A longeron is structural member of the airframe that runs in the longitudinal direction, i.e., from propeller to tail. It's principal function is to transfer loads from the skin to the frame or vice versa. For the RV-7, the longerons come as 3/4" aluminum angle about 15' long. They must be bent to match the curve of the fuselage around its widest point at the cockpit. Behind the cockpit, and ahead of it as well, the longerons are straight, but are bent into a tapering angles that follow the double cone shape of the fuselage. Time to get to it and bend some longerons. First thing is to mark the points along their length where the curve starts and ends.
While the pieces are still clamped for marking, I cut the aft ends according to the plan.
A hole drilled into the vertex makes a nice radius. Inside corners like this should always be radiused like this to discourage cracks from forming there. It works like this: If you're stress, you like to hang out at the corner. But if there is a nice gentle curve to the corner, you're not sure where to stand. Consequently, you and your buddies (more stress) can't get a crack started there because you're too spread out.
To bend the angle, clamp the angle into a vise and find a BFM. It takes a considerable force to bend the angle. This is not a job for the timid. It surprises me how frequently the project that otherwise employs digital calipers and measurements of angles to a tenth of a degree should also require the use of a big mallet.
I gave it three good whacks and then moved the angle in the vise forward about an inch. I remember thinking that this is not so difficult. I should be done in an hour! Ha Ha Ha. The instructions say to repeat as necessary and so I did. Back and forth along the section to be curved until the curve matched the template printed on the plan. It was at this point I realized that although my curve matched the plan, the longeron was also bending in the vertical dimension. Well I just won't stand for any unauthorized bending so I flip the longeron 90 degrees in the vise and pound the bow out of it. Now its flat in that plane as it's supposed to be, but making it straight there undid some of the curve. And so it goes, back and forth for two or three hours. Just when you think its about done, you notice that it is also twisted. %&$#! A large Crescent wrench is good for undoing the twist, but it also affects the curve at the same time. More pounding.
Eventually the angle tires of all of this wrestling and will submit.
Eventually it gets done and its time to bend the forward section of the longeron. This is the part that is in front of the cockpit and tapers downward as the fuselage gets smaller toward the engine. So I level the longeron on the bench and get the digital protractor ready.
At the point 28.25" inches from the front, the longeron makes a sharp 5.6 degree bend downward. More pounding, but this time it goes quickly.
With the angle bend downward all that is left is to add a 17 degree twist that will allow the longeron to follow the curved front section. The big Crescent wrench makes another appearance. To get the proper angle, I had to rotate the angle more than 90 degrees before it would snap back to 17. It's a bit scary since I've got so much time invested in the part.
Almost there.
Now to bend one more just like it. Only reversed, of course.
And there they are. I'll just add that they really don't look like much laying there on the floor, but
it's great relief to be past this step all the same.
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