by Robert Osorio - The Flying Penguin
Okay, let's talk smoke systems. I know, most of you are thinking "hey, I haven't got a giant scale plane with a gas engine, so I can't make smoke." NOT!" I don't fly anything bigger than a .90 four-stroke, all my engines run on glow, and many of my planes have smoke systems onboard. I just love to burn up a clear blue sky with some smoke, and we're not talking about a piss-ant dribble that can be mistaken for a rich engine run. No sir! I'm talking nice, thick, white smoke - thick enough to lay a haze across the runway on a low pass. I've rigged many of my planes with smoke and I've learned a lot from it. I'll be happy to share some of my hard earned insights with you (feel free to ignore them if you're feeling smug and superior, won't bother me). The most important thing is this: be prepared to experiment! If you don't like to experiment you're in the wrong hobby anyway - take up stamp collecting. If you don't like to fiddle around with a plane once it's airworthy then forget this. Trust me, you'll do nothing but tinker with a smoker. Get into that Zen Mad Scientist Mode, and prepare yourself for the grim truth that nothing really comes easy in this hobby, but you can have a whole lot of fun getting there if you're patient. As with most suggestions you're likely to hear about this hobby, take anything you read here with a grain of salt. None of this is the gospel truth, it's not written in stone or any such thing. If you ask four different people at the field about smoke systems, they'll probably tell you four entirely different things. It's not that any of them are entirely wrong (and probably they aren't entirely right either), but that smoke systems, like many things having to do with modeling, are an engineering challenge that can be approached from different angles. No two engines, installations, planes, prop/engine combos, mufflers, etc. are the same. What worked for me and your friend Joe Blow, may not do squat for you. I'm here to give you a starting point by telling you what's worked for me, and hopefully this can save you some aggravation. I've had luck getting engines as small as .40 size to smoke, but .60's are better. I've found four-strokes smoke better than a comparably sized two-stroke. My personal theory on this is that the smaller muffler on four-strokes in combination with the longer pause between exhaust cycles allows the smoke fuel to get hotter. You engineering types out there are more than welcome to comment.
All smoke systems work essentially in the same way: pumping a smoke fuel from a separate tank into the muffler. The smoke fuel, on contact with the hot exhaust gases and muffler walls, burns creating smoke. Same thing is happening in that wreck you got stuck behind on the interstate the other day. You know the one, belching big clouds of smoke for your lung's appreciation (you gotta wonder how some of those things pass inspection), except in that case engine oil is unintentionally (I hope) leaking into the exhaust system through a worn seal, bad ring, or cracked engine block (and the slob wonders why he has to pour two quarts of motor oil in it each week!).
In general, a smoke system consists of the following: a separate smoke fuel tank and associated tubing, a smoke pump, a smoke valve that can be controlled via a spare servo channel on you radio system (needless to say, you need at least a five channel radio), a one way check valve, a needle valve to control the flow of smoke fuel into the muffler, and an optional fuel filler valve (if the pump isn't buried under a cowl you could just pull the intake line and fill it from there). This basic system diagram in shown in Figure 1:
The smoke fuel is drawn from the tank by the pump, and forced through the lines until it gets to the smoke valve. When the smoke valve is opened via remote control, it allows the smoke fuel to continue on down the line through the one way check valve and needle valve. The one way check valve allows fuel to flow in only one direction (Duh!) and is necessary to prevent muffler pressure from forcing the smoke fuel back up the lines. Many other modelers and some pump manufactures consider the needle valve unnecessary or optional, but I consider it a must. It's the simplest way to control the flow, and it's just not worth the hassle to do without it for the sake of a few dollars. Keep your fuel lines as short and straight as possible. Long lines make it harder for the pump to prime itself and reduces the flow rate, leading to long delays between the time you turn on the smoke, and when it actually starts coming out. Excessively long and twisty lines can render a smoke system totally useless.
