Fly cutting is a great way to put a “finish” on a flat and true face. The sweeping tool marks are not only instantly recognizable, they are, to my eye, infinitely more attractive than the swirly patterns made by an end mill. I covered the basics of fly cutting on plane surfaces in the February 2015 issue (“Fun with Fly Cutters”). But as this month’s project will demonstrate, fly cutting can also be used for coving. Because I was after a precision radius, I used a boring head, which allows you to precisely dial in the radius of the cutter path. For more on boring heads, see the October 2015 issue (“Boring on the Vertical”).

The task at hand was to make a tool to gently press a piston pin onto a connecting rod. I made this tool for my friends who are not yet 100% sure they’ll need it, but since removing the piston pin required a makeshift drawbar to pull it out, there’s logic in the idea that some gentle persuading may be necessary to reinstall it.

The engine in question was down because of a cracked exhaust port on one of the cylinders near the firewall. Not a common issue, but it happens. The typical fix is a replacement cylinder assembly, which includes the cylinder with valves, the piston and rings. You reuse the rockers and piston pin at your discretion. Lately, anybody who’s needed a replacement cylinder assembly for any of the popular legacy engines knows that the wait times have been excruciating. There was nothing to do but bide our time. So when the word came down that delivery was imminent, the teardown could finally begin!

It was a Saturday afternoon when I got the text: “Do you have, or could you make, a piston pin removal tool?”

“No and maybe” was the answer. I grabbed my calipers and some graph paper and headed to the hangar to see what was what.

There are two types of piston pin removal tools: push and pull. For aviation, the predominant type seems to be the ACS push tool sold by Aircraft Spruce and Aircraft Tool Supply. No one seems to offer a pull-type tool for aircraft engines. The ACS tool is nice, but it is for rebuilding or refurbishing complete engines. In our situation, where we were removing and replacing one cylinder assembly, there’s simply no way to make it work if an adjacent cylinder is in the way.

A few chin rubs and head scratches later, we concluded that we could put together a shop-made puller with some all-thread and tubing. While we had to be careful not to damage the pin or connecting rod, the piston was going to be replaced, so there was no need to worry about it getting dinged during removal. As luck would have it, I had a foot-long section of 7/16-14 all-thread with a couple of nuts and washers, but no suitable tube to provide clearance to make a drawbar sleeve. Then the light went off: a socket! Sockets come in handy all the time when you need a right-size sleeve or bushing on the arbor (or hydraulic) press. All we needed was a 1/2-inch drive (deep) socket big enough for the pin to be extracted. The OD on the pin was 1.125 inch, so any socket 1-1/4 inch or larger would work!

Once the pin was removed our attention turned to putting it back. The fit is not a tight press fit, but definitely tighter than you could possibly remove by hand. Reinstalling the pin carried the burden of doing absolutely no damage to the new piston.

Which bring us to the main feature of the press: a Delrin pad machined with a cove to ever-so-gently cradle the piston while the pin is pressed back in place.

The deadline to turn in this column was before we had a chance to see how it works. The hope is, everything should slide together without much, if any, force. But, just in case, we’ll have our shop-made press ready to go. I’ll post an update on the KITPLANES® website to let everyone know how it turned out.

That’s it for now. Time to get back in the shop and make some chips.

The post Fly Cutting a Cove With a Boring Head appeared first on KITPLANES.

I had seen a jig for drilling the wedge on a popular Van’s tool supplier website, but I wasn’t in the mood to drop the asking price for the item or wait five days to get it. So, I thought I would try fashioning one of my own. Using a piece of scrap aluminum angle, some hand files and a bit of patience, I came up with a tool that made the countersinking process easy and effective. I was also able to scratch the OCD, tool-builder and stingy itches all at once.

Using the AEX wedge as a guide, start off by marking the wedge outline on your scrap piece of aluminum angle (Image 1).

Secure the workpiece and then begin working the shape with a set of files (Image 2). I used a small Nicholson triangular file and a Vixen file (Image 3). Be sure to check the AEX against the piece of angle (Image 4). After about an hour and a half, you’ll get the desired shape.

Since I used scrap angle, I needed some way to make the plate sit perpendicular to the drill press. I decided to mount it to a wood block. I ripped a 2×4 lengthwise and routed a channel that matched the wedge profile (Image 5).

