Tom-Thumb turbines power radio-controlled jets

Bob Webster
Vice President Upperspace Corp.
Pryor, Okla.

After a half century of R&D, the turbine jet engine is now more powerful, efficient, and relatively quiet. Development has also pushed the envelope in one other dimension: jet engines are getting smaller — a lot smaller. Hobbyists and model enthusiasts now have reliable turbine engines widely available for radio-control (RC) model jets, turboprops, and helicopters.

SIZE AND FUEL
Model turbojet engines are generally 3 to 5 in. in diameter, weigh 2 to 5 lb, and produce between 12 and 45 lb of thrust. And adding gearboxes makes the engines suitable for propeller aircraft and helicopter models, providing excellent power-to-weight ratios. Prices for jet engines range from $2,000 to $4,500 and up, depending on size and accessories.

Like their full-sized counterparts, model engines take air into a combustion chamber using compressor blades. There, fuel burns and the escaping exhaust turns a power turbine and provides thrust. A shaft connects the turbine and the compressor and spins it as well. Manufacturing economics seem to limit model jets to single-stage compressors and power turbines. The power turbines and combustion chambers or "burner cans" are usually cast from heat-resistant Inconel (NiCrFe).

Smaller engines also mean smaller Reynolds numbers, even if airflow is the same. In other words, the air seems thinner or less viscous in smaller engines. To deal with this, the engines employ centrifugal compressors, often using off-the-shelf centrifugal compressor wheels, such as those in automotive turbochargers, rather than axial compressors, such as those found on full-size jets. Lower Reynolds numbers and reduced size also mean turbine blades are fewer and larger, relative to engine diameter.

To produce enough power with such thin air, model engines need to spin at 110,000 to 175,000 rpm, three to five times the rpm of full-size engines. The shaft turns on ceramic hybrid ball bearings — ceramic balls between stainlesssteel sleeves — which can handle such high speeds. Earlier engines used a separate oil reservoir to lubricate the bearings. Recent designs have done away with the extra lubrication system and instead divert a small amount of fuel into the bearings for lubrication. The fuel is a mix of ordinary jet fuel or kerosene and 3 to 5% turbine oil.

Jet engines might seem a bit contradictory when you consider they require compressed air to run, and have to be turning fairly fast to compress air. So jet engines must reach critical rpm before they start running. This takes external power. In earlier model engines, this was done with compressed air from an outside source such as a scuba tank or leaf blower.

Over the past two to three years, however, manufacturers have developed selfstarting systems that consist of an electric starter motor hooked to a small tank of starting fuel such as propane or butane. The motor turns the engine and the starting fuel is ignited. (Propane and butane are more combustible than jet fuel, letting the engine start at a lower rpm.) After the engine reaches a higher rpm and temperature, jet fuel is introduced and the starting fuel is cut off. With many model jets, start-up is fully automatic, controlled by computer, and initiated by remote control.

Fuel in model jets is usually ignited by glow plugs rather than the spark-plug igniters used in full-size engines. A glow plug is similar to a small spark plug, but instead of a gap, a coil of heat-resistant wire turns red-hot when electricity is applied.

ELECTRONICS
Fuel control is critical in full-size jets. Too much fuel at the wrong time can ruin an engine in a matter of seconds. This is also true for models. In most models, fuel flow is electronically controlled based on inputs such as exhaust-gas temperature, rpm, and airspeed. If an engine overheats, fuel flow is reduced.

Exhaust gas temperature (EGT) of model jets ranges from 500 to 700°C, close to that of full-size jets. If EGT rises much beyond this level, it can damage the engine. The Electronic Control Unit, which monitors and adjusts fuel, may take action, such as reducing the throttle setting to 75% of maximum for a period, and then further to 50% if EGT is still not within acceptable levels.

Many hobbyists plug their models into laptop computers or small, ground-support terminals, usually right before flight, to monitor EGT, rpm, throttle position, fuel-pump status, and other operating data. Some manufactures let users adjust engine parameters such as fuel flow, operating limits, and fail-safe settings using these terminals.

Many engines include data-recording modules that let enthusiasts download engine operating data, airspeed, altitude, even GPS data, after each flight. This provides insight into engine and airframe operation, and can help spot engine problems such as overtemp, fuel-pump failure, and electrical anomalies.

Fuel consumption depends on engine size, ranging from around 7 oz/min for engines with 12-lb thrust, to about 12 oz/min for those in the 25 to 30-lb thrust range. While this is reasonably efficient, fuel economy of model jet engines is much less than that of model piston engines. This is not unexpected, as the same is true for full-size engines. Average fuel load for model jets is 1.5 to 3 quarts/10 lb of thrust, with most carrying between 2 and 4 quarts/flight. Fuel tanks are generally made of Kevlar and stored in the fuselage or wings.

With engine thrusts of up to 45 lb, the performance of model jet aircraft is impressive. Many fly at over 200 mph and can accelerate in vertical climbs. As you might guess, it can be a challenge to control a model jet from a fixed location on the ground. At 200 mph, the aircraft can shrink to little more than a dot on the horizon in 10 sec. It's also difficult to land one. Takeoff and landing speeds are about 35 to 45 mph, and stall speeds are in the 30 to 35-mph range. A few hobbyists install drag chutes to shorten landings.

For safety, the Academy of Model Aeronautics, the governing body for most model flying clubs in the U.S., has a 200-mph speed limit, a limit of 45-lb thrust for single engine planes, and requires automatic engine shutoffs if radio communication is lost for 2 sec.

ON THE HORIZON
Model-turbine development is being done not only by manufacturers, but also by individuals and small teams working to expand the envelope. Ewald Schuster, for example, has developed a model-sized turbofan engine, a ductedthrust system for vertical takeoffs and landings, and a small turbojet weighing only 6.5 oz with a 4-lb thrust. Jet engines with multistage compressors and turbines have also been built.

Educational and government institutions are also involved in small turbineengine research. Under a grant with the Defense Advanced Research Projects Agency (DARPA), M-Dot Co. has designed, built, and tested a 1.4-lb thrust turbojet engine the size of a D-cell battery. Naturally, the military is looking at these small motors to power UAVS. And in England, an RC-modeler equipped his mother-in-law's wheelchair with a small jet engine, giving it a top speed of 60 mph. (He uses it to raise money to combat Parkinson disease.)

More serious researchers are leveraging advances in Micro Electro-Mechanical Systems (MEMS) into turbine engines smaller than 10 mm in diameter. At MIT, parts have been constructed and independently tested for turbine engines 4 mm in diameter. These tiny engines, the size of a button, could be used to generate electrical power in the place of batteries.