The layout shown in Figure 1, while not critical, is ideal. As a general rule of thumb, you want the pump as close to the tank as possible to reduce delay times and make it easy for the pump to self prime. The smoke valve should be located after the pump so that the pump never goes dry, again, to prevent priming problems (it won't hurt most pumps to pump against a closed valve and the slight buildup in pressure gives the pump an assist when the valve is openned). The one way check valve is next, followed by the needle valve which should be the final stop before the muffler. I do not recommend the plastic valves that come with some pumps instead of needle valves. The hot exhaust will damage a plastic valve, they're not very accurate, and I've never seen one that doesn't leak anyway. I personally prefer to mount the needle valve right to the muffler, and some specialty smoke mufflers actually come this way. Don't bother pressurizing the smoke tank with muffler pressure like you do on the fuel tank. Gut instinct would tell you that pressurizing the tank to the same ambient pressure as the outlet of the system would help, but I've found it makes no difference. Just have it dump out underneath the engine somewhere so that when filling the tank, the overflow will just drain out onto the ground or into a reclamation tank. I've started saving my own overflow. That smoke fuel is expensive and probably eco-unfriendly, so is glow and gas for that matter. With all of us becoming more environmentally aware (are we?) we should all think of ways to clean up our acts before some government agency comes along and uses this as an excuse to close down fields. I understand this practice is already very popular in many parts of Europe. (Okay, I've said my green thing for the day, you can all tune back in now).
There is a simpler smoke system that eliminates the need for a smoke pump, although it can only be used on two-stroke engines, and it has some drawbacks. This system necessitates the installation of a pressure fitting on the back crankcase cover of the engine (this pressure fitting is also necessary when using a smoke pump that runs off crankcase pressure, mentioned later). The diagram for this simplified smoke system is shown in Figure 2:
With the downward movement of the piston, an engine draws the air and fuel mixture through the carburetor, the crankcase and, via the ports, into the combustion chamber. The pressure fitting provides a means of tapping into the substantial positive air pressure created in the crankcase which can be used to pressurize the smoke tank to force the smoke fuel through the system. A four-stroke engine does not draw fuel and air through the crankcase and most four-strokes don't even have an airtight crankcase, so this method can't be used on a four-stroke engine. The check valve is still required, but now it's installed immediately after the engine pressure tap to prevent smoke fuel from backing up into the crankcase due to gravity or while fueling.
The major drawback with this system is that when the smoke valve is closed, the tank swells because of the unrelieved pressure. This makes it necessary to mount the tank loosely so the swelling doesn't crack open your fuselage (I've seen it happen!). Occasionally a tank will burst a seam, spilling smoke fuel all over the interior of your aircraft (I've seen this happen too). This problem could be eliminated by installing a second smoke valve between the pressure fitting and the tank, but the valve would have to be a more expensive air pressure valve, and the additional mechanics aren't worth it (you'd have to use both valves as the air pressure valve alone wouldn't prevent smoke fuel from siphoning out of the tank under gravity). With this system you must also make certain not to throttle back to idle while the smoke is on (something hard to keep track of during some aerobatic maneuvers) otherwise it will kill the engine, as engines don't idle too well with a compression leak (this is not a problem with a pump driven by crankcase pressure as in that case the pump provides a closed system which prevents the crankcase pressure from leaking away). The only benefit of this system is that it saves you the cost of the smoke pump, saving you anywhere from $35 to $60. I've used this system successfully, but I only use it on throw away planes, never on a scale model. I used to have a 40 size Ugly Stick (yes, I actually did own an Ugly Stick once, we all go through that phase) that used this system when I was experimenting with wing-tip smoke. The plane was rigged with tubing that would carry the muffler exhaust through the wing and out the wing-tips - looked cool, but I didn't want a complex smoke system, and there wasn't much room in the plane anyway. To keep it simple I attached the smoke tank to the outside of the fuselage just behind the engine on the side opposite the muffler (ala control line style), with velcro and rubber bands. To keep it cheap and simple I didn't even use a smoke valve, I'd just pinch off the line with a pair of forceps and have someone hold the plane for me on the runway. When I was ready to take off, I'd run up the engine and signal my helper who'd remove the forceps and let go of the plane. I didn't even use a one way check valve since the smoke was never turned off, and I'd take great care not to let fuel get into the crankcase. Needless to say, without a reliable idle, all landings were dead stick, but it did work.
