No matter if you are using gas or glow engines you may use a smoke system depending on the available space in the aircraft’s fuselage, the engine’s capacity and the smoke system’s cost. Useful informations can be found below about the various smoke systems either simple or little more complicated.
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 thus creating smoke.
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 and is necessary to prevent muffler pressure from forcing the smoke fuel back up the lines.
Many modelers and some pump manufactures consider the one way valve unnecessary or optional. Smoke pumps manufacturers such as Powerbox do not suggest the use of a one way valve because of the tight and sealed gears in the pump that eliminates the engine’s back pressure phenomenon. Programmable pump’s speed also provides flexibility as in Powerbox, Emcotec and Sullivan pumps whose speed can be automatically adjusted by binding their control channel to the engine’s one (master-slave) thus eliminating the need for a needle valve.
Points of concern.
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 opened). The one way check valve is next, followed by the needle valve which should be the final stop before the muffler. 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 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.
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. Occasionally a tank will burst a seam, spilling smoke fuel all over the interior of your aircraft.
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.
What is smoke fuel? In most cases it’s a light fuel oil.
Honestly speaking its 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.
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. 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. There are lots of different smoke fuels available. Diesel fuel (straight from the gas station pump) works, but it doesn’t smoke very much.
If you’re running a big, cow led in engine, you may be able to get some decent smoke out of diesel alone, but adding a bit of kerosene will make it smoke a whole lot better.
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. Epoxy based paints would work better. 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. It doesn’t matter how well you think you’ve sealed it, smoke fuel will always get into the wing. Make sure all edges are ironed down tight as this stuff attacks the glue holding down the covering as well, and soaks into balsa.
If you ever need to get oil out of balsa, and you will someday, spray on some spot remover.
Let it sit overnight, and vacuum up all the white powder the next morning. Repeat again if necessary. 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. There is an even better way to remove oil from wood. The heat gun works wonders on oil-soaked wood.
Hit the wood with full heat, close up. The oil will bubble out of the wood. Wipe away the oil that surfaces with a paper towel.
Smoke fuel filter
Remember to use a filter when you fill your smoke tank connected to the fuel line that comes from your smoke fuel container.
Smoke pumps are easily clogged, and smoke fuel seems to have even more junk floating around in it than glow fuel does.
You already have one, but you want to be sure you’ve got the right one. A smoke system adds a good deal of extra weight and 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 the airplane’s manufacturer suggests that 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. Don’t forget to insure the rigidity of the landing gear going to hold up under the extra weight. Spend the money on a good quality engine. A smoke system puts additional stress on an engine, making it work harder and run hotter. Don’t skimp on your glow fuel either. Buy a good quality fuel with castor or castor/synthetic lubrication.
Smoke fuel tanks.
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.
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.
Do not use neoprene here, though, as it’s too soft and crimps easily. The yellow Tygon gasoline tubing works the best, but it’s a little stiff.
You may need to use a piece of neoprene tubing, 10 to 15 cm long to make the final connection at the muffler, as exhaust’s heat can melt Tygon tubing.
The tank needs to be mounted, physically, as close to the engine as possible. The ideal location is right alongside the glow tank.
In some occasions 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.
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.
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. Motor mounts that flex some allow the pump to do a better job. 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. 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 most probably 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 back plate, usually held on with four 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 sells a pressure tap fitting that fits a threaded hole.
Drill a hole in the back plate, usually drill it low and to the side. For you drill novices, use some 3-in-1 oil – just squirt a good job on the spot you’ll be drilling – it’ll help make the bit last longer. Use a tap to cut the threads. Be careful to tap the hole straight. Aluminum is soft, just keep the tap straight and steady. Wash off the back plate 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 back plate, otherwise it may interfere with the crank. Just use a Dremel sanding drum and grind that puppy down flush. Wash it off again, and bolt it back onto the crankcase (don’t forget that gasket!).
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.
As said before, the needle valve is a must if your are using a non programmable smoke pump. 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. You can purchase a remote engine needle valve, which almost all engine manufacturers make, from nearly any hobby shop.
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.
One-Way check valve
Powerbox smoke pumps tight manufacturing tolerances eliminate the need for a separate cut-off valve and TME claims you can do without one with their electric pump, but recommends you use one. Except them, no other 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.
Most check valves have an arrow drawn on them to indicate flow direction.
You should be able to blow through it in one direction, but not the other. Also make sure that is not defective and that you can you can blow through it hard, without any resistance. The check valve should be the last thing in line before the muffler, or it can be installed right before the needle valve. The exact placement of the check valve can be very critical. 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.
High exhaust temperature
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. 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. 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. Needless to say that before you flip over that wonder switch to activate the smoke pump for the first time during flight, thorough tests should be performed while the airplane is still securely parked on the ground. Is much safer and easy to adjust the smoke system, check for possible leakages and run an engine and smoke system performance check.
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. Remember smoke looks best against a clear blue sky and when lit by the sun from the front and don’t waste it during cloudy days.
Smoke also looks better against the darker azure blue directly overhead than against the light aqua marine sky on the horizon. You’ll be surprised what you can do with a smoke system. Low passes look more impressive with smoke.
Some aerobatic maneuvers that you perform are impressive but look ok 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.