The principle of how model turbine engines work or any
turbine engine is very simple. They suck in lots of air, compress it,
add fuel to the compressed air, ignite the compressed air/fuel
mixture, and blow it out the exhaust nozzle/s. Basically the exact same sequence that is followed by all internal combustion engines (suck-squeeze-bang-blow). SSBB
What is different however with all turbine engines is this is all happening continuously so there are no cycles in the SSBB sequence where power is not being produced such as in a 4 stroke engine where out of the 4 strokes of the pistion, only one (the bang) stroke is actually producing power. A turbine engine is producing power all the time and is why they are both very powerful, and very thirsty. Not only that, they compress the air to higher levels than conventional internal combustion engines so more fuel can be added to give the same fuel/air mix ratios providing yet more power output.
Igniting this already highly compressed air fuel mixture causes it to expand very quickly and seeing that there is highly compressed air at the front of the engine, this hot expanding air takes the path of least resistance – out the back end of the engine.
As the hot expanding air exits the back of the engine, it is forced by the turbine blades. This causes the turbine to spin and since the turbine blades are connected directly to the compressor blades by means of a shaft – the compressor spins and the whole cycle starts over. In other words the entire process is self sustaining.
The more fuel that is added, the hotter and greater the expansion of gas will be, causing the turbine to turn even quicker, thus sucking in and compressing even more air. The end result is lots of hot compressed expanding air exiting the back of the engine = THRUST!
You might be scratching your head right now thinking... "I understand all that, but how do you get this self sustained process going. If you just add fuel into the combustion chamber and ignite it, the hot expanding air will exit both the front and back of the engine?" That is a great question and you are absolutely right. The engine has to be spinning fast enough to create enough of a pressure differential between the intake and exhaust (higher pressure at the front so the hot expanding air exits out the back and past the turbine (path of least resistance).
For model jet engines, this "pre-spinning" is accomplished by either blowing compressed air into the front of the engine by say a leaf blower, spinning the engine with an external high speed electric starter, or by far the most commonly used method these days - using a permanently installed small electric starting motor that is mounted onto the front of the model turbine engine.
This electric motor uses something called a "Bendix engagement clutch" so the only time it's mechanically "connected" to the output cone on the front of the compressor shaft is when the electric motor is powered up and spinning. The torque inertia from the spinning motor forces the Bendix clutch's engagement rubber friction ring to "wind up" and press/seat against the small aluminum cone on the turbine shaft and then starts spinning the compressor/turbine shaft. Once the turbine starts, the electric starting motor shuts off and the Bendix disengages as there is no more torque intertia to keep it pressed up against the cone. This prevents the small electric motor from spinning with the turbine after it has started which would destroy the small electric starting motor considering the speeds the turbine spins at (upwards of 160,000 RPM).
As you can see from this cut-away view of a Wren turbine engine above, most of today's commercially available model turbine engines are of the centrifugal-flow type.
These engines use a single large centrifugal compressor blade to "throw" accelerating air outwards into the the convergence (compression) zone of the engine. This design is light and provides adequate compression efficiency from a single compressor blade.
Simple & light weight, with very few moving parts compared to an axial-flow compressor with multiple smaller compressor blades to achieve better compression efficiency (at the expense of complexity, cost, and weight gains).
Here is good good write up on the de Havilland Goblin full size centrifugal-flow turbo jet engine. It is a good read if you want to understand more about centrifugal-flow turbines and see how similar the scaled down model variety are (there is a good cut-away photo). Other full size centrifugal-flow turbo jet engines include Rolls-Royce's Derwent and General Electric's Allison J33 which powered popular jet fighters of the day such as the Lockheed P80's, Sabb 21R, and the Fiat G80 to name a few.
The picture below shows a Wren electric start turbo jet engine. The pod sticking out the front of the turbine encases the electric starting motor to get the turbine spinning. Also notice this engine has screening around the intake. This is to protect the model turbine engine from FOD (foreign object debris).
Once the model turbine engine is spinning, only then can fuel be added to the combustion chamber and ignited. Now the next thing to realize is that in order for the fuel to ignite, it has to enter the combustion chamber in a gaseous state, not liquid. This isn’t a problem once the combustion chamber is hot – the liquid jet fuel (kerosene or Jet A) will vaporize as it flows through the combustion tubes and is introduced to the high temperature air. It is however a problem when starting a cold engine.