What is smoke fuel? In most cases it's a light fuel oil. Let me warn you right now, friend; cleaning a plane with a smoker is a messy job. It's worth it, it really is, but it's a real mess on the plane. That mess need not be limited to the exterior of the plane either. Leaks always develop - plan for them! Fuel proof the compartment you install the smoke tank in, and make sure anything that has to be mounted along side, like the receiver or battery is wrapped in plastic. While we're on the subject, remember that you'll need a separate fuel container and filler pump for your smoke fuel, and that they have to be compatible with gasoline fuel. Most smoke fuels are petroleum based and will destroy glow pumps and silicon fuel lines. Even if you're not sure, get a gas compatible system anyway, as you can pump anything through it. For those of you fellow environmentalists out there who may feel that making smoke is going to contribute to global warming, I suggest you go fly electrics and ride a bicycle to work. Yes, partially burning a petroleum based oil is probably not great for the environment, but I doubt you'd be contributing much more pollution than the engine's exhaust is already putting out. I doubt these small engines contribute much at all to the problem, and as long as I'm choking on a cloud of diesel exhaust behind a city bus at the traffic light, I'm not going to feel too guilty about it.
Full scale planes can burn almost any sort of oil for smoke, or even just plain water. Unfortunately, our engines are too small for that trick. Model engines certainly get hot enough, but the smoke oil acts as a coolant. On a full scale plane that little squirt of smoke fuel hardly has any effect on the exhaust temperature at all. Our small engines get their fire doused in a hurry, though, and keeping the heat on is the secret to a good smoke system. As long as you can maintain a high exhaust temperature, you'll get good smoke. This is why some modelers suggest you preheat the smoke fuel by running it through a marine engine cylinder head or by wrapping aluminum tubing around the engine cylinder. I've tried all that and I've found that you don't get enough improvement to offset the money or the extra plumbing the pump has to work against. Later, I'll make some suggestions on how to keep your muffler hot.
There are lots of different smoke fuels available and some I've tried while others I've only heard about. Diesel fuel (straight from the gas station pump) works, but it doesn't smoke very much. If you're running a big, cowled in engine, you may be able to get some decent smoke out of diesel alone, but adding a bit of kerosene (some gas stations still sell it, especially near camp grounds) will make it smoke a whole lot better. From long experiment I've found that the best mixture is three or four parts diesel mixed with one part kerosene. This can smoke from okay to great depending on your setup and how high an exhaust temperature you can maintain. It's cheap and easy to make, but it can stink pretty bad and the kerosene eats up some painted finishes like dope and enamel. My guess is epoxy based paints would work best, but I've never tried. Kerosene also dissolves Styrofoam as well as some iron-on coverings so don't even think about using kerosene on a plane with a foam wing. I don't care how well you think you've sealed it, smoke fuel will always get into the wing. Next thing you know, you've got a hollow wing (I know, it's happened to me). I've found Monokote to hold up well, Black Baron melts, and I haven't tried any other coverings yet. Make sure all edges are ironed down tight as this stuff attacks the glue holding down the covering as well, and soaks into balsa. By the way, if you ever need to get oil out of balsa, and you will someday, spray on some K2r spot remover. Let it sit overnight, and vacuum up all the white powder the next morning. Repeat again if necessary. Somebody once turned me on to this, and it really works. This procedure is mandatory when you try to re-cover that plane somewhere down the road, as you'll find that somewhere fuel got under the skin.
Bennets smoke fuel is usually sold in the bigger hobby stores, at least down here in Florida. Bennets works even better than diesel and kerosene, and the smell isn't quite as bad. It can smoke pretty thick right out of the bottle, and seems to be slightly less aggressive on finishes, but all precautions mentioned above still apply, especially as concerns Styrofoam. Bennets costs about ten bucks a gallon, and in an effort to save some money, I started cutting it with diesel. Four parts diesel mixed with 1 part Bennets seems to smoke okay and saves a bundle.
I was told once by an auto mechanic that brake and power steering fluid smokes good, but this stuff is pretty expensive. I've also been told that Coleman stove fuel works good (the liquid fuel, not the propane, dummy!), but I've never tried it or compared prices.
Lately I've been using Ultrasmoke from Advanced Marketing in Wichita, Kansas, phone is (316) 721-3864. It's about forty bucks a case (4 gallons) including shipping. This stuff produces the thickest and most persistent smoke I've seen yet. It does not attack finishes (it's still slimy, but it doesn't seem to want to eat anything), and the company claims that this stuff won't dissolve Styrofoam. I haven't tested this claim yet, but I'm dying to try it out in a Hobbico Ultimate Bipe ARF. Also, this stuff is practically odor free which is worth it alone right there if you've ever spilled regular smoke fuel in your car trunk in the summer (took me weeks to air out that car, and I got dizzy every time I drove it). Ultrasmoke doesn't seem to be petroleum based, so diesel probably won't mix with it, I haven't tried yet.
