I'm a little late to the party, but I thought I'd toss in a couple cents' worth for those who seem interested. I spent some while reading up on the subject after this thread got started late last week (one of the benefits of academia, an all-you-can-eat subscription to IEEE Xplore). Lots of technical papers written on the subject. Wish I could post them on here, but a few tidbits I learned.
For an ironless stator, apparently the optimum coil shape is actually a rhomboid. Pie-shaped coils aren't optimal because the straight runs on the top and bottom are useless copper that adds resistance but doesn't contribute to torque. They claim a rhomboid is the best compromise.
Also allows a nice trick for water cooling, if you layer the coils together at a slant:
I know these are just sketches, but re: the drawings posted by LFP and Miles, you really want the coils to be larger relative to the magnets. Think of it this way: assuming that the magnetic flux is even across the entire face of the magnet, only the flux that goes through the center of the coil "sees" the entire number of coils. Flux that passes partway through the windings will only "see" the number of coils outside that point. Ideally, you'd want the opening of your coil to be the same size as the magnet. Notice the Ro and Ri markings in the coil picture above - those would mark the extend of the magnets. The end windings can be outside the flux since they don't contribute.
I think the flat wound coils LFP suggested are a fantastic idea. Dense copper fill and low resistance, plus the wide part of the copper is parallel to the flux so eddy losses are small. That's exactly how the coils in the machine from this paper are constructed.
You might also consider spacing the magnets out more from each other to minimize the flux leakage. I think a good rule of thumb is the same spacing as the width of the airgap. If they're closer than that, some of the edge flux will tend to couple directly to the magnet next door instead of crossing the airgap to do useful work.
One final note on the multistage idea. There's a lot written about that and the scaling seems to be pretty much linear - 2 stages gives 2x torque, etc. But it seems like torque density (by weight or by volume) is better if you just make the diameter larger. The consensus seems to be make the diameter as large as possible (or practical), then go multistage if you still need more power.
Finally, I'll leave you with this picture from the paper. The machine they designed was a dual-rotor single stator, ironless machine similar to those discussed here. It's rated for 70 kW in a package about 40cm x 5 cm. It's intended as an alternator, but the design is fully reversible.
Here's the citation, for those with the interest and means:
Caricchi, F. et al., "Performance of Coreless-Winding Axial-Flux Permanent-Magnet Generator With Power Output at 400 Hz, 3000 r/min," IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 34, NO. 6, NOVEMBER/DECEMBER 1998, pp. 1263-1269.
my work spaces all look teribly clutterd I need some time fix that.
Use a carbon fiber composite to replace as much of the metal as possible to get it down near the 20kg range.
