Torque, by definition, is a force applied at a distance to a turning point — Page 512, Lesson 609

I want to walk you through how we measure the power output of a turbine engine, because it's not as straightforward as it is with a piston engine. Let's start with the key idea: torque. Torque, by definition, is a force applied at a distance to a turning point. Think of it as the twisting effort. Now, if you remember the formula from Piston Engines Chapter 1, the turning force is the product of the mean effective pressure, which we call P, acting on the area of the piston, 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." N represents the number of cylinders, and that stays constant. The only other variable is E, the number of effective power strokes, which is essentially the rpm of the engine. So power can therefore also be expressed as torque multiplied by rpm. That's the fundamental relationship we're working with. Now, there are two main methods employed in measuring the torque of the engine. One uses oil pressure, and the second is an electronic device. The units of measurement vary from system to system. The indicator gauges may be calibrated to read psi, inch or foot pounds, newton metres, brake or shaft horsepower, or percentage of maximum. So you could see any of those on the gauge, depending on the installation. And importantly, torque is measured between the engine and the reduction gearbox. Let's look at the oil torquemeter system first. This system makes use of a phenomenon: axial thrust, which is movement along the shaft, is generated when helically cut gears are used to transfer power from one shaft to another. Figure 38.3 shows how this 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 that they are transmitting attempts to move them axially. This axial force is proportional to the torque that is producing it. So more torque means more axial force. But here's the problem: the gears cannot be allowed to move axially, because this would cause the teeth to 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 force comes from—passing engine oil through a filter. The oil pressure is used to hold the gears in place, and that pressure becomes our torque indication. So to tie it together: we have torque, which is force at a distance, and power equals torque times rpm. We measure torque between the engine and the reduction gearbox, and the oil torquemeter uses the axial thrust from helical gears, counteracted by oil pressure, to give us a reading. That oil pressure is proportional to the torque, and that's how we know the power being produced.