Not a lot of time tonight, but I ‘m finally back to semi-actual work; that is, doing stuff from the instructions instead of dealing with throttle stuff. Tonight I just removed the forward and mid left side skins, followed by basically everything that wasn’t permanently attached: gear tower plates, center section plates, auxiliary longerons, lower longerons. Then I did a little bit of cleaning up.

Now I think I can finally get to some deburring and countersinking and other fun stuff…

For real this time, it’s done.

To start off with tonight, I decided to try making a new support angle out of sheet stock, hoping that with a tighter corner bend, I could get away with locating it a bit lower. But after doing a test fit, during which I marked the rivet hole locations, it was pretty clear that those holes would be really difficult to dimple. So instead, I decided to investigate the possibility of simply adding a bend to my cover plate. I was able to conclusively measure the required offset, which turned out to be only a hair over 1/8”.

Based on the space I had outboard of the quadrant cutout, I figured I could make my bend without moving the outboard edge inboard too far. So I clecoed the cover plate in place, then clecoed the angle to the side skin, and traced the edge of the angle on the cover plate. This would be my outboard bend line. Then I drew a straight line aligned with the outboard edge of the quadrant cutout line; this would be the inboard bend line.

Making the first bend was pretty easy I just clamped two pieces of scrap wood at the line and pressed against the workbench to get the slight bend I was looking for. Making the second bend was less fun, as there was only about 1/2” between the two. I ended up clamping the whole assembly in the vice and carefully tapping the bend line with a piece of wood and a hammer until everything looked good. This didn’t get me the same crisp bend as the first one, but oh well. Some fine-tuning with the hand seamers, and it was time for a test fit. And it worked great!

That still left the issue of drilling the three screw holes to attach to the mount angle. I prefer to drill stuff like this in assembly, but it simply wasn’t possible here. Instead, with the angle clecoed to the side skin, I measured the distance from the side skin to each hole center, then marked the centerline of each hole on the skin. Once I had the cover plate in place, I transferred the lines from the side skin to the plate, then measured out from the skin to mark the center lines. I was a little worried about things not lining up, but it ended up fitting nicely. (well, after a few more rounds of tweaking the plate to get a nice snug fit to the quadrant itself)

Close-up look at the bend:

Boy, am I glad to be done with this. Now to resume tearing everything apart and getting some prep work done. I haven’t even looked at the instructions in weeks!

So all I had left to do before I could put this throttle stuff to bed permanently was to figure out how to drill the cover plate for the outboard angle screw holes. I wasn’t quite sure how to do this, but it couldn’t be that bad after everything else, right? Oh, how I make myself laugh sometimes…

The perceived difficulty here was due to the need to cleco the angle in place on the forward side skin, and then figure out some way (in tight quarters) to clamp the cover plate to the angle and drill the holes from below. This promised to be an unpleasant experience, but then I had a sudden bright idea. What if, with the mid side skin still off, I clecoed the original cover plate in place, clecoed the angle to the plate, and then figured a way to clamp the angle in place using the center section bulkhead and gear tower? Then I could simply swap cover plates and drill my holes with ease. What a bright idea, I thought!

So I swapped out the cover plates. Uh-oh, this does not bode well:

I can’t believe how far above the quadrant this plate sits. It doesn’t even make any sense to me – the quadrant mount holes were predrilled, so there’d be no way to mount this quadrant any higher so it would sit flush against the plate. But the bigger issue was that this implied that when clecoed in place on the side skin, the angle would be too high for my custom cover plate. Guess I’d better cleco the side skin on and verify the bad news.

Yep. Hard to tell from this blurry photo, but the angle sits something like 1/8’1/4” above the top of the quadrant. ARGH

OK, so now what? I could make a new replacement angle, but with the rivet holes prepunched in the side skin, it might be impossible to move the angle far enough down. To test this idea, I cut up a piece of scrap hardware store angle I had lying around. I used a couple more pieces of scrap angle to create a jig to hold the test angle level with the top of the quadrant for test fitting:

Next, I went to the outside of the skin and marked the rivet hole locations with a sharpie. After drilling a #40 hole in one of these locations, I was pleased to see that while the hole was close to the vertex of the angle, it seemed like there was enough room to set a rivet head in there. The only possible downside – if I was to make this piece out of actual angle – is that the good angle has a larger radius inside the vertex, which diminishes the available room for a shop head. Worse yet, when I clecoed the angle back in place using my test hole, I found that I’d drilled it a bit too low. Between needing to move the angle down a bit, and the larger radius of the angle stock, I don’t think that’s a viable option for this piece.