I created the channel in the wood using a router and router table. If you use this method, know that I also supported the back of the wood with some AEX wedge to keep the angle correct during the cut (Image 6 and 7). If you don’t have access to a router, I’m sure the task can be accomplished using a variety of wood tools (chisel, Dremel, wood rasps, sandpaper, etc.). It’s not important that this channel is hyper-accurate; it’s just there to keep the AEX wedge aligned.

Next, notch out a recess for the metal jig in the wood so that the jig top is flush to the wood face and the channel on the jig lines up with the channel on the wood (Image 8). Additionally, the countersink point requires a recess in the jig, so I fashioned that in both the jig and the wood.

You can also see in Image 8 that, after flattening some of the surfaces on a belt sander, I attached the second half of the 2×4 back with screws. I then drilled a few holes in the jig and mounted it with screws to the wood (Image 9 and 10).

Finally, I attached two small pieces of wood to help hold the wedge in the channel. I was then ready to begin drilling holes in the AEX wedge (Image 11), followed by countersinking.

The post Wedge Issue appeared first on KITPLANES.

I’ve made quite a few, too. Links are on my blog. Today I needed a 45° offset 7/16-inch open-end wrench. I just couldn’t bring myself to buy one for $8 to cut and bend it. Since I have tons of woodworking flat paddle bits from my father-in-law, it was the perfect stock. I found a 7/8-inch paddle bit with a broken tip. Perfect. A few minutes with an angle grinder and the problem was solved.

If you don’t have scrap tools, pawn shops typically have scrap bins full of wrenches and sockets very cheap. Cut, grind, heat and bend. Be creative.

The post Homemade Open-End Wrench appeared first on KITPLANES.

The deburring process should include addressing not just rivet holes but all perimeter edges, large holes and any internal openings in a sheet. I am going to focus just on deburring small rivet holes. A metal airplane has thousands of them, and this is where most of the builder’s effort is expended.

The most common tool used for hole deburring is the swivel countersinking tool. A popular model is the Avery “speed deburring” model shown here. There are other brands and variations of this design available from your favorite aviation tool supplier.

This hand tool is made of two parts: a simple handle with a swivel shaft and a cutting (deburring) fluted bit that screws into the swivel shaft. While it may be obvious to guess that this tool will be rotated after inserting it into a drilled hole, we should pay attention to what is happening as that bit cuts away at the holes. This deburring bit is a variation of a countersinking cutter bit. These are normally used to countersink holes for flush rivets or screws to be installed. They come in various diameters with specific cutting angles and grind away just the right amount of metal from a hole so that countersunk rivets and screws install flush with the metal surface. So why does a variation of a countersinking bit make a good hole deburrer?

The deburring bit diameter is sized a bit larger (3/8-inch or more) than the rivet hole. If we make just one turn of that bit (using the swivel handle) in the rivet hole while applying moderate pressure, we have just begun the countersinking process. While we do not want to countersink our holes, this operation does a good job of removing material (burrs) above the edge of the hole. Just one turn should do it. If you keep rotating the bit, you start to countersink—which is not what we want. So, proper use of this tool means no more than one rotation of the swivel handle while visiting each rivet hole.

While countersinking a hole in thick metal is desirable if you want to install flush screws and rivets, it is not acceptable in thin sheet metal. (Proper countersinking requires a bit that is matched to the specific hole size—deburring does not.) When flush rivets and screws are desired with thin sheet metal, we dimple the hole. Dimpling deforms the hole, but it does not remove material. The danger in countersinking holes in thin material is that it leaves the thickness inside the hole’s edge razor thin. What we know about razor-thin edges on aluminum is that they are great stress risers. Stress risers may attract cracks that can ruin your flight. So, one turn of the swivel tool is all that should be needed for deburring!

The deburring bit is also available with a short shank that can be used in a drill motor instead of the hand swivel. I cringe when I see deburring done this way on thin metal skins as there is little control in keeping the rotations down to one. If deburring holes in thicker aluminum L angle, for example, the concern of potential countersinking becomes less of an issue. This is the same with deburring holes in steel parts where it is much harder to remove material compared to aluminum.

The swivel deburring tool is a popular means of quickly deburring your drilled holes prior to assembly. You can literally see the burs break off from the holes and the process takes about a second per hole. I have found it advantageous to lift the sheet slightly off the table to allow the center point of the deburring bit to protrude through the hole to gain full contact. Don’t forget to deburr both sides of your holes if this is needed.

