P-Leads

Re: P-leads

This is a great thread!! Lots of good info and thoughts on the subject. The more I read and research on the topic the less clear it becomes. I think one of the issues is not all aircraft are wired the same. That is where shield grounding comes into play. I am trying to track down some more solid information. I will post if I get it.
DENNY
 
Re: P-leads

If the root question is" are the p-leads causing noise " just disconnect them at the Mag and run the engine. If the noise went away you found the problem. If it didn't go away, you have some other problem source.
 
Re: P-leads

I talked to Don at Aircraft Magnetos this morning and asked him some of the questions this post has brought up. He said I should go the the source and got me in contact with Jim Davis at Continental Motors this morning he has been involved with mags for over 30 years Slick and Bendix. I will try to relay his remarks without screwing it!! One of the first things he pointed out are that P leads are considered an Airframe part and therefore they will and do vary according to the aircraft manufacture wishes. Grounding of shield at mag only or at both ends is up to manufacture/IA depending on if needed to complete ground return or to solve noise issue. External filters on the older Bendix may or may not be needed depending on aircraft. Newer Slick and Bendix mags should not need any external capacitors/noise filters. As far as how capacitors are wired all mags capacitors are basically wired in Parallel. They do not look like it, but because it is a feed through capacitor it ends up being in parallel (early Bendix did not have feed through capacitors but the way they where wired still considered in parallel). I hope I was able to clearly express what he told me. One other thing he stressed was the need to separate the P lead wires from the rest of the harness.
Homer has pointed out the most painless way to diagnose a P-lead noise problem. I think one of the issues that causes confusion is that we have seen things done different ways and they all work. I continue to learn from and appreciate the input from all on this forum.
DENNY
 
Mag Connections

Ref Piper Dwg Nbr 14732; Univair Parts catalog Fig. 47, Items 83, 84, 85and 86

This 1959 magneto connection seems very antiquated and unnecessary. Does anyone actually try to restore this or do you just connect the mags with new shielded wire? I think the capacitors are still available at Univair for about $ 80.00 a pop and the plates found in the IPB aren't even shown on the Piper Dwg.

Chuck
 
Chuck, I merged your thread with two others on the subject. You might read through the posts and see if they help. What mags do you have?
 
Thanks Steve. Will review the thread. Engine has been covered, in the corner of the hangar for 8 years. Will have to look again to identify the mags and report....lol

Chuck
 
Re: Mag Connections

mdcf18 said:
Ref Piper Dwg Nbr 14732; Univair Parts catalog Fig. 47, Items 83, 84, 85and 86

This 1959 magneto connection seems very antiquated and unnecessary. Does anyone actually try to restore this or do you just connect the mags with new shielded wire? I think the capacitors are still available at Univair for about $ 80.00 a pop and the plates found in the IPB aren't even shown on the Piper Dwg.

Chuck
Click to expand...

Chuck, assuming that your only issue with that entire drawing is the filter capacitors, the bottom line is that they are there for EMI suppression.
Do you need them ? The cold hard the answer is that you will not know whether you need them until you start up your engine and discover that you
have radio noise. Then you may buy them and put the filter in and you still have radio noise. No cynicisim intended but Radio Ignition noise is a
patience testing effort and depends a lot on the radio, the P-leads wire, the grounds, and the magnetos. You are in the restoration effort, and
like most of us who do it are trying to save a dollar or two then skip the filter and see if it works out OK. IF it doesn't then go from there.
EMI suppression can be a lot of trial and effort so be patient.
 
Re: Mag Connections

Hi Homer....I understand the theory of trying to eliminate the EMI for radio clarity but the real question I'm trying to answer is whether this Piper 1959 technology (aircraft is a 1959 PA22/20) is necessary in today's world. Maybe for the restoration "purest" but not necessarily for a restoration to airworthy. The aircraft surly could and will benefit from updated mags, P-leads, grounds, radios and electrical wiring. Maybe I should restore a 59 Ford for an "original condition" show........not.....lol. I have plenty of additional research to do as I don't really want to reinstall the old Bendix mags that don't have internal protection. TCDS's will be my starting point to see what mags are authorized for the old 0320-150. I agree thatexternal protection is an option after everything is completed and there is still EMI in the radios.

Chuck
 
"Hi Homer....I understand the theory of trying to eliminate the EMI for radio clarity but the real question I'm trying to answer is whether this Piper 1959 technology (aircraft is a 1959 PA22/20) is necessary in today's world."

