Blowing Smoke: A Toy for My Toy

I will (probably) never rally my Triumph TR6, but it has driving lights and a four-point safety harness, which adds really no safety to a car that can be crushed by today’s traffic. I fit my Triumph Bonneville with a pedestrian slicer because they look very British—no other reason needed. I knew I would never fly an airshow (not completely true), but that didn’t stop me wanting a smoke system for my Sonex. Call them all toys for the toys. My want for the smoke system, however, went one—no, two—steps further than just wanting a smoke system; I wanted the project of designing and building a smoke system, and, as it turned out, the smoke system gave my tiny, shiny Sonex visibility when I felt invisible.

This isn’t a detailed shopping list and “how-to.” What I wanted for my Sonex may be too small for your needs. What this is is a look at the overall system and how I arrived at it. The why’s of my system are as important as the “what’s.” The why’s are where the learnin’ lives.

Trial and Error

As stated, one reason for wanting a smoke system was simply to have a project. With my Sonex done, I missed the thinkin’ and tinkerin’ that went with the project, and I missed walking the aisles of a hardware store, engineering as I went. From the onset of the project I knew a smoke system needed a few key ingredients:

• A tank
• A pump
• Plumbing
• Wiring
• Smoke oil

Along the way I learned specifically what kind of each was needed, and I learned I needed an electric shut-off valve. To that I credit the “trial” portion of the project, though the contributions of the “error” portion cannot be understated.

Smoke Tank

When choosing a tank size and style, the three primary considerations must be your aircraft’s weight and balance, which dictates the size of the tank and where it can be placed; how the tank will be filled (how much smoke oil are you willing to spill into your cockpit?); and how it can be safely mounted (will it hang on during a 6 G maneuver?) in the chosen location. Secondary considerations are venting the tank and providing a smoke oil outlet in the tank’s lowest position.

As my goal was to burp out an occasional burst of smoke in salute to friends on the ground and trace a single loop, series of rolls, wingover, or knife-edge turn, I wasn’t in need of a large tank. I also wanted to keep everything forward of the firewall. I chose a 1-quart JAZ Products Jr. Dragster Fuel Cell. It has a petcock on the barbed outlet, a vented cap, and molded-in 1/4-20 inserts, which allowed simple attachment to my firewall with bolts and large-area washers. The impact of the system on Metal Illness’s weight and balance was equivalent to my choice of footwear on any given flight.

Fuel Pump

I sourced a 12-volt Purolator fuel pump locally. (Facet is also a commonly found brand.) They come in a variety of psi ranges, and after purchasing a low-range pump and testing its output, I opted for a high-range (4–7 psi) pump. The oil’s flow rate is determined by the size of the orifice at the injection point and the pump’s pressure, and I felt having plenty of oomph from the pump would help atomize the oil. The pump was bolted directly to the firewall.

Electric Shut-Off Valve

An electric shut-off valve was added after I installed and flew the system without one. Even though the tank’s outlet was below the injection point in my exhaust system, the smoke oil siphoned through the system after I turned the pump off. I assume a low-pressure suction drew the oil through the lines, much like sucking on a straw.

I installed a 12-volt, normally closed Parker B2DX62 valve with 1/4 NPT ports. Today, the specific valve I used doesn’t seem to be manufactured, though searching the B2DX62 specification produces results—many used, many expensive. While you will have to match a valve to your specific needs (operating voltage, port size, and budget), it appears the current Parker-Skinner part number 7121KBN2NV00 is equivalent to what I used.

The shut-off valve was attached to the fuel pump with an Earl’s Performance brand straight union. The valve’s lightweight construction is easily supported by the fuel pump, negating the need to mount the valve directly to the airframe. Likewise for the three-way tee adapter on the output side of the shut-off valve, which splits the supply line into two feed lines, one for each exhaust pipe.

Injection Point

The straight-pipe exhaust of my Jabiru 3300 engine consists of two three-into-one exhaust manifolds bolted to the engine and an extension pipe for each manifold. I wanted to get the injection point as close to the hottest exhaust gases as possible, but not at the cost of removing the manifolds for modification. I wasn’t sure if the extension pipes were close enough to the heat or long enough to turn the oil to smoke. This is where trial produced error, and with it came some of the aforementioned learnin’.

I inserted a quarter-inch O.D. copper tube from a coil inside each extension pipe, placing each tube’s outlet near the top of the extension pipe, where I hoped to place an injection port. I secured the tube to the outside of the extension pipe with a hose clamp and went for a flight. When I returned, I found the copper had melted/failed during flight. All that remained was the portion clamped to the outside of the pipe and a small stub extending into the pipe. However, it had produced smoke, as well as knowledge to apply to the final design.

