Dynon HDX Hobbs and Tach time

[wcmorgan]

I expect the Hobb's to be somewhat higher but this seems to be excessive.
My current Hobb's time is: 315.9
My current Tach time is: 243.2
The difference is: 72.7
Can you explain this disproportionate difference?

 

[Bob Y]

Yes. See page 7-78 of the Skyview Installation Manual. It depends on your cruise rpm.

 

[Langt]

I expect the Hobb's to be somewhat higher but this seems to be excessive.
My current Hobb's time is: 315.9
My current Tach time is: 243.2
The difference is: 72.7
Can you explain this disproportionate difference?

Besides the equation, perhaps you (like me) often loaf around at lower power settings, or you might have an intake restriction or limited throttle travel that prevents full power. You also might want to check the reference value your system is using for "cruise rpm". I originally had mine set to the engine manufacturer maximum rpm, but my prop does not quite allow the engine to get there even at full power. These all lead to tach time being significantly less than Hobbs. My understanding is that this reference value should be set to a value like the rpm at 75% power level cruise. Perhaps there is a standard that others can cite.

 

[ve0kog]

My current Hobb's time is: 315.9
My current Tach time is: 243.2

1.3x factor for Hobbs time seems perfectly within normal range. It can be lower if the plane spends a lot of time on long XCs

I have 514.2 Tack and 662.4 hobbs.

 

[scrollF4]

Like they said. For perspective, I do a lot of formation flying (plenty of throttle jockeying). Here are my numbers as of yesterday:

 

[dbhill916]

IIRC, Lycoming engines had their mechanical tachs geared such that 1 hour at 2400 RPM would advance the device by 1.0 hours. The explanation I was given is that was the typical cruise RPM.

My Cirrus has an even odder quirk: apparently idling the engine doesn’t count. The Hobbs meter only gets power when the tach > 1700 RPM. Weird.

 

[NORDO Industries LLC]

Tach time is functionally irrelevant for Rotax engines. Per their MML, Hobbs is what everything needs to be based off of.