Wednesday, March 30, 2016

Rudder Pedals and Elevator Brackets

I've been making small amounts of progress here and there. I'd been putting off the rudder pedal swingarms, because I wanted them offset to work with the offset seating. In order to generate more shoulder room, the pilot's seat will sit 3" forward of the pax seat. That doesn't sound like much, but it causes shoulders to overlap rather than push against, which makes things much more comfortable. My shop really isn't set up for metal work, so cutting, grinding, and jigging this 4130 has been a challenge.

This was my first jig attempt, and while I did get it to work well enough, it was a huge hassle. The second jig turned out a lot better, and was much easier to build.

These were sent off to be welded, and are now ready to be cleaned and painted. I'll be cleaning and painting the steel parts of the control sticks assembly at the same time.

While I was set up for it, I went ahead and cut my elevator pushrods to length. I have a 5/8" rod from my control sticks to a T lever, which transitions to cables, which run back to another T lever and a 3/8" rod to the elevator.

The pushrods are not yet connected to the rod ends in the picture, but that's not an oversight. I wanted to hook everything up before drilling holes and riveting, to make sure the length was right!

The main reason for my slow progress has been my own work in adjusting existing designs to fit my needs. Specifically the brackets/T-lever assemblies. It takes me a while to fabricate the metal bits, so I try to make absolutely certain that I don't have to re-make any parts due to unforeseen problems.  A few of my sketches are below, showing the bracket and T-lever setups.

This is a 4x scale drawing of my forward T-lever setup. From left to right: sintered bronze (SB) bushing, 3/4" aluminum spacer, aluminum T-lever, SB bushing inside the T-lever, 3/4" spacer, SB bushing. I have SB bushings on both ends, so that's what will contact the bracket, and I have a SB bushing through the actual lever itself to facilitate smooth rotation.

Next is a 1:1 drawing of the rear T-lever, which also incorporates an arm for counterbalancing the elevator. I will preempt this drawing by saying, I did not use this design. Rather, I adjusted it with spacers.

And here's a 4x drawing of the center of the assembly, or the pivot point:

From left to right: SB bushing, T-lever, 3/4" square tubing for counterbalance, 1/4" angle, SB bushing. The problem with this design is, the cables will rub the counterbalance arm when it pivots. As a result, I'll be adding a spacer between the T-lever and the 3/4" square tubing. 

This gives you a better view of the problem. The second and fourth "nut" from the top will have cables attached, and anytime the elevator isn't slightly up the counterbalance will intersect one of the cables. Adding an 1/8" spacer to the pivot point and the pushrod point should give me enough clearance.... or maybe 1/4". I'll be mocking this up *not* in the plane, to make sure I get it right, before I start drilling holes in the wrong spots. Anyway, I hope to have most of this finished after this weekend. 

Monday, March 7, 2016


I've been waiting for a hardware order to come so I could finish putting my control sticks together for the last time. Unfortunately, once again I seem to have ordered the wrong length bolts. However, this time I have enough of various sizes to put together a formula, so (hopefully?) I'll get the right stuff every time from now on. The problem is as follows:

When a person wants to order AN hardware, they have two numbers and two yes/no options for standard bolts. The yes/no options are for a drilled head (for safety wiring) and a drilled shank (for using a cotter pin and a castellated nut.) The first number denotes the diameter, and is in sixteenths of an inch. AN3 is 3/16, AN4 is 4/16, (¼, if you will) so on and so forth. Nice and easy. The second number denotes the length, and the only way to know what it stands for is to have a lookup table. The good news is, lookup tables are readily available online. The bad news is, they only give you two lengths, and those are NOT ENOUGH.

The first number I'll address is *nominal* length. This is the total length of the bolt, (minus the head) which is important to know. Obviously you want to make sure the bolt is long enough to get through the material it's holding with enough room to spare for washer(s) and a nut. In addition, it is sometimes important to know how far the bolt will stick out beyond the nut (as you'll see when I build my control-stick-pushrod-to-cables-bracket.)

"But Matt," the reader may ask, "Why don't you just order a bolt that's a little longer than you think you'll need? You can always just grind it off if the extra length gets in the way." Oh, that it were that simple. Unfortunately, there is another number that's just as important, and it makes precision quite necessary. It's called the *grip* length.

Grip length is the length of the bolt that is *not* threaded. That's right, there's a *lot* of bolt that isn't threaded. In fact, on a 3/16" bolt, only a little over ⅜" is threaded, no matter how long the bolt is. Even that isn't so bad, though; just pick a bolt that has a slightly shorter grip length than the material you're bolting through. Unless, of course, you need a castellated nut with a cotter pin.

Castellated nuts and cotter pins are used when it would be very, very bad for something to come apart. On this plane, the main things I'll have this type of hardware on are the control systems, since they're very important, and they'll be in motion pretty constantly. It's a great system, and pretty hard to mess up. If you get the right length hardware. The length, however, is determined by adding the material thickness to the amount of castellated nut that is below the cotter pin, then subtracting the distance the drilled hole in the bolt is from the end of the threaded portion. This would be feasible, except NONE of these variables are published... at least, not anywhere I found.

If you're still keeping up, you've figured out that neither nominal nor grip length will help with this problem. Therefore, I took some measurements against several different length bolts, both 3/16 and ¼" diameter, and wrote a spreadsheet that gives estimated minimum and maximum material thickness along with nominal and grip length. I've checked it against all of the hardware I have, and it all matches up for me. I'm publishing it here in hopes that I can save some other poor homebuilder a few weeks of guessing and frustration, not to mention extra shipping costs.  Currently it only features AN3 and AN4 drilled bolts, but if I get data from other diameters I'll add that to it.

*The lengths listed for max and minimum material thickness are based on the use of an AN380-2-3 cotter pin. Minimum thickness was based on my personal comfort level of how much bolt needed to stick up and grap the cotter pin, and maximum thickness was taken as soon as it was possible to push the cotter pin through the hole.

**I should mention that I used two different sized bolts for each diameter, and noticed ~.002 inch variations between same diameter bolts. I decided that was an acceptable variation to assume standard hole placement was used throughout the entire lineup of same diameter bolts. However, I do not claim to have tested each length, and therefore my numbers may not be correct in every instance. Your mileage may vary.


Wednesday, March 2, 2016

Instrument Panel

I've been waiting for various hardware and tubing, but I haven't been twiddling my thumbs. I'm slowly acquiring the instruments for my plane, and I realized that having a plan in place would make picking up the right instruments a whole lot easier. I spent a few evenings trying to come up with panel arrangements using a photo editor, but I just couldn't get a feel for how it'd look in real life. So I had my local print shop print out a full-scale template of my instrument panel, overlaid with a ¼" grid. I then printed out full-sized instruments, as well as two glass screens I intend to incorporate. I cut all of this out, then used contact paper to make them more wear-resistant. What I ended up with was a completely customizable panel that shows me exactly how large everything will be in the plane. For some reason, this is a lot easier for the spatial reasoning part of my brain to work with. Here's a picture I took, not of a well thought-out layout, but just a quick proof of concept. (It's super blurry because of crummy lighting; the graphics are actually quite crisp. :D)