Guess I was asleep in logic and physics class 65 years ago so I have to ask;
When using a prop meter to determine RPM does it matter where on the prop the meter is aimed.
We find that there is a difference in RPM if the meter is at the tip than if midway from tip to center of prop.
Laurence

I dont know for sure, but I do know how hand held tachs work.  And they basically count light pulses between the 2 blades (even though a prop is one blade).

So, if you point it to a wide area of the prop and take a reading, compared to a reading that was taken at a more narrow section of the prop...I could see that there would be a difference in the RPM...since the distance between the two wide areas of the prop would be less than the distance between the area of the thinner section.

I would think you would want to take your readings as close to the outter edge as you can to get a good reading.  I would think that if you take it further in...you would not be accurate for the reasons I stated above.  Also...and this is me thinking without "thinking"...seems that the smaller circumference(sp?) near the center would give a false reading than out near the tip.

Hopefully someone that knows better will chime in and we both can learn something.  I dont really tach my props simply because I have a crappy tach that battery is always dead when I need it!  lol  But when I have used it...I've always took the reading near the end of the prop.

Your tachometer is probably a photocell with some signal conditioning. I'll bet your tach's signal conditioning is having trouble with reading the tips, and showing a lower reading than at midway between tip and hub.

With the engine holding constant RPM, stay behind the prop, with the sun somewhere in front of the prop. Take readings with the tach behind the tip, and several other locations closer to the hub. I'll wager your readings will be consistent, at all locations closer to the hub.

Best wishes,
Dave Olson

Yup, its a photocell that counts the interuptions, not the duration of the interuption. Since the middle of the prop center of the prop and the tip of the prop are turning at the same rate you can tach it anywhere, its not measureing the speed which we know will be greater at the tip because its got a further distance to travel in each revolution.

You may not get a good interuption of the light at the tip but its going the same RPM as the rest of the prop

Actually, the sun at the front isn't the best thing for getting accurate readings. It's not about how much light is coming in. It's about the color contrast that the sensor sees. A bright white board or an overcast sky in front of the plane while you're reading the RPM from behind the prop will give you the most accurate readings. I know this from using a rifle chronograph which works exactly the same way, and that's what you have to do to them to get accurate readings (or any readings at all on some days).