"Dave S" wrote in message
nk.net...
Marc J. Zeitlin wrote:


Everybody's got a tach, right? Is that not sufficient?


Believe it or not, NO...

If you get your tach signal from your Engine Controller, and your engine
fails because the electrical system or the engine controller goes kaput,
then it is very likely you will not have a tach signal. (this was the
scenario explicitly discussed in the flyrotary listserv)

How does the traditional a/c engine do it... hall effect on the flywheel?
Direct drive tach cable?

Some kicked around the idea of a rear view mirror someplace.. or an
optical tach of sorts..

Dave

The traditional aircraft engine has a drive on the accessory case, usually
from the oil pump drive gear or some such, that connects to what is
basically a "speedometer" cable that connects to the tach. The tachometer
is basically a "speedometer" calibrated in RPM. It is a small aluminum cup
with a needle on it and a hairspring to keep it from rotating. The cable
connects to a small armature inside the cup with a refrigerator magnet glued
to it. When the magnet turns, the eddy currents induced in the cup cause it
to try to follow the rotation of the magnet. There isn't very much torque
available, although the torque does increase as the speed difference between
the magnet and the cup increases because the magnet induces more current in
the cup if it moves relatively faster. The hairspring essentially works as
a "brake dynamometer" to measure the torque by displaceing the needle. We
lable the needle positions with numbers and calibrate the numbers to
approximate the rpm of the little magnet. Hence a tachometer.

Same meter with numbers and we label and calibrate the numbers to
approximate the mph or kph of a road vehicle by measureing the rpm of some
object in the drive train and call it a speedometer.

I you have the guts to trust one computer with everything and then want to
know something that trivial when it quits working you can buy a simple
optical hand held Tachometer that is powered with a nine volt battery that
reads prop RPM from the reflections from the blades. They are very accurate
since the count blade passages for some unit time and then do the math
digitally to get the prop RPM. They cost about $40 and we use them to check
those cheesy mechanical tachometers when we annual an airplane.

Highflyer
Highflight Aviation Services
Pinckneyville Airport ( PJY )

R. A. H Flyin at Pinckneyville is this weekend. May 19, 20, and 21. The
weather looks good.