. . . . some conversation on "welding cable versus the "good stuff" . . . . >I had occasion to talk with a professional aviation electrician today about >the proposed wiring scheme for my GS. Took the opportunity to ask your >question. This is what he said (illustrated by a visit into the hangar to >show samples of welding cable he removed from a Super Cub, as well as the >aviation cable). The main safety difference is that the insulation of >welding cable, which is black rubbery stuff WILL support combustion and >gives off very noxious, toxic fumes (think burnig rubber tires and worse) >while the aviation cable insulation will not not burn. The insulations are indeed different . . . tefzel and cousins don't support combustion but they give off equally noxious if not downright toxic fumes when the wires they contain are overheated. We're talking red-heat type temperatures here too . . . not impossible to generate in a battery cable with an RG battery but very difficult. > >Another difference is the size of the individual wire strands; much coarser >in welding cable, very fine in aviation cable. I'm thinking the "welding" cable he was comparing was not welding cable but some other material. All the welding cables I've worked with are designed to be VERY flexible and able to withstand a lot of abuse (trucks run over them, they get handled a lot). The wire stranding is VERY fine in order to achieve both flexibility and resistance to flexure stresses. If the cable he was showing you had fewer, heavier strands, it WASN'T welding cable . . . got it off a golf cart maybe? >This makes a difference >because the prefered route of electrical conduction is on the surface of >the conductor and the finer the component strands in a cable, the more the >surface area to conduct electricity and the lower the resistance. Total BS. So called "skin effect" comes into play in the megahertz frequency range and is insignificant below tens of megahertz. It doesn't happen at DC. >He couldn't offer any specific comparison in terms of Ohms per foot however. >Lastly, the connectors are available for the aviation cable but do not fit >on the #2 welding cable as well. Perfectly suitable connectors are available for ANY size wire. >Many resort to soldering which produces a >low resistance joint at first but which leads to fatigue failure presenting >as increasing resistance across the solder line until it fails altogether. Again, not consistent with the physics of the matter. See this article on soldering fat terminals onto big wires. I've been soldering things together for over 35 years including some wiring that carried over 1000 amps . . . I can't even imagine the failure mode being described here. >It seems to me that with a well planned electrical system (assuming battery >on the firewall) you ought only to need 3-4 feet of #2, so the cost >difference is only on the order of $10. Go with the good stuff. True . . . ACS gets about $3.75/ft for #2 and we get $1.75 when we have it in stock (which reminds me, I'm about out and need to chase down anohter spool). However, for airplanes with the battery in back, runs are longer. Canard pushers with battery on opposite end of airplane from engine need two long runs. Here the weight is a bigger factor. #2 welding cable is heavier and bulkier but it's very soft and much easier to work with. In fact, if you want to run #2 wiring throughout the cranking circuit, I would RECOMMEND welding cable be used for connections to the battery. These connections tend to be short jumpers between battery and ground or battery and contactor. #2 aircraft wire is so stiff that it's hard to work with on short bend radii and stresses it's terminals more. Use #2 aircraft wire everywhere else but nice, flexible battery connections are very nice to work with. The afformentioned hazards are about as remote as your wing falling off so for my money, it's a toss up. If you can stand the weight difference and wish to use REAL welding cable (go buy new stuff from a welder's supply), I don't perceive any great sins happening here. >Switching circuits here, I need a bit of guidance. Auto engines with auto >type ignition systems rely on external power so the design for such a plane >must include a separate, uninterruptible power supply that will function >right through an alternator failure. I would like to talk to people who >are flying with automotive style ignition systems to hear how they've dealt >with this. I am not on the RV net, nor any other systems. Every airplane has an un-interruptable source of power . . . called a battery. This pre-supposes that the battery is maintained well. We tend to treat batteries pretty badly in airplanes . . . run 'em until they refuse to crank an engine any more. By that time, their usefulness as a backup power source has been gone for months. Run periodic capacity checks on a battery, and replace it at 50% of capacity (or some higher number you choose). If you don't run capacity checks, then put a new battery in every two years. For airplanes with total dependence on electrical power, we suggest two half-sized batteries with alternator-out loads distributed between the two batteries depending on system needs. In this case, we suggest replacing one battery every year. This way, you always have one battery less than 1 year old, no battery is more than 2 years old. If you can go "total electric panel" there are options for replacing the vacuum pump with a 20 amp aux alternator. This permits total redundancy of power generation and storage for very little weight penalty compared to classic alternator/battery/vacuum pump installations. Comments and alternative views welcomed . . .
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