Instead of a fanciful state-of-the-art $200 optical heli tach, I got this tach, because its really a budget tach. Its a Hobbico Mini -Tach for planes. Its suppose to be only accurate to +-100, but what I found is that it actually is accurate to 1, yes, right to that single rpm. A simple test is to just tach your ceiling lamp. It should give you the frequency of your A.C. current supply multiplied by 60. Another test is to match it against governors of known rpm like GV-1.
So is this a CHEAP GOOD tach. No, those two words are oxymorons. The reason why this thing is so god damn accurate and cheap is because:
1. It takes a while to get a stablilsed reading and since plank engines rarely have a stable andconsistent rpm, its never accurate for plank rpm.
2. It need to be close to the spinning blades, i.e. you can't hover it and get someone to tach the heli from far. 30cm is probably near the max.
3. Ample sunlight is required to work its magic and you can't use it under any form of street light or electricity powered lamp, basically, what this means is SUNLIGHT ONLY.
As such, the intelligent(cheapskate) me divised a method to tach the heli with this tach and ZERO additional cost- using your blade holder and lead acid battery. The blade holder holds the tach straight while the 12v battery serves as a weight to keep the tach stable.
What I do is place the heli on the ground with the tach under the rotor disc and switch to idle up in zero pitch and let the governor do its job. I found that rpm to be accurate to the dot but there is a few seconds lag between rpm change and reading change, so you need to wait a few seconds.
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