Smoke Systems - A Primer
Updated from an April 1994 article prepared by Dave Printz for R/C Report magazine.
I WANT YOU TO SMOKE!
I'm crazy, you say? Everyone else says don't?? Yeah, well, this is different! This is the good kind! Real airplanes have gas engines and make lots of noise. Many make lots of smoke! You non-believers should attend a full scale air show where the likes of Bobby Younkin, Patty Wagstaff, Gene Littlefield, and/or Sean D. Tucker do their thing. The spine tingles when they go through their routines, and the ground trembles on close fly-bys. It takes your breath away! The smoke trails let you appreciate the maneuvers, the skill of the flyer, and the beauty of the machine. When Sean Tucker's 340 HP Pitts literally disappears for seconds that seem like minutes in a spectacular tail slide your legs go numb! Bobby Younkin's Beech D-18 creates a genuine feeling of disbelief as you watch him do an all down and dirty, full field-length pass at 100' with his smoke on. The sheer volume of smoke coming out of those twin Pratt & Whitneys, the swirling of the smoke contrails and the beauty of those long white billows of smoke are absolutely awe inspiring.
Now, for those of us into radio control (R/C) aircraft, much of the same applies. David von Linsowe will tell you of the impact smoke has had on his presentation at the Las Vegas Tournament of Champions. Watch the reactions of spectators to flyers with and without smoke systems and note the comparison. You've got to start smoking!
In this article we hope to help you plan and install a very effective smoke system on your own models. We want to get you so excited and confident that you'll march to your hobby shop or pick up the phone and order the parts and supplies you need to do it, right now! We'll share with you some of our experiences and lessons learned over the years we've been flying R/C models. As a middle-aged kid who started with control line flying back in the 50's, I'm now in my 16th year with R/C Giants, and ALL of my flying squadron smoke! Our flight line motto is: If it don't smoke, it ain't worth flyin!
Nearly all smoking aircraft, whether full-scale or model, create smoke by injecting a smoke producing fluid into the exhaust path. On our models, the procedure is fairly simple and requires the following components in addition to the completed, flyable model:
1. an on-board smoke fluid reservoir (smoke tank)
2. a smoke pump (to move the fluid from the tank to the exhaust stream)
3. a smoke muffler (to accept the smoke fluid at the exhaust)
4. a smoke control system (servo & valve or switch for on-off action)
5. tubing for linking the above
6. smoke producing fluid.
It's best to plan for the installation of a smoke system before you begin building or assembling your model. Installing a smoke system on an existing model is possible, but more difficult, and you'll usually have to compromise the installation or components. Figure 1 illustrates typical fuel system and smoke system layouts. I've shown both fuel and smoke tanks and connections to give you the "big picture" of their similarities and their differences. Now, there's no way around the fact that you'll get the best smoke, in quality and quantity, from the larger gasoline fueled engines. The same system principles apply for glow fueled engines, but you simply won't get equivalent results. For you glow engine fans, we'll tell you later in this article of a company which has had great success in maximizing smoke from glow engines.
For this article, we'll assume that you are installing a smoke system on a gas-powered giant model. Gas-powered" means use of one of the many gas-fueled engines like Quadra, Sachs, Zenoah, 3W, and others which run on a mixture of gasoline and 2-cycle oil.
Tubing for gasoline and smoke oil must be impervious to petroleum-based products and must withstand the high heat at the muffler attachments. Our experience has led us to use both black neoprene or "primer line and Tygon (yellow) tubing, 1/8 I.D. The black primer line is less-costly than Tygon. But, the Tygon line will not withstand the high temperatures found at the exhaust connections. Therefore, we use Tygon for all gas and smoke oil links with two exceptions: (1) the lines connecting smoke oil directly to the exhaust fittings, and (2) vent lines from both tanks to aircraft exteriors. We have also learned to not use the black primer line inside gas and smoke oil tanks. Over time, the black line will degrade in the gasoline and will cause a buildup of gunk in the carb filter screen, a maintenance task you do not need.
