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.

L-R: Narrow tank used vertically, flat-bottom tank for use over retract nose gear, high-pressure tank for pressurized fuel systems, standard sport square shape, square tank with "bumper" to protect fuel lines, popular space-saving "slant" tank. Center bumper tank comes with fittings installed.

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.
 

Tank's centerline is roughly 1/2 inch below fuel inlet. Usual way to get this height is to place receiver battery under tank, which is tilted slightly downward at rear to ensure all fuel is available for pickup.

   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.   

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.

Although fully padded to protect against fuel foaming, tank is placed nearest fuel-inlet side. Note fuel filters on inlet and muffler pressure line back to tank with wire ties, to keep them all connected. Fuel dot is extended on right side for illustration purposes.