A few months ago, we visited our friends at the Tempest Aero Group and watched as they made hundreds of oil filters and spark plugs every hour. Watching filter production (in the post-COVID world) was like watching wartime production in action; it was all hands to the pumps, getting as many out the door in as short a time as possible. The spark plug area was more sedate but equally interesting—a mainly automated process with a large machine centered around a carousel where the plugs grew before your eyes as components were added. As I said, very interesting…but at the same time, amusing because while there will always be a great demand for the products in the certified airplane world where aviation spark plugs are mandatory, the Experimental world is quickly leaving them behind in exchange for the ubiquitous BR8ES automotive spark plug because so many builders and owners are switching to electronic ignition systems and leaving their old Slick and Bendix mags behind.

The NGK BR8ES (or DENSO W24ESR-U) can be purchased in quantity from many sources and in a couple of different flavors. First tip: The BR8ES (also search for as BR8-ES, BR-8ES or any other variation of where the dash goes) comes in two different types—one with a screw-on tip for the spark plug wire and one with a solid tip. The first has a part number of 5422 and is very common. The other has a part number of 3961 and is less common, but it is the one you want for your airplane. Why? Because some folks have reported that the screw-on end can get loose and cause RFI that shows up as radio noise. The solid tip can’t do this. Whether this is true or just an old wives’ tail, it is not that hard to find the 3961s (as I write this, www.rockauto.com has them for $1.88 each), and there is no disadvantage to the solid tip so, why not?

Of course, you can use aviation plugs with some electronic ignitions, but with aviation plugs going for north of $30 each, it’s hard to pass up those $1.88 plugs and throw them away each time. I personally have four Lycoming engines, all with electronic ignitions, so I buy several dozen BR8ES plugs at a time and keep a bunch on the shelf. All my wonderful aviation spark plug tools (used for cleaning and gapping) sit in a drawer, used only when I have to work on someone else’s plane that still has mags. The NGKs are so cheap that while you can clean them, it is simply easier to throw them away every couple hundred hours and drop in new ones. It’s just economics.

Adapting

Of course, automotive spark plugs don’t just screw into a Lycoming engine—they are too small. Automotive plugs screw into a 14mm hole, whereas aviation plugs traditionally are 18mm. That means you’ll need an adapter for each plug, and these are usually supplied by your electronic ignition provider. They are generally made of brass, but some have been known to be steel. We’ve used both, but generally use the brass sets because they are more generally supplied.

Using an adapter means that there are two seals for every spark plug: one that seals the plug to the adapter and one that seals the adapter to the cylinder head. The first comes on the automotive plug and is generally a crush washer—hollow soft steel that crushes down when you torque the plug in place. While most mechanics will remove a plug and put it back in place again, the engineer who designed them probably considers these crush washers to be single use. I haven’t heard of problems (such as leakage or a plug backing out) from reusing them; I generally will only put one of these plugs in and out a couple of times. Again, they are cheap to simply replace.

The seal between the adapter and the cylinder head is accomplished the old-fashioned way—with a copper gasket. These gaskets come with aviation plugs (and can be bought separately by the thousands) and when new are in an annealed condition, which is to say “soft.” Using them properly (with a torque wrench) crushes them, providing a seal and adding tension that prevents the spark plug (or adapter) from backing out. Many mechanics routinely reanneal old gaskets and use them over, while many others declare that those who reuse them are just cheapskates—so why not use new ones every time? I personally reuse them sometimes because I enjoy the process of heating them up with a torch, watching them change color and then letting them cool down. Of course then you have to clean them to make them pretty…the whole process taking more time than simply buying a bag of new ones from Aircraft Spruce. It’s a religious argument; do what you’d like!

The Torque Problem

Automotive spark plugs get torqued to 18–25 foot-pounds. Aircraft spark plugs get torqued to about 35 foot-pounds. The difference is in the size of the thread (14mm versus 18mm) as well as the type of gasket used that provides a sort of spring retaining force. This means that to make everything happy in your Lycoming installation, you have two different torques in the assembly. If you torque both the plug and the adapter to 18 foot-pounds, you haven’t preloaded the copper gasket on the adapter sufficiently. If you torque the assembly to 35 foot-pounds, you’ve crushed the heck out of the spark plug’s built-in gasket. So what to do?

The answer is to torque them both separately, in series—but to do this, you need a couple of different wrenches. One thing you don’t want to do is torque the brass adapter in place without a spark plug screwed in; the soft adapter can be twisted and crushed out of true if there is nothing in the hole. The different electronic ignition builders have different techniques they recommend, but surveying the various options and simply looking at the joints has helped us build this procedure. The advantage is that it gives each “joint” the proper torque (not over, not under) and allows for easy removal of the plugs for maintenance. Ideally, once you have used this method, the adapters will stay in the engine—and we find that works about 70% of the time.

The Process—and Tools!

The way we do it is this: Take a new plug out of the box and screw it hand tight into the adapter using the appropriate anti-seize (which is the topic of an entirely different article!). Now screw the assembly finger-tight into the cylinder (same comment on anti-seize). Now take out your automotive spark plug socket, slide it on the plug and torque it to 18 foot-pounds. The net result is that both the plug and the adapter will be torqued into their holes at 18 ft-lb.

