The correct sentences are: Planes flew overhead constantly.
You want to say - "Planes flew constantly overhead "? Yes, you can.
Thunder boomed in the sky overhead.
You want to say - "Thunder boomed in overhead the sky"? No, you can't.
But can I put 'overhead' after constantly and before the sky?
Thank you.:-D
See above. I should add that although I said yes to the first alteration, it still sounds better in the original form.
In the second sentence, saying "in the sky overhead" is tautology. Unless something very strange is happening, the sky is pretty much always overhead!! You really only need to say either "Thunder boomed in the sky" or "Thunder boomed overhead".
Now that I've written that, I'm also really not sure you need "in the sky" or "overhead". Thunder simply booms! If the storm is directly overhead then perhaps it's useful to say "Thunder boomed overhead", but "Thunder boomed in the sky" doesn't work very well as thunder always comes from above!