Well in a nut shell- yes you do. it has to do with the type of capacitor that is in the Bendix Mags that is common on lycoming 0-320's:

from an article I found some time ago when I was womdering the same thing:

"Ignition filters

Ignition filters are used to suppress radio interference from the "P" lead. These filters are capacitors that short circuit RF currents to ground while maintaining an open circuit to ground for direct currents. Ignition filters are more frequently used on Bendix S-20 and S-200 series magnetos since these magnetos use a conventional capacitor that is not as effective in suppressing higher frequency interference. Bendix S-1200, D-2000/D-3000 and Slick magnetos use a coaxial capacitor (also called a flow-thru capacitor). Coaxial capacitors are more effective at radio frequencies.

Conventional capacitors, because of their higher inductance, are less effective at attenuating interference than coaxial capacitors. Inductance results from the capacitor's internal inductance and the inductance of the capacitor's lead-in wire. The higher the inductance the lower the frequency range where the capacitor is useful. Inductance can be decreased by shortening the lead-in wire."

here is a link to the entire article: lot's of good info here:
http://www.aviationpros.com/article...s-some-basics-on-electromagnetic-interference

hope this helps

Shane-o
 
gliderman said:
"Hi Homer....I understand the theory of trying to eliminate the EMI for radio clarity but the real question I'm trying to answer is whether this Piper 1959 technology (aircraft is a 1959 PA22/20) is necessary in today's world."

Well in a nut shell- yes you do. it has to do with the type of capacitor that is in the Bendix Mags that is common on lycoming 0-320's:

from an article I found some time ago when I was womdering the same thing:

"Ignition filters

Ignition filters are used to suppress radio interference from the "P" lead. These filters are capacitors that short circuit RF currents to ground while maintaining an open circuit to ground for direct currents. Ignition filters are more frequently used on Bendix S-20 and S-200 series magnetos since these magnetos use a conventional capacitor that is not as effective in suppressing higher frequency interference. Bendix S-1200, D-2000/D-3000 and Slick magnetos use a coaxial capacitor (also called a flow-thru capacitor). Coaxial capacitors are more effective at radio frequencies.

Conventional capacitors, because of their higher inductance, are less effective at attenuating interference than coaxial capacitors. Inductance results from the capacitor's internal inductance and the inductance of the capacitor's lead-in wire. The higher the inductance the lower the frequency range where the capacitor is useful. Inductance can be decreased by shortening the lead-in wire."

here is a link to the entire article: lot's of good info here:
http://www.aviationpros.com/article...s-some-basics-on-electromagnetic-interference

hope this helps

Shane-o
Click to expand...

Don't you just love it when people read articles and become experts overnight?? Homer is correct there lot more to EMI and RFI then any book/article will teach you. Twenty years of doing this **** on everything from airplanes to remote HF, VHF location setup's having everything from generator interference, power lines, fluorescent light interference from 1/2 mile away!!! to my case of an airplane having your mags eat up your radio signal like tasty crunchy cereal. I still have one plane giving me EMI grief one damn thing is certain Shane like expert Homer quoted the filters I installed and paid a pretty penny for helped 10% reduction maybe 20 just to make myself happy. So Happy I removed them as I feared the damn things would fall off and ground out, then you have some serious problems on your hand. EMI in itself is simple... but let me tell you it can be one of the worst problems to eliminate and there is at times just no easy answer or solution.

Jared
 
Last edited:
Re: P-leads

Brian said:
Can someone share a wiring document or photo of a slick mag showing the two series capacitors ?
Click to expand...

Found this illustration in the Jeppesen Powerplant book. The drawing illustrates a coaxial capacitor like Champion/Slick uses. As I see it, and I could be wrong..... but if you were to add another capacitor (RF filter, whatever you want to call it) between the capacitor stud and the ignition switch it would be in series. Sorry for opening an old post but I just found the drawing today.

Todd
 

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Funny, I was just reading this thread today as well and found a reference to what you posted earlier in Sacramento Sky Ranch Magneto book.
 
Chuck
Let me know if you want to get rid of the Bendix mags. I think they are about the best legal thing you can put on a Lycoming.
DENNY
 
Re: P-leads

Vagabondblues said:
Found this illustration in the Jeppesen Powerplant book. The drawing illustrates a coaxial capacitor like Champion/Slick uses. As I see it, and I could be wrong..... but if you were to add another capacitor (RF filter, whatever you want to call it) between the capacitor stud and the ignition switch it would be in series. Sorry for opening an old post but I just found the drawing today.

Todd
Click to expand...