To solve this cold start issue two methods are currently in use. The first method is to use propane or a propane/isobutane mixture as the starting fuel source. This "starting gas" is already in vapor form (at atmospheric pressure) so a glow plug can ignite the air fuel mixture, this gets the engine started and warmed up.
Once warmed up the propane/isobutane is turned off by the electric solenoid gas valve and the kerosene is introduced as the primary fuel source by turning on the second solenoid fuel valve and fuel pump.
The picture above/right shows the two electrically controlled starting gas and main fuel valves used in an auto start model turbine engine.
Most turbine powered RC aircraft that have auto start systems will carry some of this IsoButane/Propane starting gas onboard in a small 1 to 2 oz light weight aluminum starting gas bottle that is plumbed directly to the starting gas solenoid and may also have an in-line adjustable pressure regulator. You don't need to put much starting gas in. A 10 second fill on my particular heli is all I need for half a dozen starts (that would likely be about a teaspoon or so in volume). Adding more than that is just a waste and it generally also produces a BIG FLAME out the exhaust on start-up due to the increase in pressure.
In the photo above, I'm filling up the on-board starting gas bottle with a standard camp stove/lantern isobutane/propane mix gas bottle. You just get a special fitting for the bottle that hooks to a 3mm dia Festo hose and quick-connect that plugs into the aircraft's starting gas filler port. The gas bottle fitting I use has a simple press valve that you just press on the valve ears and it opens the valve to flow the gas. Others will have a valve you have to turn to open. Regardless of valve type, you have to hold the bottle upside down while filling so the isobutane/propane mix exits the bottle in liquid form; very much the same way a lighter would be filled by holding the lighter butane recharge bottle upside down to flow liquid butane into the lighter.
These isobutane/propane camp fuel bottles are only about $5 bucks, and one will generally last me a couple years and easily 100 starts so it's a very minor expense considering the other costs involved in turbine flight. It's also why I'm perfectly happy with starting gas and just can't justify the hundreds it would cost to convert over to a kerostart system; but that's just me...
other method of starting a model turbine engine is by using what is
known as a "KEROSTART" system. This type of starting system does away
with the starting gas and uses the main fuel source (jet A or kerosene)
to start the engine.
A small ceramic pre-heater/ignitor (Wren calls theirs a Kero-Burner seen to the right) is used to vaporize the liquid kerosene and ignite it by a high voltage electronic sparker or internal glow element.
The two main advantages to kerostart is you don't need to
have starting gas making it a little more convenient and easier (saves that 10 second or so starting gas fill step) and it sounds more realistic. There is no "propane
pop" on startup, but instead the all too familiar "tic, tic, tic," of
the sparker just like you would hear in a full size turbine powered
helicopter followed by the steady spool up of the engine.
Most model turbine engine manufacturers also have kerostart conversion kits. Pictured below is the one Wren offers for most of their starting gas engines.
kit includes a new FADEC, along with a new data terminal, fuel plumbing
hardware, and of course the keroburner, which is installed where the
glow plug used to be.
The draw backs to kerostart is does cost more than the starting gas method and in cold weather can be problematic to get started but they are improving. The ceramic heating element (kero-burner/igniter) can also fail and it costs much more to replace than a $10.00 glow plug. Both systems work well so it depends on what you deem more important. Saving a few bucks and dealing with some minor starting gas hassles, or having the convenience and ease of kerostart along with a more realistic sounding startup but perhaps run into a few starting issues when ambient temperatures are lower along with the ocasional costly replacement of a failed kero ignitor/burner).
Here's a video that shows an autostart kerostart sequence... They usually start much quicker than this (10 to 15 seconds), but I'm sure since it's a brand new build, there are some fuel ramp parameters to be tweaked or the lines needed to be primed?
Now when model turbine engines were first introduced about 20 years or so ago, all this switching of fuel sources and getting the engine spinning to a self sustaining speed had to be done manually. Once started, only then would the FADEC or ECU of the day handle the fairly simple task of controlling the fuel pump to adjust the speed of the engine and monitor engine temperature.
Today's model turbine engine ECU's control everything from starting to shut down, making model turbines much more reliable and safe. This type of complete on board starting is called "auto start". It costs more than a manual start system, but makes starting a turbine engine super easy and very safe. Very few if any model turbine engines theses days use manual start systems.
Now the next hurdle is getting all this model jet engine power to spin the main and tail rotor blades on our RC heli – our next topic .
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