Please remember to use a filter when you fill your smoke tank, the Sullivan Crap Trap works great, and you can see through it to tell when it's time to replace it. Smoke pumps are easily clogged, and smoke fuel seems to have even more junk floating around in it than glow fuel does (you are filtering your glow fuel, aren't you?)
Yeah, well sure, you already have one, but I want you to be sure you've got the right one. A smoke system adds a good deal of extra weight; you're doubling the amount of onboard fuel (and smoke usually weighs more than glow). There's also the extra weight of the pump, servo, tubing and batteries (if you're using an electric pump). If you're one of those people who likes to buy the minimum recommended engine size for a kit, learn to break that habit right now. If the box says you can use a .40 to .60, stick a .65 in it. Remember, also, that because of the extra weight of the smoke system and the greater torque of a larger power plant, you may have to go back and beef up the structure of the plane in critical areas to handle the load. Flimsy tail feathers should be reinforced with wire struts, and small wing bolts may need to be replaced with bigger ones. Is that landing gear gonna hold up under the extra weight, or will the plane go into bouncing oscillations on landing? Think ahead.
Spend the money on a good quality engine. A smoke system puts additional stress on an engine, making it work harder and run hotter, and that cheap Taiwanese copy might not cut the grade. Don't skimp on your glow fuel either. I'm not talking nitro, I never use more than five percent, and anything more will only make your engine run hotter. No, I'm talking oil. Buy a good quality fuel with castor or castor/synthetic lubrication. I always add an extra ounce of castor oil to my fuel because you never know what you're getting and it only helps. Lean runs aren't unusual with the extra burden of a smoke system.
Whatever you wind up using for fuel, you'll need a second tank to carry it in. One about the same size as your glow tank, give or take a couple of ounces, will work just fine. If you've got no room for the smoke tank, forget it and try it in a roomier bird. It's pretty tight in some of those .40 size kits, but there's tons of room in a Goldberg Cub for instance. BIG WARNING: As mentioned above, most smoke fuel is petroleum based and will make silicon swell up. You can't use the silicon tank stopper and pickup line that comes with a standard glow fuel tank. Buy a gas conversion kit which consists of a neoprene stopper and pickup. Same goes for fuel lines. I don't suggest you use neoprene here, though, as it's too soft and crimps easily. I've found the yellow Tygon gasoline tubing works the best, but it's a little stiff. You may need to use a short piece of neoprene tubing, an inch or two long, to make the final connection at the muffler, as exhaust heat can melt Tygon tubing.
The tank needs to be mounted, physically, as close to the engine as possible. The ideal location is right along side the glow tank. I've found in some situations that it's necessary to mount the smoke tank as high in the fuselage as possible, in order to give the pump a little gravity assist in priming. Sometimes you can get creative. I installed a smoke system in an Ultra Sport .40, a plane notorious for it's tight tank compartment. I used two, four ounce tanks, connected in series (See Figure 3), with the pickup line of the first tank connected to the vent line of the second. I mounted them in the radio compartment right in front of the servos. This worked okay, although there was a notable delay when turning on the smoke. A year later I crashed that plane and rebuilt it with an eight ounce tank mounted in the cockpit area, and I blacked out the canopy. This worked great, and the high position of the tank improved fuel flow. The lesson is, don't be afraid to experiment!
Again, you have to buy a pump that's designed to be compatible with gas, as glow pumps will be destroyed by petroleum based fuels. Varsane Products (546 S. Pacific St., Suite C-101, San Marcos, CA 92069, (619) 591-4228) makes a couple of pumps that can be used in smoke systems. The VP-22SG oscillating pump is for four-strokes and is designed for smoke fuel, gasoline, and kerosene. It attaches to an aluminum plate that is, in turn, bolted to the back of the engine using two of the engine crankcase screws. Sometimes you have to buy longer screws to make it work, but I know from experience that the O.S. Surpass engines have long enough bolts for the job. I've had good luck with it, but I usually have to mount the tank at the same level as the glow tank to get a descent prime, and sometimes there's no room. In these cases I've mounted the glow tank under the smoke tank and used another oscillating pump (glow version) to supply glow fuel to the engine from the abnormally low tank position. You can easily mount the two pumps back to back on the same mounting plate as shown in figure 4:
Remember not to use muffler pressure when using a glow pump, it's not necessary and you may make the engine run too rich. Just vent the line out under the engine compartment somewhere and use it as an overflow. Oscillating pumps take advantage of the fact that the engine rocks slightly as the crank turns to produce the pumping action. I've found that motor mounts that flex some allow the pump to do a better job. I couldn't get much smoke out of a bipe I had that used wooden engine mounts attached to a false profile cowl. When I sawed them off and installed a glass-filled motor mount, things improved quite a bit (I've never tried soft mounts, but I'd guess they'd work great).
