I’ve been making Native American flutes for the last few years (and a few transverse flutes as well) out of bamboo. The tone quality that can be achieved from what amounts to glorified grass never ceases to amaze me. I make a 6 hole pentonic minor which, if in the key of G would play G, A#, C, D, F, G, A#. There are some obvious differences in how a the mouth part of a native american flute is constructed, but in essence it’s still a whistle with a windway, a fipple of sorts, and a blade.
So the question is… is there something inherent in the tuning or mouth piece of NA flutes that keep them from playing in the second register (they traditionally only play an octave and a third, and that third is only achieved by placing the top hole high up on the flute, not by over-blowing as with whistles)? It’s one of the things that makes NA flutes easier to make - you don’t have to balance pitch and tone for two registers. I tried making a standard diatonic tuning, but could only get the first octave. Great tone though 
If I could make a NA style flute that played in the second register (either diatonic or pentonic), what would be the characteristics of the mouth part (I’m assuming that’s the major factor) I’d need to adjust (blade size, windway size, blade angle, etc).
The great thing about Irish whistles is you can play in two pentonic keys (in D it would be B minor and E minor). But the fingering isn’t as smooth since you have to skip notes. I’ve played Irish whistles and flutes for about five years now and have a modest assortment of clark, overton, copeland, and a bamboo olwell. Great fun! I just moved from NM where Native American music abounds but celtic is rare. So I’m hoping to take advantage of the celtic/irish culture around southern PA. Anyone going to the Maryland Festival end of August? Olwell and a few others will be there (I hope to go and plague them all with questions and hopefully make a few friends 
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