Warbird Information Exchange

Greetings all,

I'm just about to start wiring the project and I'm going all out to have a quiet, quiet, long range, vhf radio. Does anyone know which, if any, of the electrical services on the airplane can be wired with unshielded wire instead of shielded? The airplane is a 12volt, W670 powered, two-holer. Equipment will be nav lights, strobes, radio, txpdr, encoder, starter and alternator.

Thanks,
Dan

_________________
Defending Stearmans on WIX since Jeff started badmouthing them back in 2005.

P leads and strobes are shielded without question. Radio wires are dependant on what you install. Headset/mike wires are shielded for sure. The books on the radio should give you the proper ones to do if there is any requirement for shielded wire.

Above all, make sure that your grounds are good, and there are enough of them....ie engine to mount, mount to fuselage ect. Total resistance between any one point and the negative side of the battery should be under 1 ohm for optimal EMI prevention. Also watch that you don't create ground loops when you do your grounding straps.

Ground loops?

_________________
Defending Stearmans on WIX since Jeff started badmouthing them back in 2005.

Simplest explantion is that a ground loop occurs when there exsists a voltage difference between two seperate grounding locations. If you don't have a good ground between the engine, the engine mount and the fuselage, it's possible to see a slight voltage difference between the grounds. It's predominately a problem with audio and video circuits, but can show up in rf circuits too.

The best way to eliminate them is to use good bonding practices and verify that you have good bonds for everything. I would recomend using the negative cable tie point as your sole reference and make sure that any grounding straps or grounding points that you use, will give you less than 1 ohm resistance between each location and the reference point.

The grounding can be extremely critical for some systems. A number of the things I work on for my employer require us to have electrical bonding quality good enough to have a resistance level between the two parts of .0025 ohms or less. There have been times that I have had to remove and rebond equipment several times to make that spec.

One other thing to keep in mind....Use quality materials and connectors/terminations and good tools when you do the wires. Cheap/junky terminals and low quality crimpers will cost you more in time and effort chasing problems than using the good stuff from the start.

And either learn how to, or have a reputable aircraft radio shop do your terminating for shielding on wire and coax to be sure you don't have any 'leaks'. Using a bonding brush is an art as much as a technique, go slow (500-ish RPM) use moderate pressure and do the process in two or three second spurts until the area is clean and bare without galling the metal. After bonding use ALODINE 600 to prep the bare, bonded area or it will corrode under the lug or bolt you use to make the ground. ALODINE 600 is toxic and trans dermal (goes through your skin to your liver and kidneys) so use PVC gloves and a long Q-TIP, just a drop or two, don't flood the area let it air dry, make your connection and primer over the location (including the attaching hardware) after installation is completed. BE CERTAIN that the hardware you use is from the same nobility (all CRES or all plain steel) bolts, washers, nuts, etc. or you WILL be building a little battery that WILL cause you problems down the road.
And don't forget to make the run long enough to allow for flex and twist (about 1/2 inch of slack up and down is fine) a friend had radio problems with a BELL 47 in flight but checked fine on the ground, turned out to be the antenna coax was fractionally too short and flex in the airframe when supported by the rotor caused the coax to pull back enough to interfere with transmission.

_________________
Don't make me go get my flying monkeys-

Ok - I see where you're going with that. Actually I'm going to use one common ground bus for everything. Fortunately I have a very low tolerance for crap so everything is all high quality NAS/MS stuff. Thanks for the advice, I may have to pick your brain a little down the road.

Dan

_________________
Defending Stearmans on WIX since Jeff started badmouthing them back in 2005.

Thanks - that's good advice. I appreciate the help. Reminds me of an airplane that we had come out of inspection once. Everything worked when it went in the shop but when it came out one vhf kinda worked but not really, and one Nav receiver was u/s. Interestingly one coax up in the roof was really, really slack while the other was like a guitar string!

(IT WASN'T ME!)

