Aviation tachometer help

[DiscoDan]

I found this Jaeger tach in an old barn. From what I can find, it is a vintage WW1 military aviation tach, possibly used in SPAD aircraft. I also found an online article showing similar tachs in a Voisin airplane. My question is about how this thing works. I spun it up with a drill and the hand went around twice and stopped and then reset to zero and then you could hear the mechanism clicking and then it stopped after about 30 seconds. It seemed to operate similar to the way a handheld machinery tachometer works. In fact, the end of the input shaft has a rubber tip on it like a Machinery tachometer. Is anyone familiar with how this thing works and is it working correctly?

 

[woodchucker]

Well, you may have over revved it. I am sure there was a gear to reduce the rotations .

That clicking was where it maxed out and had to probably jump a gear..

I don't think it's any different than an old car speedometer.

 

[RJSakowski]

IIRC, the analog automotive speedometers used a rotating magnet driven by a fear in the transmission via a cable. The rotating magnet created a torque proportional to the rotation speed in the dial shaft that was opposed by a hairspring.

 

[RJSakowski]

Jaeger Chronometric Instruments

 

[Aukai]

I think that radial engines may have kind of low RPM, one of the planes my dad flew was a 2K max deal.

 

[pdentrem]

If it came off a rotary engine, the max rpm was around 1300.

I did play recently with a 1930s speedometer and yes there are magnets and disc to move the needle.