Next, take that automotive socket and put it aside. What you want next is a 7/8-inch deep socket that is long enough to allow it to fully cover the “flats” on the adapter, yet let the spark plug nest inside. The socket we have found that does this is a Craftsman 1/2-inch drive deep socket—ours is probably 40 years old, but it appears they still make the same thing. Note that a standard aircraft spark plug socket from Champion, Tempest or Autolite doesn’t work because it has a shoulder about 3/4 inch inside that will interfere with the automotive plug and not let the socket sit full down onto the adapter. You need a 7/8-inch socket that has “flats” all the way from the mount to the top (see picture). Now you probably don’t have a 1/2-inch torque wrench—or if you do, it will not work down at 35 foot-pounds, as it’s designed to work on big tractors—so find a “cheater” adapter to fit your ⅜-inch drive wrench onto the 1/2-inch drive socket.

Now you can slide this deep socket over the plug and onto the adapter and torque the adapter to the appropriate 35 foot-pounds to crush the copper washer properly—and you’re done. Next time you pull the plugs, you should theoretically be able to use an automotive plug socket and simply remove the plugs since they are torqued to a lower value than the adapters. This means you’re going to need a 14mm compression tester spud, of course. And as we noted, sometimes the adapters will still come out—so you’ll need both spuds for compression testing…maybe.

The Baffling Question

OK, so the process described above works fine until you get to a spot where the baffles (think top plugs) get in the way of that long 7/8-inch socket. Sure, it fits in place, but you don’t have room to put the torque wrench on there! What you need is a much “shorter” way to drive the big socket—and for this, you can make a special tool—if you have a lathe and welder (or know someone with the right skills and tools). The idea is to make something short that plugs into the ½-inch drive and allows you to use a crowfoot on the end of your torque wrench.

What we did was take a cheap 1/2-inch to 3/8-inch drive adapter and a short 1/2-inch bolt from the hardware store. We chucked the bolt up in the lathe and cut off all the threads, leaving a 3/4-inch hex head with a cylindrical 3/8-inch diameter shaft. We then chucked up the adapter and drilled out the inside about 1/4 inch deeper using a 3/8-inch drill bit. We cut as much off the open end of the adapter as we dared while leaving enough material to create a sturdy assembly, then cut enough of the “shaft” off the ¾-inch hex head so that it nested fully down into the adapter. Then it was a simple stop at the TIG welding table to join the two things together, and we had a 3/4-inch hex head to pop into the big socket. Use the 3/4-inch crowfoot on your torque wrench, and you’re ready to go.

An alternative is to use a mill to put “flats” on the end of the big socket and use the appropriate (larger) crowfoot directly on the socket. Essentially, it duplicates the configuration of a standard aircraft plug socket wrench that has both a ratchet drive socket and a hex head (usually for a 7/8-inch wrench) as well. This is easier if you don’t have access to a welder, but you have a friend with a mill—or you have a vise, a mill bastard file and some patience. You can start with a Craftsman CMMT47528 (1/2-inch drive, 7/8-inch deep socket—available at Ace Hardware or your favorite online retailer) and use a carbide end mill to do the work. The socket is very hard and it simply laughed at our regular (cheap) end mills that we use for aluminum. This solution gives you the best fit for top plugs where cooling baffles might be in the way, while also preserving the option of using a 1/2-inch-drive torque wrench directly. The spark plug and adapter fit snuggly inside this particular Craftsman socket.

One final solution is the cheapest (but won’t always work with baffles). Harbor Freight sells a set of square-drive socket caps (part number 67011) that give you the same thing as the welded adapter we described above—but where’s the building fun in that?

The Simple Solution

Automotive plugs are a simple solution for experimental Lycomings when using any of the popular electronic ignition systems. They fire great with a factory gap of about 0.032 inch, and they are cheap so that you will never worry about maintaining them. I have heard less discussion about lead fouling with these plugs than in the old days, when everyone used massive-electrode aviation plugs. I don’t know if this is because people do a better job of leaning on the ground, the plugs are less prone to fouling or people just throw them away before fouling becomes an issue!

It’s hard to break the habits of decades of aircraft maintenance, but not having to clean and re-gap plugs is a boon to those of us that have spent years of our lives digging out little lead balls and sandblasting expensive plugs. I first used the BR8ES in two-stroke jet skis, where the tuning process was to run the engine hard, then pull the safety lanyard off at high rpm, killing the motor instantly and preserving the combustion signatures on the plugs. You pulled the plugs, looked at the color of the deposits, threw the plug away, retuned, put in a new plug and did it over. Yeah, we went through a ton of plugs, but they were even cheaper back then.

Experimental aviators now have the option of using and throwing away plugs the same way—but we should always remember that the consequences of a blown-out plug in an airplane are much more dire than having to swim your jet ski back to shore or coast your late-model car to the side of the road. So use proper techniques and torques when installing plugs, and enjoy the convenience and savings of the modern ignition world!

Photos: Paul Dye and Marc Cook (sidebar).

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Just like every other metal-aircraft builder, I have had to develop fiberglass skills—doing layups, filling pinholes and smoothing out contours with resin and micro (or other fillers). It’s a love/hate relationship. I actually love the fact that you can do pretty much anything with fiberglass—and if you aren’t happy with the results, you can “erase” it with a sanding tool and start again. But I simply hate the mess and the repetitive nature of sanding, filling, sanding, filling…ad infinitum.

