HP-18 and RS-15
Wing Spar Unriveting Tool

By Bob Kuykendall

Once upon a sad day last year, I finished riveting the last shear web panel of my right wing spar. The sad part of what should have been a happy occasion is that when I checked the spar for twist, I found that there was about 1.5 degrees of twist to what should have been an untwisted spar.

For some reason or other, there was a bag of Basalite concrete at hand, and it was only by the thinnest of margins that I resisted installing the spar in the front yard as a flag pole. Fortunately, cooler heads prevailed, and the spar went back into the garage to await its fate.

The first step in the rehabilitation of the spar was to map the twist from root to tip. With the spar laid on the font face, I placed straight steel bars across the rear shear web and measured the twist between various spanwise locations and a reference location (the station where the root rib attaches). This showed that the inboard nine feet was absolutely twist-free, and that the twist was concentrated in between stations 108 and 156, with another twist occurring between stations 204 and 288. Between stations 156 and 204 it was relatively twist-free.

With a good idea of what the twist was, I formulated a plan that would remove most, but not all, of the twist, and would give me a spar that was twist-free at several key locations. I decided that I would remove the outboard 14í of rivets from the forward shear web, and replace the rivets while the spar was clamped in the proper twist-free orientation.

I removed the first few rivets using the techniques shown in AC43.13. This technique involves drilling almost all the way through the head of the rivet using a #31 drill, breaking the head off of the rivet using the root of another #31 drill bit, and then hammering the rivet shank through itís hole using a 1/8" drift.

The standard hammering technique caused rather more trauma to the rivet holes than I wanted. It also assaulted my ears and fingers, and threatened imminent violence to my wing spar. There had to be a better way.

Temporarily disregarding the instructions in AC43.13, I tried hammering out a few rivets using a 3/32" drift. This went wrong exactly like they said it would; the drift would poke into the rivet stem, expanding it and making it even harder to remove. I gave up on that idea pretty quickly.

The next thing I did was to build the unriveting tool shown in the attached sketch, but with one difference. My first version of this tool used 1/8" drive pins like the drifts that had worked best with the hammering technique. The unriveting tool worked great, pushing the rivet stems out of the holes just as easy as you please. However, the drive pin was still coming in contact with the edges of the rivet holes and marring them slightly. Also, the screwing motion of the drive pins didnít help, adding their own contribution to the trauma to the rivet holes.

My second version of the unriveting tool used 3/32" drive pins instead of the earlier 1/8" pins. This seemed to work best, and represents the final configuration of the tool. The smaller drive pins center easily in the conical depression left when the head is drilled off the stem, and do not contact the rivet holes. When applied with a steady push instead of a hammer blow, the smaller pin does not cause the rivet to expand, thus making it easy to push the rivet stem out.

To use the tool, start by drilling the rivet head and breaking the head off as described in AC43.13. Then position the tool over the rivet and carefully screw in one of the drive pins so that it bears on the conical depression left in the rivet stem by the drill bit. Finally, screw the drive pin in to press the rivet stem all the way through the hole. Be careful not to tighten the drive pin so much that the shoulder of the threaded rod bears on the shear web and damages it.

The tool has two drive screws so that it is not necessary to remove the tool in order to switch between removing upper and lower rivets. When using the tool, be careful to avoid scratching the shear webs with the points of the drive pins (or any other part of the tool), since any scratches in the shear webs or spar caps can become crack initiation sites. As instructed in the Introduction of the Building the HP-18 articles, carefully file and polish out any scratches that you do inflict.

After removing the 336 rivets from the outboard 14 feet of the forward shear web with the unriveting tool, it was a straightforward job to clamp the spar in a straight jig and re-rivet the web. Not that it was easy, mind you; closing the HP-18 box spar isnít easy, just straightforward. Thanks to the unriveting tool, I didnít have to buy a new spar cap set or make new shear webs.

Of course, the standard disclaimers apply; use the unriveting tool with caution and at your own risk. Your mileage may vary. If in any doubt about the tools or techniques described here, consult a qualified A&P mechanic or aerospace engineer.