I’ve almost finished my tunable aluminium low D whistle, and it looks and sounds great. At least it sounds great in the low octave. The high D is pretty much in tune as well, but after that it gets progressively worse as I ascend the scale, unless I blow unreasonably hard to bring up the pitch. Very annoying. Are there any standard errors I might have made (and can unmake)? Suggestions for getting a stronger high octave and making the higher tones of it less flat?
Constricting the bore at the end of the tube will help a low whistle stay in tune with itself. It sort of simulates a tapered bore. The Burke low whistles have a ~2 cm long piece of pipe the next size down, reducing the bore diameter by a couple of mm, attached at the end.