Checking Propeller RPM

[Zoner]

I suspect my aircraft tach is erroneous. If I point the nose at the ramp lights, which should be 60 hertz (USA), shouldn't the prop appear to stop at multiples of 60? i.e. 600 rpm, 1200 rpm,1800 rpm, etc.

[Onan the Clumsy]

There used to be a unit you could buy that sat on the glare shield and was battery powered. It sent out an IR (?) beam and measured the reflections. You could switch it between two and three blade and it would tell you the rpm accurately.

Try sporty's I guess. It must've ben in AOPA Pilot or Flight Training.

What you suggest sounds right though. What speed does it stop at?

[Genghis the Engineer]

The use of an optical tacho is a standard technique, cockpit tachos often misread very badly.

Any Rotax engine dealer should carry them, or a model aircraft suppliers - aeromodellers use them a lot.

If it is wrong, there's not much you can do to correct the problem, just make a note in the logbooks and placard the correct (indicated) RPM limitations and cruise settings.

Incidentally, if you have a Rotax or similar engine which uses a gearbox, you need to multiply the propeller RPM by the gearbox ratio to get the engine ratio, or it'll never make sense.

G

[Zoner]

Thanks guys, I haven't tried it yet as we are in the middle of the annual. It should be ready for engine run in a few days. I looked up the optical and laser tachs and they seem pricey. One of my pals suggested I contact the local RC guys, as they use them a lot. Another friend suggested I just buy a new tach as they are cheaper than the optical tachs. I think I'll try the ramp lights, just to see if it works!

[FlyingForFun]

Make a note in the logbooks and placard the correct (indicated) RPM

When I was a student PPL at solo stage, I flew one of the club's PA28s solo - the first time I'd ever flown this particular aircraft. I was just doing some circuits, but something didn't seem right. I remembered my instructors words - if something doesn't seem right, the chances are something isn't right, check and re-check everything.

So I re-checked. I had no flaps deployed. The RPM was exactly what it would normally be at this stage. I was flying level. But my speed was much too high. That was when I noticed the sticky label under the tack - "Tach over-reads by 200rpm". D'oh! It was actually underneath a bit of a ridge on the instrument panel, so that anyone taller than about 5'8" wouldn't be able to see it from the "normal" seating position. Everything was fine once I'd taken the error into account... fortunately, I hadn't been doing any dives which would have oversped the engine

If you're going to placard something, make sure the placard is where people can see it!

FFF
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[inbalance]

I used to do lot of formation flying in single engine aircraft. I noticed an effect witch can help you.
looking thrue your turning propeller the propeller of an other aircraft seem to stop when both have the same RPM.