what rpm is tach time set to?

Salty

Salty

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Salty
I never noticed this before, but in my last cross country, by my stopwatch, my engine ran for 5.5 hours, but my tach time was up more than 6.1 hours.I was running at 2500 for the cruise portion of my flight, which indicates that the tach timer is set to about 2250 rpm, which seems really low, considering that's the very bottom end of the green arc. In fact, it's lower than that because I certainly ran below 2500 for more of the trip than I ran above 2500, counting taxing and approach, etc.
 
Usually 2200-2300 or so.

If you run 2500RPM, your tach will be higher than real time.
 
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I looked into that a few years ago. What I think I remember is 2400 gives you an hour tach time for an hour clock time. I wouldn't bet more than a cheap beer though that I remember right or that what I read was right
 
Bonchie said:
Usually 2200-2300 or so.

If you run 2500RPM, your tach will be higher than real time.
Click to expand...

It depends of the tach. On an aircraft I flew a lot, 2400 yielded 98% of real time. The red line was 2400.
 
A tach hour is a tach hour regardless of Hobb or running of engine hours. So they are unlikely to agree unless the RPM is set to some specific value.

All right, all of that was so I understand the question. I'm sure it varies by engine and what causes the Hobbs to start counting.

I my case I run smsll Contientals, 75 hp on one, 85 hp on the other, and at 2200 RPM tach hours always lag Hobbs hrs, usually Hobbs = 1.2 Tach. So I'd have to run at RPM higher 2200 RPM for them to be equal.
 
most aircraft with fixed pitch props 2250 .
 
So, I was just thinking about this a little while ago. I took a short hop today and my comparison seems to be just the opposite of Salty. My flight today (Cherokee 180, cruise at 2,400) was 1.3 on the hobbs and 1.12 on the tach. I'm on a small grass field so I don't spend much time taxiing or waiting.
 
When I had to replace the tach on my Maule, the Instrument shop asked what RPM I wanted it calibrated for...apparently it can vary. ;)
 
But I'm only supposed to turn 2050 after takeoff ....
 
Great info. Thanks
 
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