Another way to make use of this deburring technique without this tool is to use a drill bit instead. (The bit should be about 3/8-inch or larger diameter.) Wrap some tape around the flutes so you don’t cut your hand. Simply rotate the bit in the hole (one turn for thin sheets) and the deburring process is completed. If you have lots of holes to work on, you may find the swivel handle tool to be less stressful on your hand. Your choice! Plane and Simple.

The post Those Rivet Holes appeared first on KITPLANES.

Then a wonderful thing happened out of the blue. I had just moved from San Diego to my dream house in the northern Sierra foothills when this fellow showed up at my front door asking if I was the college teacher from San Diego. I allowed as to how I was, and he introduced himself as the department chair of the electronics department at the local Sierra Community College. That was the start of a wonderful 40-year career teaching part-time community college electronics. But there was a hiccup.

I found out quite early, like the first day, the hardest thing about teaching was remembering that at one point in my life I knew absolutely nothing about electronics. And that these students, bright-eyed and bushy-tailed just out of high school, also knew nothing about electronics. I mean, I started off the first day of class talking about working with a 12-volt circuit. One timid freshman held up her hand and said, “What’s a volt?” Another one, emboldened by the first, followed with, “What’s a circuit?”

I decided then and there, this is not going to be easy. And ever since then, I’ve started every semester with a basic—I mean really basic—discussion of electronic terms.

Now, after nearly 38 years of writing about electronics, I’ve also come to realize that I’ve been spouting terms like “op-amp” (operational amplifier) and even the relatively obsolete “transistor” without ever explaining what they are. So here we go with some of the stuff I’ve neglected to tell you for nearly four decades.

Electron: A tiny subatomic particle. It has a negative charge. It is the foundation of the science of electronics.

Voltage: The amount of pressure we put on an electron to do our work for us. V is the symbol for voltage. (Named after Italian physicist and chemist Alessandro Volta.)

Current: The number of electrons passing by a particular part of a circuit at any one time. Measured in amperes, 1 amp equals about 6 billion billion (6,000,000,000,000,000,000) electrons moving past the circuit per second. (Named after French physicist and mathematician André-Marie Ampère.)

Resistance: The property of a circuit that limits the amount of current in a circuit. Measured in ohms (Named after German physicist and mathematician Georg Ohm.)

Power: The property of a circuit to do work. Measured in watts. (Named after Scottish inventor and mechanical engineer James Watt.)

Conductor: An electronic component that permits current to be transferred from one circuit to another. All conductors have resistance. The conductors with the least resistance and therefore the least lossy to lose power to heat in accordance with the equation (P=I2 * R) are silver, copper, gold, aluminum and they get worse from there.

Resistor: An electronic component that limits the amount of current in a circuit. Small resistors used in electronics are generally made from carbon. (You can make your own from pencil “lead,” which is graphite [crystalline carbon], but at less than a nickel for a real resistor it’s not generally worth it.) For medium- to high-power dissipation they can be made from lossy nichrome (nickel-chromium) wire for watts to hundreds of watts dissipation. Measured in ohms.

Capacitor: An electronic component that attempts to keep the voltage in a circuit constant. They are made of hyper-thin sheets of conductors (like aluminum) separated by hyper-thin sheets of insulators (like plastic) that store electrons on the conductors and release them into the circuit as the need requires. Measured in farads (F) or more commonly millifarads (mF) to picofarads (pF). (Named after English scientist Michael Faraday.)

Inductor: The inverse of the capacitor in that it attempts to keep the current in a circuit constant by means of storing energy in a coil of wire that forms a magnetic circuit and releasing it as voltage to keep the current constant. One inductor we are all familiar with is an ignition coil. Voltage is fed into the coil to create the stored energy and then at the right time the circuit opens (breaker points) and the inductor attempts to transfer all that energy into voltage to keep the current constant. This produces a horrendously high voltage that eventually is fed into a form of capacitor called a spark plug, which is designed to transform all that energy into a little tiny lightning bolt that dissipates all that energy as a spark. Measured in Henrys (H). More commonly used as millihenrys (mH) to microhenries (µH). (Named after American scientist Joseph Henry.)

Transformer: Made from two magnetically connected (but electronically isolated) inductors. An AC signal in one inductor (the primary) is magnetically transferred to the other inductor (the secondary), and by adjusting the number of turns of wire on the primary and secondary, the secondary voltage may be made to increase, stay the same or decrease the AC voltage transfer from primary to secondary. For example, if we connect the primary to the wall socket (120 volts AC) and wind 10 times the number of turns of the primary inductor as the secondary (10:1 ratio), we will create a 12 volts AC voltage at the secondary. By the magic of semiconductors, we can change that AC to DC to run a 12-volt aircraft radio on the test bench from a wall socket supply (T).

