The objective with a helicopter is to keep the main rotor rotating at a more or less constant speed. The main rotor is connected to the free turbine, or the N2 speed. Rotor speed and N2 speed are directly proportional. So, that means the objective will be to keep the N2 speed more or less constant, or within the range dictated by the helicopter manufacturer.
(Step 3) Checking the datum
To do that, it needs to check for a pre-programmed value (called datum). The computer compares actual conditions to the datum and makes a decision as to whether we need to increase or decrease fuel flow. It makes this comparison electronically, and it then sends that signal as to whether it needs to increase or decrease it to the next decision point.
But it's not quite this easy. The datum, in addition to having some pre-programmed parameters, is altered by various operating conditions. For example, the position of the collective pitch, or what we call anticipation, biases that decision. That is to say, we've told the computer we want a certain fuel flow to maintain a specific N2, but if I happen to be moving the collective pitch, that's going to affect the rotational speed — so, I have a collective pitch position signal transmitted electronically to the computer saying "you need to bias that quantity of fuel flow I just asked for based on rotational speed to also make allowances for the load (increased or decreased pitch).
Then, there's an N1 speed signal coming from the other engine, which is compared to the N1 speed on this engine, which makes a load sharing decision as to what needs to be done to fuel flow to keep the N1s matched. Keep in mind that the N1 is directly related to Power on these engines.
So, the computer (DEECU) takes all of these things into consideration, and it makes a decision as to what it wants to do to N1 to get the N2 speed that's required based on these conditions.
N1 is the gas generator speed. So now, we're sending a datum as to how much N1 we want to an N1 datum selection memory in the computer. It then takes this information and compares it to the selected control mode in step 1 to check for compatibility. If these numbers are compatible with the control mode, it passes on the information. This information is called the Raw N1 Datum. In other words, this is what we want N1 to do without taking any limitations into consideration.
(Step 4) Checking the limits
The next step for the computer, is a verification step in the decision process. It checks what the pilot is asking for against limits. It looks at the torque limit and the amount of torque. Also checked is the N1 speed, the control lever position, and it also calculates standard day information to correct for atmospheric conditions. The computer then takes the value that is lowest, the limit, or the raw datum, and passes it on through the logic process. For instance, if what I'm asking for is not as high as the limit, that data will go on through. If what I'm asking for is higher than the limit, it will only let the limit go through.
From this point, the signal is referred to as the "limited N1 signal."
(Step 5) N1 control
The limited N1 signal then goes to N1 control. Remember that the N1 is what needs to be changed in order to control N2. You can think of the N1 control and N2 control as governors, but in this case they are electronic memory. In any case, this N1 control takes this limited N1 signal and compares it to the actual N1 — it checks to see if actual N1 speed is higher or lower than the requested value. This determines if we need more or less fuel flow to increase or decrease the speed of the N1.
(Step 6) Fuel flow selection
Having determined what it needs to do, the N1 control sends a signal to the fuel flow selection. The fuel flow selection is made based on the amount of fuel flow requested, the control mode selected, and whether or not we're starting the engine. In every case it checks the control mode, making sure the selections are compatible with what we're requesting. If we're starting the engine, the DEECU will send a different fuel flow signal than if it's operating. Having verified the amount of fuel flow requested and insuring that it is compatible with the control mode selected, it passes the signal on to the next step.