In bar 35 Köhler introduces trills into the study for the first time.
Even though I have never particularly enjoyed playing trills, they do bring a quite unique character to the musical line and as such, are worth practicing in their own right.
When working on these embellishments it is worth remembering a few points.
When playing a trill that is a whole step, or tone, the upper note of the trill should be in tune and this fact is more often than not ignored by flute players. The general thought is that as long as a finger is moving up and down, the work has been done! However, if it isn’t in tune, the trill sounds somewhat lifeless, dull and sad.
I find that most (but by no means all) whole step trills on the flute need to be very slightly lipped up, for the upper note to be both in tune and vibrant.
I also find trills to sound more true, if for the first one or two movements of the trill, the notes in question are played with the full fingerings. If the listener’s ears have heard genuine notes from the start of the trill, they are less likely to notice any changes that you might subsequently make to the trill for ease of performance (once again, this is particularly important for trills in the top octave, where unusual and awkward fingerings often come into play).
As an element of speed is associated with a trill, it makes sense to keep the moving fingers as close to the key work as possible, in particular towards the resolution of the trill.
Finally, a trill is part of the music, so always try to give it shape. In this exercise I have written a crescendo to the middle of the trill and then a decrescendo towards the end. If played this way, the trill will hopefully have character and elegance.