I have had to make composite tips (tail and wing) fit by cutting and re-bonding. I have had to fix warped parts with  filler and lots of sanding. And I have performed major surgery on cowls and fairings to get the air to move past discontinuities properly. It’s just part of the road to finishing—a road with few shortcuts. Unless you know a good glass airplane builder who will trade you their skills for a nice set of engine baffles.

Today’s Challenge

So what prompted this particular set of musings on the “joys” of composite work? Well, it turned out that the cowling for our eXenos project needed to be lengthened as part of the process of mounting a completely different powerplant than what the original design called for.

Why? It’s the pain of being an early adopter. As one, I received an engine mount for the electric propulsion system that was longer than intended. Bad news for me but the good news is that all of the production versions will be the correct length to allow use of the standard, unmodified Xenos cowling. Progress!

When all was said and done, with the early, too-long motor/battery mount and installing the electrics, the stock cowling was about 2.5 inches short of reaching the firewall when the front was butted up against the back of the propeller. Rather than wait for the new mount, we modified the cowling.

While we could have attached the cowling to the firewall and done considerable re-contouring up front, the thought of forming shapes in foam and reworking contour lines lost out to the simple task of bonding in flat strips on the sides and bottom to match the slab nature of the fuselage. We had to do some work to make the curved top fit properly, but at least we could do that without making molds.

So let’s take a walk through the process we used to lengthen the cowling for this project. You will be tempted to say, “Hang on, this simply isn’t relevant to me. I am going completely stock and my factory cowl will fit perfectly!” Let me point out that a lot of metal airplane builders over-trim their stock cowl and end up making it too short—so they need to add some back in. Why does this happen? Composite cowlings are a little like octopus skins—they keep changing shape until they are in their absolute final position. So trimming a little that seems just right when the cowl is not in its final position often means you end up a quarter-inch too short. It doesn’t fit until it, uh, doesn’t fit! Love/hate, remember?

Establishing “Truth”

In many airplanes, both the fuselage and the cowling are tapering toward the nose, so getting a good fit is not always easy. Tapers and compound curves are hard to fit. Add to that the fact that most cowls are split—either top to bottom or side to side (ours is side to side). As a result, you quickly find that the cowling doesn’t want to hold any particular shape until it is actually mounted—but you can’t mount it until you freeze the shape. Catch 22. (If you caught Larry Larson’s multipart series in our March, April and May 2023 issues on fitting the RV cowling, you know part of the story already.)

The way to deal with this is to figure out how to establish a baseline, a “truth,” if you will. Once you pin the cowling in a specific position, you can then adjust from there to get a good fit. With a cowling that is too short, the problem is compounded because it is literally floating in space until you lengthen it somewhere.

For our project, the slab side of the fuselage provided a good place to establish truth, so the first point of attack was to add some material to the back edge of the sides. Once we added material to the sides, we could trim and fit those to where it could be Clecoed in place. Then we could deal with the curved top or the flat bottom—whichever seemed more appropriate when we got to that point. But once the sides we pinned, we had truth and everything else could be adjusted to fit.

Lengthening With Scarf Joints

How do you add material? Well when it is flat, it’s pretty easy—you start with a scarf joint. What this means is that you want to taper the surface to which you want to attach the new material. An aggressive sanding disk on an angle grinder or hand sander will generally do the trick. We laid the cowl on the workbench, outside down, then ran the sander along the edge, tapering it to a fine knife edge. With a layer of gelcoat on the outside, this was easy—you could tell that you were down to the knife edge when you saw the white appear. We then tapered the joint in about an inch, uncovering the layers of the original cowl like sedimentary rock on the edge of a reservoir. This taper is the key to a good scarf joint—the added material will be tapered in the opposite direction and the overall thickness will remain the same all the way from the old part to the new part, with the outside edges lining up due to the knife-edge taper.

With the joint prepared, I now needed a mold—this was nothing but a backing surface that was straight and flat, covered in something to which the fiberglass resin wouldn’t bond. Plastic sheeting or clear packing tape is good for this. I found a nice flat piece of particle board in my scrap bin, laid it on the workbench, then laid the cowl “side down” on that, leaving sufficient board sticking out to add the 3 inches of glass that I needed. I then drilled a couple of #40 holes in the cowl and through into the board and used silver Clecoes to hold the cowl to the board.

Yes, I’m drilling extra holes in the cowling I won’t need later. Not a big deal. You are going to be doing a lot of finishing and filling on the cowl anyway so filling a couple of tiny holes is in the noise. Make your job easier—fasten things down now and fill the holes later. It’s really better this way.

With the cowl and mold prepared, it was time to cut some fabric and make a mess! I used a roll of BID (bidirectional) cloth and cut progressively narrower layers—because I wanted an overlap of 3 inches, I started with two layers 3 inches wide, then one layer 3.5 inches wide, another 4 inches wide, one more 4.5 inches wide and then, finally, one 5 inches wide. This gave me a taper to match the scarf on the existing cowl. This bundle of layups went on plastic sheeting to be saturated by activated resin, another layer of plastic, complete wetting out of the matrix and finally, application to the cowl and the mold sheet. A layer of Dacron peel-ply finished it off, with a final squeegeeing of excess resin.

