As far as intonation goes, what should one be aiming for?
Well, this is going to sound silly, and self-evident, but one should always be aiming to play in tune in the given situation. In other words, being “in tune” is situational, not absolute. So for example this issue of Equal Temperament versus Just Intonation: if you’re playing along with fretted strings and/or keyboards (which are in Equal Temperament) you would only be “in tune” if you played in Equal Temperament, but if you’re playing along with an uilleann piper (which will most probably be in Just Intonation, more or less) you would only be “in tune” if you were playing in JI.
The upper and lower range of my Generation seems very unbalanced, the low D plays very sharp unless at near-inaudible volume, yet the second octave tends to play rather flat unless pushed, then it gets really loud.
Yes that’s how the octaves of Generations and many other whistles are tuned: the 2nd octave is intentionally tuned a bit flat, meaning that you must blow the 2nd octave fairly strongly, and somewhat underblow the 1st octave, to keep the octaves in tune. This might be thought of as the traditional octave tuning, and not only Generations and other “cheapies” but also high-end whistles like Overtons were tuned this way. The trouble with this design is, as you point out, that it tends to exacerbate the already-present volume differential between the octaves (on whistles the 2nd octave is louder than the 1st).
The other extreme is the octave tuning of MK whistles, on which the 2nd octave is tuned much sharper, meaning that to play the octaves in tune you have to blow the 1st octave very strongly, nearly to the point of the notes breaking, and you have to rather underblow the 2nd octave. (I’ve owned six different MK Low Ds and they have varied a bit in their octave tuning, but even the one with the flattest 2nd octave had a 2nd octave far sharper than an Overton, Goldie, Dixon, Generation, or any other make I’ve had.)
In the middle are Reyburns, Burkes, and Susatos, and I’m sure others.
But a good Generation shouldn’t be as bad as you describe! Good Generations have full, round 1st octaves and sweet pure 2nd octaves and can easily be blown into tune by the player. Quality control with Generations seems to be lax or nonexistent and you might have to play through dozens of them to find a really good one.
is that why flutes have a taper headjoint?
You can read about the “why” of Boehm flute design in the inventor’s own words, by reading his book:
The Flute and Flute-Playing
by Theobald Boehm
1871
In which he says
"I had prepared, in 1846, a great number of conical and cylindrical tubes of various dimensions, and of many metals and several kinds of wood, so that the relative fitness of each as to pitch, ease of sounding and quality of tone, could be investigated.
The most desirable proportions of the air column were soon found. These experiments show:
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That the strength as well as the quality of the fundamental tone is proportional to the volume of air set in vibration.
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That a more or less important contraction in the bore of the upper part of the flute tube, and a shortening or lengthening of this contraction, have an important influence upon the production of the tones and upon the tuning of the octaves.
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That this contraction must be made in a certain geometrical proportion, which is closely approached by the curve of the parabola.
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That the formation of the nodes and segments of the sound waves takes place most easily and perfectly in a cylindrical flute tube, the length of which is 30 times its diameter, and in which the contraction begins in the upper fourth part of the length of the tube, continuing to the cork where the diameter is reduced one tenth part. "
The notable thing about Boehm is that all of this stuff was arrived at by experimentation, not by the application of pre-existing theories. He also had a flute-tube made with movable holes so that he could arrive at the ideal location of each hole by experimentation.
Also are whistles generally tuned to equal temperament? I’ve seen posts on this site sugesting that they are closer to just intonation.
Pretty much all of the different whistles I’ve owned have come more or less strictly tuned to Equal Temperament. However the F# on some of my Generation D’s has come rather flat (where it should be, in JI) and I’ve carved that hole out a bit. I need my whistles to be in ET for the sorts of gigs I do.
There’s really not all that much difference between ET and JI. Take your ET D whistle, throw a bit of tape on the F# and B holes, and voila! you now have a JI whistle.