I've also used the Varsane two-stroke pump that runs off crankcase pressure with good results. Using this pump will require the installation of a crankcase pressure tap on the engine to power the pump. Please keep in mind that this modification will probably void your warranty, but then again, so will flying your plane into a concrete runway. If you ever want to use the engine in a plane without smoke, in the future, just attach a short piece of plugged fuel line to the pressure tap to prevent leaks.
If you don't like the idea of drilling a hole in your engine, take it down to your hobby store and I'm sure they'll do it for you for a reasonable price. It's a ten minute job if you've got the right tools and have done it before. To install a pressure tap, remove the engine backplate, usually held on with four metric hex socket-head bolts. Some engines use a clear plastic gasket between the back plate and the crankcase, take care not to tear it or lose it. Du-bro Products (480 Bonner Road, Wauconda, IL 60084) sells a pressure tap fitting that fits a 6-32 threaded hole. Do yourself a favor, if you haven't got a tap and die set go out and buy one. It's a must-have tool for fixing bad threads or cutting new ones. Most hobby stores sell individual taps along with the proper size drill bit to go with it, if you don't want to shell out the dough for a complete set. Drill a hole in the backplate, I don't really know if the exact location makes a difference, but I usually drill it low and to the side. For you drill novices, use some 3-in-1 oil - just squirt a good gob on the spot you'll be drilling - it'll help make the bit last longer. Use a the tap to cut the threads. They sell some neat alignment blocks out there that help you tap the hole straight, I just go for it. Aluminum is soft, just keep the tap straight and steady. Wash off the backplate so there are no metal cuttings left (you don't need any of those going through the engine), and screw in the pressure tap. It's very important that you make certain the pressure tap doesn't extend past the inside wall of the backplate, otherwise it may interfere with the crank. Just use a Dremel sanding drum (you do have a Dremel, don't you?) and grind that puppy down flush. Wash it off again, and bolt it back onto the crankcase (don't forget that gasket!).
With either Varsane pump, you have to set the flow adjustment for maximum. On the four-stroke pump, the whole inner barrel is the adjustment screw, and it must be unscrewed out almost all the way to get maximum flow (don't go too far, or it'll come out of the pump). On the two-stroke pump, there is a small set screw that must be screwed in almost all the way. BE VERY CAREFUL! I've been told that tightening the screw in all the way will damage the demand regulator. I believe it will still work as a smoke pump, but it will no longer be any good as a gas engine fuel pump. You should not go in past the point where 1/16" of the screw shaft is showing. The Varsane pumps have to be manually primed the first time they're used. It's easy, just connect your smoke fueler container to the inlet of the pump and pump a bit of fuel through it. It's also a good idea to connect the two ends of the pump together with a short piece of tubing when not in use, to keep it from drying out. Both of the Varsane pumps cost about $35 each.
I've recently been trying out the TME Simple Smoker (TME, PO Box 340608A, Tampa, FL 33694, (813) 968-9510) in my Aeromaster Bipe. This pump greatly simplifies installation because it eliminates the need for a smoke valve and servo. The pump plugs directly into a receiver channel, but runs off a separate battery pack. The only downside is the weight of the extra battery pack, but in my experience most planes can carry it comfortably. Also, you'll have to invest in a fast charger as I've found I have to recharge the pump battery after every few flights (if you fly all day like I do!). A fast charger can set you back anywhere from $40 to $90, but is well worth having. I fast charge all my radios at the field, and I never allow myself to get into the situation where I want to fly, but my battery voltage is iffy. TME claims the pump will run off a four cell receiver battery, but I've found it works better with a five cell pack. The TME pump costs around $60. A little pricey, but I like it so far, it's very reliable.
The smoke valve is a mechanical valve, connected via a linkage to a servo, that actually turns the smoke fuel flow on and off by remote control. The simplest one is made by Du-bro. It's a plastic assembly that pinches a neoprene tube to shut off the flow. The more expensive ones look like reactor valves on nuclear powered subs. You can also make your own if you've got the time and inclination (I have neither), and save a little money. It's by far the simplest part of a smoke system, but can cost from $10 to $20.
