Autopilot Porpoises when Mike Keyed

[jls]

Also instrument needles go crazy.

Aircraft is a Lancair with antenna hidden in the horizontal stab. How
'bout directing us to some reading material about how to stop it.

Our local avionics guy says we may have to live with it, or install
external antennas, which will dirty up a Lancair in a hurry.

Jim Weir?

Thanks and happy holidays.

[[email protected]]

jls wrote:
> Also instrument needles go crazy.
>
> Aircraft is a Lancair with antenna hidden in the horizontal stab. How
> 'bout directing us to some reading material about how to stop it.
>
> Our local avionics guy says we may have to live with it, or install
> external antennas, which will dirty up a Lancair in a hurry.
>
> Jim Weir?
>
> Thanks and happy holidays.

Check your com antenna connections. Sounds like a bad cable
shield ground allowing RF to escape and disturb sensitive circuitry.

Dan

[RST Engineering]

Google "ferrite balun". You could have saved us some time telling us a
little bit about what sort of antenna was hidden in the horizontal
stabilizer. Second question is why you put it in the HORIZONTAL stabilizer
when com antennas were meant to live their lives VERTICAL.

Jim




"jls" > wrote in message
ps.com...
> Also instrument needles go crazy.
>
> Aircraft is a Lancair with antenna hidden in the horizontal stab. How
> 'bout directing us to some reading material about how to stop it.
>
> Our local avionics guy says we may have to live with it, or install
> external antennas, which will dirty up a Lancair in a hurry.
>
> Jim Weir?
>
> Thanks and happy holidays.
>

[[email protected][_1_]]

> Aircraft is a Lancair with antenna hidden in the horizontal stab. How
> 'bout directing us to some reading material about how to stop it.

A friend of ours had this exact problem in his SX-300. I helped him
troubleshoot it ("OK, key the mike!" for hours on end...) while he
stood on his head fiddling with antenna wires.

He found that the culprit was the antenna wiring going up into the
tail. Since replacing that wiring would have required opening up the
vertical stab, he found that he could stop it from happening by
shielding it with strategically placed pieces of aluminum foil. He also
minimized it by re-routing the cable, and replacing some of the
connectors that were "leaking".

By doing this he was able to eliminate the porpoising on one of the two
frequencies it was happening on, and he minimized it dramatically on
the other.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

[jls]

wrote:
> > Aircraft is a Lancair with antenna hidden in the horizontal stab. How
> > 'bout directing us to some reading material about how to stop it.
>
> A friend of ours had this exact problem in his SX-300. I helped him
> troubleshoot it ("OK, key the mike!" for hours on end...) while he
> stood on his head fiddling with antenna wires.
>
> He found that the culprit was the antenna wiring going up into the
> tail. Since replacing that wiring would have required opening up the
> vertical stab, he found that he could stop it from happening by
> shielding it with strategically placed pieces of aluminum foil. He also
> minimized it by re-routing the cable, and replacing some of the
> connectors that were "leaking".
>
> By doing this he was able to eliminate the porpoising on one of the two
> frequencies it was happening on, and he minimized it dramatically on
> the other.
> --
> Jay Honeck
> Iowa City, IA
> Pathfinder N56993
> www.AlexisParkInn.com
> "Your Aviation Destination"

OK, men, very helpful info so far, and I'll find out more tomorrow.
Present owner (and hangar-mate) did not build the airplane and now he
finds out more about the antenna tomorrow when the builder sends
pictures and drawings.

With your gracious help we'll solve the problem.

Thanks to all of you.

[Mark Hickey]

"jls" > wrote:

>Also instrument needles go crazy.
>
>Aircraft is a Lancair with antenna hidden in the horizontal stab. How
>'bout directing us to some reading material about how to stop it.
>
>Our local avionics guy says we may have to live with it, or install
>external antennas, which will dirty up a Lancair in a hurry.
>
>Jim Weir?

The easiest thing to try would be to clamp ferrite cores around the
input leads to the AP computer (even better if there's enough slack to
loop the cables through a larger core a couple times).

Next might be to use antenna cable that's more shielded (assuming that
the cable runs quite a bit closer to the autopilot cabling than the
antenna). Might be easier (and certainly cheaper) to just re-route
the antenna cable away from all other cables as much as possible.

And I'm sure Jim's gonna want to know a bit more about that horizontal
dipole (?) antenna you're running.

Mark "RF stands for Random Fluctuations" Hickey

[rpellicciotti]

jls wrote:
> Also instrument needles go crazy.
>
> Aircraft is a Lancair with antenna hidden in the horizontal stab. How
> 'bout directing us to some reading material about how to stop it.
>
> Our local avionics guy says we may have to live with it, or install
> external antennas, which will dirty up a Lancair in a hurry.
>
> Jim Weir?
>
> Thanks and happy holidays.

I'll take a completely different tack. Since your instrument needles
move when you key the mike, I suspect a ground loop. I would be
interested in knowing if you engine needles move when you turn on and
off other consumers like the strobes or nav lights.

I just finished working on a Lancair that had a similar problem. You
could turn the nav lights on and off and the CHT's would go up and
down. Keying the mic caused the trim indicators to change. We ended
up pulling out most of the wiring and redoing it for the plane's new
owner. Not cheap, but everything works now.

Rick Pellicciotti
Belle Aire Aviation

[jls]

rpellicciotti wrote:
> jls wrote:
> > Also instrument needles go crazy.
> >
> > Aircraft is a Lancair with antenna hidden in the horizontal stab. How
> > 'bout directing us to some reading material about how to stop it.
> >
> > Our local avionics guy says we may have to live with it, or install
> > external antennas, which will dirty up a Lancair in a hurry.
> >
> > Jim Weir?
> >
> > Thanks and happy holidays.
>
> I'll take a completely different tack. Since your instrument needles
> move when you key the mike, I suspect a ground loop. I would be
> interested in knowing if you engine needles move when you turn on and
> off other consumers like the strobes or nav lights.
>
> I just finished working on a Lancair that had a similar problem. You
> could turn the nav lights on and off and the CHT's would go up and
> down. Keying the mic caused the trim indicators to change. We ended
> up pulling out most of the wiring and redoing it for the plane's new
> owner. Not cheap, but everything works now.
>
> Rick Pellicciotti
> Belle Aire Aviation

Thanks so much and thanks to all the others who responded with help.
I'm going to advise hangarmate to rewire his airplane.*

The com antenna is in the vertical stab, at least one of them. The
airplane was built by one of those shops that the owner sends the kit
to. We're trying to get in touch with the builder to find out where
the other com antenna, if any, is.



*Just kidding. We'll try all the other avenues first, but fact is the
more troublesome of the two radios does do a number on the egt and cht
readouts, as well as the autopilot and some of the other instruments.

Top o' the season to you. It's summer here in WNC at 70 deg. F. and
warm sunshine.