dumb tacho question
 

hi all,

if i were to tacho a ducted fan impellor, that has x number of blades, and i use a tacho and set the tacho to read y number of blades, how do i convert the reading properly?

i am confused.

i thought it might be

[reading x (Y/X)]

this appears to be incorrect, but i'm unsure.

the problem is that when the tacho reads a two blade setup, whenever two blades cross the optical sensor, it registers a rev right?, so theoretically a 7 blade impellor registers 3.5 rpm per rev when measuring on the 2 blade setting....? right?...

so 30k revs in this instance would read as 3000 x 3.5 = 10500.

is this correct?.. or is it [reading x (Y/(X-1))] is it only the 6 blades inbetween the 7th?, or does it include the 7th, so effectively being 4 rpm per rev.

it appears to be picking up all the blades on my fan, as the numbers are repeatable.

Tim.

RE: dumb tacho question
 

You paint a white strip on the centre cone about 6mm wide. The tacho is pointed at that, not the blades!

Hope this helps.

RE: dumb tacho question
 

G'day Gray,

You know what, i just came in from the shed after doing just that!

it's gotta make more sense to get confidence in the contrast eh?.... i told you it was a dumb question.

i feel foolish, as i do know of others who do it this way...

the thing is that i was able to successfully tach other rotors using this method.

there is something about the data that doesnt make sense though, as i have three engines of sizes small, medium and large, and two rotor types, and unless it is a function of the rotor load curve versus engine power curve, then the numbers dont make sense.

i'll give it another go tomorrow.

Happy New Year!

Tim.