I’ve had an idea which may or may not be new, but if it is new I don’t want to patent it as it could do immense harm for one manufacturer to dominate the market using it. Here’s the plan then:-
Think about how half-holing flattens notes. If you have a tube within a tube with the same holes in them and in perfect alignment, when you rotate the inner one (by turning the ring at the bottom of the tube), you can partially close all the holes and flatten the instrument across its entire range - you just need the bell note to be vented with an open hole in the side of the tube so that it is tuned along with all the rest. So, instead of changing the length of the instrument to adjust the tuning, you change the sizes of all the holes. This has a huge advantage in that the tuning can then be correct across the range instead of having the higher notes in the octave go flatter relative to the lower notes as you lengthen the tube with a traditional slide. (The disadvantage is that the instrument will become a little bit quieter as you tune it flatter, but that’s so trivial that it likely doesn’t even qualify as a problem.)
There’s no issue with air leaks and tight seals, so this would be low maintenance - the seal is made on the outer tube by the finger/key pressing in, and any air finding paths through between the tubes won’t find shorter escape routes to the outside.
There is an issue of holes not all being the same size, but if you make them all the same width and have them closer to rectangular than circular, it should be possible to make the tuning shift the same for all the notes so that you don’t need to make separate adjustments for each hole.
This system should be able to provide good tuning across a range of a semitone using big holes at one end of the range and small ones at the other.