So tomorrow I guess I’ll see about forming my own angle piece out of sheet stock. I don’t think this piece needs to be particularly strong, so I may be able to mitigate the issues by using fairly light sheet stock, but I’ll just have to see how that goes. I’ll also throw this tale of woe up on VAF and see if I get any interesting ideas from other builders. Another possible solution – which I haven’t really thought through at all – could be to fabricate an angle piece that rivets directly to the center section bulkhead and gear tower flanges, instead of the side skin itself. Then I could just fill the side skin holes with rivets that attached to nothing – though I wonder if there’d be any concern of the skin flexing and rubbing against the angle mount.

I dunno. I’ll figure something out, as usual…

Well, I think I can finally put this throttle hilarity behind me. Funny how this started out as “let me take care of this one thing before I move on with prep work.” And that was about three weeks ago.

I started out today with a few more iterations of gradually enlarging the passthrough holes. After the second attempt, I was close enough that instead of actually removing the quadrants again, I just moved the linkage out of the way and filed the edge of the hole with the linkage in place. And finally, I got what I wanted: full movement of the throttle with no rubbing of the linkage.

Whew.

OK, so the final piece of the throttle puzzle was fabricating a new cover plate for the forward throttle. As I probably mentioned before, there’s a cover plate included with the kit, but it seems to be pre-cut for a three-lever quadrant. For whatever reason, the cutout in the thing is significantly wider than my quadrant. Making this piece was actually kind of fun, though there was some tedium towards the end.

To start with, I cut a form block out of scrap wood, using the original cover plate as a template to lay out the cut lines. I also used sandpaper to round the corners a bit where I’d be bend material later, to help promote an appropriately gentle bend:

Checking fit of the form block in the throttle quadrant mount:

I used the block itself to make the initial marks on my sheet stock, then marked off ~7/8” for the flanges:

For the corner of the plate what will have flanges, I punched and drilled a #12 hole to ensure there’d be no sharp corner here. This also adds some relief to account for the bend in the mount plate, which is a bit rounded. Then I used the edges of the hole to mark cut lines for the ends of the flanges:

And here’s the piece after making rough cuts, filing down edges, and smoothing everything with the bench grinder. I’ve also drilled two tooling holes, which are used to positively locate this piece on the form block. These two holes are in the area that will be later removed for the quadrant itself:

Here I’ve bent the flanges and made notches to fit over the throttle mount spacers. I’ve also test-fit this piece with the quadrant attached to the mount (sans levers), and drilled the screw holes where it will attach to the mount. And finally, I’ve marked the cutout for the quadrant. The outermost lines are the actual perimeter of the quadrant faceplate; inside that are my expected cut lines, inset 1/8” on the sides and 1/4” on the ends. Inside the cut lines at the corners are center marks for #12 holes that I’ll drill to serve as the corners of the cutout:

This was where things got fun. After drilling the corner holes, I started making cuts with a Dremel cutoff wheel. I made the short cuts at the ends with no problem, but I was just starting one of the long cuts when the cutoff disc jammed up and completely stopped the Dremel. Before I could switch it off, it turned off on its own – and notably, refused to turn on again. Not sure if I blew some internal fuse or what – after a quick Google session, I checked the motor brushes and they seemed OK, but I never could get the thing to turn back on.

So I just proceeded with my rough cuts, using snips instead. I used a unibit to open the tooling holes up to the largest possible size so I could get the snips in to work with. This worked decently, but I wasn’t able to cut as close to my cut lines as I could have with the Dremel, so when it came time to file the cut lines out to their final locations – well, let’s just say there was l-o-t-s of filing to be done. That was the tedious part I mentioned earlier. It doesn’t help that it’s hard to figure out how to clamp this piece for filing some of the locations.

