A Jet turbine is such an awesome engineering feat, not to mention how cool they look, sound and perform! Being able to have the opportunity to use these engines in our Radio Control models is totally awesome.
A typical jet turbine used for RC is capable of spinning over 100,000RPM in order to produce the thrust force they generate. RPM ranges between about 117,000RPM for a large model turbine and 245,000RPM for a relatively small turbine. Thrust ranges between about 20N (4.5lbs) at the small end to over 220N (49.5lbs) at the larger end of the range. The 220N turbine would weigh less than 2kg (4.5lbs). This kind of performance is incredible!
Fuel Source for a Model Turbine
Turbines in general can run on just about anything. Model jet turbines use either Jet A fuel that you can purchase at an airport, diesel fuel that you can buy at the local gas station or Kerosene that you can buy at several locations locally. There are advantages and disadvantages of each fuel. Primarily, my preference is to use Kerosene because of one factor. Odour! Kerosene is very clean and the odour that it gives off is quite minimal. Since the fuel is used to heat homes/cottages and such, it needs to be not too overbearing in terms odour! The disadvantage is that it costs a lot. About $3 canadian per litre as opposed to deisel and Jet A that is much cheaper. About half the cost or more.
Fuel must be mixed with turbine fuel in order to properly operate the turbine. Mixture is usually between 2.5% and 5%. More on this below under the lubrication heading.
Deep Dive in to the Jet Turbine Inner Workings
The big question is How does one of these model jet engines work? We dive deep in to the model of a turbine to get a better understanding as to how one of these turbines is able to operate. Check out the video for full details:
Turbine Starting Sequence
It all starts from the transmitter. The starting sequence is initiated by performing a sequence of control input specific to the manufacture of the turbine. Typically it involved moving the throttle and throttle trim in a certain manner. From here the ECU (Engine Control Unit) begins the starting sequence.
Firstly, to get the model turbine started, the igniter must be pre-heated. At this point there is a delay to allow for the igniter to warm up. Once the delay is over, the starter motor which is an electric motor on the front of the turbine is used to spin up the turbine to a pre-determined speed. At this point fuel begins to flow in to the turbine usually through a starting fuel line. (Can be seen externally on some model turbines) The fuel for starting ignites and when a certain temperature is reached, the EGR (exhaust gas temperature) sensor reads this and initiates the next part of the starting sequence.
Transfer from Starting Fuel to Main Burner
Once the engine is at the next temperature stage, fuel is then brought in through the main burner. During this part of the sequence, the starter motor is slowly ramping up to the idle RPM as the fuel flow rate is increased. At some point during this ramp up stage, the starting valve is closed and fuel only enters the combustion chamber through the main burner fuel line. When the model turbine ramps up to idle RPM, the automated process is complete and all control is now transferred back to the radio. During the entire starting sequence, the ECU handles everything and the radio does not have control.
At this point you have an idling turbine ready for operation.
Mechanical Workings of an RC Model Jet Turbine
Mechanically, the amount of precision required in these engines is very high. Balance of the compressor wheel, shaft and turbine wheel is very important as the engine spins a significant amount of RPM. Much of the mechanical workings of the engine is discussed in the video with visual of a turbine model.
Lubrication of the Bearings
Lubricating the bearings of a model turbine is a very important function. The fuel that is burned in the turbine contains the lubricating oil for the bearings. unfortunately, all oil used in the fuel is 100% consumed. Oil making its way within the inside of a model turbine is required to find the bearings otherwise there is no lubrication. It is only a small percentage of oil that comes in contact with the bearings, otherwise the rest of the oil simply exits the nozzle of the turbine.
The most important part of the lubricating oil is not the lubrication part. It is actually most important for the lubricating oil to control the temperature of the bearings. Maintaining the proper temperature for the bearings is how we are able to get maximum life out of them before the time to rebuild expires. If the bearings were not correctly cooled during operation of the turbine, lifespan would be drastically reduced.
Internal Combustion Stages in a Turbine
I hate to say it like I did when I was in high school but for the record, it makes me remember the sequence actually quite well. A turbine follows that same internal combustion engine logic that we all know. Suck, Squeeze, Bang, Blow. However, it just does all of this at the same time!
There is a compressor wheel that operates centrifugally to intake the air (suck). The air is then compressed by the wheel (squeeze) as it enters the turbine. From here, the compressed air enters the combustion chamber as it ignites and expands from the heat of the flame front. (Bang) This expanding air can not stay in the combustion chamber. It makes its way to the rear of the engine out of natural easiest route or path since air on the other side is compressed. As the hot gases exit the turbine, they pass over the turbine wheel spooling up the compressor wheel to start the process over again.
Importance of Reliable Operation – Flame Outs
Now just imagine that all of what we have just covered is happening at the exact same time. If the flame were to go out by having a tiny air bubble in the fuel line, the engine quits and needs to be restarted using the entire starting sequence again. Any type of issue that can result in the flame burning out will stop the continuous combustion process. This is known as a flame out and a big part of the reason we need to be able to operate a turbine reliably through its entire on time.
RC Jet Turbine Conclusion
The operation and precision required to operate a turbine is very high. I hope this has given you some insight as to how these marvels work. If you haven’t seen one of these in person, my recommendation is that you check it out. Simpy speaking, they are just impressive.