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 materials—not 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, an 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 Loop—an outside loop performed from level, inverted flight—fuel 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 inlet.
    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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