That pretty well wraps it up for what we call the most common “passive” components. Next month we will talk about the really interesting stuff like transistors, op-amps and other “active” components in the electronic toolbox. Until then…stay tuned…

The post Terms of the Trade appeared first on KITPLANES.

The post Out-of-Date Hoses 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.

Building a metal aircraft? You’re probably going to be cutting a bunch of sheet metal parts from time to time. If you are not building a metal aircraft, there is still a good chance you will be called upon to create some parts using sheet aluminum. What we all need is a tool to make clean cuts in aluminum sheet. However, the best tool for cutting sheet aluminum may not be so obvious if we take our cues from online aircraft tool suppliers.

The cutting tool we need goes by several names: aviation snips, tin snips, metal shears and more. Aside from aircraft construction, these cutting tools are used with all types of metals in various industries (think of fabricating heating ducts for your furnace as an example). As aircraft builders, we have a specific use for these multi-material cutting tools—usually cutting thin sheet aluminum. This specific use of these tools means that not all tin snips are created equal for builders.

Viewing online aircraft tool catalogs would lead one to believe that the cutting tool of choice is the one shown on top in the picture below—the ubiquitous aviation-style hand snips. The “aviation” part of the name signifies that it is constructed with a dual pivoting design. This provides power for cutting thick materials by way of the leverage gained from the pivots. If you look really, really closely at the blades, you will see a light serration (saw-like appearance). This also aids in cutting tough materials. With very careful scrutiny, you can see that a tooth-like impression has been left on the metal these snips cut. Aviation snips come in left, right and straight versions, with color-coded handles for easy identification.

If you are already familiar with using this style of cutting snips, then you know that the difference in choosing a right or left version has to do with what side of the metal gets curled away while cutting. If cutting a narrow strip of metal from a wider piece, we typically want the narrow piece to curl out of the way while cutting. The shape of the blades controls the direction (up or down) of this curling process. If you choose the “wrong” tool (left instead of right, for example) the cutting experience can be awkward as you hold the sheet.

It turns out there is a much better tool for cutting thin sheet aluminum. Notice the large scissors-style shear next to the aviation version in the picture. You can usually find this model at your favorite aviation tool vendor, but you may have to really search for it. It is made by Malco as well as Klein Tools, among others (also available at Amazon). It has no official name that I am aware of, other than snips or aluminum snips. After I started using this tool 20 years ago, I rarely use aviation snips when cutting thin sheet aluminum. Here is why.

The blades are more than twice as long as the other variety, so each motion of your hands provides for a longer, straighter cut (less starting and stopping to get to the other side). There are no serrations on the blade—so your work is not left with a bumpy ridge that needs to be filed down. That is a big plus! Your cut material is ready for use and the edges look very nice. There is no concept of left and right, so the curling of the cut material is intuitive. It operates just like a scissors—we need no training for using them!

Are there any downsides to using these big scissors snips? Yes. I have not thrown away my aviation shears because if you are cutting anything other than thin aluminum sheet, you better hold onto those conventional snips. Once the aluminum sheet gets thicker than about 0.035 inch, you will need hands stronger than average to operate. You cannot cut materials other than aluminum without potential problems. There is a good reason that aviation snips have serrations, multi-point pivots and small curved blades—they really work good on tough materials (steel, fiberglass, thick aluminum, etc.). But if you are cutting a large panel from aluminum sheet that needs a professional, straight edge with no further dressing, then these large scissor snips are tops.

I normally would not make a big deal about comparing tools for cutting if I didn’t think there was a significant difference between these models. It is surprising to see so many fellow builders not have these snips available for their sheet cutting work. Almost every tool supplier seems to push the aviation style first and foremost in their online marketing. After years of working on metal aircraft projects I can attest that you should appreciate the difference in results between these two styles of cutting tools. Give them a try! Plane and Simple.

The post The Straight Cut appeared first on KITPLANES.

But what really happens when AoA and airspeed disagree with each other? Can we really force ourselves to trust the AoA system when we’ve been flying via indicated airspeed for most of our flying career? I got to witness this firsthand this past week.

