The whole wing is bound with a similar membrane (Which is much thicker and stronger than their bodily skin) from to sides of the body to the wing finger tips on the trailing edge, with additional membrane running on the top, stopping at the alula. The alula fingers are bound together but other wise free, and are fully moveable like its counterpart in birds, and the wing fingers are similarly bound together (But still attached to the wing).
6. Are obligate carnivores, but began agriculture as a way to keep prey in one area year-round, and do eat some plant material (Their overall diet is doglike; They need meat, but some plant matter for supplemental nutrition is helpful).
7. Evolved from a species of flying predator that mostly hunts on the ground (Like a secretary bird), so flight would be mostly for travelling, not high-speed stunts/hunting in thick forests (Although they can do this, it takes practice, like gymnastics), with travel speeds being about 48 miles per hour at a normal pace (Their cruising speed, if you will).

Other biological traits include:

1. A highly mobile (But not prehensile) lizard-like tail with a large vane at the tip for communication;
2. A large tail fan near the top of the tail, used for a fair amount of steering in flight;
3. Three-toed digitigrade legs, with hooves on the toes (Like some early horses); 4 Six-fingered hands (Held at the side while flying unless holding something, in which case a streamlined holding method is used);
4. And great color vision, hearing, and olfactory senses (On par with hawks, cats, and non scent-bred dogs, respectively).
5. Their distant ancestor is a six-finned placoderm counterpart that eventually evolved onto land with six limbs.
6. They also have 6 pairs of tendrils used for various uses: 2 pairs of breathing/scent collection tendrils for breathing and scent aid; 1 pair for taste and drink absorption (Think a butterfly's proboscis)

They should be able to fly with an extra two-thirds of their weight, like other flying animals.

Is my species able fly with 24 foot wings, or is it too small/big for my world? If it's too small/big, what's the minimum wingspan?

Answer 1

I'm going to say that your creature can actually fly very easily, given your world's conditions. 180-240 pounds for average weight gives a mass of about 81-108 kilograms (metric is much easier to work with, and mass figures don't change when gravity does). A 24-foot wingspan translates to about 7.2 metres. I'll note that your height figures might need to change (I'd advise about 2.5 metres as your low end: my rule of thumb is that each metre in wingspan translates to a minimum of one foot in height/body length), since the wings need to be able to fold up against the body effectively, and they can't do that if the folded wing is still taller than the rest of the body.

As noted by KerrAvon in his answer, Quetzalcoaltus is our best practical comparison point for big flying creatures and their feasibility. Its mass is still debated, but current estimates are around 200kg, with a wingspan of 10-11 metres. Half that mass with two-thirds the wingspan looks eminently plausible to me (albeit your top end of 360 pounds or 162 kilograms is probably an excessive size range to have), so your creature could probably fly on Earth without much difficulty.

With your world, you've specified a lower gravitational pull. This helps with flight for the obvious reason that less lift is required to stay aloft. However, you've also specified that air pressure is tripled at sea level compared to Earth, which is a mixed bag. On the one hand, higher air pressure means that each beat of the wings has more air to "push" against, allowing more force to be applied; it thus takes less wing surface to provide the same lift, as long as muscle strength is equal to the task. On the other hand, it means more air resistance acting against you (drag) and more energy being required to beat the wings: both of these can be overcome with sufficient strength, but this is likely to shorten practical flight range due to exhaustion. Short flights will be easy enough, but flying for extended periods will be much more draining than on Earth.

Verdict: the endurance of your creatures in the air is questionable, but they could fly even on Earth: on your world, it will be very easy indeed.

Postscript: Your fliers having both arms and legs as well as wings means a lot of drag for dubious (pre-intelligence) use, and evolution is especially ruthless about energy/mass efficiency when it comes to flying creatures. It's not impossible to have usable arms and legs, but their strength is questionable. The arms in particular won't have much muscle (wings take so much that there just isn't much room in the chest area to support separate arms).

As for the legs, you'll need to figure out if your creatures are more avian or mammalian in build; feathered wings imply an avian build, but you could potentially tinker with their wing structure. Avians have to be able to launch themselves with their legs, since their wings can't contribute until they get far enough off the ground; the wingtip feathers would bend on contact with the ground rather than provide useful propulsion. This, obviously, requires strong legs, which will raise your mass estimates for your fliers. It also means that your creatures can be practical bipeds, however, as their wings will only be necessary for flight.

If your creatures are built more like pterosaurs (your creatures can't be built like bats unless you eliminate the arms, as a bat's wing membrane runs from the shoulders to the hips and beyond), their wings can be used to impart practical contact force with solid objects, as the wing membrane has muscle and bone to support the edges and prevent it from deforming. Their wings would thus be their forelegs (they would walk with wings folded, probably on their wrists or the equivalent joint) as well as the main force involved in launching. In this case, the legs would likely be incapable of supporting their full weight for more than a few seconds, because they would walk on four limbs: the rear legs simply wouldn't need that much muscle, being more for balance than for taking their full weight.

Answer 2

Wingspan is OK, but...

The standard fallback comparison for "is my big flying creature plausible" questions is to look at Quetzalcoatlus. This is slightly problematic, since there is a lack of certainty about how much it weighed (translation: everyone is making educated guesses) but it does provide a baseline. In comparison to this baseline, your creature looks plausible, especially on a world with significantly less surface gravity than Earth. However, there are some minor nitpicks, and one major problem with your world.

First, the minor nitpicks. While you want your flying creature to have a low mass, for obvious reasons, it seems a bit too low for a creature that is so large and has so many "extras" - fully functional arms, fully functional legs, a head large enough for a sapient brain and all sensory equipment at better-than-human levels (which requires more brain for processing the inputs). Even with bird-like hollow bones, it still seems to me that it should mass a bit more. Additionally, the airborne cruising speed may be a bit high for a creature operating in such thick air (3x Earth sea-level density), though I could be mistaken on this.

Second, the big problem is the oxygen content. As noted in this question, as soon as the oxygen percentage goes above 35% any fire that starts will not stop until there's nothing left to burn. This is an undesirable quality in an inhabitable world. (It also makes iron tools rust if you so much as glance sideways at them, but that will only worry you if you survive the inferno long enough to reach the Iron Age.) I definitely suggest you reduce the oxygen percentage by at least 3 points.

Other than that, I would consider it plausible if I were reading a story with these creatures. Good luck with the worldbuilding!