I have a tach I use for my fixed wing. I assume it will work for the copter.
But how do you physically do it? Do you clamp the heli down? Does some poor slob get elected to hold it while you're checking?
I've read that clamping is not a good idea as oscillations can shred a chopper. What is the safe way to do this?

Clutchbob,
I spin mine up on the ground and get it so it's just light on the gear but stays down. I just hold the tach right over the blades. Then I pitch it negitive to see what happens. Yes, it makes me a little nervouse. Yeah, there are probley better ways to do it.
As far as clamping it down... don't. I learned the hard way. The gear snapped off and it took control of my living roon and then got smashed in to pieces...

If you were afraid of accidentally taking off, you could program one of the idle-up modes you're not using with a flat pitch curve at a very slightly negative pitch (to keep it on the ground) and a linear throttle curve.

Of course!! Thanks guys, you so smaht.

The SAFE way of measuring head speed on a helicopter is to do it optically.

Get youself one of these:
[link]http://www.modelavionics.com/tach.asp[/link]

You look thru a window and match the headspeed and read the number on the face of the device. I don't even leave my chair at the flying field when my buddy asks me to measure the headspeed on his helis.

Here is a picture:



Rafael

My advice is don't bother. Of what practical use is the data?

No matter what you do, taching in ground effect is not the same as what is happening in clean air. So why bother. Theoretically, you know the rpm per volt of the motor and the gear ratio so if you really have a need to be a rocket scientist, do the math. If not, just go fly!

I have little knowledge on how electric motors really work. I have no clue on whether an electric motor can overrev. Read below for the explanantion on nitro engines.

I like to set-up my Trex to a certain RPM and leave it there. I do this checking the temperature of the motor, battery and speed control. If any one of them gets too hot, I back down the headspeed. Longevity is the name of the game.

I do the same thing on my nitros and gasser. Fuel engines can be overreved and you run the possibility of damaging the internal components of the engine. Understanding where the peak performance of an engine / motor is, will prevent it from damaging itself.

When a helicopter explodes in your face because you were running too high of a headspeed you will understand the use of the data. I know some people are running 3000 rpms on TRex 450's and metal heads. Go too far over the limit on a bigger helicopter's plastic head and you will remember this thread.

Rafael

I have a couple of those Eagle Tree inflight data loggers. They add a little bit of extra weight to the heli, but I think it's worth it. With one of those you can see what happends at every point of your flying. It logs pack voltage and amp draw, and with with some periferals you can log rpm (optical, magnetic or brushless sensor), temp, air speed, height or servo amp draw. togheter with a vid of your flying, you can really learn something about how you treat your heli/battery while actually flying.

I use one of those tachos' as well. I spool up and hold it under the main rotor blades facing up with the pitch at very slightly negative. Since I fly only in governor mode the headspeed should be the same at all times.
I think a tachometer is a must. It's the only reliable way to see what YOUR motor/pinion combination is producing.

Well - Here's a little something that will illustrate exactly what Rafael is talking about - You can see what overspeeding your rotors can really do. All of the damage happened in the air, and was strictly due to excessive head speed.

one way to keep your heli down to check rotor speed in all ranges is to have a metal ruler go over both sides of the skids and have bricks on each side of the ruler to hold the heli down, this will keep the heli from lifting when you want to check the rpms @ points 4 and 5 on the throttle.

Optical tachs are more expensive but You can tach Your heli from a safe distance and they are ACCURATE! !
Another plus is that someone else can tach it from underneath while in hover or just flying around from any angle. You just press the shutter speed button up or down until the blades look like they are stationary.

I bought a 29.99 sensor type and the instructions specifically say that the Sun has to be shining to take an accurate reading from the sensor and You have to be very close. Needless to say, I don't use it.

Gary