Part of the simplicity of Figure 1 is due to the fact that most of the gas engines use pumping carburetors (Walbro, Tillotson, etc.). These carbs use the engine vacuum and an integral diaphragm pump to pull fuel from the gas tank. When the engine is not running, fuel does not flow thru the carb. The fill/drain connections therefore allow for maximum simplicity. There are only two gas tank fittings required, one for filling and/or draining the tank, and one vent fitting. The vent is always open and allows the tank to breathe. The fill/drain fitting is plugged when not in use. We use a pointed, 3/4" length of 5/32" or 3/16" wooden dowel as the plug, and allow both the vent and fill/drain lines to simply hang from the bottom of the cowl. No special or extra cost fittings are required. The T-fitting is a 1/8 I.D. plastic fitting available from your local auto parts store. The fill/drain line inside the tank in Figure 1 is the one with the tubing and pick-up clunk. Use an external, in-line fuel filter during filling, not one inside the on-board tank. When filling the tank, you remove the dowel plug and attach the gas line from your external fuel container. When full, the line is disconnected and the plug inserted. When finished flying for the day, the fill/drain line is used to drain the remaining fuel from the tank back into your external fuel container.
In Figure 1, the smoke tank is identical in that only two fittings are required and they are used for the same purpose and in the same way. Again, the T-fitting is used to allow for filling and draining the tank. The smoke tank hardly ever needs draining, as it's usually emptied in flight before landing! On our Zenoah G-62-powered Stinger, we use a 32-ounce tank so we get eight minutes or more of smoke time during the flight.
In Figure 1, the smoke oil will be pulled from the tank by the on-board smoke pump. For this illustration, the smoke pump is also a diaphragm-operated mechanism. It's basically a low pressure fuel pump as may be found on snowmobiles or other small engine applications. There are three fittings on this smoke pump; one for oil entry, one for oil exit, and one for the pressure line which activates the diaphragm operation. The pressure line is connected to a crankcase tap on your engine. The pulsing of the crankcase pressure "powers" this low pressure pump continuously during engine operation. Advantages of this pump are that it is powered by the engine crankcase pressure, it is always on, it requires no additional power source, and its output is proportional to the engine rpm.
From the pump, the oil passes thru the smoke valve which is your On/Off control for the smoke system. There are a number of smoke valves on the market, all of which are effective. The smoke valve has only two fittings, "in" and "out and a connection for the servo to move the valve to "on" or "off". Any servo will effectively control the typical smoke valve. From the control valve the smoke oil goes to the smoke muffler where it enters the muffler thru a fitting at or near the engine exhaust. As soon as the oil enters the muffler, the heat of the exhaust stream and the hot muffler cause the oil to vaporize.
If it is necessary to drain the smoke oil tank, you do so just as you would drain the fuel tank. Just remember to end each flight with your smoke valve in the OFF position! When filling or emptying your smoke oil tank, you want only the vent line open. The smoke valve must be closed to prevent pushing oil into the muffler during filling and to prevent sucking air from the muffler during draining. An alternative smoke pump is a battery-operated electric pump which will be turned on/off via a servo-operated micro switch. The benefits of an electric pump include elimination of the on/off control valve and not having to tap your engine for a pressure line to a pulse-driven pump. A disadvantage to the electric pump is the requirement of an additional battery pack to power the pump.
My personal preference for smoke tanks, pump, valve, and fittings is Dick Bennett's B&B Specialties, of Granger, IN.
There are many smoke mufflers on the market. Odds are that the muffler you have for your engine right now will make an effective smoker. You can obtain brass smoke fittings which may be installed in your muffler. On our custom mufflers, we place two such fittings near the exhaust entry point. Muffler design, style, and features such as injection fittings, internal baffles, and pre-heater coils, may have some effect on smoke performance. Our experience has shown that smoke oil pre-heaters are not necessary on the gas-fueled engines. Also, an L-shaped path for exhaust flow through the muffler, such as in the Pitts-style mufflers, is superior to the straight-out mufflers.
