RPM Cruise

[jethound]

Since Most maintenance is done on Tach time. I find that if I set the tack Cruise RPM setting at lets say 1000 rpm... then the tack time is way up there pass the Hobbs time by about double. .. If I set it near 2000 rpm is is lower than the Hobbs but still faster than a clock flight time.

How do I get this to be accurate to represent an old time tachometer that would record tack time above 1ooo rpm.

I record my logged time by the tach in the aircraft log book so I can set my maintenance schedule. but this is really messing up my log keeping

I will give you one more example.
I had the cruise tach set at 1900 rpm.

I flew from Atlanta to Ft. Pierce in 3.6 hours

The tach time said 4.3 hours.

Now you see the problem.

Thanks
Jack Hunt

 

[dynonsupport]

Jack,
An old tach did not read when above a certain speed. It "normalized" the speed to a certian RPM. It works like an odometer on a speedometer- if you go 30 MPH it would take an hour to read 30 miles, but if you go 60 MPH it only takes half an hour.

Our tach time works this way too- you set your "normal" RPM and we accumulate an hour at this speed. So set it at 2,450 RPM, and we will do exactly an hour per hour at 2,450 RPM. If you're at 1,225 RPM, it will take 2 hours.

A simpler way to think of this- the tach time is actually a measure of how many times the engine has turned over, no matter what RPM you were at.

It's only poorly implemented EFIS systems that just do one hour per hour above X RPM. The whole point of tach RPM is to show wear on the engine, and the wear is clearly a lot more at 2,700 RPM than 2,200 RPM. Our real tach time does this right, while a system that records time above 1,000 RPM is off by quite a bit.

This is covered on page 7-57 of the install guide.

 

[jethound]

I understand what you are saying but during a lot of flight instruction the Tach would normally be pretty accurate regardless if we were doing takeoffs and Landings or up in the air doing airwork. I do not think that the old tachometers were graduated. The hobbs as you know ran on the master switch and generally were about 10 percent above the tach time.

I will set the Tach to 2450 and see what happens after I get to my next oil change. I have been keeping up with the flight time recorder on the engine that you can reset every time and it is really accurate.

thanks
Jack Hunt

 

[dynonsupport]

Tach timer from Wikipedia and other places:

http://en.wikipedia.org/wiki/Tach_Timer
http://www.airlinepilotforums.com/flight-schools-training/4134-tach-time.html
http://studentpilot.com/interact/forum/showthread.php?31139-hobbs-vs-tach-time

You'd be surprised how much it matters. Spend an hour in the pattern and you'll often only rack up 0.6 tach time. I cruise at 2,300 RPM so I only get 0.93 tach hours per hobbs hour, which is why my plane has accumulated 350 hobbs hours but only 300 tach hours.

A good hobbs timer is wired to the oil pressure switch so renters can't just turn off the master and fly for free

 

[jethound]

Sorry to take so long on this but if what you state is true why don't we just set the tach setting at 2700 which is max rpm for the engine, then anything under that will read less.

At that rate my hobbs reads about 130 hours now and my tach time is only 85 hours.

So being a mechanic... do I perform my oil changes on tach time every 50 hours's...Tach time or flight time or hobbs time.

Jack

 

[Dynon]

You could do that (set it high), but most people don't. There's interesting information in both of those numbers. Whether you use tach or hobbs times for maintenance varies by engine, mechanic, and the engine religion you subscribe to. Pilots with Lycomings and similar often use tach time for engine, but on a Rotax, all that counts is the time the engine has run (hobbs).