two engines that are nearly identical in power and responsiveness, we must be able to regulate the engines independently so that their output (torque) can be identical. On the older helicopters that I worked on, this was done by mechanical linkage adjustments and a droop compensatory. Because you are trying to control two engines with one control (collective stick), you must have a mechanism gives each engine just the input it needs to accelerate appropriately. It is a fact that an older, worn engine, requires higher N1 RPMs to produce a certain power level while a newer, stronger engine can produce the same power at a lower N1 RPM. This means that, theoretically, with the same movement of the fuel control lever to each engine, the stronger, more responsive engine will make power faster than the older, worn engine. Again, torque splits. Now, considering that our controls to the engines are made up of rods, bell-cranks, adjustable “Lollypops” and levers, we can play with the adjustments of these mechanical devices to get just the amount of “throw” on a fuel control that is needed. The longer the throw on a lever, the longer it takes to move the same amount of degrees at the axis compared to a shorter lever. Do we all agree? Combined with the electro-mechanical trim motors in the linkage, the pilot can usually fi nd a comfortable setting for the engines to fl y together. This is great — once the pilot is in cruise mode, he or she doesn’t have to do much. It’s when the pilot is in take-off or landing mode, when he or she is doing all of his collect inputs, that things get busy. This condition not only makes the pilots job more diffi cult; it makes the job of the mechanic much more diffi cult because we need to make all of this work at all settings. What’s good at a hover may not work in cruise, take-off or landing. We must fi nd acceptable settings for all. As I said earlier, manufacturers have come up with a
few diff erent ways to take some of the pressure off of the pilot. Some of the larger, heavier duty helicopters incorporate a combining gearbox that synchronizes the engines for smooth power. I don’t really know much about them because my experience has been with intermediate-sized helicopters. I worked for many, many years on MBB, Eurocopter or Airbus Helicopters such as the BO-105 and the BK117. They have the type of control linkages that I described above. For the last 3-4 years I have been working on the Airbus EC135 that incorporates a FADEC system that I will talk about later. I have just been introduced to the EC145 that uses a control system called a VARTOMS. (Variable Rotor Torque Matching System). Basically, instead of the pilot trying to control the rotor speed and torque matching using aircraft mechanical linkage and trim motors, an Electrical Control Box is in
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HelicopterMaintenanceMagazine.com August | September 2019
charge using the same linkages and trim motors. Before VARTOMS, pilots were monitoring all of the instruments and making adjustments for the conditions they were seeing. The VARTOMS Control Box takes all of the inputs from various aircraft sensors such as: OAT, airspeed, altitude,
torque and trim motor position and
addresses these inputs before the pilot sees them. All right now. It’s time to go high-tech. Enter, the FADEC. (Full Authority Digital Engine Control).
EC135 FADEC Control Computer
EC135 FADEC Control Panel
FADEC Wikipedia Defi nition: The FADEC is a system consist- ing of a digital computer, called an Electronic Engine Controller (EEC) or Engine Control Unit (ECU) and its related accessories that control all aspects of aircraft engine performance. The goal of any engine control system is to allow the engine to perform at maximum effi ciency for any given condition. The FADEC not
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