Engine Instrumentation — Page 524, Lesson 613

I want to walk you through engine rpm measurement, because it's the heartbeat of engine monitoring. The measurement of engine speed is of vital importance, since together with other parameters, accurate control and monitoring of the engine can be achieved. On piston engines it is crankshaft speed that is measured, whilst on gas turbine engines it is the speed of the compressor. The rpm indicator is called a Tachometer, or Tacho for short. There are three basic methods of measuring engine rotational speeds. First, the Mechanical, or Magnetic, Tachometer. Second, the Electrical Generator System, which is called the Tacho Genny. And third, the Inductive Probe System. There are no firm guidelines as to the application of each of the tachometer systems, although engine and aircraft design will dictate which system can be best utilized. Let's look at the Mechanical Tachometer first, because it's the oldest. It is now only found on older piston aircraft. It consists of a Flexible Drive Shaft that is connected to the flight deck Tacho-indicator. The input drive causes a magnet in the indicator to rotate. That magnet rotates inside a copper or aluminium drag-cup. This induces Eddy Currents in the drag-cup, which oppose the magnetic field of the magnet. A torque is established which turns the drag-cup in the same direction as the permanent magnet. A shaft extends from the drag-cup and is connected to a pointer. The turning motion of the pointer is against the tension of a Hairspring, which controls the drag-cup position and hence the position of the pointer. The flexible drive is driven at reduced speed, but true speed will be shown on the indicator. The indicator incorporates compensation devices for change in temperature. Let me make sure the operating principle is clear. The engine turns the flexible drive shaft, which spins a permanent magnet inside a drag-cup. Because the magnet spins, its magnetic field sweeps across the copper or aluminium cup, and that induces eddy currents in the cup. Those eddy currents create their own magnetic field that opposes the magnet's field, and the interaction produces a torque that drags the cup around in the same direction as the magnet. The cup is restrained by the hairspring, so the cup settles at an angle where the spring tension balances the magnetic drag torque. The pointer, connected to the cup through that shaft, then shows a reading proportional to engine speed. The drive may be geared down, but the indicator is calibrated so it still shows true speed. And because eddy currents and magnet strength change with temperature, the indicator has built-in compensation devices to keep the reading accurate across temperature changes. I want to show you the layout of this system, because the geometry matters. Here's the mechanical tacho arrangement. Now, the other two methods — the Electrical Generator System and the Inductive Probe System — we'll take up next, along with how they differ from this mechanical approach.