What is in this article?:
- Tom-Thumb turbines power radio-controlled jets
- MODEL TURBOJET ENGINE MANUFACTURERS
Vice President Upperspace Corp.
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.