To
make smoke you only need to do one thing: Get
oil to reach its Smoke Point. The bad news is:
That's a lot harder to do than it sounds. You
only have one source of heat - your engine, and
you can only access that source at one place -
the muffler. The problem is that the exhaust is
constantly moving through the muffler, so the
oil doesn't get much time to get hot before it
is blown out into the atmosphere, so you need to
keep the oil inside the muffler for as long as
possible. This is one reason why small 2-Stroke
engine don't usually work well with smoke. Each
stroke of the piston blows the contents of the
muffler out, so the oil doesn't get very much
time to get hot. Plus, the type of plane that
would be powered by a .40 size 2-Stroke would
probably suffer under the extra weight of a
Smoke System. With larger (Gas) 2-Strokes, the
amount of "Time-In-Muffler" is not a problem,
due to their excessive heat. I have also had
much success with Glow 4-Stroke
engines.
Let's
take a look at what is involved. The major
components that we need are:
- Tank
- Oil
- Fuel or Gas Tubing
- Pump
- Flow Regulator
- Check Valve
- Muffler
Now
let's look at each component one at a
time:
TANK
Nothing too tricky here. I use a standard
10 oz Fuel Tank, but the oil I use is
non-petroleum based, so I can use the regular
stopper that comes with the tank. If you use a
petroleum-based oil, you'll have to use a
stopper that is approved for petroleum use (More
on Oil later). One thing to note is you should
probably consider a 3-line system (Or at least
give yourself access to the feed line before it
reaches the pump). You may find that due to
Murphy's Law, you will often land with a full
oil tank and need to empty it by hand.
OIL
There
are lots of Smoke Oils on the market. Super-Dri
is one of the popular brands, but I must confess
that I have never tried it. Be that as it may, I
have heard many good things about it. Many
people also make their own by mixing 3 parts
Diesel Fuel with one part Automatic Transmission
Fluid. This is a classic example of Petroleum
Based Smoke Oil. Personally, after having tried
a few Commercial Oils, as well as a few
"Home-Brews", I have settled into a product
called "Ultra Smoke". It gives great Smoke, and
it is not Petroleum based, so I can use any type
of fuel lines, and the airplane doesn't stink up
my car and house afterward. The bad news is,
last time I checked, the distributior of
Ultrasmoke was out of stock and having a hard
time finding the ingredients, so I may be
looking for another soon (Fortunately Last time
I called I bought a case!)
FUEL TUBING
The
only thing that needs mentioning here is that if
you use Petroleum based Oil, you need to use a
fuel line that is approved for Gasoline, as the
Petroleum based oil will eventually deteriorate
standard silicone tubing.
PUMP
Now
we get to the heart (no pun intended) of the
system. There are 3 methods of pumping the oil
that I know of.
- 1) Electric Pump
- 2) Crankcase-Pressure Activated Pump
- 3) Crankcase Pressure alone
Now,
let's take a look at these three methods, along
with their pros and cons.
The
Electric Pump is by far the easiest setup. It
simply plugs into an auxiliary channel on your
receiver, and into a separate (usually 4.8v)
power supply. There are many commercially
available pumps on the market, but I use a windshield washer replacement
pump from the J.C Whitney website and turn
it on and off with a servo-activated
microswitch.
- Pros: Easy to set up and use.
- Cons: Requires separate battery
(which will also require charging before each
trip to the field), and it can cause radio
interference if placed too closely to radio
components.
The
Crankcase-Pressure Activated Pump is in essence,
a fuel pump from a motorcycle. They work very
well, and do not require a battery, and they
work best on Gas Engines where you have a lot of
crankcase pressure.
- Pros: Delivers good oil flow. No risk
of radio interference. No need to charge extra
Battery pack.
- Cons: Requires an extra servo to
operate on/off valve, and they only work on
Gassers.
