Blade Tacho

[AlexC]

Is there a DIY version similar to the Magic Mirror Helicopter Optical Tachometer?

[bluejets]

Just about everything comes on a phone app these days
https://www.youtube.com/watch?v=OgQD2IWp3Bs

[Martin]

The cheap LCD shutters (aka light valves) you see on the Chinese sites, switch fast enough to make reasonable model helicopter tachos: switching times are about 10 milliseconds, both for opening and closing, but get slower at low temperatures. So they're okay up to about 3000 rpm. For the display readout, you want something easily readable in bright outdoor light, so regular, non-backlit, monochrome LCD displays are best. You can buy all the parts you need for about thirty dollars.
Hacking some of the 3D glasses that were used with 3D televisions (remember those ?) might be a good way to go.

[AlexC]

Thanks Martin,

Are you aware of any projects using an Arduino with the Pi Cell Shutters?

[Martin]

I've not seen any such projects, but I've not searched for any either! I'm tempted to have a go myself, but I've got lots of other projects on the go already...

[AlexC]

Just wondering if they are reliable as the original manufacturer Sky-RC discontinued their design as did HobbyKing and the Turnigy version.

[Martin]

I don't know about the reliability. The LCD shutter is a thin glass panel, so can easily be broken, but should be okay if mounted in a protective enclosure. The glasses for 3D televisions used these things, and so do modern welding masks - and they seem robust enough. You could 3D-print an enclosure and have protective clear plastic sandwiching the LCD shutter (with air gaps). You could use polaroid film for the protective plastic, which might help give a better contrast ratio too.

I suspect it's more to do with a declining market for the devices. Most upmarket electric helicopters are fitted with governor 'constant speed' ESCs, so you don't really need a tacho, and don't need to set up fancy pitch and throttle curves. Not so many pilots fly ICE helis any more, and even those ICE (nitro) helis can be fitted with governors. Those display and log the head speeds, as well as attempting to hold them constant at whatever demand RPM you set.

For ungoverned ICE helis, or just those who want to know what RPM their electric helis are running, the tacho can be useful, but you can't fly and use it at the same time - as a pilot, you need a knowledgable helper to operate the tacho for you while you fly.

And experienced fliers don't really need to know the exact RPM (as long as they're not in danger of overspeeding the head) - they're (usually) just trying to set throttle and pitch curves that keep the RPM fairly constant when flying in a chosen mode. You can hear when the RPM rises or falls - you don't need a tacho to tell you, and with experience you learn when the RPM is too low or too high, just from sound, and the feel of the way the model flies.

I've seen audio tachos used at flying fields - they measure the engine speed just from the exhaust note. Of course, they're affected by doppler when you fly towards or away from yourself, but you can average those errors out. They only work well on calm days, with only one (noisy) model in the air.

[Martin]

It would have more of a market if it worked up to the speeds we run props at on fixed wing and quadcopters, but the cheap LCD shutters aren't fast enough. It's probably easier and cheaper to use a mechanical shutter.
That gives me the idea of using a cheap small 'drone' motor, probably brushless because they hold speed better. Use that to spin a plastic disc with two or more shutter slots inside a little enclosure with holes both sides to sight through. A photodiode could accurately measure the disc speed, and display it on a cheap watch/calculator style LCD. A small ESC to drive the motor, a couple of buttons to alter the speed, a battery and an Arduino to control everything, and you're done. Probably more robust and reliable than an LCD shutter, and easy to trade off contrast and brightness, by just varying the width of the slots in the spinning disc.
Actually, because the Arduino is measuring the speed, and controlling it, a brushed motor would be fine, and you could drive it (hardly any load, because it's only spinning a disc) with a little MOSFET, to save the cost and bulk of the brushless ESC.

[bluejets]

Had a similar mechanical shutter idea some 20 odd years ago when flying r/c choppers.