Engine Instrumentation — Page 524, Lesson 614

I want to walk you through engine speed measurement, and I'm going to start with the oldest electrical method still in use on large aircraft — the Electrical Generator System, which is shown in Figure 38.6. Here's the setup. We have a small three-phase generator, called a tacho-generator, that is driven directly by the engine. The output of that generator is fed to an indicator. Inside the indicator we have an asynchronous motor that turns a drag-cup assembly, and that assembly moves a pointer over a scale — mechanically, the same way the older mechanical system did. So the electrical generator replaces the mechanical drive, but the final pointer movement is still a mechanical drag-cup arrangement. Now, what does that pointer actually show? The indicator, in Figure 38.7, can display one of two things. It can show the actual revolutions per minute — though that's not very common — or, much more commonly, it can show the speed as a percentage of maximum engine speed. That's why you'll see it called a percentage tacho-indicator. Let's talk about the terminology you'll see on multi-spool engines. On twin or triple spool engines, the speed of rotation of the high, intermediate, and low pressure compressors can each be displayed. These are termed N3, N2, and N1. The N is the SI symbol for rotational speed. So N1 is the low pressure compressor speed, N2 the intermediate, and N3 the high pressure. And here's a key point for thrust management: N1 and EPR — that's Engine Pressure Ratio — are the parameters used to measure thrust in turbojets. Now, there's a very useful feature called an overspeed pointer, also known as a trailing or limit pointer. It's fitted concentrically with the main pointer — meaning it's mounted on the same axis, behind or around the main pointer. Initially it's positioned at the appropriate maximum rpm graduation on the scale. Here's how it works: if the main pointer exceeds that position, it carries the limit pointer along with it. When the speed is reduced, the limit pointer stays at the maximum speed that was reached. So it records the peak speed for you. To reset it, you apply a separate 28 V DC supply to a solenoid inside the indicator, which pulls the limit pointer back to its starting position. Now, there's a practical limitation. There's always provision on the HP compressor spool — the high pressure spool — for driving a tacho-generator through the high speed gear box. But facilities may not always be available for driving tacho-generators from the intermediate and low pressure compressor shafts. When that's the case, we use a Speed Probe, which is shown in Figures 38.8 and 38.9, and it works to very good effect. Let me explain how the speed probe works. The probe is positioned on the compressor casing, in line with either a phonic wheel — that's a toothed wheel — or, alternatively, in line with the actual fan blades themselves. As the spool rotates, the magnetic flux in the probe, or sensor head, is altered. That change in magnetic flux changes the current flowing in the coil fitted inside the probe. And here's the key relationship: the frequency with which that current changes is directly related to the spool speed. So the frequency is fed to an indicator in the cockpit, and that shows the spool rotational speed. Now, both the tacho-generator and the speed probe can do more than just drive the speed indicator. They can also provide a signal that illuminates a warning lamp on the engine start control panel. This lamp tells the pilot two things: first, that the engine is turning, and second, whether the engine is turning in the correct direction. That direction check is particularly important during engine start, because it's used to inform the pilot when to open the HP Fuel Cock — the high pressure fuel cock. And note this: the lamp is only illuminated during the start cycle. Finally, let me give you the advantages of this system. The big one is the reduction in moving parts required in the engine. And second, a number of separate electrical outputs, additional to those required for speed indication, can be provided — for example, for automatic power control and for flight data acquisition systems. So to tie it together: we have two ways to measure spool speed electrically — the tacho-generator driven through the gearbox, and the speed probe sensing magnetic flux changes from a phonic wheel or fan blades. Both feed a frequency or signal to the cockpit indicator, both can drive the start warning lamp, and both give us the N1, N2, N3 speeds we need for thrust management and engine monitoring.