In a few pieces there are trills between the middle register F and D. I try to trill it with the normal fingering, but the trill ends up very clumsy and the notes, not well defined. How do I trill between these notes?
There is an alternate fingering for this tremolo, but you will still end up moving two of your fingers. I believe you are trying to do a D5-F5 tremolo (correct me if I’m wrong). The easiest way to do this would be to finger a regular D, except place the first finger on your left hand down as well. Then you would lift your second and third fingers on your right hand to change to an F. It is not much of a fancy alternate fingering, but it is the only one that works for this tremolo as far as I know. Hopefully that will make it a bit easier for you to do.
You can also do it this way, which works better for me:
F - LH T123, RH 1-3 D - LH T-23, RH 123
No RH pinky for either note
That way, LH 1 moves, and RH 2, but the flute is stable and the D doesn't break as easily. The F is slightly less full, but in a trill it's not as big a deal.