I searched the Forum, and didn't find a thread on this, though I'm sure it's been discussed here.
Anyway, here's the situation: In response to a recent bout of frustration with the D pennywhistle, I checked all of my whistles/flutes/recorders against an electronic tuner. The results: The Boehm flute is consistently on pitch from the bottom to the top of its range. The tenor recorder is consistently on pitch from the bottom to the top of its range. The two alto recorders (one plastic, one wooden) are consistently on pitch from the bottom to the top of their range. My wooden soprano recorder can't be brought up to pitch no matter what I do, but its flatness is consistent from the bottom to the top of its range, so I think it's tuned to A=420 or something. My plastic soprano recorder is consistently on pitch from the bottom to the top of its range. My plastic low G whistle is consistently on pitch from the bottom to the top of its range. My plastic low D whistle is consistently on pitch from the bottom to the top of its range. My plastic C whistle is consistently on pitch from the bottom to the top of its range.
When I say "consistently on pitch from the bottom to the top of its range," I mean that, once I've got the lowest note to pitch by adjusting the tunability of the instrument, without my making any conscious effort, I can play all the notes to within fewer than 5 cents of variation. With a bit of concentration on my breath, I can put them all "right on" and keep them there.
HOWEVER--every one of my standard D whistles seems to be all over the place--18 cents flat on one note, 13 cents sharp on the next. The only note that is "on" for every D whistle is low F#. This is the case with a Sweet, a Susato, an Oak, a Feadog, 2 Generations, and a Clarke Sweetone. The only two I seem to be able to get close are the Sweetone and the Sweet, and that's with significant concentration on my breath--blowing as usual on some notes, very hard on other notes and much softer on still other notes.
Is the D just the hardest whistle to play? or is it that I've got a bum bunch of whistles? OR, (and I realize this is entirely possible) I just can't play the D whistle???
I'm at the point where I'm going to throw out all my Ds and go buy one strictly on whether it can be played at pitch.