But al that filing paid off, as the end result fits quite nicely:

The only thing left to do was to reassemble the throttle and try fitting everything in place. But first I had a modification to the throttle. Since I’d moved it outboard a bit to accommodate the linkage, the pivot bolt was now too short to fit the friction lock lever. The bolt is retained by a little clip riveted to the inboard quadrant plate, so I had to drill out those two rivets. I had just the right length AN bolt lying around, but I noted that the original bolt had a much longer threaded shank than the AN bolt. So after riveting the new bolt in place, I got out the tap & die set and added another inch or so of threads to the new bolt. And finally, the whole thing went together. (…again…)

Test-fitting everything in place revealed that I needed relief notches in the outboard corners of the cover plate, to accommodate the flanges of the center section bulkhead and the gear tower. Pretty simple modification really…and then finally, I had everything in place!

The only thing left to do on that cover plate is drill the screw holes on the outboard edge. There’s a piece of angle mounted to the forward side skin that the cover plate will attach to – the fun part will be figuring out how to mark and drill those holes. Next time, I suppose I’ll cleco the forward side skin in place again, both to try and get those screw holes drilled, and to perform a final check of the linkage with the additional bracing of the skin.

But finally, the end is near!

Really, I should be more upbeat, because I’m finally closing in on the end of this semi-farce. I did a couple more iterations of enlarging the passthrough holes, and finally got to a point where I was comfortable committing to drilling the linkage mount hole in the forward throttle arm. (which required disassembling both throttles…again…) Then I did two test fits of the linkage; the first test fit showed that the holes needed a little more work, so I filed away at them some more. The second test fit is looking pretty good, but still not perfect. The linkage is still making light contact at the bottom of two of the holes:

Just needs a little bit more filing…another day…

Here’s the linkage in place:

And a close-up of the front throttle attach point:

Once I get those holes taken care of, I’ll be pretty much done here. The only other thing to be addressed is the friction lock for the front throttle; since I moved it away from the mounting plate, I’ll probably need to swap out the bolt there to account for the difference. Oh, and I still need to fabricate a new top cover plate for the front throttle.

So after examining my options for dealing with the linkage mount hole location – in particular taking a close look at potential edge distance issues on the center section bulkheads – I decided that I could proceed by simply enlarging the passthrough holes in the bulkheads. I carefully drew lines on each bulkhead so I’d know where my edge distance limits were, then went to work enlarging the holes.

Here’s the enlarged forward hole. The vertical edge distance guide line can be seen, along with a horizontal line showing how far downward I wanted to enlarge the hole:

Meanwhile, I disassembled both throttles (…again…) and used the rear throttle arm to mark the hole location on the forward arm. Blue marker doesn’t show up very well on the black arm, so I added some blue masking tape around the perimeter of the hole location to provide a better visual guide. Then I reassembled both throttles (…again…) and mounted them both in place (…again…).

Getting there:

At this point, the linkage was interfering with the rounded “corners” of passthrough holes, not the absolute bottom, so after removing both throttles (…again…), I filed the holes outward and downward to provide some more clearance. Then I reinstalled both throttles (…again…). Even closer this time (sorry, no photo), but now there’s a new issue. This is the current interference point, preventing the linkage from going lower:

To get more clearance here, I’d have to file the hole basically straight towards the rivet hole, but I’m not comfortable removing any more material in this area. Both passthrough holes have ample clearance on the outboard side of the linkage though, so I think what I’ll do to address this issue is move the forward throttle outboard a bit. That should move any potential clearance issues to the outboard side of the passthrough holes, where I’m much more comfortable removing material.

But that requires removing and disassembling the throttles (…again…), and I’ve had enough for one night.

I should just stop thinking that, on any particular day, I’m going to finally nail down this throttle stuff. You’d think I would have learned this lesson way back with the empennage, but apparently not.

With the linkage roughly laid out, I figured tonight I’d mark the throttle arms for the linkage connection holes, mark the linkage for shortening (it was still a little long), and finalize the necessary spacer to locate both throttle quadrants correctly. I started by pulling the linkage and shortening it, removing about 1/4” from each end. Then I drilled a little deeper in each and and re-tapped the holes so I could have proper thread depth for the rod ends.

