# How much could heaviest flying animal weight?

OK I'm not sure if I put this in right subforum.

I remember once reading in some science book by Isaac Asimov that the heaviest any flying animal can be is 22 kg, but I read on Wikipedia that Quetzalcoatlus may weighted up to 250 kg. So how come there's such a great difference in that assumption just few decades ago?

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Well for one, Isaac Asimov was not an aerodynamicist, so I hardly believe he is the authoritative source on the matter. There is no theoretical maximum limit. We make airplanes that fly that weigh many tons, and they are less efficient than birds. The limit is how large the animal can be, because the heavier it becomes, the larger wings it needs (and the more it needs to eat to provide energy).

davenn
A.T.
The limit is how large the animal can be, because the heavier it becomes, the larger wings it needs (and the more it needs to eat to provide energy).
There is also the the issue for all big animals, that mass scales cubic, while area just quadratic.

A.T.
I read on Wikipedia that Quetzalcoatlus may weighted up to 250 kg.
Yes, but there are also debates on what kind of flight they did. Soaring in updrafts after taking off from an elevated position (like a hang-glider) is different from take off from level ground in still air. These different criteria might be the reason for the discrepancy.

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We make airplanes that fly that weigh many tons, and they are less efficient than birds.
Less efficient then birds? Animals only have muscle power. So there is no weight limit to muscle power?

Andy Resnick
OK I'm not sure if I put this in right subforum.

I remember once reading in some science book by Isaac Asimov that the heaviest any flying animal can be is 22 kg, but I read on Wikipedia that Quetzalcoatlus may weighted up to 250 kg. So how come there's such a great difference in that assumption just few decades ago?
There's a delightful discussion in Thompson's "On Growth and Form". We need to distinguish *gliding flight* from *active flight*. Larger birds (and pterodactyls) primarily rely on gliding.

For active flying, the work required by a bird of length 'l' to fly varies as l3.5; an ostrich requires 5 times as much effort to fly than a sparrow. This is because the momentum of the bird scales as l3v and the momentum of air air deflected downwards is proportional to l2v2, where v is the speed of the bird. Setting them equal gives the scaling relationship above. Now, comparing the work expended on flying to the rate of power generation from muscles (which scales as l2v) probably gets to Asmiov's estimate

Considering gliding flight is more difficult, because 1) it depends on the relative speed between bird and air and 2) depends on the details of the wing geometry, and wings are both movable and flexible. For aircraft, the minimum speed scales as the square root of the length- if we say the sparrow's minimum speed is 20 miles/hour, the ostrich must fly at 100 mph- and here again, birds (and flying insects) display a range of behavior, including swooping- a griffon vulture can swoop at speeds up to 180 mph. By contrast, terns have been observed to fly at 15 mph. Clearly, gliding flight accommodates masses greater than 22kg- consider hang-gliders. and sail-planes.

I read 40 lb is the upper limit. The musculature to support more than 40 lb at takeoff would be cumbersome. I'm not sure of this as I have never considered the question in depth.

The limit is how large the animal can be, because the heavier it becomes, the larger wings it needs...

Nugatory
Mentor
For active flying, the work required by a bird of length 'l' to fly varies as l3.5; an ostrich requires 5 times as much effort to fly than a sparrow.
Seems like a really big sparrow or a rather small ostrich to get just 5 times the effort out of that l3.5.

Gold Member
There is also the the issue for all big animals, that mass scales cubic, while area just quadratic.
Less efficient then birds? Animals only have muscle power. So there is no weight limit to muscle power?
Sure there are some practical considerations, and at some point you'll definitely run into problems of energy consumption in order to fly, but since we are talking about the theoretical limit, and there is no theoretical reason why you couldn't have a 500 kg animal flying. It just probably would never evolve because of the energy/food requirements to sustain such a creature.

256bits
Gold Member
OK I'm not sure if I put this in right subforum.

I remember once reading in some science book by Isaac Asimov that the heaviest any flying animal can be is 22 kg, but I read on Wikipedia that Quetzalcoatlus may weighted up to 250 kg. So how come there's such a great difference in that assumption just few decades ago?
Well there is the Kori Bustard of Africa, up to 40 pounds that can fly, but it spends most of its time walking around.
https://en.wikipedia.org/wiki/Kori_bustard

The 22 pounds seems to be a reference to the Albatros, which can sustain flight for days and travel 1000 of miles without doing a wing beat.
Long wing span and use of soaring can keep it in the air with no more energy expenditure than if it was resting.
https://en.wikipedia.org/wiki/Albatross
http://www.dailymail.co.uk/sciencet...le-journey-WITHOUT-flapping-wings-solved.html

A.T.
Asimov that the heaviest any flying animal can be is 22 kg,
Well there is the Kori Bustard of Africa, up to 40 pounds that can fly, but it spends most of its time walking around.
https://en.wikipedia.org/wiki/Kori_bustard

The 22 pounds seems to be a reference to the Albatros,
The OP states 22kg = 48.5 pounds, so the Kori Bustard is within that, and can even takeoff vertically:

The Albatross is more optimized for long flights than quick escape, so it runs (ideally downhill) or uses the updraft at a cliff.

256bits
Andy Resnick