Some versions of the TUT programs sense the engine or motor speed.  This is how you mount the bits.

The system uses a Hall sensor mounted on the plane, and a 3mm rare earth magnet mounted in the spinner.  The top picture shows the complete system mounted in a plane (with a partial spinner).

The next three pictures show the Hall effect pickup, glued to a piece of aluminum -- this was glued together with normal 15-minute epoxy glue about five years ago, and is holding up well through perhaps 100 flights.

The final picture is a spinner backplate, with the magnet mounted.  The hole was made with a 1/8" diameter end mill so that it doesn't go all the way through.  Then the magnet was glued in so that it is captured by the epoxy.  This has lasted about as long as the pickup.

Any noticeable increase in vibration as even that little magnet would unbalance the spinner? Could add another 180 deg. opposite to balance out and then divide the count by 2 for correct rpm?

I have never seen one of these on an IC.  What does it hook up to, what does it do and how is it better than a simple Tach?

Ken

Originally I was doing it so that I could record engine speed in flight, and see how it varied as I did maneuvers.

I've got someone who wants to use a TUT to time landing gear retraction and extension; we've decided that the normal sequence will be to extend the gear on a timer, but we want to sense the motor speed and extend the gear if the motor cuts prematurely.

Because the only thing more distressing than an electrical problem that leads to an early landing is an electrical problem that leads to a belly landing.

Now it makes sense. Good idea. I would assume that you would want the gear to extend if the speed drops below some preset RPM for the occasional battery that goes wobbly on you but still keeps the motor running.
My only issue would be what good does extending the gear do me since mine always seem to quit when I am inverted.

Ken