A customer was bringing in his RV-8 for a condition inspection, and he lives close enough that I agreed to fly him back in his RV-8. It’s a gorgeous airplane, very nicely equipped, so I suffered through it. Sure enough, at takeoff the AoA system started yelling at us right about liftoff. I was in the rear seat and tried to look over his shoulder to see the airspeed, and it looked a little high, but RVs accelerate so quickly it was really hard to come to any conclusion. However, he mentioned it had started yelling at him recently, and he did perform a stall at altitude and the airspeed seemed about 10 knots high. On final approach, the AoA system was again yelling, and the pilot was using normal indicated approach speeds. It was a little rough outside due to winds, and the approach felt mushy to me. The landing was not one of his best. (In his words, his worst one all year.) I was convinced it was due to low actual airspeed. I remarked to him that I think he had a static leak.

As I was taxiing out for the return trip, I came up with a plan. RVs accelerate so quickly at takeoff that I usually just make sure the airspeed is alive, and then I really don’t look at it again until I am in the climb attitude. In this case, with 5500 feet of runway ahead of me, I used much less power and slowly accelerated to liftoff speed. There was no doubt that liftoff speed was higher, and the AoA was yelling at me quite early.

Back at the shop I connected the pitot-static tester to the aircraft and it showed a massive leak on the static side, just as I had predicted. It took a while to find it, as it was hidden on the back side of the tee connector in the aft fuselage that went to the static ports. The Nylo-Seal tubing had split where it had been forced over the plastic tee. They should have used a Nylo-Seal tee instead of forcing the tubing, but another possibility is to heat the tubing prior to installing it on the tee. That will usually prevent the cracking.

So, let’s talk about the reason for the disparity between airspeed and AoA. AoA is a measurement between the relative wind and the angle of the wing chord. It can be done using a vane (you can usually see this type on commercial jets) or via a special pitot tube, as in this case. The pitot tube has the normal forward-facing hole and then has another hole below it at a different angle. Then, through fancy algorithms and math, calculations are performed to arrive at the AoA value presented to the pilot. With either method, when the critical angle of attack is approaching, which is usually around 17° for most normal aircraft, a warning is announced. The bottom line is that this measurement is not affected by external forces. By the way, I say “most normal” aircraft as there are military aircraft that due to sheer power are able to break some of the rules regarding angle of attack.

If you remember from your primary flight training, the airspeed indicator is the only instrument that receives both pitot pressure and static pressure. The primary purpose of the static connection is to correct the indicated airspeed for altitude. Simplistically, this means that at higher altitudes there are fewer air molecules striking the pitot tube, so without the correction, the indicated airspeed would be lower. The wing only cares about indicated airspeed. At 1 G, it takes a given amount of air molecules flowing over it at a specific airspeed to create the necessary lift. Add more G’s and it takes more air molecules, hence more airspeed.

So, what happens if we were to decrease the static pressure at a given airspeed? Well, decreasing the static pressure is the same as climbing, so the indicated airspeed would show an increase. This is exactly what was happening with this RV-8. The inside of the aircraft, and especially the aft tail cone area, is at a much lower pressure than the outside air, due to the lift created by the fuselage. The cracked tubing was allowing the static system to sense this lower pressure and correcting the airspeed indication, just as if it was at a higher altitude. So, the result was a higher indicated airspeed.

That brings us back to the first question: Which one should we trust—AoA or indicated airspeed? Personally, I am of the opinion that AoA has a better chance of being right than does the indicated airspeed. At least, that has been my experience across the amateur-built fleet. Most static systems are never checked for leaks unless they are certified for IFR flight. Almost every static system that we check for the first time does have leaks.

One last point here. I get a kick out of how many owners brag about their particular aircraft being so much faster than everyone else’s. Invariably, every single one of them that has come to our shop, especially during the prebuy, has had a static leak. So, if yours seems to be a whole lot faster than the average model, you might want to have the static system checked. I’ll even offer up a poor man’s quick check, with a word to be careful. Set your altimeter to zero, or the nearest thousand feet if you are at too high of an elevation to use zero. Take off and then make a low pass down the runway at 10 feet or so. Take someone with you, or use a GoPro in the cockpit, as you want to be focused outside the aircraft for the low pass. If your altimeter doesn’t read pretty close to your low pass level, then you have a static leak.

Sorry if I’ve busted your bubble on your speedy aircraft, but accurate altimeters are good for low approaches in IMC, and AoA will be your best friend for those short field trips.

The post Angle of Attack vs. Airspeed appeared first on KITPLANES.

