Before anything else, let’s review the range of possible pitches on a standard set of timpani:
(The 20″ drum is less common and may not be in every set)
Keep in mind that every drum can vary by an inch or so in either direction, which can affect its overall range. This is what I feel is the most common safe tuning range of each drum, with the note in parenthesis being sometimes just out of reach. It is also possible to detune a drum a little to achieve a lower pitch (Low C’s aren’t uncommon the lowest drum). That being said, note that the drums have the clearest pitch in their high range, so be careful writing any quasi-melodic passages on the drum’s low end.
On modern timpani, these pitches are changed by manipulating a tuning pedal, with one pedal on each drum. There are several different types of tuning pedals, which I don’t feel like writing about so here’s the wikipedia on it if you want to read up. For the purposes of this post, the type of pedal is inconsequential.
While not usually visible to the audience, tuning changes can be a virtuostic feat depending on the demands of the composer (shout out to harpists). As a general rule of thumb, the more the feet move the more difficult the music is, whether the tuning is changing or the feet are just moving from one drum to another. While writing, try to keep track of where the feet might be and make sure you aren’t writing any sort of crazy feet crossing moments.
The ability to gliss is the double edged sort of timpani playing. It is objectively cool to be able to do a large change of pitch on the drum (achieved just by a continuous pedal movement). That being said, they can sometimes be hard to avoid in an undesired moment. If a tuning change happens in an active passage without time to mute, a gliss from the previous pitch to target pitch may be audible. Consider the passage below:
In this case, while not notated, a glissandi would likely be audible. If playing in a standard 4-drum setup, the outer drums would be stuck on their E’s, with the middle drums never holding their pitch for more than a moment. Without any time to dampen (unless the drums have been prepared), all of the tuning changes would be audible. This, along with the fact that it’s an insane amount of footwork, makes this passage an example of not good writing.
Some effort should also be made to clarify whether to not the drum is to be struck at the end of the gliss. I’m not aware of a standardized way of doing this. An obvious answer would be to put an accent on the note, however this might not be musically appropriate, meaning some additional performance notes may be necessary.