Most of us are familiar with the light bulb test to check the accuracy of a tach. Just point your tach at any A/C generated light source (USA 60 cycle) and the tach should read 3600 RPM. That's 60 cycles/second X 60 Minutes=3600 RPM.

Now here is the puzzle. If you point your tach at a rotating 2 blade prop, the tach reads one pulse per blade or 2 pulses per rotation. The tach is designed to display on the read-out screen only half of the pulses read or one-third if using a 3 blade prop. If the tach is making these calculations then why doesn't it read only 1800 RPM when doing the light bulb test.

You might think of it another way, if the tach reads 3600 RPM on the bulb test then the RPM indicated when checking a rotating 2 blade prop in sunlight should be only half of that indicated on the read-out or one-third for 3 blades. Of course, we know that is not correct. Puzzle???

We have this discussion quite often in our club with no valid answers. Help us solve this puzzle. . . . . . . . . . . .

Alternating current produces what is essentially two periods of "on" per cycle.

BTW accordding to my screen, my answer to your post was before you even asked the question...that is real brain teaser!~

(Post No. 1) 4/18/2004 12:14:37 PM your question

(Post No. 2) 4/18/2004 10:01:56 PM my answer

wish someone can explain that logic[:@]