Flying Model Aircraft Power sources

Powered models contain an onboard powerplant to propel the aircraft through the air. The model is usually powered by an electric motor or small piston engine, but other types of propulsion include rockets, small turbines, pulsejets, compressed gas engines and twisted rubber bands.

Rubber Band
An old method of powering free flight models is Alphonse Pénaud's elastic motor, essentially a long rubber band that is wound up prior to flight. It is the most widely used powerplant for model aircraft, found on everything from children's toys to serious competition models. The elastic motor offers extreme simplicity and survivability, but suffers from limited running time, an exponential reduction of thrust over the motor's operational cycle, and it places substantial stress on the fuselage. Even so, a competitive model can achieve flights of nearly 1 hour.

Stored compressed gas (CO2), similar to filling a balloon and then releasing it, also powers simple models. A more sophisticated use of compressed CO2 is to power a piston expansion engine, which can turn a large, high pitch prop. These engines can incorporate speed controls and multiple cylinders, and are capable of powering lightweight scale radio control aircraft. Gasparin and Modella are two recent makers of CO2 engines. CO2, like rubber, is known as "cold" power because it becomes cooler when running, rather than hotter as combustion engines and batteries do.

Steam, which is even older than rubber power, and like rubber, contributed much to aviation history, is now rarely used. In 1848, John Stringfellow flew a steam-powered model, in Chard, Somerset, England. Hiram Stevens Maxim later showed that steam can even lift a man into the air. Samuel Pierpont Langley built steam as well as internal combustion models that made long flights.)

Internal combustion
An internal combustion powered model aircraft.For larger and heavier models, the most popular powerplant is the glow engine, a form of internal combustion engine. Glow-engines appear similar to small gasoline motorcycle-engines, but glow-engines are considerably simpler in operation. The simplest (and cheapest) glow-engines being a two-stroke cycle engine, using a glow plug to ignite fuel, which is a mixture of slow burning methanol, nitromethane, and lubricant (castor oil or synthetic oil.) Initial ignition is from an external electric current but once reciprocating, the engine heat, pressure and catalizing action of a platinum metal coil in the glow plug are sufficient to keep igniting the fuel. The four stroke glow engines (see below) with alternate intake and exhaust cycles also rely on the same fuel and ignition system. The reciprocating action of the cylinders applies torque to a crankshaft, which is the power-output of the engine. Vendors of model engines rate size in terms of engine displacement. Common sizes range from as small as 0.01 cubic inch (in3) to over 1.0 in3 (0.16 cc–16 cc). Under ideal conditions, the smallest .01 engines can turn a 3.5" (9 cm) propeller at speeds over 30,000 rpm, while the typical larger (.40-.60 cubic inch) engine will turn at 10-14,000 rpm.

Jet and rocket
Miniature jet turbinedevelopment is the use of small jet turbine engines in hobbyist models, both surface and air. Model-scale turbines resemble simplified versions of turbojet engines found on commercial aircraft, but are in fact new designs (not based upon scaled-down commercial jet engines.) The first hobbyist-developed turbine was developed and flown in the 1980s by Gerald Jackman in England, but only recently has commercial production made turbines readily-available for purchase. Turbines require specialized design and precision-manufacturing techniques (some designs for model aircraft have been built from recycled turbocharger units from car engines), and consume a mixture of A1 jet fuel and synthetic motorcycle-engine oil. These qualities, and the turbine's high-thrust output, makes owning and operating a turbine-powered aircraft prohibitively expensive for most hobbyists. Jet-powered models attract large crowds at organized events; their authentic sound and high speed make for excellent crowd pleasers.

Pulse jet engines, operating on the same principle as the WW II V-1 flying bomb have also been used. The extremely-noisy pulsejet offers more thrust in a smaller package than a traditional glow-engine, but is not widely used. A popular model was the "Dynajet".

Rocket engines are sometimes used to boost gliders and sailplanes, such as the 1950s model rocket motor called the Jetex engine. Solid fuel pellets were used, ignited by a wick fuse. Flyers mount readily-available model rocket engines to provide a single, short (less than 10 second) burst of power. (US?)government regulations and restrictions initially rendered rocket-propulsion unpopular, even for gliders; now, though, their use is expanding, particularly in scale model rocketry. Self-regulation of the sport and widespread availability of the 'cartridge' motors ensures a future.

Electric power
In electric-powered models, the powerplant is a battery-powered electric motor. Throttle control is achieved through an electronic speed control (ESC), which regulates the motor's output. The first electric models were equipped with DC-brushed motors and rechargeable packs of nickel cadmium (NiCad), giving modest flight times of 5-10 minutes. (A fully-fueled glow-engine system of similar weight and power would likely provide double the flight-time.) Later electric systems used more-efficient brushless DC motors and higher-capacity nickel metal hydride (NiMh) batteries, yielding considerably improved flight times. The recent development of lithium polymer batteries (LiPoly or LiPo) now permits electric flight-times to approach, and in many case surpass that of glow-engines. There is also solar powered flight, which is becoming practical for R/C hobbyists. In June 2005 a new record of 48 hours and 16 minutes was established in California for this class.

Electric-flight was tested on model aircraft in the 1970s, but its high cost prevented widespread adoption until the early 1990s, when falling costs of motors, control systems and, crucially, more practical battery technologies came on the market. Electric-power has made substantial inroads into the park-flyer and 3D-flyer markets. Both markets are characterized by small and lightweight models, where electric-power offers several key advantages over IC: greater efficiency, higher reliability, less maintenance, much less messy and quieter flight. The 3D-flyer especially benefits from the near-instantaneous response of an electric-motor. As the size of a model aircraft increases, the cost of electric-flight increases much more rapidly than traditional glow-engine flight. As of 2005, an electric-flight conversion for mid-large scale-models (above 0.60in3(10cc) glow-engine) is prohibitively expensive (greater than 400 USD.) Most such models remain powered by the venerable glow-engine, as their pilots prefer the sound and smell of a genuine 2 or 4-stroke IC-engine.

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