What is in this article?:
- Tom-Thumb turbines power radio-controlled jets
- MODEL TURBOJET ENGINE MANUFACTURERS
MODEL TURBOJET ENGINE MANUFACTURERS
There are dozens of scale jet kits, both with and without engines, available through several manufacturers. There are also a variety of kits for turbojet engines. Model jets with an engine range in price from $5,000 to $15,000. They typically have wingspans of 60 to 80 in. and weigh 18 to 25 lb. The price, size, and necessary expertise tend to restrict RC model jets to the more avid hobbyist.
Custom-designed jets are common, particularly for scale models, and some of these scratchbuilt models are quite advanced. A 1/8-scale B-52 with eight turbojet engines, for example, was recently flown in the UK. And a scale model of a Harrier VTOL jet, complete with a ducted turbofan engine, is currently being developed and tested. Obviously designing and building a model jet is hardly a trivial task. The hobbyists must understand high-G stress, flutter, and other aerodynamic problems.
Model turbine engines are also used to power propeller aircraft, boats, and helicopters by attaching a gearbox to the turbine shaft, like fullsizeturboprop engines. This provides lightweight power at the expense of some fuel economy. For example, the Simjet 1200 TCP turboprop engine produces 7 shaft hp but weighs only 4 lb.
Manufacturers of model jets and engines include:
AMT USA LLC, (304) 375-3777, usamt.com
BVM, (407) 327-6333, bvmjets.com
FTE Inc., (863) 607-6611, franktiano.com/turbines.htm
Jetcat USA LLC, (818) 781-2300, jetcatusa.com
SimJet (Denmark), (45) 86 36 46 67, simjet.com
SWB Turbines, (920) 725-3721, swbturbines.com
Turbojet Technologies (Australia), (61) 089 478 1877, tjt.bz
Wren Turbines Ltd. (UK), (44) 562 467 0260, wren-turbines.com
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.