Easily 100 RPM. I have found them nearly 200 RPM low. In Canda we are required to check magnetic-drag-type tachs (which is most of them) annually and they must read within 4% of true at a mid-cruise setting. At 2500 RPM 4% is 100 RPM. The magnets in these things weaken with age and so they don't pull the aluminum cup around far enough. The cup is attached to the needle shaft. That magnet generates an eddy current that creates its own field that interacts with the magnet's field.
we have no requirement here in the US.
I know that. I was just making the point that the tachs get old and start under-reading, and I've found enough of them doing that.
Are we still talking about a Cessna 150-G?
It’s my understanding that old tachs over read rather than under. There is a calibrated spring that holds the needle back, and as that spring weakens the tach over reads.
Most often the tach leaks and the oil makes it sticky, then the disks stick
I have found only one tach that over-read, and at least 20 or 25 that under-read.
Same principle. Magnetic eddy-current.
not really
And while it can be calibrated incorrectly either direction, an aging spring will cause that design to read higher than actual.A more common variation of this type of mechanical tachometer uses a magnetic drag cup to move the pointer in the indicator. As the drive shaft turns, it rotates a permanent magnet in a close-tolerance aluminum cup. A shaft attached to the indicating point is attached to the exterior center of the cup. As the magnet is rotated by the engine flex drive cable, its magnetic field cuts through the conductor surrounding it, creating eddy currents in the aluminum cup. This current flow creates its own magnetic field, which interacts with the rotating magnet’s flux field. The result is that the cup tends to rotate, and with it, the indicating pointer. A calibrated restraining spring limits the cup’s rotation to the arc of motion of the pointer across the scale on the instrument face. [Figure 3]
From https://www.aircraftsystemstech.com/2017/05/mechanical-movement-indicators.html
I had one cut open for my students. It was from a 1970s 172. They were all like this.
A more common variation of this type of mechanical tachometer uses a magnetic drag cup to move the pointer in the indicator. As the drive shaft turns, it rotates a permanent magnet in a close-tolerance aluminum cup. A shaft attached to the indicating point is attached to the exterior center of the cup. As the magnet is rotated by the engine flex drive cable, its magnetic field cuts through the conductor surrounding it, creating eddy currents in the aluminum cup. This current flow creates its own magnetic field, which interacts with the rotating magnet’s flux field. The result is that the cup tends to rotate, and with it, the indicating pointer. A calibrated restraining spring limits the cup’s rotation to the arc of motion of the pointer across the scale on the instrument face. [Figure 3]
From https://www.aircraftsystemstech.com/2017/05/mechanical-movement-indicators.html
I had one cut open for my students. It was from a 1970s 172. They were all like this.
To demonstrate eddy currents to the class I used to take a powerful supermagnet that had a slot in it (it was from an old hard drive) and a strip of aluminum that fit through that slot loosely. One could slowly push the strip back and forth through the magnet, but if you tried to speed up it resisted the movement. If I held the magnet so the strip fell vertically though it and just let the strip go, it would move slowly through the magnet then fall rapidy to the floor once clear of it. It's the same principle the NDT techs use when they put their eddy-current probe on an aluminum part to look for cracks.