Next, I pulled the rear throttle apart (for the 900th time or so). Going the rod end route here means that I need some extra clearance on the outboard side of the throttle, so I fabricated a 1/8” thick spacer and added it to the assembly. I also had to add washers to the pivot point, which ended up with me realizing that I’ll need yet more bolts from Spruce. I can get the stop nut started on the pivot point, but it doesn’t have sufficient engagement. It’s enough to hold everything together for now, but not acceptable as a long-term solution:

I also went ahead and drilled the new linkage connection point on the rear throttle before reassembling the entire thing. Here’s the new assembly:

Next, I removed the front throttle, disassembled it, and fabricated spacers of aluminum tube to move it outboard to the appropriate location. I also fabricated another 1/8” thick spacer to give the extra space needed for the rod end on the outboard side. The only thing left to do was to drill the linkage connection hole at the spot I’d marked (using the actual linkage in place). As a sanity check, I compared this mark to the hole in the rear throttle, and…uh-oh. The mark on the front throttle is a good 1/4” higher than the hole on the rear one. That’s not good – I want those to be the same distance from the pivot point so both throttles have identical travel. Having the point higher on the front throttle would mean that the front throttle would have reduced travel as compared to the rear one.

So now, the question is: how do I handle this? I’ve worked out a couple options so far:

The simplest option would be to simply drill the hole in the front throttle as marked. Based on my math, this would result in the front throttle travel being reduced by about 6.5° from the rear (43.44° vs 50°). I’m not sure this would make a huge difference, but the bigger question regards how the throttle rigging itself works. I don’t know if proper throttle rigging depends on the lever having the proper travel, or if this is easily adjustable using a linkage at the engine. I need to do some more research before I know whether or not this is a feasible option.

Another option would be to use the rear throttle arm as a template, drill the front arm to match, and enlarge the bulkhead passthrough holes as needed. By my calculations, I’d need to enlarge the center section bulkhead holes by roughly .2” downward. This would put the lower edge of each slot roughly vertically in line with a rivet hole shared with the bulkhead cap. I think this could be down without causing edge distance issues, but it would be close. This would probably be the most preferable option, assuming it causes no edge distance issues. I’ve only done rough measurements regarding edge distance, so I need to go measure precisely to determine the feasibility of this option.

The third option would be to drill the front arm as marked, and then red rill the rear throttle arm to match. This would allow me to basically leave the passthrough holes in the center section alone. The passthrough in the armrest bulkhead would need to be expanded, but I’m far more comfortable enlarging the hole there instead of the center section. The big downside here is that I’m 99% sure this can’t be done without replacing the throttle arm. I could probably just fabricate a new arm from 1/8” bar stock. The only other consideration would be finding somewhere local to black anodize the arm so it would match the rest of the quadrant.

Off the bat, #3 sounds the best to me. #2 might work, provided edge distance isn’t an issue. #1 seems like a bit of a hack that requires justification. I’ll have to think this over before deciding how to proceed.

In the meantime, here’s a photo of both throttles mounted with the linkage sitting in place:

So I didn’t get a whole lot done today. We spent most of the day doing a bit of kayaking around Lake Houston. I still coaxed myself into doing some work in the garage afterwards, mainly because my stuff from Spruce came in and I was kind of excited to move forward with the throttle stuff.

After inspecting all the stuff and verifying that the rod ends were the same width that I used for laying out the holes for the linkage, I decided that I was happy with the layout. So it was time to take the big step and start making holes. I started out with #20 pilot holes, then broke out the Unibit and enlarged each hole to 5/16”, which I figured would be a good baseline size for the 1/4” tube. Next, I clecoed the forward quadrant back in place so I could verify that the positioning looked good. Whoops, turns out there was one thing I didn’t think about: the forward quadrant mount.

That blue at the end of the far hole is the quadrant mount:

Even better, the positioning is such that the hole lies right on a bend on that quadrant mount:

The good thing is that the mount is quite beefy and there are no potential edge distance issues, so I didn’t have to be extremely precise when making this hole. I decided to start by making a 9/16” hole, drilling square to the angled portion of the mount (that is, the part that doesn’t lie against the center section bulkhead). I figured this would give me a hole that was roughly oval in profile when viewed from directly behind – and it did.

