im new to tachs
 

wont the prop size/distance from the hub affect the reading, or am i wrong? also, how do you tach helis?

RE: im new to tachs
 

no, you are only counting the number of times the prop passes in front of the sensor. size won't make any difference to how it reads. Helis are tached just the same, just make sure to not get the flybar in the the reading also, it will show 2x the true rpm in that case.

RE: im new to tachs
 

Yes, I think it is due to the shadows on the propeller and the direction of the sun/light.
I find that it is more accurate if the sun is shining through the propeller. Having it reflect off of the blands usually gives me 2x the RPM as a reading.