Tach time

[Shadrach]

Since installing the davtron in my yoke, I’ve really noticed the delta between tach time and run time. I taxi at 900rpm, climb at 2700 and cruise at 2500. It looks like the tach is adding about 12%-15% over actual. I’ve noticed this on the last several flights. It would seem to me that a mechanical tach either works or it doesn’t so I’m surprised to see what appears to be an increase. Have any of you made similar comparisons?

[Hank]

I taxi at 1000,  limb at 2700, cruise and descend at 2300 low, 2400 mid and 2500 above ~7000msl. I record tach time and time off ny yoke clock for every flight, the tach is usually at or behind the clock. Let me look for a picture of my little notebook. 

[Hector]

I read somewhere that in our planes the tach time is roughly equivalent to run time at 2500 rpm or so. If you are spending a portion of the flight at 2700 and the rest mostly at 2500 then total tach time may be a little over Run time

 

 

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[Shadrach]

16 minutes ago, Hector said:

I read somewhere that in our planes the tach time is roughly equivalent to run time at 2500 rpm or so. If you are spending a portion of the flight at 2700 and the rest mostly at 2500 then total tach time may be a little over Run time

 

 

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I would think that the taxi time at 900-1000 would offset it more.

[Jim Peace]

My JPI is set to equal real time at 2400 rpm. Not sure if that was done by EDM or my shop. 

[Shadrach]

I don’t know where mine is set, but I don’t like that It’s accelerating the time on my engine and airframe. 

[wcb]

+1 for JPI is set at 2400 and my total Hobbs vs Tach is always a little different accounting for time when the JPI is on but the engine is not running.  A couple weeks ago 3 hr flight Houston to Memphis and on the return trip Memphis to Houston.  Hobbs and Tach were equal on the JPI.  Standard climb and cruise settings.

[PT20J]

Stewart Warner (now Mitchell) made mechanical recording tachometer models calibrated for several cruise RPMs, with 2300 and 2566 being the most common. Mooney used the 2300 RPM model. So, at 2500 RPM it will read 2500/2300 = 8.7% fast. I discovered this when I had my '78 J tach replaced. The electric tach that drives a "Hobbs" meter on my '94 J seems to be set closer to 2500 RPM, but I haven't tried to calibrate it. Maybe someone knows.

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