Thursday, December 24, 2009

The Tail Skid Bracket

This must be what it's like to build one of the traditional RVs where it's not uncommon to see a step in the manual that says something concise like "Fabricate the Engine Mount."

In this case, it's nothing quite that complicated. After all, it's just a piece of extruded aluminum that has to have a hole tapped to allow a bolt with a hoop at one end threaded into it. This will provide a way to tie the tail down when the airplane is parked outside. There will be two more of these, eventually. There will be one in each wing. Or so I assume.

There's a little more to the fabrication than simply tapping the hole, though. Not that there needs to be, mind you, as the additional steps seem to be more about the aesthetics of the part than anything else. When you consider that this part will be trapped inside the fuselage where no one but an accident inspector is ever likely to see it, I have to wonder if there is another issue at play. You see, the RV-12 was designed in the midst of a years-long FAA reappraisal of the rules that experimental airplanes are built under. During that time, there was an intense focus on the issue of fabrication. The FAA was trying to put more of the work back into the hands of the builder as they saw a trend towards kits that were so complete that the builder was just assembling a relative handful of parts into a six seat, pressurized business jet.

One of the more frightening proposed rule changes was to mandate that the builder fabricate some percentage of the kit. I believe the value of 20% was proposed at some point. Some wags wondered aloud if builders were going to be tasked with mining their own bauxite to make the aluminum.

This leaves me wondering if some fabrication make-work was built into the design to absorb some of that regulatory burden in non-critical components, should it ever become law. Look at it this way: if you were ever going to fly with me in this plane, would you prefer that I fabricate a tie down bracket or the wing spar? The percentage of fabrication could be made up of meaningless tasks performed on benign parts, or it could consist of parts that are absolutely critical to the flight of the plane. As a designer, which path would you choose?

Motives aside, the fabrication effort is pretty straight forward. As we saw at the tail end (heh!) of the last post, I had tentatively marked a pair of lines that were tangent to the provided holes. What I had not yet noticed was the importance of the somewhat off hand direction to cut tangent to the 1/4" holes. The holes in the template were not 1/4", nor were they actually in the part to be fabricated. So, Step One was to ignore my previous Step One and drill the 1/4" holes, then mark the tangent lines. It made quite a difference:

You could be asking yourself why we need 1/4" holes here. The directions insist on a kind of blind obeisance to the beneficent motives of the designers at times, but eventually it becomes clear as to why you did something. In this case, you can see that the 1/4" holes will be mostly cut away, leaving a nice 1/4" radius on the flange of the skid bracket:

See? It just takes a little faith!

We're also instructed to cut a diagonal off of the main body of the part to create... what? I don't know! This is either a weight saving effort (every .1 ounce matters, I guess) or to provide clearance for an obstruction that we will see later. That, or it's make-work. In any case, it looks like it's going to be a bit of a chore without a band saw:

I decided to save that step for last. I'd concentrate on the most functional step first: the drilling and tapping of the hole.

There's already a hole running the length of the part, so the drilling is simply to increase the radius of the hole to a size suitable for tapping. Even with the existing hole there to help guide the bit, I decided that the drill press would be the safest way to ensure that the hole remained straight:

Section 5 of the manual has a nice paragraph on tapping holes, and one of the suggestions is to also use the drill press for the actual tapping. They have you chuck the tap into the drill press, then manually turn it to tap into the metal. I tried it that way, but it didn't work very well. I needed one hand just to hold the tap down into the hole against the spring action of the drill press that wants to lift it back out, while trying to turn the chuck with the other hand. When the need for a third hand to hold the part steady became apparent, I figured that I was just not correctly configured for the job. And, having no third hand to borrow, well, it just wasn't going to work. There was another problem, too, in that the direction the the chuck needed to be turned to advance the bite of the tap was the same direction that loosens the chuck. I decided to give up on that whole approach and just use the handle that came with the tap & die kit:

The failure of the drill press method of tapping which, to be fair, was probably intended for tapping much smaller holes, didn't dissuade me from using a couple of other pieces of advice. First, they suggest using a lubricant. Second, they advise that you make no more than two or three complete turns before backing the tap back out and cleaning away metal chips. Using that method, it took awhile to get the tap all the way down in the hole:

Here you can see the metal chips that impede progress if you don't remove and clean the tap every few turns:

Here's the tapped hole:

With the intricate machining done, I shifted over to the brute force hacksaw to remove the excess material on the flange:

The hacksaw is the proverbial bull in the china shop and leaves quite an unsightly mess - not exactly what you want when making an aesthetic modification. This is, of course, why we spend $55 on a Scotchbrite wheel. I made both cuts with the hacksaw and then used the wheel to smooth everything out. As long as I was grinding, I added the notch on the side shown on the template (I looked ahead - it's for clearance from another part) and rounded the corners on the end where the flange of the part remains unmolested by the rapacious hacksaw:

Note the beautiful symmetry of the notches on the sides, a symmetry that would be even more beautiful if both of those notches were actually supposed to be there. Unfortunately, one of them is extraneous. The template that provided the radius for the rounded corners only provided it on one side, so I flipped it over to do the other side. I then forgot to return it to the correct orientation before cutting the notch. It won't matter in the long run, but it was a bit of a frustration if only because of how much of the expensive Scotchbrite wheel was wasted in its creation.

There was nothing left to do but to make the final, most difficult cut. This is a perfect example of why I should buy a band saw:

Yuck! The hacksaw is never going to be confused with a scalpel (or me a surgeon, for that matter):

The Scotchbrite wheel was able to clean most of that up, but there are some unsightly gouges that will remain. So, here's the final tail skid bracket, viewed from its good side:

Good thing it will be invisible!

No comments:

Post a Comment