Curing time complete, it was a simple matter of separating the peel ply from one side, then popping the cowl with its new 3-inch extension off of the mold. I now had 3 more inches to work with on the sides (I repeated the process for the other half of the cowl of course).

Now came the usual fit/trim cycle—using the back of the prop as “truth,” I trimmed the new back edge (on each side) until it was close enough to Cleco to the cowling’s attach points (in this case, hinges). Two holes drilled for silver Clecoes held each side rigidly in position so that I could continue trimming and fitting, eventually drilling all of the holes to the attaching hinges. I now had the cowl positioned so that I could work on the top. The bottom was going to be a simple job and its final position might depend on how the top turned out, so I left it for last.

Getting in Deeper

Creating the curved extension on the top was similar to the process for the sides, except that the “mold” needed to be curved to follow the contours of the existing cowl. A curved backing isn’t all that hard to create, however, so long as you’re dealing with a single curvature—or something close to it. I suspect that there is a bit of compound curving going on with the Sonex cowl, but it was small enough that we could get away with using a piece of heavy poster board for the curved mold, taping it securely to the outside of the top portion of the cowl, then laying the material into it. Care had to be taken in trimming because this truly is a case of “it doesn’t fit until it fits,” so you have to sneak up on the final trim.

Once the sides and top were more or less in place, we fit the center seam—which, in all the perversity of working with fiberglass, now had a gap that had to be filled. We did this to a final fit by building a “bridge” of scrap aluminum, Clecoed in place on the outside after covering it with packing tape. Working inside the joined cowl, I laid in the necessary glass to re-joint the two halves. When it was all set, the entire cowl gets Clecoed back in place to check the fit, then split the cowl down the middle, leaving the specified 1/16-inch gap. You’ll see in the pictures that we made a strip of aluminum match-drilled to our hinge and used that on the outside instead of trying to use the hinge on the inside during all this fitting. The same thing was done on the bottom.

Finally, the bottom needed to be extended and, again, this was as simple as the sides. With some rough trimming and aluminum splices holding the bottom center seam together, we simply added 4 inches to the back of the bottom, then trimmed to fit. Since an exit is required for cooling air, we simply didn’t do the extension in the middle area as shown on the plans and finished the back with hinge material as per the standard Sonex design. We now had a cowl that fit all the way around and matched the fuselage nicely.

One Step at a Time

When first faced with a project like lengthening a cowl, it seems like a daunting challenge and trying to figure out how it is all going to fit is a puzzle. But just like all aircraft building, you have to stop trying to look at the whole picture at once—you need to break the project down into single steps. Determine how much you have to add all around and add some margin for error—maybe 20%–25%. Figure out the easiest place to add material so that you can “pin” the project in position to figure out the next step. Scarf your joining surface. Prepare your layout. Saturate with resin. Lay it in place. Let it cure. Then mount and trim to fit. Now begin the next area.

It isn’t all that hard, just messy. But so is drilling lots of repetitive holes or spraying metal primer—or if you’re doing tube and fabric, sanding all that area again and again! If you’re a wood builder, tell me about all those rib pieces and gussets! Airplane building is messy, no matter the material. In the case of fiberglass, wear old clothes, cover anything in the shop that doesn’t react well to dust and find yourself a good set of safety glasses and a dust mask. Then have at it!

And as I said at the start—if you don’t like the results, sand or cut them away and start again. That’s the beauty of non-structural fiberglass work—it’s all art and it can all be fixed.

The post Love and Hate appeared first on KITPLANES.

Integral fuel tanks, like those on RVs, are generally riveted together and made leak-proof using a polysulfide sealer. (Commonly Pro-Seal.) This two-part chemical sealer is bad-smelling and known for getting everywhere, but does a good job of keeping fuel inside—most of the time. The problem comes with rivets—all of those little holes in the tank just waiting for a tiny spot that wasn’t sealed well to give way.

While big leaks around fuel senders and access panels make up the majority of fuel tank problems, it is not uncommon for tanks to develop leaks at rivets that are so small they don’t drip—they just tend to seep (or weep) fuel at a rate equivalent to its evaporation. The process of evaporation usually leaves rings of color (the blue dye in 100LL, for example) around the offending rivet. If the weeping is bad enough and the rivet is on the bottom, the fuel might actually run down the tank and leave streaks back to the spar.

Weeping rivets are not really an issue in terms of losing substantial amounts of avgas. But no one likes a fuel leak, even if its only downside is a stain. Most owners see these things develop slowly over time, usually well after the airplane is painted and often years into the life of the machine. The first response is denial, followed by occasional cleaning—and finally acceptance that, yes, you have a weeping rivet. Even with acceptance, most owners continue the cleaning ritual (acetone works well) for a while before deciding to address the problem.

There are really only a few ways to deal with such a weeper. The most difficult method is probably the surest—remove the tank, cut an access hole, blob the rivet with Pro-Seal, then seal up the access hole and re-install the tank. This process is highly invasive and adds an access hole that needs to be sealed properly—or you have introduced a much larger leak than you started with.

Well, This Sucks

A simpler but far less reliable fix is to empty the tank of fuel and pull a vacuum on it, then wick some green Loctite into the rivet, allowing it to be pulled in by the vacuum and seal the microscopic leak. (Loctite 290 is the common “green” version, made for wicking into threads by capillary action.) While this method sounds elegantly simple, it also does not reliably work—especially in the long run. Try it if you wish but eventually the weep will come back, especially on a bottom rivet.

