Excellent post.
Don't forget those of us with three bladed props. Or four; whatever.
Your point illuminates elegance in thinking.

My son the musician could call RPM from the (musical) pitch.
(BTW he soloed last Monday)

Data are where you notice them; all around.

When the shock wave from the Trinity test passed,
Enrico Fermi dropped a piece of paper,
noted how far it was moved,
and got the yield right to two digits.
Regards
H.
N502TB

BTW What a choice!
Buy both.

"EDR" wrote in message
...
Oh, come on folks!
Poor man's tach check...
go out at night
fire up airplane
taxi to place on airport with mercury vapor/fluorescent light
park with light behind you
adjust engine rpm for 600/1200/1800/2400 rpm
if prop blade is not "standing still", adjust until the blad does not
move (strobe effect)