HELICOPTER ACCESSORY GEARBOXES ...THEY GET NO RESPECT
There I was, 1,320 feet and a little more than 12 seconds away from getting my coveted trophy. I had 450 throbbing, ear splitting, pavement pounding horses in the form of a 350-cubic-inch “small block” under the hood of my ‘57 Chevy, eager to get me to my destination. I slowly came up to the line, staged and set, waiting for my competition. (A Ford? Ha! No competition for me today!)
Bring the rpm up to 2,100 and hold it ... pull the special hand brake which allows me to keep the rpm constant and prevent the car from moving while slightly disengaging the clutch. The Ford is staged and the Christmas tree begins. Yellow, yellow, yellow, GREEN ... release the break while shoving the gas pedal to the floor and working the now-warm clutch so I get off the line straight and the tires are smoking — now we are going. The tach needle is at the north end of 7,100 rpm and my right hand is applying a little back pressure on the shift handle of my “Muncie” four-speed transmission. Now pull the lever, keep gas pedal to the floor and shove the clutch just enough to suck the tranny into second gear (a speed shift), and then it happened … the horrible, metallic sound of a transmission in excruciating pain. Screaming from below the floor boards, my transmission was hemorrhaging its internal organs and gears, bearings all coming apart in a cacophony of noise akin to 1,000 garbage disposals trying to grind 1,000 knives and forks, all accompanied by my 450 horses protesting in a high rpm pitch.
I rolled to a stop and watched the Ford finish the race. To really rub it in, the win light in his lane shone brightly … expletives deleted. All that horse power was defeated by an over-stressed, over-worked group of gears. It was then that I realized the importance of a transmission. An expensive lesson for sure, but one I haven’t forgotten — a nice segue into today’s article topic, the helicopter accessory gearbox, the most reliable but the least thought about component in the helicopter required to sustain flight. It’s the least thought about until a gearbox failure that causes both the owner’s wallet and the gearbox to have to come out.
Of all the flying machines flitting about the skyline, the helicopter is the only aircraft that relies so heavily upon the proper operation of boxes full of gears:
• The engine accessory gearbox
• The main rotor gearbox (aka main rotor transmission)
• The tail rotor gearbox
Failure in one of any one of these will result, in the best case, to a nice safe auto rotation. In the most catastrophic case, it will result in a complete loss of control of the helicopter an end in a crash.
The engine accessory gearbox is our subject du jour. Since I am most familiar with the accessory gearbox from the ubiquitous RR model 250, we will use this component as our study subject.
The Basic Gearbox
Let’s begin with a basic description as I paraphrase from the Rolls-Royce 250-C20 Series Operation and Maintenance Manual:
The main power and accessory drive gear trains are enclosed in a single magnesium cast gear case.
This gear case serves as the structural support for the engine which means that all engine components consisting of the compressor, the turbine and engine accessories are attached to the case. Three points on the gearbox are used as attachment points to the airframe. A two-stage helical and spur gear set is used to reduce rotational speed from 33,290 rpm at the power turbine (N2) drive, to 6,016 rpm at the output drive spline.
The accessories mounted externally to the gearbox and driven by the N2 gear train are the airframe furnished tachometer-generator and the power turbine speed governor. The accessories mounted externally to the gearbox and driven by the gas producer turbine (N1) at an rpm of 52,000 are the fuel pump, an airframe furnished gas producer tachometer-generator, gas producer fuel control and airframe furnished starter/generator. Internally mounted is the engine oil pump. Finally, mounted on either side of the gearbox are the compressor and turbine assemblies.
Now that we have covered the basics, let’s discuss the vital role the accessory gearbox plays in the total operation of the engine. All the components have a symbiotic relationship between themselves in order to produce power to sustain flight. The gearbox is literally and figuratively the center of this relationship. The compressor must entice the air to enter so it can be put to work. The turbine must use the energy from the air to produce power after it is heated. The accessories gearbox that holds everything together.
Everybody is concerned about foreign object damage (FOD) affecting the compressor, or a “hot start” affecting the turbine. People go to great lengths to mitigate those occurrences. Unless the gearbox complains by generating metal (which is rare), nobody thinks about the gearbox. What exacerbates this line of thinking for the RR 250 engine is that its overhaul life is stated as “on condition.” If this gearbox begins life with all the gears and bearings in alignment with each other and you keep it well lubricated with clean turbine oil, this magnesium box full of gears will happily turn its gears all day long in support of the activities of the compressor and turbine in the helicopter airframe for at least 4,000 flight hours.
Let’s look inside and see how the gears all interact with each other. Using depictions from the Rolls-Royce training manual, we can see there are two styles of gears. There is one set of multiple gears which all interact with each other and have straight-cut teeth. This is the N1 gear train. Then we have the N2 gear train, wherein the major gears are noticeably more robust with gear teeth that have a helical cut. These gears have different jobs within the gearbox.