We are, of course, biased on this subject. We believe that we have the best aviation smoke oil on the market. It's called "Super*Dri Aviation Smoke Oil" and was developed after much experimentation and many failures with other concoctions, including blends of kerosene, diesel oil, form oil, various thinners and reducers, and even --- perish the thought --- gasoline! It's used by both full scale and R/C fliers. And it's very effective. It is clear, non-volatile, will not attack foam and does not contain reducers. We'll ship anywhere, and we have an international following. "Super*Dri Aviation Smoke Oil" gives you the billows of white, intense, and persistent smoke to best show off your equipment and performance. I'll also tell you that colored smoke for our engines is not possible. When you see colored smoke from the full scale birds, it's likely from a flare of some type, or an injection of certain metal powders into the engine exhaust. Beware of those who attempt to sell you color smoke. The dye in the oil may damage your plane or your skin but you'll still get only white smoke.
We spoke earlier of a company which has developed a good smoke system for glow engines. American Model Products, Inc. of Baxley, GA, offered complete setups for 2-cycle and 4-cycle glow engines. Note the past tense. AMP is no longer in business. You can see the AMP system and its effective operation on glow engines on the Smoke On video tape referenced in this article. AMP's advantage was their "Pre-Heater, where the smoke oil is heated prior to its injection into the muffler. You might be able to find an AMP system on the used market or might craft your own equivalent. Other manufacturers are on the market with smoke mufflers and heater coils for glow engines. We've not seen these in operation and cannot endorse their operation.
ON FLYING AND SMOKING.
Now we'll talk you through a typical flying session so you'll better understand what and how it all happens and the impact. Among my best flyers are a G-62 powered Lanier RC Stinger, and a 3W70B-2 powered Pirate Models Extra 300. Both carry 32 oz. of fuel and 32 oz. of smoke oil, and use a smoke system just as already described. After assembling the bird, we fill both tanks. Since I have a habit of always leaving my smoke valve "off at the end of each flight, filling the smoke tank proceeds without problem. Before takeoff we walk the flight line to identify and talk to any other fliers on my frequency. When we get the frequency pin we perform a full ground check of all systems. Starting a good gas engine is a piece of cake. With or without spring starters, after confirming that the engine kill switch is on, I engage the choke, go to full throttle and full throttle trim, and crank the engine. With the choke fully on, the engine will not run, but it will fire. Once it fires, I go to low throttle with the choke off. One more crank usually gets it running. There's seldom any need to "tweak" the carb needles, either. Gas engines are much like your lawn mower and snow blower --- just set 'em and forget 'em.
I check for takeoff clearance, taxi onto the runway, and begin my takeoff roll. With full throttle applied, I quickly turn the smoke system on (I normally use the retract channel switch). As smoke begins to billow out, I pull back on the elevator stick and go straight up! A column of smoke led by a rapidly departing airplane is quite an impressive sight to behold! And then there are the aerobatics. On calm days, against a clear blue sky, the smoke is just awesome! Rolls, loops, Lomcevaks, spins, snaps ..., all look better with your super smoke system on. The smoking tail slide gives you a real adrenalin rush! If the wind cooperates, and you've got it down just right, for several seconds you won't even see your plane! You can hear the engine ..., the whole sky seems to stir ..., the smoke cloud boils violently ..., but you can't see the airplane! Then it's there ..., dropping tail first ... smokin' up a storm! Then, just as you're sure your heart will fail from the excitement, the plane's nose drops and you begin the recovery. DYNOMIIIIIITE ! You try it again, and again, and again, and... Soon you'll discover that smoke-on touch'n gos are neat, too. Eight to ten minutes into the flight the smoke tank is exhausted and it's time to land and let the next guy have the frequency clip. It's also time to enjoy the comments and questions from the spectators and other fliers.
TO LEARN MORE.
To learn more about smoke systems and to have your questions answered, look for the VHS video called, "Smoke On". Produced by Gulf Stream Video in 1994 and distributed by Goldberg Models, copies of it are likely to be found in hobby shops and libraries.
This wonderful hobby just gets better and better as you experiment with new and exciting aspects of it. Smoking is one of the best! Happy Flying, and Super*Dri Smokin'