As we blow into the fipple, our breath is directed down a narrow slot and directly on to a sharp edge: the fipple blade which splits the air stream, causing it to vibrate as part of the air stream goes off the top and part goes down the tube. It is this vibrating stream of the air reflecting off the inside surface of the body that we hear as our tone. The width of the blade has a lot to do with how the fipples performs. Wider fipple blades demand a lot more air, however too narrow, the whistle will tend to chirp, that is give off a high-end harmonic unexpectedly( this is also caused by blowing too hard). The balance is so delicate that ever 1/16 of a inch makes a noticeable difference.
I am sure you have noticed that whistles come in a variety of sizes and are made of a variety of materials. It is the distance from the fipple blade to the tube end, relative to the tube's diameter, that determines the basic tone or Pitch, of the whistle. A lot of whistle makers use 7/16 inch diameter brass tube for the body of the D and the eflat whistles. This is the perfect length to diameter ratio. Getting your whistle to play a 2 full octave range, is as much about air speed as it is about fingering and with too wide a body you simply cannot accelerate the air stream enough to get the highest notes , blow hard as you will. So, correct design is very important.
You will have noticed the bodies of the various whistles are made of different materials, folded sheet metal, plumbers brass tubing, nickel-plated brass tubing, even PVC. Each surface will react differently to the air stream, sound differently and play differently. The thickness of the body's material also has a huge bearing on the sound and performance, a thick wall, nickel-plated whistle will have a smooth but heavy sound. This thick, smooth, shiny sound is like the thick, shiny tube that produced it. The Hudson D Nickel deluxe and the Hudson D Brass PRO have quite a difference character and it is only by playing both that opens the imagination up to a broader spectrum of tonal qualities to make use of.
You will find that you prefer some tunes on one whistles and others on different one.