After thinking over a few ways to enlarge and finish the passthrough holes in the bulkheads, I decided to just lay out the required vertical size of the holes and go ahead and enlarge them with a Dremel cutting bit. Originally I’d had thoughts of carefully enlarging the holes so they were just large enough, but that would require a ton of trial and error. Furthermore, I could enlarge to the expected vertical size without getting anywhere near edge distance issues, so I figured there was no need for the tedious trial-and-error.

Here’s a (horribly blurry) photo of the enlarged holes in the center section bulkhead:

I’d previously removed the forward and mid side skins so I’d have access to make the center section holes; at this point, I left the forward side skin off but reattached the mid side skin to the center section bulkhead. With the forward throttle quadrant in place, there’s sufficient rigidity to work on the linkage, and this way I have much better access to the inside of the forward quadrant:

I finished the night by laying the linkage tubing out, cutting it to length (being conservative, since as always, removing material is much easier than adding it back), and drilling and tapping the ends for the rod end bearings. Tomorrow I should be able to work out final positioning of the forward quadrant, and drill the new linkage holes in both levers. Once positioning is finalized, I can fabricate the new top cover plate for the forward throttle. That should be fun, as it’s a bit more complex of a piece than the rear throttle mount plate that I spent so much time on the first time around…

Overall, a much more productive session tonight. I think I’m finally over the hump of all the throttle head-scratching now (famous last words…). I put in another Spruce order today for a pair of rod ends and some other associated hardware, for v2.0 of the throttle linkage.

One thing I did last night that I didn’t mention was a test run at tapping the ends of my linkage. The linkage is 1/4” steel tube, which will be cut to length, and then the ends will be drilled to size and tapped for 10-32 threads. Originally I was going to put threaded rod into each end for the (female) clevises to attach to, but with the new setup, the rod ends will thread directly into the linkage. Anyway, last night I cut off a short section of the tube, with the intent of seeing how the tapping worked out. Based on some quick Googling, it seemed that the proper drill size was #21, so I drilled the tubing using that, then went to town with the tap.

It went pretty horribly. It was almost impossible to turn the tap, to the extent that I was worried I was going to break the thing a couple times. I ended up with a mangled piece of tubing and something of a foul mood. Some further research turned up a more detailed chart, which called out different drill sizes for, say, aluminum vs. steel. Specifically, it called for a larger pilot hole for steel, perhaps to account for the harder material. According to this chart, #18 was the proper drill size.

So I started out tonight with some tapping experimentation. I had an extra little piece of 1/4” aluminum bar lying around, and I decided to drill three different hole sizes and try tapping them. For the first hole, I used the same #21 as before. I didn’t have a #18 drill bit on hand, so in its place I used a 11/64 bit, which is .024” larger. I did have a #17 reamer, so I used that for my third hole. (#17 is another .011” larger than 11/64) As expected, tapping the larger holes took much less effort than the #21, though none were especially difficult thanks to the soft aluminum.

Next, I wanted to get a ballpark idea of the relative strength of the threads. Presumably the downside of a larger hole would be reduced thread depth, and thus reduced strength. To test this, I decided to thread a bolt into each hole, snug it down, and then proceed to tighten it with the torque wrench until the threads failed. To be clear, this is in no way intended to be scientific or representative of real-world strength; I was just looking to compare the three holes relative to each other.

The results were quite interesting. The #17 hole gave up the ghost at only 30 in-lbs, while the 11/64 hole made it to 60 in-lbs and the #21 to 70 in-lbs. It would seem that that extra .011” from 11/64 to #17 makes a substantial difference. Based on these results – and keeping in mind that the steel rod should be significantly stronger, along with this being a fairly low-stress application – I believe that 11/64 is a perfectly acceptable pilot hole size here.

Here’s that aluminum test piece after I finished ruining the holes:

Next, I moved on to my favorite topic, trying to locate the bulkhead holes for the linkage. I decided to go ahead and lay out the holes; since I know the dimensions of the rod ends I ordered, I can determine where the linkage’s centerline will sit relative to the throttle arm (and by extension the side skin and other stuff) in the horizontal plane. I decided to use the rivet holes in the center section bulkhead to locate the linkage vertically; maintaining edge distance to those holes is the most important consideration. Since I’ll be drilling new holes in the throttle arms for the linkage anyway, I’m free to locate it vertically where it best suits me, and then drill the throttle arms to match later on.

