LAST MONTH I wrote
about four-stroke model engines and compared their many design, but few
operational, differences from two-stroke engines. Throughout this series
I have covered starting, maintaining, and getting the most from your
engine as you run it, but in the real world, model engines require
support equipment to operate. I took this for granted in previous
installments, but this month I'll cover onboard and fueling equipment.
The most basic piece of engine-support equipment is the onboard
fuel tank. Without someplace to store fuel in the aircraft, flight times
tend to be short. Fuel tanks designed for RC models are usually
blow-molded using fuel-resistant synthetic materialsnot metal. Metal
fuel tanks are usually designed for and used in CL aircraft, although
many CL modelers also use "plastic" tanks.
In RC's early days, the metal tanks could sometimes interfere with radio
reception. They can also be dangerous if they come into contact with an
electrical charge from the receiver battery.
Today's RC fuel tanks come in many sizes, styles, shapes, and
construction materials. A photo shows just a few of the options. Most
trainer models use some form of 8- to 16-ounce square tank.
The fuel tank's size depends on the engine's displacement. The .25 cu.
in.-displacement engines use 4- to 6-ounce tanks, .40-size engines use
8- to 11-ounce tanks, and .60-size engines work best with 12- to
16-ounce tanks. Size does matter with fuel tanks. You will see why
shortly, but first there is a concept you need to consider.
In all of my previous engine-theory writings, I treated the fuel as if
it were just waiting there at the carburetor, ready to jump into the
engine's fire to be burned for our modeling pleasure. That is not quite
the way it is. Many forces are at work to help the reluctant fuel flow
into an engine and meet its fate, the most obvious of which is gravity.
Click on photo to view large image with caption
However, gravity is tricky for several reasons. To begin with,
aircraft in flight is its own center of gravity. I am not referring to
the famed CG, but the fact that an aircraft creates its own "gravity"
field whenever it changes direction. Without getting too technical,
Newton's laws of force, momentum, and acceleration are at work.
For instance, in a sharply banked, tight turn, fuel would flow toward
the aircraft's bottom, away from the turn's direction and the engine's
fuel inlet, and not toward the side facing earth's gravity. At the top
of a reverse outside Loopan outside loop performed from level, inverted
flightfuel would flow toward the aircraft's top rather than toward the
earth below its bottom. Again, this would be away from the engine's fuel
If you doubt this concept, hold a cup of water while riding in a light
full-scale aircraft. It is fascinating to see the water stay firmly
inside the cup as the aircraft loops and barrel rolls. (You better make
that wine instead of water; you might want it to calm down after the
maneuvers are over if you are not the pilot.)
But in straight, level flight, the earth's gravity does pull the fuel
toward itself and therefore toward the engine. And most important, the
earth's gravity is fully at work on the ground where we set high-
and low-speed mixtures. These mixture settings stay constant despite the
changes in fuel-flow directions once in flight. Somehow we must include
the effects of a constantly changing "gravity" on fuel flow.
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