Multi-engine wiring, part 2

Model Airplane News, Mar 1999 by Gimlick, Greg

Last month, we looked at parallel and series wiring of twin engine airplanes and discussed why we might choose one method over the other. This month, I'll build on that and look at ways to use those methods with each other to solve some interesting challenges. Using this knowledge, I'll show you how to make some decisions on wiring planes with perhaps four or more motors and still use normally available equipment. I know there are some electrical-engineer types out there who build their own controllers for dealing with some of these problems, but most of us want to use what's on our shelf, and this will allow us to do just that, so let's take a look at some scenarios.

Remember last month we talked about doing a twin-engine aircraft using Speed 400 motors and decided we may want to use a parallel setup, since the combined current might only be 20 amps at full throttle. With a four-engine plane with Speed 400s, the combined current in a parallel circuit would still only be about 40 amps (remember we plan for about 10 amps per motor for Speed 400s), and that falls well within our current limits for many speed controllers. But there is another way to do that yet get lower current demands on the controller and battery pack-a combination of series and parallel circuits in one system (Figure 1).

"Where's the advantage?" you ask. If you were to do this in one series circuit, you would have 28 cells and draw only 10 amps, but it would take a lot of room for the batteries, and they would have to be very small cells or the plane would be quite heavy. To do it in parallel the circuit would have only 7 cells, but the current would be 40 amps, and maybe we don't want that much current. Using a combination of parallel and series circuits takes advantage of the best of both systems. By wiring the motors as pairs in series and then wiring the two motor pairs in a parallel connection with the controller and motor pack you provide half the cell count of a completely series system and yet have only half the current of a completely parallel system. It's a compromise to get some of the benefits of each system. This may not seem necessary with smaller motors like the Speed 400, but for larger motors, it's a very workable solution to a problem.

Imagine a giant-scale plane with four Astro 25Gs on board requiring 16 cells each. To use a series system would require 64 cells, and since each motor will draw around 25 amps, we have to use the larger cells like the Sanyo 2000RCs; that's a ton of cells (OK; maybe only about 8 pounds, but that's a lot of battery weight). If we do to it as a parallel circuit to reduce the weight, we use only 16 cells, but we're drawing about 100 amps (four motors at 25 amps each), and that just isn't a workable solution. In a series/parallel circuit, we'd have 32 cells and a 50A current draw, which is workable with normally available equipment. Those of us who are sport flyers still tend to think of SO amps as a lot of current while those of us who are into competition might see it as low compared to the current required by our FSB planes, but let's remember that we'll probably only be seeing this full-throttle current level on takeoff and will spend the rest of our flight at lower throttle settings.

You can make these four-motor setups more reliable by wiring the two outboard motors together in one series and the inboard motors in the other and using a fuse to isolate them from the whole circuit in case of a malfunction in one of the motors. I must admit I've never experienced a motor failure in flight (thrown brush holder, open comm, etc.), but I have seen a couple of single-motor planes quit due to one or more of these malfunctions. By wiring them this way, you'll still have an engine on each side of the plane to help you nurse it back home and avoid any asymmetrical-thrust problems at the same time.

OK; what if we decide that the 50 amps is still too much, and the weight of 64 cells really isn't a problem in our super, giant-scale transport plane? First, we bump into the fact there isn't a controller that will handle 64 cells because of the liability issues involved with the manufacturers. This is a serious amount of voltage, but there is a way around it, and that's what we'll look at now.

Obviously, this plane is a very large project and requires some "alternate thinking." Why can't we just wire two motors on one circuit and the other two on an independent circuit? We can. By doing this as a dual series system (see Figure 2) we avoid all the problems of the high voltage or high current found in our last method when doing high-cell-count motor systems. Each circuit consists of two motors (25s in our example), 32 cells and a speed control. With these two independent series circuits, all we have to do is use a "Y" cord to connect the two speed controls to the receiver.

This might also be a good time to address some questions I've gotten regarding the use of brushless motors in multiengine projects. The main question is, "Can it be done?" and the answer to that is "Yes, but ...." The brushless motors that most of us use are Hall sensor motors, each requiring its own dedicated controller, so there's no way to wire two motors to one controller. You can do a multi-engine project by wiring two complete systems as dual series circuits and tying both controllers to the receiver with a "Y" cord. It's a shame, but I'm afraid that's the answer for the time being, although there is some hope on the horizon. Sensor-less controllers are now being manufactured; they supposedly allow two brushless motors to be driven through one controller, but I've yet to see it done by anyone. Rumor has it that there are such systems operating in Europe, so it's only a matter of time before we see them. For further information on the Konitronics sensor-less controller, I suggest you contact Sal at Northeast Sailplane Products*, which is its U.S. distributor.