I’ve been building low A and G whistles (the A and G below the D of a high D whistle). I’m using a Sweetone C head and experimenting with different bodies using the tin whistle calculator to locate the holes. My problem is, I keep getting a very weak low octave (low breath pressure/low volume or it breaks into the upper octave) and a somewhat flat upper octave. The length to bore ratio is fairly high: 27 to 1 on the A whistle which uses 5/8" copper pipe for the tube. Can our resident whistle smiths give me any ideas on how to correct the weak lower octave. The upper octave sounds fine other than being a bit flat.
Sounds like you need to make the window bigger longitudinally. Here’s a crude little diagram:
