BRYAN AIRCRAFT, INC.
Williams Co. Airport, P.O. Box 488
Bryan, Ohio 43506
CUTTING AND FINISHING THE SPAR CAPS
Be very careful to avoid scratching 7075-T6 extrusions. This material is very hard; scratches and nicks weaken it in the same manner that they would weaken glass.
Two tapers are used in the spar caps. One edge is straight and is aligned on the edge of the extrusion, full length. We will mark all extrusions to show where they are to be cut. Two caps are cut from each extrusion.
Cut on a bandsaw so that none of the line is removed. File out all cutting marks and excess material up to the line.
The finished caps will usually curve out of straightness in the plane of their width due to relieving of extrusion and stretching strains. Caps can be returned to a perfectly straight condition by placing on a smooth heavy steel bar 1" x 2" in cross section (or larger) with the concaved surface against the bar and hand hammering. A hammer with a smooth, slightly convex surface should be used. (A good quality carpenter's claw hammer will suffice.)
Firm blows are struck along the inside curved edge on the top surface about 1/4" from the edge at 1/4" to 1/2" intervals, depending on the severity of the curvature. Extreme care must be exercised to avoid nicking or denting the surface of the aluminum with the edge of the hammer head. Proper workmanship will result in visible spots about 1/4" in diameter that are shallow enough so that they cannot be felt with the fingers.
This hammering spreads the aluminum enough to expand the shrunken edge so that the cap will return to original straightness. Narrow sections near the tip naturally straighten easier then the wider portion at the root.
The finished spar cap does not follow a straight line at the point where the box section drops of to an "I" beam. The proper angle at this point can be obtained by hammering near the aft edge of the cap.
Remove all scratches and nicks with a vixen file.