And
finally, there are a few systems that work by
tapping the engines crankcase and applying the
pressure directly to the oil tank. I have never
tried this method, but I have doubts as to its
reliability to deliver steady oil pressure
(which is a MUST).
- Pros: Probably the lightest of the
systems. Least complex.
- Cons: Steady oil supply questionable.
Requires servo-operated on/off
valve.
Flow Restrictor
The
amount of oil that is pumped into the Muffler is
critical. Too little oil means not enough smoke,
and too much will cool the exhaust below the
Smoke Point of the Fluid. Ideally, the pump will
put out a steady, constant flow of oil in a
larger quantity than is needed, which can then
be regulated by a restrictor - similar to the
way a Needle Valve on your engine
works.
The
right combination of heat and oil will give a
beautiful stream of Smoke.
Not
enough oil will produce too little
Smoke.
Too
much oil will cool the exhaust so the oil can't
reach the smoke point.
 |
A
Flow Restrictor can be as complicated as a
needle valve, or as simple as the one I use,
which is simply a hole drilled in a 3/8" dowel,
with a pinch screw that clamps down on the line.
Some commercial pumps have adjustable flows
built right into them, which can be regulated at
the
transmitter. |
Check Valve
A
Check Valve is needed because a muffler contains
a lot of pressure, and we only want that
pressure to go one way - INTO the muffler, and
not OUT from it.
MUFFLER
Last
but not least, we have the Muffler. This need
not be near as fancy as you might think. All
that is really needed is a way to get the oil
into the exhaust stream. I have tried adding
copper tubing to the inside to "Pre-Heat" the
oil, but in most cases, I have found this
practice to be detrimental to making good
smoke.
Why?
Let's think about it. When you heat the oil with
muffler heat, you are at the same time COOLING
THE MUFFLER! Heat is the key to good smoke; so
don't rob your heat source.
Now,
I'm not saying not to preheat, anything that
makes your system work better is good, I'm just
saying that IF you need to preheat, try using
the engine head instead of the muffler. I have
found that in both of the engines I use a Smoke
System on (YS 91 AC and Zenoah G-62) I don't
need to preheat.
SMOKE MUFFLER?
You
can buy a commercially available smoke muffler,
but in many cases, it's easy enough to customize
the one you are using now. The trick is in
knowing where to introduce the oil. There are
two factors to consider:
- The amount of time the oil will spend in the
heat
- "Bernoulli's Principle"
The
first one is easy, introduce the oil in a place
where it will spend the longest amount of time
in the muffler (I.E. Near the engine's Exhaust
Port)
The
second is not nearly as complicated as it
sounds.
Whether you remember it or not, back then
you were taught about a thing called
"Bernoulli's Principle". In a nutshell, this
states that if a fluid is moving at a high
velocity, it will have low pressure, and if it
travels at low velocity, it will have high
pressure.
So
what does that mean? Take a look at the diagram.
I remember seeing many diagrams like this as a
kid in my Science books in school. It didn't
mean much then, but as I got older, I saw many
practical examples of Bernoulli's Principle in
action.
Simply stated, if you have a pipe that
has wide and narrow sections, the narrow
sections will have lower pressure that the wide
sections (Ever notice how the tank pressure
fitting on your muffler is always located on the
fattest part of the Muffler?).
Now,
let's look at two mufflers; a 2-stroke, and a
4-stroke.
The
taps are placed where they are for two
reasons:
- This position gives the oil the longest
amount of time inside the muffler
- This area has lower pressure that the fat
area, which means your pump won't have to work
so hard.
For a
gasser, you need to get a little more creative,
but it's still not tough. All you need to do is
drill a hole and insert a piece of copper tubing
So that it ends right at the exhaust port. If
you can open the muffler, it's a lot easier, if
not, you can do it with the muffler removed from
the engine so you can see the tubing through the
exhaust port (Or just take a guess if you're
feeling lucky). Once in place, hold the tubing
secure with JB Weld.
|