Wiring Made Simple
Terminal strips versus point-to-point
Stewart Fields, EAA 494795
Helicopter pilot and builder, engineer, designer, musician, writer, photographer, sailor, boat refitter, welder, and all-around good guy Stuart Fields (Stu) spent three years building his Safari, electing to finish it at the factory. The reason this project took so long was the engineer in him always wanted to make it better or the best or “right.” He began flight training in June 1997.
At about 80 hours’ flight time, Stu’s earlier fixed-wing training came along and bit him—he crashed his Baby Belle. (In 1999 the name changed to Safari.) Since the rebuild in ’98 and completion of flight training in 2000, he has accumulated about 200 hours on his red/yellow ship.
Stu Fields lifts off from the Trona, California, airstrip.
Stu’s first helicopter article appearing in Rotorcraft (2000) helped move him and his wife, Kathryn, into the editing and publishing business, by prodding them to publish their own magazine, Experimental Helo (pronounced hee-low). The first issue mailed in August 2004.
In the latter stages of building your project, regardless of which airframe you have chosen, you will be faced with electrical installations and wiring decisions. All this must be thought out and planned before you install those fascinating gauges. The following will give you contemplative action to occupy all those empty hours during the winter—as if you needed any more concerns.
If you have never heard of or seen a schematic drawing of an electrical system, you will want to check out these resources for basic definitions and explanations. They look more complicated than they are. Essentially, a schematic drawing shows the layout of an electrical system in electrical shorthand—a sort of map complete with a lot of labels. With an electrician’s legend (as in the legend for a map) you’ll be able follow your ship’s schematic as you would a road map.
Wiring Your Ship
Whether you built it or bought it, you are going to want an adequate schematic showing how the system is wired. If you are building and have reached the wiring part, the first thing to acquire is a schematic showing all of the things that need to be connected and what size wire is needed. It makes sense to allow for future growth here.
Hopefully the kit you bought, assuming you aren’t designing and building from scratch, has a schematic included. Quite often these schematics are really just bare-bones layouts. In one kit I know there was one sheet showing the electrical hookup. By the time a set was done to allow wiring, complete with test points and wire identification, there were seven sheets.
However there is some philosophy involved here. And this is where you’re going to want to start thinking of your needs before making a knee-jerk decision.
Point-to-point wiring that goes from sensor to gauge, or switch to instrument, without having any test points, can be done with a minimum of planning and resulting schematic sheets. This minimal wiring is fast if an immediate project completion is the goal. However, long-term maintenance and troubleshooting problems are built in with this “quick and dirty” method.
Terminal Strips Versus Point-to-Point
Point-to-point wiring takes up less space and often uses just connectors for attaching wires to sensors and gauges. Mil-spec connectors are available and are relatively reliable. Standard aircraft methods use a connector to go through the firewall. These connectors allow for disconnecting for fast removal, and re-connecting is relatively foolproof. I recall a U.S. Air Force maintenance officer class that was able to cross-connect some Cannon plugs on a C-119 engine, allowing the prop to change pitch when the boost pump was turned on!
When problems show up and it’s necessary to make voltage or continuity measurements, point-to-point wiring, even with connectors, is problematic. Here is where intermediate terminal strips that make test points readily available become very nice to have.
Side issue: Anytime you are troubleshooting and you think of a test, but the test needs a bit of prep work, there is often a tendency to want to find another test not requiring so much effort. Called “gumption trap” by Robert Pirsig in his book Zen and the Art of Motorcycle Maintenance, this mentality perverts your troubleshooting procedures and can cause frustration and a waste of time. The book contains the best description of the troubleshooting traps of any I’ve ever read (available for less than $10 at Amazon.com and BarnesandNoble.com).
Voltage and continuity measurements on a point-to-point type wiring system usually involve sticking pins through the insulation of the wire to make a contact for the volt/ohm measurements. This is a hit-or-miss method, and if you miss the wire with the pin, you can get false information in your test, which then can send you down the wrong path. This stick-the-pin-in-the-wire method drains my gumption faster than anything, so I prefer the terminal strip layout.
Modifications that require you to tap into a bus voltage or a signal are also a bit of a bugger in the point-to-point layout, while the terminal strip usually has a pick-off point nearby that only requires lifting a screw and attaching a new wire on an existing terminal point.
Types of Terminal Strips
- Contact screw (European style)
This type uses a set-screw directly on the bare wire and doesn’t need any wire connector.
Pros: It is fast and easy to use and doesn’t require much space.
Cons: It is more prone to vibration failure as the screw-compressed wire end can wiggle loose. Personal experience here with a false zero oil pressure indication—I call these terminal strips “auto loosening.”
- Barrier strip variety
This type uses staked-on connectors at the end of the wire. Both spade and ring types are available.
Pros: With the ring-type connector the connection is more assured.
Cons: Crimping is required, and good crimpers are a must and can be costly. The spade variety are not advisable in an aircraft environment.
Note: Solder-on connectors are also available but not advised since the solder can wick into the wire upstream from the connector and cause a stress point. Also there is a big difference in quality from the Radio Shack variety of staked-on ring connectors to AN quality.
Here is an example of the use of terminal strips to make the points that could assist in troubleshooting a failed instrument or system.
Stewart Fields, co-Editor and co-Publisher of Experimental Helo
Stuart & Kathryn Fields
P. O. Box 1585
Inyokern, CA 93527
760-408-9747 for general questions and information about the magazine, including subscriptions, renewals, back issues.
Call 760-608-1299 for technical questions and information concerning aviation.