There's a quick and quite accurate way of getting at least a few
accurate tach check points. I've only used in here in the States on
direct drive engines, but with a bit of calculation it should work
anywhere:
1.) Figure out, based on the gear reduction ratio of your engine, the
engine RPMs at which the prop turns a multiple or submultiple of 60
times per second (here in the States) or 50 times per second elsewhere.
E.G. for Europe, 2500 _prop_ rpm, 1250 rpm, etc.
2. Park the airplane, at night, on a ramp illuminated by mercury or
sodium vapor lamps. Failing that, and with the possible ire of your
neighbors, in your driveway with the garage door open and fluorescent
(_not_ incandescent) lights on.
3.) These lights flicker at line freq. If your tach is correct, a prop
blade will appear to "freeze" at or near the correct rpm.
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