TWIN-ENGINE HELICOPTER TORQUE SPLITS OR


A LAZY MULE ON THE WAGON TEAM TERRY PEED | CONTRIBUTING AUTHOR


Ok, I know this is a helicopter magazine and I may be taking this analogy thing a little too far


in my writing, but if it works, I’m all in. I’m going to get into some very basic helicopter technology in this article and I don’t want to lose half of my readers to technical writing induced coma. We know that there is a big diff erence between


airplanes and helicopters. Just look at them. Well, one diff erence that I notice right away is that there are no airplanes that I know of with two engines hooked up to one propeller. I believe the rule of airplane design (and I don’t know for sure) is one engine, one prop — unless you are talking about turbojets, and that is a whole other subject matter. No, I’m talking about an event that can occur on twin-engine helicopters. It’s called engine torque splits. Before I get too far into this, we should start with some basic technology information so that we are on the same page. I’m going to use turbine engines as my example for this discussion. A helicopter gets its lift from the angle of the rotating


blades mounted on the mast of the main transmission. The main transmission gets its drive from the turbine engine attached to it by a main driveshaft. The main driveshaft is attached, through a free-wheeling unit and a reduction gearbox to the N2 (power) turbine shaft and turbine wheels. The N1 compressor and turbine wheels is controlled by the fuel burn and the fuel control to maintain a constant speed of the N2 turbine wheels, no matter the demand put on it. Depending on the power demands put on the main rotor blades via the pilot collective control, through the swashplate, more or less power (torque) is required of the engine. This immediate power demand can put a momentary strain on the engine and cause a loss of rotor RPM (Nr Droop) and also an engine N2 droop. To counter-act this degraded condition, a droop compensator system, of some kind, must be incorporated into the engine control system to add a little fuel to the engine, (much like stepping on the accelerator of your car when you start to climb a hill) so that you can recover the RPMs


that you lost. Depending on the type of helicopter you are working on, this can be accomplished by governors, droop compensators in the mechanical linkage, FADEC (Full authority digital engine control) and even in the case of piston engines, a twist grip on the collective to control the carburetor. All of these systems are pretty simple and work great. Now, let’s talk about the lazy mule on the wagon


team. In the case of a multi-engine helicopter, we are faced with the dilemma of getting the two engines to work together and share the work load of providing equal torque to the main gearbox. Much like the mule team, you may have one lazy mule that is just walking along with the other mule that is doing all of the pulling. Now, the mules may have an agreement that one pulls for a while and the other one takes a break to rest. That’s ok as long as they are both sharing the load. But, if only one mule does all of the work, he will soon wear out and you are going to be stuck with just one mule. It’s the same with a multi-engine helicopter. You


don’t want just one engine pulling all of the torque during fl ight. You want the work load to be shared equally. Now, if you know which mule is being lazy, you can just crack the whip on the ass of the ass (sorry, I couldn’t resist) and it will catch up to the other mule. Usually though, it will jump and overtake the other mule and the result is a transfer of work load. On a helicopter, this diff erence in work load is called a torque-split. Instead of cracking a whip, like the mule skinner, the pilot can beep the trim up on the lazy engine so that it can catch up to the working engine but, like the mule, the slower engine typically overtakes the other and the torque load shifts to the faster responding engine. None of these scenarios are ideal for the longevity of the mules nor for the turbine engines. This is also too much work to put on the pilot because he or she has other things to worry about. The solution to this condition has been addressed


over the years in numerous ways by helicopter manufacturers. Since it is nearly impossible to get


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HelicopterMaintenanceMagazine.com August | September 2019


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