On Thu, 05 Apr 2007 21:50:24 -0700, TheSmokingGnu
wrote:

How exactly does a synchrophaser work?

On a piston-pounder it is changing the propeller blade angle/engine
RPM via the prop governor. Some use a coil on the slave engine to vary
the spring pressure on the flyweight governor, some use a step motor
and a jack screw to change the OAL of the prop control cable to the
slave governor.

Have worked on turbo-prop and turbojet systems that use the same step
motor and jackscrew assy. On the jets, it varies the length of the
"throttle" cable...

What is it measuring to make its adjustments?

Again, on a piston-pounder there is typically a small pick-up coil on
the base of the governors, or in some cases sensing units inline in
the tachometer shafts.

On one set-up that I am familiar with the units on the tach shafts are
aligned with the propellers in a specific relation ship to each other,
I would assume to reduce "drumming" on the fuselage.

Why does it have to be off for takeoff and landing?

It has the potential to reduce the RPM slightly on the slave engine if
the master has an RPM sag.

Again, on the systems I am familiar with, the total "authority" of the
system is only around 100-150 rpm, so it isn't going to reduce it
much.

What voodoo is contained therein (and what's the difference between a
mechanical and electronic version)?

Have no idea, AFAIK all the box is doing is trying to match the
frequency output of the pickup coil on the master engine to the output
of the slave engine by tweaking the blade angle/RPM of the slave
engine.

TC