Let me start with the definition, because everything else hangs off it — Page 512, Lesson 609

I want to walk you through how we measure the power being produced by an engine — specifically, the torque. Let me start with the definition, because everything else hangs off it. Torque, by definition, is a force applied at a distance to a turning point. Think of it as a twisting effort. Now, the book ties this back to the piston engine formula you'd have seen in the Piston Engines chapter. The turning force is the product of the mean effective pressure, which we call P, acting on the area of the piston, which is A, at a distance L — and L is the stroke, which is twice the throw of the crank. So P, L, and A together can be replaced by the single word "torque." Then N represents the number of cylinders, and that stays constant. The only other variable is E, the number of effective power strokes, which is effectively the rpm of the engine. So power can therefore also be expressed as torque multiplied by rpm. That's the key relationship — power equals torque times rpm. Now, how do we actually measure torque? There are two main methods. One uses oil pressure, and the second is an electronic device. And the units of measurement vary from system to system. The indicator gauges may be calibrated to read psi — that's pounds per square inch — or inch or foot pounds, or newton metres, or brake or shaft horsepower, or percentage of maximum. So the same physical quantity can be displayed in several different ways depending on the installation. One important point: torque is measured between the engine and the reduction gearbox. That's the location where we tap into the system. Let me focus on the oil torquemeter system, because that's the one the excerpt develops. It makes use of a phenomenon: axial thrust — that is, movement along the shaft's length — is generated when helically cut gears are used to transfer power from one shaft to another. Helical gears are cut at an angle, so as they mesh and rotate, they tend to push each other along the axis of the shaft. That end thrust can be utilized to provide an indication of the torque output of a turboprop or turboshaft engine. Here's the mechanism. As the gears in the propeller reduction gearbox rotate to drive the propeller, the amount of torque they are transmitting attempts to move them axially. And this axial force is proportional to the torque that is producing it. So more torque means more axial push. But here's the catch — the gears cannot be allowed to move axially, because if they did, the teeth would no longer mesh with each other and the drive would fail. So the axial force has to be counteracted to maintain the gears in alignment. And that's where the oil comes in. The force comes from passing engine oil through a filter — and that's where the excerpt cuts off, but you can see the setup: the oil pressure is what resists the axial thrust, and that resisting pressure becomes our torque indication. The harder the gears push axially, the more oil pressure is needed to hold them in place, and that pressure is what the gauge reads. So to tie it together: torque is force at a distance from a turning point; power is torque times rpm; we measure torque between the engine and the reduction gearbox; and in the oil torquemeter, helical gears generate an axial thrust proportional to torque, which we counteract with oil pressure — and that pressure, read on a gauge calibrated in any of those units, gives us our torque indication.