"The moment you doubt whether you can fly, you cease forever to be able to do it" - J.M Barrie
Let's make a person glide.
We have a man wearing a wingsuit on a platform.
Let's increase the area of the wingsuit to be proportionate to a flying membrane of a flying squirrel.
Then, make the man run as fast as he can to the end of the platform, and jump off.
At this point, the airflow speeds up on top of the suit as the result of the suit catching air from below.
It creates lift.
If you run as fast as Usain Bolt with this suit, it will support 2.8kg.
So, naturally this happens.
For a normal person to glide in a wingsuit like this, the gravity will have to be small and the atmosphere dense.
Titan, a satellite of Saturn, is one such example that is suitable for gliding with our wingsuit.
Titan is a harsh environment with a temperature of -179°C and methane rain.
But its gravity is 1/7th that of Earth, and its atmosphere is four times as dense as our atmosphere.
We can manage to glide horizontally.
If you do it on Earth, you'd have be running at 200 km/h in order to glide.
This person can also enjoy horizontal gliding.
If you can't run at 200 km/h, you'll need this much suit area.
Now, let's think about making humans fly by flapping wings.
A small hummingbird hovers by moving its wings in a "figure 8" motion 80 times per second.
Let's put some light wings on a 60kg person and make him do the same motion.
And increase the rate of flapping.
Try flying at 80 times per second, the same rate as a hummingbird.
When you are as large as a human, such flapping will only support 3.1kg.
Ten times that of a hummingbird, 800 times per second, still looks like this.
At 1540 times per second...
Our human can now hover.
Estimates from the data of other birds, a man-sized wing should be 7 meters long.
You could glide with this wing.
It can hover at 70 times per second, which is less than that of a hummingbird.
So, as long as you don't doubt our ability to fly, we can fly.