The Jabiru exhaust extensions are stainless steel, so I sourced two stainless steel weld-on bungs to weld just below the slip-fit expansion of each extension pipe. A variety of bungs are available from aftermarket automotive suppliers. Bungs for oxygen sensors seem an obvious choice but often have a metric thread. I wanted to use common AN and NPT fittings, so I went with a bung with 1/8 NPT female threads. In today’s market, I found a DeatschWerks weld-on fitting, part number 6-02-0751, to be close to what I used.

After the bungs were welded in place, I drilled a 1/16-inch-diameter hole through the pipe, centered within the bung. As I was drilling stainless steel—curved stainless steel—more learnin’ took place. I learned I should have drilled the holes before the bungs were welded in place.

Plumbing

I am covering plumbing broadly, because everyone’s needs will be different. I had a rule for my airplane: no hose clamps. I violated that rule for my smoke system, however, because the tank came with an integrated, barbed shut-off valve. I used standard fuel hose, secured with hose clamps, from the oil tank to the input side of the fuel pump. (If it failed, the worst that would happen is I would have a quart of smoke oil to clean from the bottom of my Sonex.)

My copper tube experiment taught me to use stainless steel near the exhaust, yet I wanted to avoid having all rigid tube between the manifold and the engine. To transition from the heat of the exhaust to the flexibility of braided hose, I made a 3-inch-long, quarter-inch O.D. stainless steel tube assembly with a brass NPT-to-tubing straight adapter on one end—the end that attaches to the exhaust pipe—and an AN816 nipple, AN819 sleeve, and AN818 nut on the other end to accommodate attaching a -04 braided hose assembly. I coated the NPT threads with anti-seize before installing each assembly in its exhaust bung.

The final step in plumbing the system was to make the -04 braided stainless steel hose assemblies and install them between the manifold and each transition tube.

Wiring

Anticipating a smoke system early in my build, I ordered a Ray Allen stick grip with a three-position switch to accommodate activating the system. Center was “off,” forward was “momentary on” (the pump ran for as long as I held the switch in that position), and aft was “on.” The stick grip switch was wired to a relay that activated the pump. After I found the system needed an electric shut-off valve, it was an easy matter to provide 12 volts to the valve from the adjoining pump.

Smoke Oil

Smoke oil is expensive. Super-Dri Skysmoke oil is $98 per gallon. Per gallon. I used it briefly, back in the mid-2000s. It was pricey then, too, but not nearly so. BlueSky White Lightning is $90 for 5 gallons. Other dedicated smoke oils are only available in 55-gallon drums; their price equal to my annual fuel budget. They all claim to be environmentally friendly (largely paraffin- or mineral oil-based, rather than petroleum-based) and produce long-lasting, billowy white clouds behind your biplane. That’s fine for sponsored acts, but for those of us who only smoke recreationally, it gets prohibitive. I ended up using diesel fuel. It’s affordable and accessible, but certainly not good for the environment. I wouldn’t advocate that the Titan Aerobatic Team use diesel fuel, but the amount I smoked in a year wouldn’t propel a pickup truck 30 miles.

In addition to the failed copper tube test, I made another error you will want to avoid. Don’t use vegetable or other cooking oils. You and your airplane will smell like a Wisconsin fish fry. In my excitement to ground-test my newly installed system, I ran a few ounces of vegetable oil through the exhaust and attracted hungry Catholics from far and wide.

Tuning the Injection Ports

The amount of smoke a system produces depends on the quantity of oil it is fed and the oil reaching a suitably hot gas to turn it to smoke. If the oil is injected too near the exhaust exit, it will have less time, therefore less heat, to turn to vapor. If too much oil is injected anywhere in the exhaust pipe, it can be self-cooling, thereby reducing smoke output. Think of it like putting out a campfire. If you dribble water on the hot coals, it will steam for a very long time, every drop turning to vapor; if you dump gallons on all at once, the fire will give a heavy sigh and die while water runs from the pit. Any oil that exits as a liquid not only did not produce smoke, it probably cooled other oil and prevented it from turning to smoke—a chain reaction, of sorts. As my system was simple, the only adjustment I had available to me was altering the size of the injection ports; I could drill the hole larger, if I felt it needed more oil, or choke the holes smaller, with stainless steel safety wire inserted through the hole and bent on both sides to keep it in place, if I felt it needed less. I used the TLAR method to adjust orifice sizes: That Looks About Right. A more advanced system could include a valve to make metering the fluid more precise, more controllable.

This smoke system was a fun project to plan and implement, and fun to employ during flight. It was neither expensive nor heavy. The specifics of my system would fit nearly any airframe, and the overall concept can form the basis of larger systems for those wanting, or needing, more.