Why Humans can't fly like birds???

Although mankind has conquered the skies with airplanes, we have yet to match up to our winged animal counterparts who fly on their own. And now, scientists have determined that we never will: it is mathematically impossible for humans to fly like birds.

 The arms and chest of a human do not have anywhere near enough muscle mass to provide the necessary power. And it’s unlikely that we will achieve flight by flapping wings powered by our legs. In theory, human legs do have enough strength to do this, but only if the wings’ span is large enough — at least 80 feet or so — and if they also weigh significantly less than the human. 

 This large-size/low-weight combination is extremely difficult to achieve with current mechanical technology, and no one has yet succeeded or even come close to realizing it, although many have tried. But, we have achieved one type of human-powered flight: using an aircraft with a fixed wing and propellers driven by leg pedaling. Because the wing is fixed, it can be built both long enough and light enough to permit flight. 

A bird can fly because its wingspan and the wing muscle strength are in balance with its body size. It has a lightweight skeleton with hollow bones, which puts a smaller load on its wings. A bird also has air sacs connected to its lungs, which makes it even lighter and allows for easy passage of air through its lungs during flight.

On the other hand, calculations of the ratio between human size and strength reveal that our species will never be able to take flight unaided. As an organism grows, its weight increases at a faster rate than its strength. Thus, an average adult male human would need a wingspan of at least 6.7 meters to fly. This calculation does not even take into account that these wings themselves would be too heavy to function.

There is a reason that a 6-year-old girl can do more pull-ups than her 40-year-old father — she may be weaker, but her strength-to-size ratio is still greater than her father’s ratio. In other words, humans are not too large to fly, but our strength simply cannot support our weight in flight.