Dan

_________________
Defending Stearmans on WIX since Jeff started badmouthing them back in 2005.

Dan: There are a couple of problems with trying to make a common bus for grounding...If you go with a single bus, then it has to be enlarged to handle the max current load that you could ever produce. More load means a bigger bus capcity which means a physical increase in size. That means you are adding weight.

Also, going with a single ground bus means that you are adding a significant number of feet of wire to your harnesses. As the harness size goes up and the constant load within the bundle increases, you now have to watch that you could be approaching thermal limits of the wiring that you are going to use.

With the increase in wiring due to a homerun grounding system, you now have to add in the length of the ground to your wiring calculations. An example: If it's a 19 foot run to a powered item and you use the homerun ground, your wire size calculation for the given load now has to factor in that the total wire length is now 38 feet instead. This then can cascade into a major thing.

Here is little rundown on the cascade can happen and where it can lead to. These are simply made up numbers for the sake of an example.

Original wiring run was 19 feet in length and a 22 ga wire would handle the required current and had an acceptable voltage drop.
Doubling the length now takes the voltage drop just past the acceptable limit for 22 ga and therefore the current capacity out of limits for the length. Got to move up in wiring size...we are now to a 20 ga wire. Bundle diameter has now increased a bit for each wire. Oops...total bundle is now over 50% larger in diameter and is now tightly packed....Well, the tightly packed has now moved us out of free air temp rating of the wiring and we now have to move to the tight bundle/conduit rating. That derates the load capacity of the 20 ga wire to less than what we need so now we have to go back and up the size again. We're up to a 18 ga wire now. Oops. our bundle just got bigger again and now we have a routing problem because the original 1" diameter bundle has now swelled to over 2" with the grounds that we had to run and with the new wire diameters that it ended up needing. Now we need to look at the routing because you can't stuff that much wiring through the available hole size in the bulkhead. Up the hole size and now you have changed the structural integrity of that bulkhead and you need to do some engineering analysis....and so it goes.

With tube and rag you have a lot of options to routing and bundle sizing and usually lots of free space to work with, but it isn't always that way. Wood, metal and composite structures often end up with very little available space in some areas and have large wiring needs. Usually, the way the production wiring was set up is going to be the most efficient, for the least amount of weight and be flight safe. Unless you have very little wiring, creating a single bus ground system is probably going to be a an excersize in massive frustration. The stuff that I work on has miles and miles of wire, hundreds of connectors and hundreds of grounding points and we still have the occassional grounding problem as well as miswired connectors.

Food for thought...

Also, keep any bends or direction changes in your wires pretty gradual, don't let wires rub on structure or in lightening holes edges (how do they get lightning to go thru those holes??) tight corners lead to heat build up and line loss. Remember, electrical currents flow along the outside of each individual strand, not through them, that's why most aircraft wire is silver plated to promote electron flow so tight turns promote amperage and heat build up in each strand and could cause failure or arcing. Few things are more fun to chase with the crew on its way than to have a stray AC signal bleeding over from another system (particularly into a comm system) so it's a good idea to keep AC and DC stuff separated and 'butterfly' clamped where bundles cross and do not put all the big load stuff in one bundle-big heat build ups can happen. A good rule of thumb is clamp about every 12 inches ands leave a tiny bit of 'loop' or 'sag' between clamping points for two reasons-1) flex in the airframe 2) moisture has somewhere to drop off the bundle. Also always leave a 'drip loop' where ever a cannon plug attaches to a socket again so moisture can drop off the bundle and not into the socket where really, really unhappy things can happen. A drip loop is nothing more than a downward deflection of the bundle just before it inserts into the cannon plug of and inch or so. Cannon plugs should go in either horizontally or best upward so moisture cannot run into the connector jack/component. Keep your bundles as 'combed out' as you can, if it looks like macrame` or a 'birds nest' it'll be trouble.
Obviously, (but ya never know about some folks) NEVER clamp to, or route wires near or under a fuel line, fuel can drip but rarely do electrons.

_________________
Don't make me go get my flying monkeys-

And then you toss in UHF and higher frequency coax and fibre and it gets really interesting....

