The International Cessna^® 170 Association

Certified, schmertified... it’s not attached to or installed in the airplane. Set it for two blades and point it at a fluorescent light; if it reads in the vicinity of 3600 rpm you’re good to go.

cessna170bdriver wrote:Certified, schmertified... it’s not attached to or installed in the airplane. Set it for two blades and point it at a fluorescent light; if it reads in the vicinity of 3600 rpm you’re good to go.

I believe you are referring to the factor of 60 Hz?

gahorn wrote:

cessna170bdriver wrote:Certified, schmertified... it’s not attached to or installed in the airplane. Set it for two blades and point it at a fluorescent light; if it reads in the vicinity of 3600 rpm you’re good to go.

I believe you are referring to the factor of 60 Hz?

Yes, the 60Hz frequency of the power grid is held to about +/- 0.02Hz, or 1 part in 3000. https://en.m.wikipedia.org/wiki/Utility ... tion_(TEC)

I measured the prop, it is 74" as its supposed to be. I'll get my mechanic to check my tach and cable. Anything else I need to try?

What MIles is referring to is the stroboscopic effect of fluorescent lighting. Fluorescent lighting (true...not LED copies) will behave similarly to engine timing strobes and actually “flicker” at 120 cycles per second (twice each time the AC current reverses in each second in the U.S. standard of 60 cps. Human eyes ordinarily disregard that on/off cycle and smear it into a steady appearance of lighting, but some folks can distinguish occasional “flicker”.)

Anyway....If you operate your engine in the presence of fluorescent light (such as nighttime on a ramp that uses fluorescent lighting) at a multiple RPM of 120 the propeller will appear to “stand still”. Therefore, if your tachometer is reading 2400 and is absolutely accurate, the prop will appear to stop.
Unfortunately, ANY multiple of 120 will make the same appearance, which is why Miles’ reference to 3600 will also work (event tho’ we should not be running that RPM, of course.).

This is a crude gauge tho’, because it becomes subjective as you observe your prop to slowly “advance” or “retreat” forward/backward in any minor difference and you’ll not know the actual amount of error. And there are exceptions where commercial facilities utilize 3-phase outdoor lighting that rids the stroboscopic effect. (Common household AC current is single-phase and will work as described making the prop appear to stop at multiples of 120 cps. This is why commercial machine shops sometimes use 3-phase power ...not only because it’s cheaper to operate heavy electrical equipement on 3-ph, but also because of the danger of rotating machinery appearing to be not in motion thereby offering hazards to operators in noisy environments.)

I was just offering a simple method for checking the calibration of a PropTach.

cessna170bdriver wrote:I was just offering a simple method for checking the calibration of a PropTach.

A-HA! I see the divergence. In my previous post was not discussing a “certified” tach-check... I was meaning a certified airplane. I didn’t realize you were thinking of a way to simply check accuracy of a cheap prop-check. Thanks for the suggestion!

Just an update, I borrowed a friends Tru Tach to make sure my tach is working correctly. It was spot on every time I checked it (idle, static run up, and at cruise in flight). So my tach is working correctly.. Still not sure why it goes to 2700 RPM during runup.. argh!

You mentioned earlier you thought maybe the prop was just “wore out”. That evokes thoughts that imply a fixed pitch prop loses efficiency based upon operating time alone... and that is not correct. While minor erosion will occur with a fixed pitch prop...unless it’s been operated in an abrasive/sand environment and/or improperly modified by an aggressive “dressing” file... that is not what I’d suspect.

As ghostflier suggested, have it inspected by a prop shop for airworthiness for it’s model and type design. Otherwise, it’s possible your tach is still the culprit at higher rpm. A tach can be accurate at lower rpms and “jump” as internals fail at higher rpms. The tach might also be sent to an instrument shop for testing, but a new tach would likely be more cost effective.