Control Line - Power

Power

Control-Line airplanes usually have a power plant of 0.049 cubic inches (0.80 cm3) to 0.60 cubic inches (9.8 cm3), although engines can be as large as .90, or may have electric power. Two-stroke glow engines are most common, but almost any form of model engine has been used, including both pulse jets and turbojets. Control-line models tend to have very high power-to-weight ratios compared to R/C models or full-scale aircraft. The size of the engines and the models are significantly limited by the maximum line length of 70 feet (21 m) used for competition, although very long lines (as much as 150 feet) have been used on rare occasions.

The competition categories that need high power output and speed can turn at very high rotational speeds for a reciprocating engine. A 0.15 cu in (2.5 cm3) engine used in the FAI Speed event may produce as much as 3 hp (2.2 kW) at rotational speeds in the range of 45000 rpm - faster than some full-scale turbojets. The specific output is around 1200 hp/liter which is far in excess of racing motorcycle engines or Formula 1 auto racing engines. Many breakthroughs in two-stroke engine design (both model and motorcycle) can be traced back to C/L speed models, as the small size makes it easy to experiment with new designs at low cost.

Control line models tend to run a varying mix of fuel however 10% nitromethane, 20% Castor Oil and 70% methanol is common. Castor oil is sometimes replaced by synthetics however as control line aircraft typically run at high throttle settings for the entire flight, castor oil generally provides better lubrication and cooling and is considered generally safer for the engine. It is however somewhat viscous and the resulting oil drag can rob some power compared to synthetic oil, and can also lead to "varnishing" of the cylinder. Some older-technology engines commonly used for control line can be very quickly damaged with typical R/C fuels because of low oil content.
Pulse jet models use gasoline, a variety of flammable liquids like acetone, methyl-ethyl-ketone, and other similar fluids. Pulse-jet models are started by applying a continuous spark device (e.g. a "buzzer coil" as used on a Fordson tractor) to a spark plug in the side of the combustion chamber, and then using a bicycle pump or pressurized air to blow air across the fuel injector and into the engine. When a flammable mixture is present in the engine, it will detonate, sending a shock wave down the tail pipe and creating suction at the intake end of the engine, sucking in more fuel/air, and creates another explosion. Once started, the engine becomes hot very quickly and no longer requires the spark. The spark box and air source are disconnected and then model launched as quickly as possible to prevent the tremendous heat generated by the engine from causing the airplane to catch fire. The engine is extremely loud in operation and cannot be muffled, and can be heard for miles under the right conditions.

The propellers used for control-line models are commonly made of wood (usually maple), fiberglass-reinforced plastic, or graphite/kevlar/fiberlass and epoxy. The propeller pitch and diameter are chosen based on the engine size, type of performance desired, and cost. A typical 61-sized piped Stunt engine uses a 3-bladed propellor around 12-13" in diameter and around 4" of pitch, and is usually made of graphite/epoxy. A 20-sized sport model might use an inexpensive 8" diameter, 4" pitch propellor made of fiberglass-reinforced plastic. The graphite stunt propellers are usually made in small production runs or even by hand, and can cost as much as $50. The small GRP sport prop is made by injection molding and may cost as little as $2.

The fuel for the engine is usually held in a metal or plastic fuel tank, shaped so that fuel is drawn from the outside edge of the tank, as the fuel tends to be thrown to the side by centripetal acceleration as the airplane travels in a circle. A "clunk tank" as used in R/C is satisfactory, but dedicated tanks with wedge-shaped cross-sections are frequently used and tend to have better characteristics as the fuel runs out. A tank with a vent on the inner edge, or multiple vents, is usually called a 'suction' tank. The pressure of fuel delivery with a suction tank changes as the fuel runs out, causing the engine mixture ratio to become leaner as the flight proceeeds. Tanks vented to only permit air to enter at the outside edge ("uniflow" tanks) provide constant fuel pressure over the duration of the flight and a constant mixture ratio.

Combat and some speed models use rubber tubing ("bladder" tank), baby pacifiers, or fountain pen ink bladders, inflated with fuel in a veterinary syringe, to hold the fuel under fairly high pressure. The fuel line is pinched off to prevent fuel loss until the engine is started. The high pressure of fuel delivery permits the use of a larger intake on the engine, allowing more air flow than would otherwise be possible, and thus more power. This type of fuel delivery is by far the most steady until the very last bit of fuel runs out.

The carburetion on most control-line engines is a simple fixed-size orifice (venturi) with only a mixture ratio adjustment. The engine can be run over a very wide range of mixtures and adjusting the needle valve can be used to adjust the engine speed over a small range. Once released, the engine runs at a more-or-less constant speed until the fuel runs out, or, if equipped, the fuel-shutoff is activated. Altering the size of the venturi used can be used to adjust the gross power. Two-stroke glow motors can be made to run in a 4-stroke mode where the engine mis-fires on every other stroke, and changes mode of firing based on load of the propeller. A tremendous degree of control over how the engine runs in flight is possible by altering the fuel contents, propeller size, pitch, and pitch distribution, venturi size, compression ratio of the engine, and the length of the tuned exhaust, if used.