Today I went again flying since it was one of these postcard like FL
   days. Wanted to test if I still had the static like noise in the
   headset after takeoff. I had installed capacitors on the prop wires
   as suggested. On runup and taxi there was no noise. After takeoff it
   came back. Made it to the ocean and turned back since the ATC was
   getting bad transmissions from me.
   Had a head scratching session with an electronics engineer and more
   or less we zeroed on the alternator. Here is the scoop:
   1 On taxi and runup 3,500 rpm before and after landing there is no noise
   2 Activating the prop pitch switch on the Ivo stops the noise when
   pressed
   3 I'm receiving ATC transmissions very clear
   4 Looks like the noise appears around 5-6,500 rpm

   My alternator is from '86-88 RX7 just rebuilt. Noticed that once I
   activate the alt. field switch, the output comes up as it should. But
   later even if I turn it off, it does not affect the output. Looks
   like the alternator feeds the field after that?
   Couple of people suggested that the built-in regulator may be gone bad?
   Any comments?
   Buly



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Questions, not answers...

Hmm...  Why does activating the prop motor quiet the noise if it's the
alternator?  Does the system need load to quiet down?  Does activating
other system loads quiet the noise (landing light, nose gear motor, nav
lights)?

Most internally regulated alternators get the field excitation current
internally.  The sense wire just tells the alternator to turn on, and
likely provides the seed energy to the field to get it start reliably.
Once it's turned on most internally regulated alternators can't be
turned off save stopping the engine or cutting the belt.  That's the
major gripe about them.  There are, of course, exceptions.

Can the engine turn 5000 RPM while on the ground (flat pitch)?

Is the noise conducted to the intercom or radiated to it?  If you power
the intercom from a separate battery pack does the noise go away?  Is
the noise the intercom or the radio?  Does the noise go away when
transmitting with the intercom turned off?

Does a handheld transciever pick up the noise at the same time the panel
mount system does?

Does the prop controller have any circuitry/smarts besides just moving
the pitch setting?  Is it pseudo constant speed in any way?  I am
wondering if an electronic governor circuit is making noise.  Can you
safely pull the breaker on the prop, or otherwise completely
electrically disable it while operating the airplane?

Does the noise come on suddenly when reaching a particular RPM, or does
it steadily get quieter when slowing the engine.

How is the alternator wired to the battery?  Both ground side and B+
side...  How is the engine grounded (to the battery)?

Are you a Nuckolls head?  :)


Matt-


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Copyright 1998-2006 All world wide rights reserved.