Piston Engines - Propellers — Page 186, Lesson 241

I want to walk you through the next piece of the propeller story, and it splits into two ideas: synchrophasing and reduction gearing. Let's start with synchrophasing, because it's a refinement of something you already know about. You've already met the idea of synchronising propellers — getting all the engines turning at the same rpm so the noise they generate beats together in a steady, less annoying way. Synchrophasing goes one step further. It's a further significant improvement in noise levels, and it works by ensuring that adjacent propeller tips are separated by some optimum angle. The point is to prevent noisy interference. Think about it this way: if two propellers are turning at the same speed but their blades pass the same point at slightly different moments, the pressure pulses from each blade can either reinforce each other or cancel each other out. By holding the blades at a specific angular separation, you get the quietest result. And here's the practical bit — some aircraft give the pilot a means of manually "fine tuning" this angle. So the pilot can adjust the phasing by ear, or by feel, to find that quietest setting in flight. That's synchrophasing in a nutshell: same speed, but a deliberately chosen angular offset between adjacent propellers. Now, the bigger topic: reduction gearing. Let me set up the problem first, because the purpose of a reduction gear is really a design conflict. A powerful aero-engine needs a large propeller to convert its power into thrust. But here's the catch — if the propeller is too large in diameter, and you rotate it too fast, you run into two specific dangers. The first is sonic compressibility, which is the blade tip approaching the speed of sound and the airflow over it compressing into shockwaves, which destroys efficiency. The second is blade flutter, which is a vibration of the blade itself. So a big propeller cannot be spun fast. But the engine, at its maximum rpm, is spinning fast — far too fast for a large propeller. So the engine cannot be directly connected to the propeller. The drive speed must be reduced to a more suitable level, and that's exactly what the reduction gear does. It sits in the driveline, between the engine and the propshaft, and steps the rpm down. Now let's look at the two main types of reduction gear you'll meet. First, the parallel spur gear. This is mechanically simple and relatively cheap to produce. But it has a real drawback: it takes up a lot of room at the front of the engine, because the axes of the gears are parallel. That means the input shaft and output shaft are offset from each other, so the whole assembly is bulky. It's been used mostly on V-type, in-line, water-cooled engines — the classic examples being the Rolls Royce Merlin and the Griffon. Those are the big V12s from the Second World War era. The second type is the epicyclic reduction gear. This is the one you'll see on most modern installations, and it's quite compact. Its big advantage is the concentric layout — everything rotates about the same centre line. So the input shaft, the gears, and the propshaft all share one axis, which makes the whole package much shorter and neater at the front of the engine. Now, the gears inside an epicyclic unit can be cut in different ways. They may be straight cut, bevelled, or helically cut. And that choice matters, because a helically cut gear imparts a degree of end-thrust — a force pushing along the axis of the shaft. That end-thrust is proportional to the torque passing through to the propeller. And because it's proportional to torque, it can be used to provide a torque indication system in the engine's instrumentation. That's the bridge into our next topic. Let me introduce the torque meter. Its purpose is to give the pilot information about the amount of power he is deploying from his engines during any phase of flight. So it's not just a curiosity — it's a direct readout of how much power you're actually using. The torque meter may be calibrated in torque units such as pounds feet, which is written lb.ft, or newton metres, written Nm. It can also be calibrated in percentage, or in pounds per square inch, psi. Or any other suitable unit of power. So the gauge face can vary, but the job is the same. Now, how does it work? There are two main varieties of torque signalling systems. The first is electronic. Here, the twist of an intermediate drive shaft is measured electronically. That twist is proportional to the transmitted power — the more power you're transmitting, the more the shaft twists. The angle of that twist is measured, and that angle signal is used to drive the torque meter. This electronic system is inherently lighter and more reliable than the other types. The second variety is oil pressure. Here, the end-thrust of a helically cut planet wheel, or the torque reaction of a ring gear, is used to alter the oil pressure of the torque transmission system. That pressure is then read off on the torque meter gauge. So instead of measuring twist electronically, you're converting a mechanical force into an oil pressure, and reading the pressure. Let me walk you through the ring gear system in detail, because the geometry is important. When the engine is running, the pinions — and those are the planet gears — are being driven around the stationary gear by the central input shaft from the engine. So the input shaft drives the planet gears, and they orbit around the stationary gear. Now, the thrust reaction to the pinion's movement will try to rotate the stationary gear backwards. That's the key. The planet gears are pushing against the stationary gear as they orbit, and that push tries to spin the stationary gear in the opposite direction. That reaction force is what you harness. It's proportional to the torque, so by measuring how hard the stationary gear wants to rotate backwards, you get a measure of the torque being transmitted. And that's what drives the oil pressure change that the gauge reads. So to tie it all together: the reduction gear lets you use a big, slow propeller with a fast engine, and the way the gears are cut gives you a free torque signal. The torque meter then turns that signal — whether it's shaft twist measured electronically, or gear reaction measured as oil pressure — into a cockpit readout of the power you're deploying. I've got the figures here that show exactly what I mean. shows the synchrophasing positions — you can see how the adjacent propellers are held at that optimum angular offset. And shows the two types of spur type reduction gear arrangement, so you can see the parallel layout side by side.