I attached my wings this weekend.
I used a 6" 1/4" black oxide bit then switched to this one:
http://www.fastenal.com//web/products/details/0350457
- double_margin_bit.jpg (50.13 KiB) Viewed 19180 times
Here's a picture I took while doing a practice run that better highlights the stability provided by this drill bit. The contacts with the hole wall are a lot wider than I was expecting.
- double_margin_bit2.jpg (43.71 KiB) Viewed 19157 times
The bit was very stable in the hole due to the double margins. I used an electric drill and stopped about 5 times to remove chips. The last time I removed chips I was almost bottomed out in the hole, so I unchucked the bit and slid it about 1/4" out and was able to make it all the way through without having to switch sides. Both holes seem great.
The biggest "gotcha" I had was my rear spar attachments were only pilot drilled when I started. I thought I was supposed to drill to 1/4" with the wings attached to the fuselage, but was unable to get the drill positioned without interfering with some riveted angles on the fuselage walls. I then decided to remove the wings and attach them resting on sawhorses. This allowed me to attach the main spars solidly together with bolts in the 3 alignment holes. I was then able to attach the rear spar channel and drill up to 1/4" without interference. This is probably a step I skipped somewhere.