BUILDING THE HP-11 SPAR
By Richard Schreder
Start by band sawing the extrusions. All four of the 14 ft. angles are cut the same, namely, a straight cut from 12 1/8 inch from one end to 9 inches from the other. Since it is difficult to draw a straight line of this length, it is suggested that a mark be made midway between the points 5/16 inch in from the edge of the angle. By passing the trim line through this mark, a better accuracy in insured.
Saw outside of the trim line (1/2 x .025 x 14 tooth blade) and hand file sawed edges smooth. 7075-T6 aluminum is very hard and therefore is seriously weakened by any nicks or sharp cornered cutouts, so proper smoothing is very important.
Use the same procedure for cutting the 35 ft. 10 inch angles. Clamping the 1 inch legs together when marking each pair of angles will insure that they will be cut in the necessary pairs and will aid in holding them straight. Marking, of course, will have to be done on the inside surface of the wide angle.
Use a pair of dividers a scale graduated in 1/100s of an inch or a drill guide to lay out the rivet holes. As shown on our tool list, we can provide a hardened drill template for the 0.975 inch spacing. Whichever method is used, do not lay out all holes consecutively without using the 9-3/4 inch rib location check points as an error of only 0.001 inch on hole spacing will result in a 3/8 inch error in the overall spar length.
Drill all holes in the angles with a no. 30 drill. Holes for 3/16 rivets can be reamed out later with a no. 12 drill. The holes can be laid out with a scale marked in hundredths or 0.975 hardened drill guide can be used.
If one long angle is drilled first, it can be used as a drilling template by clamping it to each of the other mating angles. One of the mated angles so drilled, can then be used as a template for the remaining angles.
Cut the shear webs to exact width. Check original sheared edges by placing two sheets together on the floor. If both check straight, use one as a straightedge to mark all others. Snip or bandsaw close to trim lines. Hand file to the lines.
Lightening holes can be put in now, but there will be less chance of error if this operation is delayed until the spar is riveted together.
Clamp one set of angles to the exact edge of the full web. Web splice plates may be left off until angles are riveted in place. Run drill through angle holes to pierce web. Bolt assembly together by using 3/16 bolts in one of each of the 2 vertical stiffener holes. Repeat this operation to attach the angles on the opposite edge of the web. Be sure that angles are aligned exactly with the edge of the web, as this will determine the accuracy of spar width. Sight the spar or use a stretched nylon thread to verify perfect straightness of the spar before drilling this second line of holes.
After drilling, take the components apart to clean out the chips, place the mating angles together and check the radius of the resulting contour; both edges should match a 30 inch template. If variations of the extrusions do not produce this radius, appropriate filing of the outer surface of the 1 inch legs will correct the contour.
Rivet all holes except those for stiffeners and ribs. Drive 3 rivets along the tip edge and then three along the bottom as riveting progresses down the spar, to prevent building in a bow due to stretching of the angles by the bulging rivets.
Vertical stiffeners should now be cut to fit as closely as possible between the parallel top and bottom angle legs and rounded to fit the angle radius. After clamping each in place, a drill can be run through from the reverse side. Rivet the stiffeners.
Web access holes can now be cut with a fly cutter. Insure that each hole comes at that edge of the rib attaching flange to provide proper access during rib skin riveting.
Place both spars on a flat surface with webs horizontal and rivet heads up. Block as necessary to insure that both webs are straight and in the same horizontal plane. With connecting pin installed, clamp one set of splice blocks to the bottom flanges on both spars, so that the end of each spar is aligned with the land between fingers of each attached splice plate. Adjust the spars until the included angle formed by the top flanges is 176 degrees.
Make sure that the 3 finger blocks are both on the left wing spar and both 2 finger blocks are on the right spar. Both spars are identical until attachment of the splice plates. For best appearance, we orient the round rivet heads on the forward side of the spars.
Drill the holes through the spar flanges by placing a steel drill bushing in each splice plate hole and drill with a number 12 drill. Remove bushing and drill all holes with a 23/64 inch drill. Ream all holes to 0.370. Drive 3/8 inch bolts in place and install nuts.
Clamp the second set of splice blocks in place so that the center line of the connecting pin is centered on the line bisecting the 176 degree included angle and the center of the lower pin.
Drill and bolt the second set of blocks as explained above.
Check completed spars for straightness by placing each spar on the floor or bench with the shear web horizontal. Stretch a nylon thread full length along the full length cap angles. If any bow exists, it can be removed by hammering the flanges on the concave surface. Back up the leg of the angle with a heavy bar and use a good hammer having a smooth, slightly rounded head surface. Avoid making any nicks or sharp dents in the metal surface. This hammering will stretch the angles enough to restore spar trueness.