I’m looking at making a whistle, I’m a mechcanical engine and have access to different tubing too. I have looked at several web site that show how to make a PVC or Copper whistle, but I haven’t found one on the theory of design, calculating the tubing size, distance and size of holes, or fipple design.
Or as I suspect is it a lot of trial and error ?
Bob
No, it’s mostly magic… 
There are whistle plans out there: do a search for “bloody hand whistle” and you should find some.
Daniel Bingamon ([u]Jubilee Music Instrument Co.[/u]) has an [u]Instrument Builder’s Page[/u] with some utilities and resources.
Yep! There will be a great deal of trial and error. But I’d recommend you follow some plans, as Paul suggested. This past weekend, I had my first attempt[s] at a low whistle. The first finished product was horrible: large toneholes, odd spacing, and VERY VERY quiet! But, the next day, I tried again, and I must say, I’m happy with the product: fairly loud, better offset holes, and they were normal-sized. A tad weak in the high octave, but nonetheless, a good homemade whistle.
If Interested, I used these plans…
http://science.univr.it/goccia/whistle/
Have fun! 
In addition to the links already posted, you might be interested in this introduction to flute acoustics.
The technical info on Jubilee’s site is great, especially the twcalc program. All the theory turned me into a particularly well-educated failure on the first few whistles, but then I copied an existing design and succeeded. The theory then became great for modifying the design to suit specific needs, and TWCalc probably saved me from making a dozen sub-standard tubes.
Do see Gaelic Crossings Builders forum archives: loads of links, ideas etc.
I’ve considered trying to build one but it’s the whole “fipple” thing that bothers me. It seems it would be easier to build a flute since you just need a hole to blow in. Albiet a special hole.
Check out http://www.dougsparling.com/irishmusic/whistle/whistlemake/
and http://www.cwo.com/~ph_kosel/flutomat.html
Angelo
Thanks to all for info on whistle making, it was exactly hat I was looking for.
Bob