Some additional thoughts about Lip Trills
I have never found the lip trill to be very productive for most students even those that have no difficulty getting their lips to “flubber”. And the reasons for this is that lip trills encourages excessive air flow which is not the desired description of the well phonated vocal fold oscillation. The air flow needed is little more than what is produced when you moisten your glasses with your breath.
Breathing and producing a tone through a folded handkerchief folded over your forefinger and placed at your lips such that the lips seal the flow of air is a far more accurate measure of air needed to produce an efficient vocal tone.
The advantage of any of the exercises that I outlined earlier is that they help the singer sense how little airflow is necessary and they create a back pressure in the vocal tract which assists the vocal folds in their oscillation by preventing them from having to create an excessive amplitude of vibration.
All these exercises are easily taught and the student experience success very quickly. The best of both worlds.