All machined things (parts, tools, etc.) eventually have to be “finished” in one way or another. This may include tumbling, sandblasting or burnishing, but if rust or corrosion is to be prevented, the options start with paint and advance from there toward chemically applied finishes, such as plating and passivating, or in the case of aluminum or titanium alloys, you can add anodizing to the mix.

The chemistry of metal finishing has always been a fascinating, yet mysterious, subject to me. I have a grasp of the basics, but beyond that, it’s all magic. I gave up trying to figure out how it all works and contented myself by focusing on where and when the various methods are best applied.

Although there are DIY procedures for at-home plating and anodizing, I have avoided them mostly because of the noxious chemicals involved. I know people who do it and I know the process can be safe. It’s just not something that appeals to me.

I make an exception for one type of chemical finish: black oxide. Black oxide is used on all kinds of hardware and tooling. Virtually all commercially made tool holders for lathes are finished with black oxide. For years, I have been using a DIY product called Tool Black from Precision Brand. Tool Black can be found in better hardware stores and at most tool suppliers. The kit, which costs about $160 to $180, consists of pint bottles of cleaner, blackener and sealer solutions. Although pricey, it’s a fast (about 5 minutes for most small parts) and convenient way to corrosion-proof shop-made steel parts and tools without affecting the dimensions. Application consists of cleaning, then swabbing with the Tool Black solution. After about 30 seconds, rinse with water, lightly buff and then seal with a penetrating oil. Depending on the type of steel, surface finish and exposure time, the resulting finish will be anywhere from dull brown to blue to glossy black. It’s worth noting that most “gun blue” finishes on firearms are a light application of black oxide.

A long time ago, I made a quick-change tool system that included several tool holders for my bench lathe. I treated all of them with black oxide. In addition to corrosion protection, a black oxide finish no doubt enhances the curb appeal of shop-made tools. Sometime later, I augmented my collection of holders with a shop-made boring bar holder. Unlike the original tool holders, which were steel, this one was made out of 7075 aluminum. Since my black oxide kit is for steel only, the shiny aluminum holder stood out as an odd duck in the tool drawer. “Someday,” I thought, “I’ll get that anodized.”

Time went by and I never got around to anodizing that tool holder. Then, one day while at a local sporting goods store, I discovered Birchwood Casey Aluminum Black in 3-ounce bottles. At only $10 for the blackener and $6 for a bottle of degreaser/cleaner, it was worth a try!

The process is basically the same as steel: clean, swab on the blackening solution and let it do its thing. Rinse, buff and seal. It’s worth mentioning that Birchwood Casey also sells a steel blackener called Super Blue in the same 3-ounce size, as well as Super Black touch-up markers for both steel and aluminum. These seem like a good choice for treating small parts in the home shop as well as for restoration and repair. If you really get into it and plan on applying black oxide to dozens and dozens of jobs, then it makes sense to get a Tool Black kit from Precision Brand (Insta-Black from EPI is another option), but if you are only doing a few small items here and there, the 3-ounce sizes make more sense.

How did it work? I was a little disappointed with the finish on my tool holder. After two tries, it still came out a bit uneven and splotchy. I suspected the issue was a combination of the part not being perfectly clean and possibly the alloy itself. The procedure requires the surface to be both clean and oxidation-free. Oxidation can be problematic on older parts such as my boring bar holder, which was made several years ago. The instructions suggest using steel wool to brighten the surface and remove any oxidation. I used Scotch-Brite abrasive pads and, after the first application, it was obvious where I didn’t get all the oxidation off. So I scrubbed all the black off and started over. The second try went better, but wasn’t 100%, which I attributed mostly to the uneven removal of surface oxide. That made me suspect that the ideal time to apply Aluminum Black would be immediately after the machining process and before any surface oxide has formed. To test that theory, I turned a test piece on the lathe and, after cleaning with the degreaser and a quick polish with a fine Scotch-Brite pad, swabbed on the Aluminum Black. The results were much darker and much less splotchy, though not 100% perfect.

My conclusion is that it is tough to replicate the success of professional plating or anodizing in the home shop, but if you spend the time cleaning and deoxidizing, you can get reasonably decent results.

That’s it for now, time to get back in the shop and make some chips!

Additional Reading: Before anodizing aircraft parts, I suggest reading “Anodizing and Fatigue Life” by Stuart Fields in the November 2016 issue.

The post Chemysteries appeared first on KITPLANES.