N1 Gear Train
The gears of the straight-cut N1 gear train are driven by the N1 gas producer turbine assembly and support the N1 operation. This gear set turns the fuel pump, fuel control, oil pump, N1 tach generator and the starter generator. Let’s put this gear train into action. The starter/generator is energized by the pilot. Through the rotation of the starter, the N1 gear train is set into motion. The compressor begins to turn, drawing in ambient air, compressing it and sending this compressed air to the rotating gas producer turbine. Eventually, the N1 turbine extracts the energy from this air after it is heated in the combustion section by flame supported by a liquid kerosene fuel. Along with these major engine components, this gear train is causing the engine N1 accessories (consisting of the fuel pump, fuel control, N1 tach generator and the oil pump) to respond and begin their jobs.
The fuel pump now delivers fuel to the fuel control. The fuel control manages this fuel and sends it to the combustion section fuel nozzle for ignition. The N1 tach generator rotates and sends an electronic signal reporting N1 rpm to the pilot. The oil pump begins distributing oil throughout the engine where lubrication is required. All this happens simultaneously. At a certain N1 rpm, the pilot will introduce fuel into the combustion section and ignition takes place. Now the N1 turbine is powering the compressor and the N1 accessories, and is no longer being driven by the starter. We should also note that the N2 gear train and turbine are not mechanically tied to the N1 gear assembly. How does the N2 gear train join the party? It works independently of but concurrently with N1.
N2 Gear Train
Take a look at the depiction of the N2 gear train and turbine assembly. While the N1 is churning away the N2 section is also slowly joining the party. By design, the N2 turbine wheels are turned by the energy of the expanding hot air delivered to it by the N1 gear train. The purpose of this helical gear train is to convert the kinetic energy produced by the motion of the N2 rotor to usable shaft horse power to sustain helicopter flight. This is why when you watch a RR 250 start, you hear the engine running at a higher rpm which is the N1 assembly, but see the main rotor blade rotation moving slowly at the beginning of the start sequence. Again, referring to our depiction, you can see coupled to the N2 rotor is the helical power train drive gear (pinion gear). This gear is matched to and drives the helical torque-meter gear (whose additional function we will review soon). Geared to and driven by the torque-meter gear is the helical power take off (PTO) gear. The PTO gear, as you will notice, has a straight-cut gear as part of its assembly. This drives the gear shaft, which turns the N2 governor and the N2 tach-generator.
Now let’s talk about the helical torque-meter gear. When you look at the N2 gear train relationship, the torque-meter gear is located between the pinion and the PTO gear and is part of the reduction system, reducing the 33,000 rpm of the pinion gear to a manageable 6,016 rpm for the PTO gear. The pinion gear drives the torque-meter gear and the torque-meter gear drives the PTO gear. The torque-meter gear is subjected to a twisting moment which, when measured, produces a torque value. Without going into great detail, engine oil flows into a chamber within the gear and is evacuated via a calibrated orifice. Because of the helical-design of the gear teeth, the torque-meter gear moves as torque (engine power) increases, allowing the quantity of oil in the chamber to increase (or, as torque dissipates, the oil quantity will decrease). This change in oil quantity is transmitted to a gauge in the cockpit so the pilot has a basic idea of the engine power compared to the turbine outlet temperature (TOT) which is also shown on a cockpit gauge.
We have looked at the gearbox and its contents as a whole. I hope you will have gained more insight into and respect for the importance of the accessory gearbox and its support role regarding overall engine performance. In future articles we will go over the function of the oil system, torque system, and even the fuel system for this series of engines. Time permitting, we will do some troubleshooting along the way.
Oh yeah, I almost forgot — I replaced my Muncie tranny with a new one, and managed to win a couple of races before Uncle Sam called and I sold the car ... but you know, I never did find that Ford again. Oh well.
Mike Broderick has been an A&P technician since 1971. He has worked as a shop and hangar technician, field technician, customer support representative, and owner of a Part 145 engine overhaul facility. His specific experience is in turbo-shaft engines in light to medium helicopters. The one he is most familiar with is the Rolls-Royce (formally Allison) 250 series engines. He is currently employed at H.E.R.O.S Inc. & HYE-Tech Manufacturing LLC. His role within these two companies is VP of business development. H.E.R.O.S Inc. is a Part 145 repair station as a full service Rolls-Royce 250 engine and Honeywell fuel system overhaul agency. HYE-Tech Manufacturing LLC holds more than 300 PMAs for the Rolls-Royce 250 engine and the Honeywell fuel management systems for the 250 engine.