Mistakes Are What You Make of Them.

As soon as I kinked my second tube I knew I would need more to get two good wingtip bows, so I placed an order for three 7' long pieces of tubing. I only needed about 6' to make one bow, but there is almost no leverage for the last of the bend unless you add some excess. I bought three in case I messed another one up beyond saving. Happily, Wicks Aircraft still had the tubing I needed for $1.05 a foot. About $50 and two days later I had my tubing and borrowed a conduit bender from work.

I took a scrap from my kinked tubing and tried bending it with the conduit bender; INSTANT KINK.
Ooookay... now what?
 I read some more from Tony Bingelis' Sportplane Construction Techniques, quoted below.

"Thin wall tubing is more difficult to bend successfully because it is very quick to flatten and buckle"

The wingtip tubing is .028" wall; the tubing used in the rest of the fuselage is .035" wall or greater.

"The larger the diameter of the tubing, the greater its excess length should be. Allow and extra 12" on each end for tubing up to 3/4" in diameter."

"Before you attempt to bend any large diameter tube (3/4" or larger) you should fill it with densely packed sand."

Ta-da. I never packed my tubing with sand before trying a bend. I'd heard of it being done, but didn't think it would be necessary for something like this. Clearly I was wrong.

 Armed with this new information I embarked on. I'll let "Uncle Tony" describe the process.

"Plug the bottom end of the tube with a wood plug and pour in the sand. Tap the bottom end of the tube repeatedly against a solid surface (concrete floor). After the tube is full of sand, the continued tapping will cause it to settle and pack more densely. Add additional sand. After the sand shows no further sign of settling, drive a wood plug into the top end of the tube, making sure it bottoms solidly against the sand. Your tube is now ready for bending. The risk of a flattened bend will be considerably reduced."

  I tried my first sand-packed tube bend with scrap tubing in the conduit bender. It kinked, but not as badly. So, I cut off the kinked bit, repacked the tubing, and tried again with the plywood bender I had made before. Success! I proceeded to bend up the scrap piece, matching my wingtip pattern as I went to get a feel for the process. I almost had enough tubing for a usable part, but not quite.

  With one attempt mostly successful, I went on to bend two of my three new tubes into wingtip bows. Each successive bend went faster and smoother than the last.

 The one bit of information I would like to add to Tony's instructions: If you have a tight bend near an end, make that at what was the bottom end when you were packing sand. The bottom end has the most densely packed sand because of the weight of the sand packing in above it. 

I had some slight wrinkling at the inside of the tightest bend, but not enough to be concerned with. Below is a picture of the too-short test piece and the two good bows. There is extra material at each end that will need to be cut off and trimmed to fit the wing's leading and trailing edge. The test piece will be very helpful in practicing these cuts as well.

 It turns out those "wasted" hours making a plywood bending form weren't a waste after all.

Total Time: 214.8 hrs.