Impossible Flight

It seems easy to imagine a planet where gravity was too strong, and the atmosphere too thin, to allow the evolution of flight. The principle of aerodynamic lift that allows air flowing over a wing to create sufficient lift to overcome gravity and create flight is dependent on the givens of the gravity of earth and the weight it gives to objects, and the density of the atmosphere and the mass of the air flowing over the wing. There must be a point where where the speed of airflow caused by flapping a wing would create insufficient lift for flight: a jet engine might still create enough thrust for flight, but birds and flying insects could not evolve.

If you assume that life must have evolved elsewhere too; that somewhere in the trillions and trillions of other planets in the universe there must be other sentient beings (but so impossibly far away that we will never, never know them), then it seems probable, and perhaps a certainty, that somewhere, sometime, a 'person' or 'people' have lived their entire history without even considering traveling through the air.

One can easily imagine the circumstances, but not so easily the mindset of the person who lives in those circumstances. It is difficult to believe that flight would be non-existent in their thoughts, but without the examples of birds and insects, how would they think to try it? And if the thought ever did cross their minds, the jump required to Jet Engine amounts of thrust would be impossible to make and simple experiments would fail so miserably, the thought must die immediately.

Perhaps imagination might bring to them the idea of a Superman style of fantastic flying, akin to weightlessness, but would flapping ones arms ever enter their head? No. As long as they lived solely on that world, winged flight would never occur to them.

Conclusion: One of the best parts of believing solely in evolution and natural causes is that it then becomes perfectly reasonable to think that Earth might NOT be the perfect setting for sentient life. We often assume it is a garden of Eden specfically designed, or luckily formed, to be the perfect environment for our existence. But what if it isn't? What if Earth is only marginably habitable and on different planets sentient life forms can do things we on Earth can only imagine in the crudest of ways, if at all? If our imaginary denizens of the flightless planet can evolve to intelligence and think themselves complete and powerful without flight, what equally impactful possibility for ourselves are we failing to even imagine because local conditions don't support it?

Questions: Is it true that the heavy world inhabitants wouldn't imagine winged flight? The principle of lift would still be evident in a sail or rudder of a boat - would they discover the principle and apply it to the idea of flight?

What are the boundary ranges of gravity and atmospheric density required for a bird to fly?