Reference RPM for tachometer time
 

Would anyone happen to know the reference RPM for the tachometer time in a Cessna 172 (preferably N, R (180 hp) or S, if that makes a difference)? I.e., at what RPM is the tachometer time equal to actual time?

My reason for asking such a bizarre question is that I am trying to work out the cost/nm in a club that charges by tachometer time... :8

Tacho hours
 

To the best of my knowledge a 'tacho hour' is 2300rpm constant per hour but I could be shot down on this.

Often there is a water decal on factory original tachs, within which the reference RPM is given. It could be different for different tachs, as the tachs are manufactured for different engines, with each engine's specific characteristics in mind.

A look at the tach back case behind the panel might yield this information for you.

The other way to find out would be to stopwatch the tach while you're flying the plane. To the right of the ot the tenths digit, you should be able to see the hundreth's "wheel". It may be half black and half white. Experiment with RPM, and find out which RPM causes this wheel to turn at that rate.

Pilot DAR

Thanks! Will try those suggestions and see what I come up with...

Not a bizarre question at all. In fact I just had a discussion with the owner of a maintenance outfit the other day re this. While it may not help you directly as we discussed a C182 his answer was that tach = time at around 2400 RPM.

The AC tach-meters came in about 3 or 4 different flavours.
http://www.aircraftspruce.com/catalo...mech_tachs.php suggests that a C172 may be 2566 RPM gearing, but it would need a bit of investigation.

Reference RPM for tachometer time
 

The Reference RPM for tachometer time is the RPM for Max Rated Power at Sea Level, so its 2700RPM.

172N with Lycoming O-320-H2AD, Max Rated Power is at 2700 RPM
172R (180HP)/S with Lycoming IO-360L2A, Max Rated Power is at 2700 RPM

tp

I have seen many Cessna and Piper examples where the engine's maximum RPM is much higher than the tach's reference RPM. This would appear to be an effort to record the correct time of operation at a realistic cruise power setting, not full power.

Pilot DAR