Ok..
This question has been asked before in various different ways, but the answers have always confused me even more!
So let me try to ask it my way
The tacho is in the centre of the rpm guage. The rpm guage refers to the rpm of the prop (not the engine). Therefore am I correct in saying that the tacho effectively counts revolutions of the prop?
Let me clarify this with an example.
If the prop is set at 2,400 rpm, and it continues at this setting for 1 hour, then it will make 2,400*60=144,000 revolutions in 1 hour. My (ignorant) belief is that the tacho is callibrated to record "x" number of revolutions of the prop as being 1 hour. Lets say 150,000 prop revolutions is calibrated as 1 hour. So in the case of a prop set at 2,400rpm, then 1 hour later the tacho would have recorded 144,000/150,000*1 hour = 0.96 minutes.
Is this understanding correct, or am I completely wrong? If I'm correct, then the MP in no way affects the tacho reading.
For the sake of clarity, I'm only thinking of cruse settings, and not the low rpms that might be used on the ground while taxiing.
Thanks
dp