As I said before, the needle valve is a must, and don't let anybody tell you otherwise. Once you've got your smoke system installed, it's going to take a few flights to adjust it properly. You want to set the needle valve for a compromise between the thickness of the smoke and the longest possible run time. You're going to be switching the smoke on and off during flight, so you don't need a full flight's worth of smoke. Mine usually set up with seven to nine minutes of continuous smoke, which is fine for a twelve minute flight.
You can purchase a remote engine needle valve, which almost all engine manufacturers make, from nearly any hobby shop. I personally like to use the replacement needle valve assembly for an Enya TV60 engine. It costs about $9, and the bigger hobby stores around my area stock them for the ducted fan crowd, who have to use remote needles on their engines. It's business end (the outlet that normally screws in the carburetor) is threaded for an 8-32 hole. I drill and tap a hole this size and screw the needle assembly right into the side of the muffler, this nicely solves the problem of mounting it. Most mufflers have thick enough walls to let you do this. On four-stroke mufflers, drill into the angled flanged area where the pressure tap is usually located - it's thicker there than at the sides. You can also mount the needle valve away from the muffler. In this case a strap, and a big blob of PFM glue is usually enough to hold it in place. Remember, you will have to add an extra pressure tap in addition to the one already on the muffler, unless you're using a pump for the glow fuel system as well, in which case the existing pressure tap will be available for your use. If you do decide to use the TME electric pump, throw away the plastic valve it comes with, it's worthless.
No pump can fight the substantial back pressure from the muffler's exhaust, so you need a one-way check valve to keep exhaust gases from backing up the line. A one-way check valve allows fuel to flow in one direction, but prevents fuel or exhaust gases from traveling in the other. TME claims you can do without one with their electric pump, but recommends you use one. Use it, it really is necessary. Don't use the ones made for high performance pattern engines with pressurized fuel systems, as these are intended for glow fuel and smoke oil will destroy them. Varsane makes a check valve to go with their smoke pumps, and so do a couple of other manufacturers. A check valve will set you back about $5.
Most check valves have an arrow drawn on them to indicate flow direction. The Varsane check valve is shaped like an arrow, and points in the proper direction. If you can't tell, just blow through one end. It's a good idea to do this anyway to make sure it's not defective, and to loosen up the flap inside which is sometimes stiff. You should be able to blow through it in one direction, but not the other. Also make sure you can blow through it hard, without any resistance. I've gotten a few defective ones that seemed to work okay when I blew through them, but they wouldn't work under the higher pressure from the pump. Spent a day taking an entire smoke system apart, until I narrowed it down to the check valve, which seemed to be working, until I blew through it real hard.
The check valve should be the last thing in line before the muffler, or it can be installed right before the needle valve. I've found that the exact placement of the check valve can be very critical, especially with the Varsane pumps. It's best to have a few inches of tubing between the valve and the muffler otherwise the higher back pressure nearer the muffler can force the check valve to stay closed (the valve is actually only opening between combustion cycles when the exhaust pressure drops briefly). If there's no smoke, but there's plenty of smoke fuel squirting out of the check valve when you disconnect it from the muffler (and you're certain the check valve is working), you can bet your check valve is too close to the muffler. Check valves also go bad over time, and hot exhaust gases can melt the internal parts during a lean run, so pickup a spare and keep it in your flight box.
As I said earlier, the secret to a great smoke system is maintaining a high exhaust temperature. This isn't always easy to do, as a good muffler is usually designed to keep the exhaust temperatures down below where we want it. If the engine is going to be cowled in then all the better, just make sure there aren't too many holes that allow cooling air to pass over the muffler. Do make sure, though, that the engine's cylinder head is getting plenty of cooling. Since we're running up the exhaust temperature a bit, the engine is going to tend to run hot. Remember, also, that you may not get as much smoke during test flights with the cowl off as with it installed. If the muffler has cooling fins on it, you may want to grind them off with a Dremel tool. In general what you want to do is to try and keep cooling air off the muffler entirely, or if that's not possible, at least make that cooling ineffective. I like to slip a tight sleeve of silicon tubing (the kind used for connecting tuned pipes to headers) over my four-stroke mufflers, to help keep them hot. Remember that the engine will be running hotter, so you want to run it a little richer than you normally would to keep it from overheating. As I mentioned earlier, I like to add an extra ounce of castor oil to every gallon of fuel I buy. Our engines can always use the extra lubrication, and the extra oil helps carry heat away. Most hobby stores sell castor oil, and it usually costs around $8 for a ten or twelve ounce bottle.
