Just don't explain why there is no flight.

Just write your book or whatever you are doing without flight. If when you are done you think you need an explanation, check out the other great answers here. 

For example, I read somewhere (on this site I think) that the creators of Groundhog Day tried to explain the repeating day with some voodoo witch stuff. Obviously they decided to leave it out, leaving no explanation, but the end result is still a great movie. 

Maybe you could leave some vague references to past events, like a failed test or something bad that happened, to make it a mystery. J. R. R. Tolkien often did this, telling of peoples/countries that disappeared/were destroyed without fully explain everything (like the Entwives) to keep things mysterious.

 So you don't necessarily to explain why there isn't flight.