A compromise method that works almost all of the time and solves the problem for good is to remove the bad rivet and insert a new, sealed pulled rivet using Pro-Seal gooped on the end to seal the hole. This method is marginally invasive and can be scary until you have done it once. But it is far simpler than removing the tank and overall much more reliable than Loctite.

Case Study

In the case demonstrated here, the weeping rivet was on the bottom of an RV-8 tank and had been seeping fuel for about a year. It was time to fix it before the blue stain permanently discolored the white wing paint. The bad rivet was in a rib/skin hole and was a 3/32-inch countersunk solid rivet. Unfortunately, research of the usual (and more unusual) vendor sites showed no sealed pulled rivets available in that diameter. (A sealed pulled rivet is actually enclosed on the back side so that nothing can leak through the stem hole after the rivet is set.) There are, however, quite a few sealed 1/8-inch pulled rivets available, so that is the direction we had to go.

You can buy sealed rivets from various suppliers—the easiest to find from an aviation source being the AD42H, available from Aircraft Spruce. Unfortunately, this is a dome-head rivet, not a countersunk one. Since the rivet being replaced was a flush head, this means the hole is already dimpled, making a dome-head rivet impractical. However, this is where a little shop miracle can be applied, because the AD42H is an aluminum rivet—and therefore soft. You can reform the head!

This is actually a much simpler process than it might sound. Find a small steel block—an old steel bucking bar will work. It’s what I used because it was the first thing I saw while looking around the shop. Drill a 1/8-inch hole perpendicular to a face, as deep as the unpulled rivet. Drill a little deeper, because you’re going to countersink the hole. Now pull out an aviation countersink tool and countersink the mouth of the hole until a -4 flush rivet sits flush. This is now your female mold. Next, take another piece of steel—I used a 3/4-inch diameter piece of 4130 rod I had in the drawer, about 6 inches long—and cut the face square. Drill a hole in the face that is deep enough to take the entire unpulled rivet shank, plus a little more. I did this with my lathe, but you can do it by hand just as well if you’re careful.

With these two pieces of steel, your little tool is complete. Place the female block on something firm, like your bench vice. Insert the dome-head rivet in the block. Place the other piece over the shank. You’ll notice that there is a little space (the height of the dome head) between the two pieces of steel. Now whack the thing with a hammer—the gap should be gone! Remove the rivet and look at the perfect 100° countersunk sealed, pull rivet you have made! Make a few of them, just for fun—it really is easy.

Wrapping It Up

With your new rivets all set, you can now go to the airplane. Drain the fuel tank completely and let it air out until you don’t smell fuel. Drill out the offending rivet with a #40 drill. Now upsize the hole to #30. You’re still left with a dimple that is sized for a 3/32-inch rivet, but in the case of an RV tank, the skin is 0.032 inches thick, allowing for a little bit of aluminum removal. Carefully take a #30 countersink in a cage and countersink the hole a little at a time; check with one of your rivets to see when you’ve gone deep enough. Remember, you’re not doing a line of these rivets. So while you’ll reduce the strength of the joint a little, it won’t be a big deal in the great scheme of things.

Once you have the rivet sitting flush, pull out your Pro-Seal and mix up a little bit of the nasty stuff. Force a little into the hole and then dip the head of the rivet in more, insert the rivet and pull it normally. Pro-Seal will squish out. Wipe it off flush, clean any remaining Pro-Seal off the surrounding area with acetone while it is still soft and walk away for a few days until the unused Pro-Seal on your mixing pad has cured. Refill the tank and you’re back in business!

Is this a long-term fix? Well in my case, it took about 1800 hours and a dozen years for the rivet to start weeping in the first place, so I don’t think I can give you a definitive answer for a decade or so. But I have heard positive reports from other experienced builders that this is the most certain way to fix a weeping rivet. And you might want to save your tools, or just make up a batch of the countersunk AD42Hs in case someone else needs one in the future. After all, you’re now a specialist in weeping rivet repair.

Photos: Paul Dye; illustration courtesy of Van’s Aircraft.

The post Weep No More appeared first on KITPLANES.

Our RV-3 (Tsamsiyu) has been flying since day one with a Whirlwind 151 three-blade prop (the one with the yellow tips). Fabulous propeller—light weight, constant speed, great aerodynamics—that gave us tremendous performance, both in cruise and climb. The airplane will climb 3000 fpm at sea level or run down the ocean beach flat out faster than the airframe was designed to go (if you let it). But nothing lasts forever.

The 151 was one of Jim Rust’s early designs and he has learned a lot in more than a decade, refining both aerodynamics and mechanics to the point where the 151 has been discontinued in favor of the newer 300 series (the one with the white tips). And since the 151 was discontinued, he doesn’t have any parts for routine servicing or overhaul. So it was only matter of time before it became an expensive paperweight. And since I’ve been very happy with the 300 on our RV-8 for a year, Jim made us a good deal on a new prop for the RV-3.

The swap was simple—we didn’t even need to take the spinners off! The old prop (with the yellow tips) was added to Jim’s collection, while the new prop (white tips and beautiful carbon fiber-weave blade faces) was carefully balanced and proved to be smooth as silk. Since the performance is more or less comparable and we were swapping constant speed for constant speed, we simply did a new weight & balance calculation and made a logbook entry—it’s not a significant enough change to warrant a new Phase I.