Some muffler manufacturers make a special muffler, or sell a retrofit kit, that adds a coil of tubing inside that the smoke fuel has to pass through before being injected into the exhaust. This allows the fuel to stay in the muffler longer and, subsequently, get hotter, giving better smoke. I've never bought one of these, but I built my own once by modifying a J-Tec muffler. It works, and it's worth it if it can make a difference between a lame and a great smoker. Don't forget, though, you're also adding more plumbing for the smoke fuel to travel through, which will increase delay times when turning on the smoke, and make the pump work harder. These mufflers aren't cheap either, so you'll have to decide if your budget can afford it. Some modelers also suggest soldering a piece of tubing to the pressure tap, on the end inside of the muffler, and crimping it and cutting small notches in it with a razor saw. The idea is to make an atomizer of sorts that sprays the smoke fuel into the muffler as a fine mist. I've never had any luck making one of these, but I can see why it should help. Experiment!
Before installing a smoke system, I like to have an idea of how well a particular engine/muffler combination will smoke. I connect the filler line from my smoke fuel storage container directly to the muffler pressure tap through a needle valve, and pump a small quantity of fuel into the muffler with the engine running (I use a Du-bro squeeze pump, so it's pretty easy to meter it out). Ideally, it shouldn't take much fuel to make it smoke. Despite the fact that smoke fuel has the side affect of cooling the muffler, some engines want a large quantity of fuel before they'll smoke worth a damn. All that fuel in there, I'm guessing, loads the engine down, which increases the engine temperature, which also increases the exhaust temperature. So sure, you could put a sixteen ounce tank in there if you've got the room, and increase the flow, but it's a waste of smoke fuel, you'll really slime up the plane now, and all that puts a heck of a load on the engine. While you could drown the muffler to achieve your goal, you're only going to choke the engine on it's own exhaust, and performance will suffer. You should always strive for fuel efficiency by doing anything you can to keep the exhaust temperature high (within reason), while keeping the required fuel flow as low as possible.
The smallest details can make the difference between good and bad smoke. If the fuel is chilled, it'll cool your muffler and won't smoke very well. Keep your fuel storage container in a warm place for a few hours before flying (car trunks in the summer are just fine). Weather conditions can affect smoke thickness and persistence. Smoke looks best against a clear blue sky and when lit by the sun from the front. Smoke also looks better against the darker azure blue directly overhead than against the light aqua marine sky on the horizon. Don't even bother packing the smoke fuel on an overcast day, unless you like to do simulated crop dusting (which is actually a lot of fun). Each flight, I spend the first few minutes flying around looking for the best patch of sky to make smoke in. I make most of my impressive (at least to me) aerobatic maneuvers in that "sweet spot" where the sun hits the smoke just right, and the sky is clear blue behind it.
You'll be surprised what you can do with a smoke system. Low passes look even sexier with smoke. Some aerobatic maneuvers that you know are impressive but look ho-hum normally, suddenly stand out as a crowd pleaser with smoke. Try a flat spin with smoke, and just listen to the oohs and ahhs from behind you. That tail slide really looks like a tail slide when you can see the plane actually back into the smoke. Smoke can also make your mistakes stand out. That four point roll that you thought was perfectly axial, might prove to have a little burp in it that shows up in the smoke trail. This is good, it helps make you a better pilot. I set up a Skyward .60 ARF trainer with a smoker once, and I found that the smoke trail helped student pilots tell their orientation easier.
I hope you've found this article useful, or at least entertaining. I hope I haven't scared any of you away from trying smoke, because that would be a shame. Yes, a smoke system does require you to do some extra fiddling at the field, but so do retracts or, for that matter, any scale feature you add to a plane. If you're just starting out in the hobby, you've got lots of other fun and interesting challenges ahead, don't worry about smoke right now (you want real fun, wait until you tear your first set of retracts out of an EZ kit on a grass landing!). But, if you're like me, and feel like a new challenge, then smoke may be the ticket for you. With some experimentation, advanced planning, and a whole lot of patience, you can enjoy the thrills of a smoke system. Go on, show off!
Good luck, and keep Smokin!
Updated March 1997
Robert Osorio - The Flying Penguin