We had to replace some fibre in some new FMS F-16's... each fibre was about 13 feet in length. Repair process at the time required us to run full insertion loss tests prior to uncoiling for installation. If it passed, we could string it in the bundle. Then we had to run the test again and thereafter once we had reinstalled every 3 clamps on the run. Once it was competely clamped, rebundled and inserted in the connectors, we had to do it again...The first bird we did took us almost 19 hours to complete....that was just the fibre run and didn't include tearing out two complete avionics bays to get to the routing.....by the time we did the last jet, we were down to about 10 hours...

What I work on now, lets just say that I really despise coax.....and I'm glad that coax termination is not one of the certifications I carry at work.

Dan, it all looks complicated at first, but within a short while it becomes very easy to figure out and know how it should be done. Probably the hardest thing to learn about wiring is working neat and being consistant, especially if you have to troubleshoot anything you wire

Boeing is encountering problems with fibre optics cable on the 787. The minimum size of a stowed coil or loop is 10X the diameter of the BIGGEST cable in the bundle (and not a bad general rule to follow with any wire bundle runs if you have to coil and stow). FO stuff comes into Everett installed @ the 'partners' facilities (usually done by people who the partner is still trying to break to shoes and showing up @ a set time every day) and it's wound and panduited in a loop tighter than your first girlfriends dad. People being the lazy creatures they are, use hanging bundles and wires as 'hand holds' going through the airplane if the floors are out, or to help themselves get up off the floor, each and every instance fractures the glass rod inside and requires the entire cable be replaced (it CANNOT be repaired), some over 200 feet long @ 'ain't cheaper by the foot' prices causing production delays. If you are doing any Fibre Optics routing please DO NOT apply light of any kind to one end and then look in the other, one Boeing Instructor did and now has a permanent blind spot burned onto one of his corneas.

If you do a first rate job of bonding your mechanical grounds and protecting them with primer or paint, the airframe becomes your return path (think in terms of a 6 volt, positive ground old car) and everything should flow smoothly and evenly to your ground block without the added weight and trouble of running two wires (check your grounds with a multi meter, they should read about .0002 ohms or less for an average light aircraft ground bond, one probe on the screw or bolt head, the other at least 1/2 inch away on bare structure). ALODINE will not work on steel, only aluminum.

_________________
Don't make me go get my flying monkeys-

Thank you to all for a refreshing look at wiring. I still keep my hand in the avionics world by working on the weekends with our CAF birds but a PT-22 and OY-1 don't get real complex. However, everything you have said holds true for them as well.
The reason I said refreshing is that after a day of pulling T-150 (150 mil, 3 conductor) cable at a length of 270 feet in one of the Main Machinery Rooms of CVN-71, it is nice to be reminded that people still work with normal size wires out there. As a side note, the power generation system for CVN-78 USS Gerald Ford will be 13,500 volts and the minimum bend radius for those cables are measured in feet instead of inches.

All that power to run the Chiefs coffee pot and reading lamp? you're not pulling that stuff by hand are you?

_________________
Don't make me go get my flying monkeys-

Hey hey, easy on the Chiefs' coffee pot, I drink from that pot. The power increase is not the coffee pots or reading lights. The electro-magnetic cats and weapon's elevators are responsible for the biggest boost. Up till the Ford, the cats were steam and the weapon's elevators were hydraulic/mechanical. I am sure the scope dopers will have a few more bells and whistles as well but the cats are the big draw.

Thanks for all the help, guys. Fortunately I'm working this all out on paper (well...electrons) first. Being steel tube construction (a Stearman) I can use the airframe for the ground return but the wooden wings will require a dual wire installation. It should be a fairly simple 12 volt system with just nav lights, strobes (one power supply), a very minor instrument light setup, starter, alternator, one VHF comm, one txpdr/encoder, and a gps. I'm trying to build the quietest/rangiest radio setup I can and I was concerned about rf noise bleeding back through the airframe ground. Going with a dual bus system (main & avionics) so any other help or advice really appreciated.

tailwinds,
Dan

_________________
Defending Stearmans on WIX since Jeff started badmouthing them back in 2005.