FADEC is an acronym that stands for ”Full Authority Digital Engine Control”. The term is used in the real and
model turbine engine
world to describe the engine control unit or ECU as many people might call it.
The F-A-D-E-C is a small computer that monitors and
controls various functions on the model jet engine to start it, keep it
running safely and efficiently, and shut it down.
As was already
discussed in the
how a model turbine works
section, we know a turbine engine’s speed is regulated by the amount of
fuel it receives. Therefore, the FADEC’s main responsibility is
controlling and regulating the fuel pump giving us safe and controllable
If you want an auto start model turbine engine, your FADEC
computer will have to control the startup and shut down sequences as
well. There is fun example of how this works further down this page.
In order to perform these responsibilities, the ECU monitors
various engine functions to ensure the engine is always running within
the user programmed parameters. It will also detect any problems with
the engine or corrupted radio signals from the transmitter to perform a
failsafe shut down procedure. This failsafe function is explained in
greater detail in the
model turbine safety
There are five basic functions the FADEC monitors:
The throttle command and position from the RC
receiver – it plugs into your throttle output channel on your receiver
just like a speed controller
Turbine engine speed or RPM
Fuel pump battery voltage
Exhaust gas temperature (EGT)
Engine run time
These measurements can be viewed in real time by
plugging a laptop or a hand held data terminal into the FADEC output
The hand held data terminal is small enough to fit in your
RC field box
and allows a field friendly way to readout engine data and program
various ECU parameters to customize the ECU for different model turbine
engines. You can also program the unit with the laptop.
These parameters include:
Position of the throttle and trim settings at off and idle
Start up minimum temperature
Electric starter motor type and sustain speed
Glow plug power start up/shut down sequence
Propane/Butane valve sequences
Fuel pump start up temperature point
Fuel pump start up ramp to idle
Electric starter motor shut down speed
Idle RPM speed
Speed of acceleration and deceleration (spool up and down times)
Position of the throttle control at maximum power
Full power RPM /maximum engine speed
Maximum exhaust gas temperature
Position of the throttle and trim control for shutdown
Stop or shutdown RPM
FADEC Sequence of Operation Example
To get a better understanding of how a FADEC works and make this a little more fun, let’s pretend we are a model jet engine ECU, and go through a cold auto start sequence.
We first get told by the receiver that the pilot
has just given a start up command sequence on his/her radio. We check
the engine temperature to confirm it is not too hot or too cold and
start our stop watch.
We turn on the electric starter motor to get the
engine (compressor and turbine) spinning to create air flow though the turbine.
We keep checking the engine speed – once up to the minimum start
speed, we power up the glow plug and turn the propane and/or butane
valve/s on – time still looks good. If this is a kerostart turbine engine; we open the electric fuel valve, power the the fuel pump at a very low delivery rate to provide just enough fuel for the staring sequence, power the fuel vaporizer/heater, and electronic sparker.
We now watch the exhaust gas temperature very carefully and the turbine speed.
Once the temperature starts rising and the speed
starts increasing, we know combustion is taking place the turbine is starting to run under its own power –
we turn off the electric starter motor and power to the glow plug. Check
the watch, the time it has taken to get to this point and spool up time
is within tolerances – let’s keep going.
We continue watching the exhaust temperature rise and the engine speed.
Once the temperature is high enough to vaporize
the kerosene, we start up the fuel pump and start feeding kerosene into
the engine and turn off the propane/butane valve/s or turn off the fuel vaporizer on a kerostart equipped engine.
Now we watch the model jet engine speed very
carefully and adjust the fuel pump output voltage to stabilize our
engine speed at about 50,000 RPM – hooray we are at idle. We check our
stop watch, about 12 seconds have gone by since we got the command to
start – an eternity for an ECU, but just about right for a model turbine engine.
We have to keep checking the speed and
temperature several hundred times a second to make sure everything is
running as it should according to our programming/algorithm and continue making
small corrections to the fuel pump voltage to keep things running safely
and smoothly. We also have to keep an eye on that throttle signal from
the receiver in case "jet jock" has enough courage today to bring the
throttle up and actually fly, or if he gets yelled at by the wife or neighbors
to turn that noisy, stinky thing off and gives us a shut down command.
Not the most exciting life is it, but it demonstrates what the FADEC does and just how important it is.
This is a very simple explanation but it's sort of a fun way to look at model jet engine operation from the prospective of
the ECU. It should also give you a much better understanding of how a
RC turbine engine works and whet your RC turbine helicopter appetite.
The video below demonstrates a FADEC controlled auto start sequence.
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