So with all that in mind, and once again using my trick of clecoing some angle in place as a guide, I got my hole center locations laid out:

The second photo is of the center section bulkhead. Here I’ve clecoed the bulkhead cap in place to help evaluate the placement; I’ll definitely be needing to remove some material from the cap here for clearance. But even if I made a 1/2” hole on this center point (which would be a very generous size), I’d still have 5/8” edge distance to the rivet holes. I’ll need to be very careful here to make the passthrough hole only as large as necessary; I’d prefer a good margin over minimum edge distance in this location.

I briefly considered going ahead and drilling pilot holes in these locations, but I’m going to wait until I get the hardware from Spruce (hopefully on Saturday) just so I can double-check the dimensions of the rod ends. Considering how much time I’ve put into puzzling over this setup, it would be pretty dumb to get head of myself now and bugger up the whole works.

For a final task, I went ahead and fabricated a second mount plate for the rear throttle. Ever since I made the first one, the more I played with this setup, the more clear it became that my two large lightening holes weren’t going to be much help in proving access to the lever for rigging the linkage. Even with the lever full forward or back, it was mostly blocked by the web between the holes. So I decided to make a new plate with three lightening holes, where the central one would provide good access to the lever.

Fabrication went pretty quickly (I think it took about an hour) since I was able to use the first plate as a template. About the only unexpected development was that the first lightening hole I cut was larger than I wanted it to be by about 1/4” –  I guess there’s a little slop in my circle cutter. Fortunately, I cut the center hole first, so when I readjusted the cutter to make the other two holes smaller, the whole thing ended up looking nice and symmetrical. And now I really do have good access to the throttle lever:

From here, I figure that once I get the hardware from Spruce and confirm the rod end dimensions, I’ll drill the pilot holes for the passthrough, mock up the linkage location, and use that to figure out how far inboard I need to move the forward throttle. Once the positioning of the forward throttle is nailed down, I can fabricate the top cover plate there and finally put this task to bed.

Ugh, slow going. The good news is that I got in my order from Aircraft Spruce, so I could at least start to move on. First I used the aluminum tubing I ordered to make new spacers for the rear throttle. I already had to replace the one I mistakenly cut, so I just went ahead and made two spacers. At least this way, I’m not using stacks of washers to space the thing out properly:

So I clecoed the rear throttle back in place (for what seems like the 500th time or so) and resumed staring fixedly at the whole setup, trying to figure out how to hash out the drill locations in the bulkheads. The problem I always had was trying to work out a way to mock up a straight line parallel to the rear throttle and extending forward to the front throttle area. I wasn’t convinced that I could rely on the skin being parallel all the way up. Tonight, though, I hit upon a ridiculously simple way to handle this. The rear throttle is parallel to the edge of the armrest, which in turn is located at each end by the bulkhead. All I had to do was find a piece of angle and cleco it to the two armrest bulkheads, and I’d have a reference all the way forward!

I didn’t have any actual angle of an appropriate length, but this untrimmed stiffener stock worked just fine:

Now I could simply measure the distance from the angle piece to the rear throttle arm, and then roughly lay out the horizontal position of the matching potential hole location in the center section bulkheads:

Well, that’s a problem. I’m quite sure there’s not enough room right between those two rivet holes to cut the hole for the linkage and still maintain acceptable edge distance. And I’m certainly not going to flirt with edge distance issues here.

I think what I’m going to have to do here is use rod end bearings at the ends of the linkage, instead of the clevises I ordered from Spruce. That would allow me to offset the linkage outboard by probably half an inch or so. Of course, if I do this, I’ll have to order stuff from Spruce…again. More money tossed in the wind for shipping costs.

Anyway, that’s pretty much where I quit for the night. I feel like I’m closing in on a solution, but at an almost maddeningly slow pace. After how quickly and easily the rest of the fuselage went together before, this is a rather frustrating change of pace.