jastu23 Posted July 25, 2015 Report Posted July 25, 2015 Just back from Oshkosh and what a great time! During the trip, I realized the tach time was reading very fast. I did a few tests. At 2.500 RPM (and around 20"), 0.1 on the tach time was anywhere from 5min 5sec to 5min 13sec. At 2,300 RPM, 0.1 tach time was between 5min46sec to 6min. I'm ignorant on this as it relates to the Mooney M20J, but I guess I was surprised. I would have thought 1:1 tach/hobbs would be closer to 2,600, but then again I'm not sure how MP and RPM factor in. Would anyone have any insight, links to information, or suggestions for me? Among others, my concern is that I'm spending extra money doing scheduled maintenance too soon based on this. Thanks! Jason N1137Z Quote
carusoam Posted July 25, 2015 Report Posted July 25, 2015 Jason, Somebody has explained this here before. The question is where and how to find it.... Use the search function, use a couple words.... Tachometer, Hobbs meter will get you close... The standard mechanical tachometer is 1:1 at exactly one particular engine speed(rpm). Unfortunately, it is not an RPM that anyone uses for cruise. Which tach do you have? Did you compare to the rpm sensed from a smart phone to see if it reads correctly? Best regards, -a- Quote
N201MKTurbo Posted July 25, 2015 Report Posted July 25, 2015 Tachs can be ordered to record at different RPMs. If you remove the tach it is probably written on it, if not you will need to look up the part number. Quote
N601RX Posted July 25, 2015 Report Posted July 25, 2015 Some of the models use 2566 rpm as the 1:1 point but I don't think that all do. 1 Quote
peevee Posted July 26, 2015 Report Posted July 26, 2015 Some of the models use 2566 rpm as the 1:1 point but I don't think that all do. No clue where ours is, never cared to check it. Would be nice if it's above 2600 where I cruise. Quote
carusoam Posted July 26, 2015 Report Posted July 26, 2015 PV Are you running incognito for a reason? Quote
jastu23 Posted July 26, 2015 Author Report Posted July 26, 2015 Carusoam Good suggestions. Thank you. My searching hadn't turned up much so I'll try multiple words. I'm not certain the tach I have. Only guessing, but it seems like it was the original shipped with my mooney in 1987. It's in the ballpark of TTAF when I bought it a year ago. I've also not tried the smartphone apps. I'm curious though, if I find it's way off, what then? Overhaul the tach, replace it, and what about aircraft logs? Seems that since it's running fast then all maintenance would have been done early (rather than late) but any other concerns I should deal with? Quote
carusoam Posted July 26, 2015 Report Posted July 26, 2015 Good questions to have with your local mechanic. How off is it? What to do next. Overhaul instrument, swap out cable, that kind of thing... Keep in mind time to OH is based on hours accumulated using the less than perfect hour meter. For part 91 flying, it is not mandatory to OH on the hours shown. Aka, on condition... I am not a mechanic. Just a PP?.. Best regards, -a- Quote
Hank Posted July 26, 2015 Report Posted July 26, 2015 Anthony, While I agree that the resolution to his tach timing should be discussed with his A&P, be aware that not all Mooneys have "the less than perfect hour meter." All I have are the magic odometer numbers on my tach, which drives all periodic maintenance that is not specified by the calendar. Guess I should look for an android RPM app or two or three, just to see which appears more accurate. 1 Quote
N201MKTurbo Posted July 26, 2015 Report Posted July 26, 2015 The hour meter on a mechanical tach cannot be wrong. It is gear driven from the cable. It has no relationship to the indicated RPM. The RPM readout can get an error if the eddy current coupling loses its magnetism or the hairspring changes its spring rate. , but the hour readout just runs an odometer off of the cable through a gear train. Quote
carusoam Posted July 27, 2015 Report Posted July 27, 2015 It is a perfect engine revolution counter/totalizer... Expressed in hours that may be different than the actual time the engine was running... Close enough for what we use it for. The O has an electronic hour meter that counts time based on an oil pressure switch. Jason, the OP, hit on an interesting way to define what a 1/10th of an hour is at different rpms. He can now build a graph of rpm setting vs actual time to click off the 1/10th. It is a linear function requiring only two points. Back out the rpm where 1/10th equals 6:00 minutes... I would bet that 2566 rpm is probably his answer... The problem for this is... His observation shows a range of times at one rpm setting to record a 1/10th of an hour. Of course, MP has nothing to do with what is being counted... MP may have a lot to do with wear. The only problem I encountered with my C's tach, was a worn cable that caused sticking and over speeding oscillations until the needle fell off... It his hard to set the power when the needle fluctuates... Back to the important stuff: Does the RPM gauge read correctly. Mechanics use a calibrated device. My iPhone reference may not be accurate enough...using sound to mathematically back out rpm should work. Using light to count blades passing by over time would be better... Jason, if you are still with me... How well does the hour meter work over larger portions of time? Repeat the experiment over 1/2 hour instead of the original 6:00 minute intervals. This is an attempt to eliminate or minimize the error caused by the human operating the stop watch. How well does you MP gauge work? My C broke the small aluminum line at the weep hole behind the MP gauge. It becomes a barometer when that happens. It shows barometric pressure, no matter what throttle setting is being used... Best regards, -a- Quote
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