I flew the airplane (from our home base near Reno) down to El Cajon (just east of San Diego) one day last week in cold temperatures (and great tailwinds), arriving about 10:30 a.m. By 12:30, the airplane was ready to fly with the new nose! Kudos to Jim and his team, as well as to painter/artist extraordinaire John Stahr, who pre-painted the spinner to match the rest of the airplane before the prop was ready. I flew the airplane back to Big Bear Lake for the night and headed home the next morning getting some qualitative performance data—it felt faster and climbed better! The winter air in the lee of the Sierra was too rough to get good quantitative data but I took it up the next day and saw a speed increase of about 3-4 knots flat out and probably a 10% increase in climb. Given that the airplane’s redline is 183 KTAS, I guess that getting 181 (WOT, full RPM, leaned for best power) is probably good enough when it comes to speed.

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I have spent 50 years working on airplanes of all types and materials—from tube and rag to metal plus a bit of composite. I have learned all sorts of fabrication and assembly skills. But one thing I have missed out on (probably because I took calculus instead of shop in high school) is welding. I have always admired those who could weld and love to see fine weld work. As a senior engineer in complex space programs, we talked about devices and components that had been welded, and evaluated their readiness for flight—but I had never really done it myself. With a bit of slack time between major airplane projects, I figured it was finally time to give it a try. (The fact that my wonderful wife gave me a welding helmet for Christmas probably gave me a clue that it was OK to explore a new skill as well.)

There are lots of different types of welding and the truth is that this is more of a sampler endeavor for me than something I am going to use in my routine airplane work. If I build a ragwing, I’ll most likely have someone else weld up the fuselage. Aluminum airplanes require precious little welding—and what there is (exhaust systems for instance) requires considerably more skill than I am probably going to develop. But there are reasons to weld around the shop—making tools, brackets, storage and fixtures, for instance. Owning my own hangar has me looking around at things that I could make, such as tie-down fixtures and workbenches. So forget the gas welding—I bought a multi-purpose unit that does stick, TIG and MIG.

Finding welding classes in a world where there are shortages of skilled welders is problematic—all the classes are full of folks wanting to get a job, so dilettantes aren’t welcome. But many of my neighbors are skilled at welding and they were happy to show me the basics, then turn me loose on scrap piles of steel to see what I could do. So far, I have pasted together a lot of scraps, cut them apart, and tacked them back together again.

So far, TIG seems to be winning out over MIG for the kind of work I need to do—mostly building small tools—but it’s nice to know I can stick bigger pieces together (like shelves or machine bases) when I need to.

So, no, welding isn’t an essential skill for the homebuilder. But it sure can come in handy—and if you’ve got slack time in the shop, it’s worth the time for “education and recreation.”

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As anyone who studied for their private pilot exam 50 years ago should know, airplane propellers should be parked horizontally—that’s so that when it rains, and the wood of the prop gets wet, both blades absorb the same amount of water, and you don’t get a heavy blade. Yeah….this is still technically correct, but really only applies if you have a wooden prop, and while there is probably a higher percentage of wood props in the E-AB world than on SPAM cans, most pilots are now flying behind metal or composite.

Wandering around our hangar this morning, with airplanes of different sizes and stances, I noticed that I have a different system for parking props—all of which are either metal or composite—that make the hangar safer and more navigable. And no, none of the props are parked horizontally.

The RV-3 sits kind of low (it’s a small airplane) but the prop has three blades and sort of long—one of those blades could stab you right in the eye if not positioned properly. One blade straight down keeps you from banging that one with your legs, but that puts the tips of the other two right at eye level, so one blade straight up seems to be the safest configuration; it puts the other two tips low, so they just take out your knees.

The RV-6 has a two-blade prop, and sits sort of like a low-rider. If you put the prop horizontally, you get stabbed in the throat if you approach the nose from the side, so vertical (or slightly canted off vertical) is good. Slightly canted is better in  case someone lifts the tail—you’d hate to stub the prop blade on the floor if it was vertical!

The Tundra sits tall on its big tires, and frankly, you could probably put the prop horizontally and few would walk in to it, but we’ve got a few tall guys in the airport who drop in once in awhile, so vertical is safest.

The RV-8 is hiding in the workshop space so no picture, but with its three-blade prop on a nose held proudly high, it is similar to the RV-3—just don’t leave one blade horizontal or it will stab you in the eye before you see it.

And lastly, there’s the propeller-less ankle-biter… the Subsonex. No prop, but that pitot tube sits right at shin-level, just waiting to trip you. Once you see it, you can’t look away – you’re like a moth attracted to a porch light… you can just see yourself walking through it. So we have to give it as wide a berth as a prop, just to keep us—and the airplane—safe.

If it’s just one airplane in a T-hangar, you can do what you like but if you coexist with other machines in group hangar, propeller parking is something to think about!

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The inspection, fortunately, isn’t all that hard as long as you have a borescope. And of course, I have a borescope but it has been getting a bit long in the tooth, is a bit buggy to connect with my i-Thingy devices and was old enough to have poor resolution. So into the shop came a new Teslong borescope designed to work with an iPhone and providing sharp, high-resolution pictures.

