Smoke Systems

[from the AMA National Newsletter, November 2001]

by Robert Osorio

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 airplanes have smoke systems onboard. I just love to bum up a clear blue sky with some smoke and we’re not talking about a 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.

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 an airplane 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 they probably aren’t entirely right either), but 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 .60s 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.

Smokin’!
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. The 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 your 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).

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 other modelers and some pump manufacturers 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.

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.

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).

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 airplanes, never on a scale model. I used to have a .40-size Ugly Stick 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 wingtips—looked cool, but I didn’t want a complex smoke system, and there wasn’t much room in the airplane 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 (a lá control line style), with velcro and rubberbands. 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.

Fuel for Thought
What is smoke fuel? In most cases it’s a light fuel oil. Let me warn you right now, friend: cleaning an airplane with a smoker is a messy job. It’s worth it, it really is, but it’s a real mess on the airplane. That mess need not be limited to the exterior of the airplane 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 alongside, 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 what the engine’s exhaust is already putting out.

Full-scale airplanes 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 airplane, 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 campgrounds) will make it smoke a whole lot better.

From long experiments 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 an airplane 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 recover that airplane somewhere down the road, as you’ll find that somewhere fuel got under the skin.

from The Flightline
Bayside RC Club
Tom Minger, editor
Fremont CA