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Started make the bends
I made all the marks called for on the longeron. Then I made additional marks every inch down the line to help me move the longeron one inch at a time for the bends. I pulled on the longeron in the direction I wanted it to go and then started whacking it with a rubber hammer. Then started the back and forth bend checking with the drawing on the cardboard. Then the instructions said to start using the F-721B aft canopy deck as a go-bye for the rest of the bend. I noticed immediately that no matter what I did, this longeron started bowing along the line. After I finished with the whackin' and checkin', I turned the longeron 90° and clamped it in the vise and bent it back straight with just my hands. When I got the bow out, I noticed that the original bends that I put in with the hammer were now off from the chart. So I went through the whole process again, eventually getting the longeron just right.Here an article from Van's about using the F-721 as a "go-by":
UPDATE FROM THE RVATOR ABOUT BENDING LONGERONS.
When bending the longerons for the side by side two seat airplanes, there are two possible templates. One is printed full size on the drawings The other is the F-721 aluminum canopy deck, which is prepunched to the correct curve. Before pre-punching, the canopy deck and the longeron were both shaped to fit the paper pattern. But we have found that the paper printing process can vary dimensionally, so it isn’t unheard of for the paper template and the pre-punched aluminum part to mis-match. Now, the aluminum canopy deck is going to rivet onto the longeron — the paper pattern is not. So it seems logical to bend the longeron to match the canopy deck. Looking ahead, you can see that differing curves on the longerons will affect the curves necessary in the canopy frame — and is one reason we tell builders that the plans dimensions on canopy drawings are a place to start, not an absolute. These are. after all, handbuilt airplanes so some variation and adaptation is inevitable.
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