Here’s a tool tip! I liked my old borescope’s straight tube (with an articulating end) because I could maneuver it precisely. The new Teslong has no stiff section, so it is a bit floppy, making it a challenge to get it to go where you want. But you can fix this with a plastic soda straw—just slit the straw lengthwise with a razor, slip it on and tape it in place. Now the last 10 inches of the bore scope is rigid and more easily maneuvered. Yes, the end still articulates.

The inspection required enlarging a tooling hole in the outboard rib of the stabilizer after moving the elevator counterweight out of the way. This was easily accomplished with a step drill. Sliding the bore scope in and angling it around for a good view was a quick task, and voila! No cracks were found on either side of the -6 or the -8. Logbook entries made, service bulletins filed until next year, when the inspection has to be repeated.

But what about the RV-3? Well, it’s also fine but took a little more ingenuity. Since there are no outboard counterbalances on the elevator, the tips are glassed in to the end of the stabilizer, providing no access from the tip end. But the stabilizer is short on such a small airplane and the empennage fairing comes off. There are only two ribs on each side of the stab—root and tip—so with nothing in between to be navigated, it was a simple task to (once again) enlarge a tooling hole and slide the borescope in, this time all the way in, and confirm that we had no cracks. “Whew!” (I dread having to repair a cracked spar, not because of the metal work, but because of the custom paint job.)

Of course, we did the RV-6 inspection as part of its annual condition inspection in February—so at least that airplane is out of the shop for awhile. But the SubSonex jet gets its annual physical in March so, once again, we’ll have a patient in the examination room!

The post Slack Time – Part 3 appeared first on KITPLANES.

I was just approaching the tops when this unbelievably sharp BANG! went off in the cockpit. It sounded like a shotgun blast. My first thought, of course, was that I had blown a jug clean off the engine. I pitched the nose over, anticipating power loss and decaying airspeed.

As I leveled off, the next thing I did was to scan the engine gauges—all good there. No oil on the windshield and the engine seemed to be running smoothly. All of this took about 5 seconds, though it clearly felt like 50.

Life remained good as I broke out on top. As my heart slowed down I started looking around the cockpit. The baggage area behind the seat had all my usual stuff and the windows were intact. But there was that paper bag from Mom. What exactly was in there? As I peeked in, I saw the cause of my “failure.” A mylar Valentine’s Day balloon—no, not so much a balloon as a flat envelope of foil. Vinyl balloons stretch as the altitude goes up but mylar balloons just build up pressure until they go bang!

I had just relearned a valuable lesson: When you think you have a failure, look for confirming cues to see if it is real. The fact that the engine kept running smoothly made it pretty obvious that I hadn’t blown a jug and the instruments seemed to verify this supposition. It works both ways, though. There are times when your instruments might be lying and you can use “other cues” to verify their readings.

Instrument Trust

What do you do when the instruments tell you one thing yet senses tell you something else? Moreover, trusting an individual instrument is not the same as trusting the picture that all your instruments paint for you. What do you do when the instruments are in conflict—with themselves or with your senses?

The answer, of course, is cross-checking. When learning to fly with instruments, we are taught to cross-check the gauges against one other. If the altimeter is unwinding with the airspeed rising yet the attitude indicator shows us straight and level—something is amiss and it is probably that artificial horizon! If the directional gyro shows us in a turn while the turn-and-bank shows us going straight ahead, as does the compass, you might want to get the bearings in that DG checked. Of course, the ultimate cross-check in VFR conditions is to look out the window.

Other Systems Checks

What about the stuff that tells us how our engine is doing? Can we cross-check that as well?

In the Experimental flight test business, with lots of instrumentation points and fancy telemetry systems, the first thing we teach young engineers is to ask themselves how the data might be lying to them. It pays to fully understand the data-gathering equipment before you rely on it to make decisions. You need to understand that when indications show a failure, you need to use some common sense and realize that anything made by a human being can have flaws. No machine—including the machines that monitor the machines (instrumentation)—is perfect. Therefore we train people to always take just a moment when they see something unusual to ask: How is the instrumentation lying to me? Is what I am seeing real?

This is where the old adage comes from: “When you see something wrong, the first thing to do is wind the clock.” Old-fashioned wind-up panel clocks had a knob on them (I know, I am dating myself) and you could calmly reflect for a few seconds while turning that knob. It gave you a moment to not do the wrong thing. It is a rare situation where you can’t afford to take a few seconds to reflect before taking action.

A classic example would be a tachometer failure. Let’s say that you’re flying along, straight and level, the engine running smoothly—and the tach starts dancing about. You see wild rpm variations, some well above redline. What’s the first thing you should do—pull back on the throttle? Well, eventually that might be a good idea. But before taking that step, how about using your ears? Does it sound like the engine is varying speed? It’s pretty easy to tell by sound that nothing is changing. If the tach is showing fluctuations it must be out to lunch, don’t you think? Now the next question, of course, is what do you do about it. Land immediately? You can do that if you like but if the engine is running fine, you have plenty of gas and nothing else has changed, how about continuing on to your destination or at least a good place where you know that you can get the tach replaced?

Let’s take a look at something a little more serious—oil pressure. You can’t hear oil pressure and the only way to tell if you really have it is via some sort of gauge or pressure switch. Most certified aircraft simply have a gauge—a single source of data. Those with advanced avionics might have software that reads the pressure and if it falls below a preset limit will turn on a light to get your attention. But in the Experimental world, you can add anything you want—and a very simple pressure switch is an easy addition. Wire this up to a light in the cockpit, and you now have the ability to independently confirm or deny the information given to you by the pressure gauge.

