A Jet Powered RC Helicopter - Part 1 Turbine RC helicopters are RC helicopters powered by jet engines. Nothing quite compares to seeing one of these monsters power up at the flight field, hearing the jet engine come up to speed and start to spin the rotors. Although they’re not very common, mostly due to the enormous cost of running a model jet engine, RC turbine helicopters are some of the coolest machines around. In part 1 of this article, we’ll look at how model jet engines work in general, and then move on to their application to RC helicopters. How RC Helicopter Jet Engines Work Cutaway Diagram of A Jet Engine So how to jet engines work? They’re not actually that complicated, its the precision machining and temperature tolerances that make them expensive. On the most basic level, RC helicopter jet engines work the same way that full size jet engines do. Air enters the jet through an intake, is compressed and mixed with fuel, and then ignited, producing thrust and spinning a turbine. The turbine is connected to the compressor via a shaft, starting the process over again. Air is compressed in the engine because it increases the thrust obtained by burning the fuel air mixture. The end result of this whole process is a lot of hot air travelling out the back of the jet engine at a great speed, which results in thrust. How does this process get started? After all, if the fuel / air mixture is burnt in the engine, shouldn’t the resulting air flow be able to go out the front and the back? The answer is simple: the jet engine compressor has to be spinning at a speed high enough to create enough pressure at the front of the engine to force the airflow out the back. In order to do this, the engine has to be spun up before starting. This is done by either blowing hot air into the front of the engine with a leaf blower, or spinning it up with a high speed electric starter. In some cases, jet engines have these electric starters built in. There’s only one more problem: in order to combust, the fuel must be in a gaseous state. Kerosene, the most commonly used jet fuel, is liquid at room temperature. To solve this, propane is used as fuel when starting the engine. Propane is a gas at room temperature, so it can enter the engine and combust right away. Once the engine is hot enough, the propane is turned off and the kerosene introduced. Once started, we need some way to control the jet engines thrust. This requires precision control of the amount of fuel entering the engine, handled by a device called the ECU – engine control unit. The ECU is just a small computer that controls and monitors various engine functions, including: the turbine speed, usually measured in RPM (rotations per minute) the throttle command from the transmitter the fuel pump’s battery voltage the exhaust gas temperature (EGT) the total engine run time Jet ECUs usually have a number of advanced features and fail safes built in for safety. Learn About Turbine RC Helicopters – Jet Power! Part 2 A Two Stage RC Helicopter Jet Turbine - The Most Common Design we’ve shown you how RC helicopter jet engines work, but how do you use them to power RC helicopters? After all, jet turbines spin at extremely high speeds. Converting this rotation to a rotation of the main rotor blades isn’t very simple, and we need some cool technology to do the job. There are basically only two ways that an RC helicopter can use the high speed motion of a jet engine to power the rotors – here’s how they work: RC Helicopter Direct Drive Turbines RC Helicopter jet engines spin their axel at far to great a speed to directly hook up to the rotors, so the most obvious way to slow them down is a gear box. In the direct drive design, the turbine shaft is directly connected to a gear box, which slows down it’s rotation speed and powers the main rotors. This is the most obvious and simple design, but it has a few problems that have caused another, more sophisticated one to gain popularity. For example: All the thrust produced by the jet engine is wasted. Because the only work done by the engine comes from it’s main turbine shaft, all the thrust from the engine is left unused. The thrust from the engine is usually channelled outside the RC helicopter. The wasted thrust can lead to unwanted motion. RC jet engines produce a lot of thrust, and forcing it outside the RC helicopter creates unwanted forces that have to be cancelled out. Direct drive jet RC helicopters are very inefficient. Only 20% of the total power produced by the rotor blades actually goes into turning the rotor blades. With all the problems present in direct drive turbines, most jet RC helicopter enthusiasts are turning to another design: two stage turbines. RC Helicopter Two Stage Turbines Two stage turbines use the thrust produced by the engine, instead of simply venting it as exhaust. Recall that a jet engine has a main turbine shaft, which is used to compress air and keep the engine running. A two stage turbine leaves the first shaft free to spin, and then adds an additional turbine to the end of the engine, where the exhaust is emitted and thrust is produced. The second turbine spins another shaft, which then drives the gearbox that powers the main rotors. What’s the benefit of using two turbines instead of just one? There are actually quite a few: A two stage jet engine is about 80% more efficient than the direct drive design. Less thrust is wasted, so there are fewer unwanted forces caused by venting it. The engine has to carry less load. Overall, it’s better if the jet engine doesn’t have to directly power anything. Harnessing the thrust produced by the engine reduces vibrations and wear. The only problems with the two stage design are that two stage turbines tend to be heavier than their direct drive counterparts, and they’re also a little more expensive.
Posted on: Sat, 10 Aug 2013 19:52:17 +0000
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