The 42 volts was put in the rules because at the time AMA rules for electrics had that limit. It was a safety thing, not a performance thing. The understanding of the rules is that no part of the electrical system should be more than 42 volts (nominal) away from any other part. So two motors each running 10S LiPo packs (which are nominally 42 volts at low current and much less at high current) is the maximum voltage permitted. These motors would be independently wired so the voltages can't add together to produce an 84 volt potential anywhere.
Now to get back to the original question: What stuff should you use? I have had good luck with the AXI 2826/08 in my Skyrays. They have been reliable and smooth and get me to 75 mph (barely) on 4S batteries. I use 4s 3900 mAh Thunderpower ProLite G6 batteries. If you were to have good performance with this airplane and go for a four minute low, you might need a total of 5000 mAh. I have to guess here that the flight time for a given battery would scale as the weight or, alternatively, that the battery size required for the same flight time would scale as the weight. So assuming you are at the maximum weight and scaling from the Skyray, 4.5 lbs/3.5 lbs X3900 = 5000. This leaves you with a flat battery after 4.5 minutes of low, not much reserve and probably not optimum for battery life, but it is a start. I have neglected the efficiency difference of a twin airplane vs. a single engine airplane, since I have no data. Another motor I use is the Scorpion SII-3026-1190. It has more power for only a little more weight, but I really run it hard and it will vibrate like crazy if I don't have an external rear bearing mounted to the airframe. I have a Scorpion SII-3020-1110 that I will try in my new Class I. It is a little smaller, shorter, lighter, and slower. It might be more stable. I have not run it yet, so I don't know. For the controllers, I love my Castle Creations ICE controllers. You will need one for each motor, so you are stuck with buying two. If you can mount them with ducting for good airflow, you can use the ICE Lite 100A for the 3026 or Ice Lite 75A for the AXI or the Scorpion 3020. These controllers have built in data loggers and they are robust with considerable reserve. The regular ICE controllers have finned aluminum heat sinks and are not quite so sensitive to cooling air flow, but that adds weight.
As others have noted, you will be struggling to meet the weight limit. At every point when building, make the decision to do the light thing. That's true for single engines as well as multi engines.
And Eric's recommendation start with a single engine model is sound advice. I would be more specific and suggest you build an electric Skyray. It's quick and easy and you can use it as a testbed for all the components that you will put in the twin. And when something craps out, as it is likely to do when you are getting started, a Skyray will be a lot easier to fix.
Pete
Topic: electric question (Read 552 times)