Yes, I can hear you saying that a man with two watches never knows what time it is! Well, this is true in principle, but if the man knows that it is daytime and one watch says it is noon while the other says it is 3 a.m., he has a way to pick between the two. Likewise, if your engine has been running fine, and the oil pressure light comes on—the pilot can check the gauge to see if it has changed. Unless you have had two independent, dissimilar yet simultaneous failures, it is quite likely that the engine’s oil pressure is doing just fine. A quick check of oil temperature should add additional data to tell you if things are heating up from a lack of oil (and oil pressure) or not. While these indications are sometimes slow to manifest, they almost always follow each other. Loss of oil pressure leads to rising oil temperature. (Yes, no oil in the engine can show low temperature but by the time that happens the grinding noises from ahead of the firewall will be a more prominent clue.)

Many years ago, we had no EGT gauges in light airplane cockpits, then we got single-point gauges. We could lean until the needle peaked and that would give us some idea how we were doing toward optimizing fuel flow. Next came all-cylinder gauges with a probe for each exhaust port and we learned that maybe we didn’t know as much as we thought from that single probe and gauge. Unfortunately, some folks have always flown with the all-cylinder EGT systems and have never had to simply have faith in the engine designers who say that for a normally aspirated engine, there is no such thing as too high of an EGT. The pilot sees a temperature and wants a limit—or they see a high value and get worried. For non-turbo engines there is no real limit. Instead, you should be looking for consistency from cylinder to cylinder and from day to day.

We teach flight test engineers some very basic interpretation skills that cover most types of transducer systems. A sudden drop-off (it is reading high, then reading low) is usually an indication of a transducer or instrumentation failure. Most temperatures or pressures simply can’t change that suddenly. A steady decline, however, is a good indication of a trending measurement that might be real—a leak, for instance. If your fuel gauge goes from 30 gallons to zero instantly, you probably didn’t rupture a tank, you probably broke a wire. If, on the other hand, you see a steady decline over time, you might very well have ruptured a hose.

You can (and should) go through the mental exercise of evaluating every measurement you have available to you. Think about what it is really telling you and how the data is gathered. Think about what you can use as an alternate cue to confirm or deny a bad reading. Determine in advance what you need to react to immediately and when you can take the time to “wind the clock”—at least a little bit. And always ask yourself the question, “How is my data lying to me?” It is far more common, in fact, for instrumentation to fail than some of the robust systems we have in our airplanes.

Learn what’s normal in your airplane by observation in flight and by looking at recorded data later. When you get an indication that’s out of the ordinary, first look at corroborating data and relationships before pulling the throttle back and looking for an emergency landing site. Maintain a healthy dose of skepticism toward the data and an attentive ear to the machinery. And, while you’re at it, beware mylar balloons.

The post Conflicting Cues appeared first on KITPLANES.

More recently, I was working on another little task that required a very narrow-nosed socket wrench. In the past, I have butchered cheap sockets on a bench grinder, most notably to have a tool that fits Lycoming connecting rod big-end bolts. But now realized I could have a whole set of such custom sockets with the new lathe skills I’d been learning. Off to Harbor Freight for the sacrificial victims.

It’s a simple matter to set the lathe up to move the tool at a 5 degree taper angle, and I proceeded to turn down the entire set of sockets (one at a time) until they were tapered to a knife edge. Given that it is a set of small sockets and torques rarely get very high, they should do the job—whatever that might be—in the future. Since things were all set up, I did regular and deep sockets at the same time!

Hey, what’s all that racket on the internet? Oh dang, a new service bulletin just came out that affects all three of our RVs… get the tools out!

The post Slack Time – Part 2 appeared first on KITPLANES.

So what about keeping busy in the shop? Well, our shop work is in transition. We managed to move the eXenos project into a neighbor’s hangar that was big enough for the 46+ foot wingspan before winter really descended—and finished assembly. The airplane was weighed, paperwork filled out and everything is ready for an airworthiness inspection (since completed). But arranging that with the holidays and a number of other constraints has been problematic. And, of course, the hangar isn’t being used for anything else and the ramp is in the shade so there is now a good foot of frozen slushy ice trapping the airplane until we get a good thaw. Even if we get the airworthiness certificate soon, it’ll be hurry up and wait….

So the shop is fairly empty, and we even cleaned things up—put away tools, resurfaced a workbench, sorted and stored a bunch of parts. But that just means that there really isn’t much to do between condition inspections on all five family airplanes. Those do come around!

But lest this sound like nothing but complaining, there is an advantage to “slack time.” It can be used for education (and recreation, I suppose). Everyone builds different skills in their lives and one of the skills I have missed along the way was welding. So that was on the list—picking up a multi-purpose welder and learning how to stick scrap metal together. It’s been a fun time learning how to MIG and TIG—a lot of random pieces of shop steel have been sacrificed in the pursuit of knowledge and skills. Lots of inadvertent holes have been burned into some pieces, but every day we get just a little better—and yesterday, I made my first actual “thing”—an adapter for my slide hammer that allows me to pull Lycoming case through-bolts. It could be prettier, but its functional!

Next time: fun with the lathe…

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