NO, the capacitor you see is illustrated by the pictorial insert, and it appears to be in Series with the P-lead to the ignition switch. The schematic representation shows what is electrically connected. The capacitance plates are connected to the Plead to the ignition switch and also to the breaker points. The casement for that type of capacitor is connected to the second set of plates and is connected internally to ground. The little capacitor in the circuit is what that big pictorial is doing so ignore the big tube in the picture and look only at the electronic circuit showing the capacitor symbol. That capacitor is providing an open to ground to steady state signals and a ground for RF or other perturbations of voltage. Now, you would not put another capacitor in series with the magneto stud and the ignition switch because that would be putting an Open circuit and you would have a hot mag all the time. Ign Switch -----l (----- Magneto Stud This is what a capacitor would look like in Series in that line. If you put an external capacitor like the ones that look like a T-Bomb firecracker or even of the type you see in the pictorial, that would be in Parallel. That would actually be defeating your purpose because capacitors in parallel are slower reacting to higher frequencies due to the increased plate size from the now two capacitors attached to the line. With this magneto as pictured you do not want external capacitors, if you put them on and it didn't help, then take them off. If you put them on, and it worked then leave them on because you fixed some elusive voltage spike from somewhere else in the engine compartment.
 
P lead question

Hello,

i think the p lead on my pacer may be miswired. I have what I think is old wiring with external brading and slick mags. Only the internal core of the wire is attached to the magneto stud and the shielding is attached to nothing.

What i I read on here was to connect the p lead to the stud and the shielding to the ground stud on the magneto. So that seems straight forward and uncontroversial.

My question is what should the other end of the wires at the switch look like. I seem to hear different answers.

First, should the shielding be connected to the ground stud on the switch or not? I have read some folks say what I learned at the eaa avionics course: the other end of the shielding shouldn’t be connected to anything to remove emf. I have read others who say connect it to the ground stud on the switch arguing that when in the switch is in the off position there’s no emf since the engine isn’t running and you wont get an emf ground loop. And when the switch is in the L,R, or B position, the shielding is disconnected from and terminates at the switch and thus only connected on the magneto end when running. Whose got this right?

Secondly, right now my ground stud on the magneto switch is connected via a short wire to a screw bolted through the panel. Not sure that’s a great ground, and I also wonder if that could fry my electronics that are also bolted to the panel as current tries to find ground. Does anyone have an opinion? Is this a good idea to ground the magnetos into the panel?

Third, does the switch ground stud need another wire connected to the frame, or panel? I read some folks say the shielding is only for emf and not part of the ground circuit, which leads me to think I need another wire to ground from the switch. I read others who said slick recommends grounding through the ground on the magneto, which leads me to think I don’t need another wire and if anything it would undermine the emf isolation the shielding provides.

For the third question, my thinking is that it depends on whether you connect the shielding to the ground stud on the switch or not. If you do, then in the off position you are grounded through the shielding back to the magneto, to the engine, to the frame. If you don’t connect the shielding to the switch then you need to get it to grounded somehow like the firewall, panel etc. maybe that explains the different opinions I read. Did I guess right?
 
The shield is for RMI only and not to ground the magnetos. I use a seperate ground wire at the switch to my common ground to a ground lug under the panel usually welded to the frame. I merged your thread with two more with some really good information. I had to read through some posts to refamiliarize myself on the subject
 
Thanks Steve. i started looking through the posts but sadly lots of the links seems dead to sky ranch, etc. So, if I understand your post the ground wire at the panel I have is what you do and you would connect the shielding at the mag. What do you do,with the shielding on the other end? Terminate at firewall or connect to the ground switch?

And , I kept researching this last night and found a post mention mag filters on the firewall. I didn’t know what I was looking at and assumed that the prongs sticking out of the firewall were a fancy way bring the p leads through the firewall but now think those are the original mag filters. I have slick mags and saw some posts say you shouldn’t use mag filters with slicks, but I co uldn’t find anything from champion on the subject or a manual. How are folks wiring slicks? Should I remove those filters and run wire to the switch?
 
I don’t recall the explanation behind it at the moment but I was taught sheilding only gets connected at one end and I don’t think I have ever seen it done otherwise.
 
As a general matter, that is what eaa taught me when I went to their avionics class. But, some say ground the switch and don't connect the shielding these are two different circuits for different purposes (Steve seems to be in this camp). Others say connect the shielding to the switch but don't ground the switch and the shielding serves double purpose (Jim seems to be in this camp). When switch is off, it is the ground. When on, it is open and thus acting as the emf sink. And, still other folks say this emf ground loop issue was an issue with old electronics which has largely been designed out of newer radios and that it is better to ground both ends now for some reason I didn't quite get. So, I count at least 3 different answers.

Then you have these firewall mounted filters. Some seem to have 'em. Some say after 50 years they are trash and should be removed. Some say you only need them for Bendix and that the extra capacitor can harm slicks due to how they are designed and/or the plugs. Others, of course, disagree.


Maybe I should have just asked if wheel landings are better than three points. :new_Eyecrazy:



Jeff J said:
I don’t recall the explanation behind it at the moment but I was taught sheilding only gets connected at one end and I don’t think I have ever seen it done otherwise.
Click to expand...
 
I just wire it like Piper did, figure as many airplanes as they build they knew what they were doing.
 
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