Setting Up Twin Electrics

Sep 23, 2011 3 Comments by

 

 

 

This A-26 from Global Hobbies is the perfect candidate for electric conversion. Plenty of room to run the wires through the wings and the batteries are located under the center hatch on top of the wing.

 

 

 

Easy tips for making a solid connection on multiple motor setups

THIS MONTH I’ll be explaining how to go about setting up planes that have more than one electric motor. It is no secret that I am a big fan of using E-power for planes with multiple motors. To me, the advantages far outweigh the disadvantages when compared to glow power. First there is the clean factor; you never have to wipe oil off of your plane when using electric power, unless of course you flew too close to the overly rich glow plane or landed in an oil spill. The other thing is reliability; you never have to worry about getting both motors started and there is only a very rare chance of a motor quitting during the flight. Glow-power twins do have one great feature, though: that fantastic sound they make as they fly by. There is nothing else like it, but I would still give up great sound over reliability any day. So, let’s see what we need to do to properly set up a good running electric multi-motor plane.

WHAT TO LOOK FOR IN A TWIN

The first thing you need to consider is having an easy-to-reach compartment for the battery pack(s). As with all electric planes, you want to be able to easily remove and install battery packs for changing. The plane just has to have an open area that would be able to fit the packs; you may or may not need to make some type of removable hatch for easy access to this area. It would be a good idea to have the battery compartment somewhere near the CG so the additional weight of the batteries will not need to be compensated by more weight to balance out the plane. Many twin planes will have a center fuselage that will be perfect for this. If the plane you are considering for E-conversion only has space for the batteries near the rear of the fuselage, you may want to consider using a different model.

The second thing to think about is easy access to the motor location or nacelles, especially for ESC placement, which should have some type of airflow going through it. There should also be some easy pathway through the wing for all the wiring you will need to run to the battery and receiver. Once your plane has met these requirements you are ready to install the E-power system.

 

FIGURING OUT WHAT TO USE

 

Generally, adding an electric motor to a glow kit is much easier. They take up less room, have fewer cutouts on the cowl, and are easier to install. Here are the motors mounted on the VQ models A-26 from Global Hobbies using an adjustable mount. 

 

One of the hardest parts of doing an electric setup is just trying to figure out what motor combination to use for your plane. Fortunately for us modelers, many manufacturers are more than willing to help you out. A simple phone call is all you need to make to find just the right motor for your conversion. Many hobby shops and manufacturers can tell you what motor will be right for your plane if you let them know what size gas or glow motor the plane is made for. You will generally have a couple of equivalent motors that would work well for your application. For me, I like to use the stronger of the selections just to make sure the motors are never over-stressed, which could happen if you pick the weaker motors and rely on their maximum performance all the time just to fly the aircraft.

 

ESC SELECTION

Once you have the motors selected the rest falls into place. The ESC needs to be the right type for the motor (brushed or brushless) and be able to handle the maximum about of current the motor will require. For example, let’s say you have a motor that normally runs at about 30 to 40 amps but has a maximum limit of 50 amps; you should get an ESC that can handle 50 amps.

 

SOME THOUGHTS ON LIPO BATTERIES

 

Most twins, such as this OV-10D Bronco from Kondor Model Products, have a roomy fuselage in the center of the wing that is a perfect place for a battery compartment.As for the batteries, that requires a little more thought. Depending on how you wire up the system, the battery pack will either be used for one or both of the motors, or you will have two packs feeding power into all motors. When using one battery for both motors (or four if you have a heavy bomber) you will need a battery that can handle the power draw of all the motors. First, consider the voltage requirements of the motors. If your motors can run on four to five cells then that is all the battery will need to be. You will not need eight to 10 cells because you are running two motors.

However, the amp draw will need to be doubled or quadrupled depending on how many motors you have. For example, if you have two motors, each one draws 40 amps continually so you will need a battery that can keep up with an 80A continuous draw. How do you find out the amp draw of a battery? You must multiply the battery pack’s capacity by its “C” rating. If I have a 4000mAh LiPo battery pack that is rated at 15C, that pack can only have a maximum of 60000mAh (15×4000 = 60000mAh) or 60 amps pulled from it at any time. This means that this battery pack will not work for an 80A setup. But, if that same LiPo pack has a 20C discharge rate (20×4000 = 80000mAh) it could support an 80A draw. The only problem with this pack combination is if the motors pull more than 80 amps, you run the risk of heating up the pack from drawing too much current and puffing it, thus ruining the battery pack. I like to pick my battery power combination with a little extra breathing room so if my power system pulls more amps than expected I have some reserve amps in the pack. In this example I would use a LiPo pack or combination of packs that would equal 5000mAh at 20C allowing me to pull up to 100 amps from the pack at any time. The advantage here is if the system does pull more then 80 amps the batteries can handle it, and because the batteries are not performing at their maximum peak they run a lot cooler.

 

WIRING FOR MULTI-MOTOR POWER

The beauty of using LiPo batteries for power is that they are lightweight, give longer running times and can be wired in any combination. This is very advantageous for us when we are running more than one motor. When connecting your batteries in series you are doubling the voltage while maintaining the same capacity rating (amp hours). This is done by connecting between the negative of the first battery and the positive of the second battery. Then run the negative wire off the open connector of the first battery to the negative of the motor, and then run the positive off the open connector of the second battery to the positive of the motor. You would use this type of connection if your motor needs 22.2 volts to operate. Simply connect two 11.1V batteries (3-cell packs) together in series to make a 22.2V (6-cell) pack.

But in most cases, you will be wiring up the packs in parallel, which would double the capacity (amp hours) of the battery while maintaining the voltage of one individual pack. This can be accomplished by using a Y-connector between two or more batteries that connects the positives of both batteries and the negatives of both batteries together. Then have a positive and negative wire go from one of the batteries to the motor. This is how we can use two batteries together to meet the current draw required by our multi-motors. In our example above we could connect two 2500mAh 5-cell (18.5V) packs together in parallel and make a 5000mAh 5-cell battery. That would be more than enough to power the needs of our two motors.

 

CONNECTING MOTORS AND BATTERIES TOGETHER

 

This B-17 from Cedar Hobbies has both the inboard motors attached by Y-harness to one battery while the outboard ones are attached to another one. Both battery packs are stored in the fuselage right above the CG point of the plane.You do have a couple of options on how you can connect the battery packs to the motors. This first option is to connect a proper-size battery to each motor. The advantage here is that each motor has its own power source and a simple wiring setup. Just connect a separate battery pack (of the required size) to each motor/ESC. The disadvantage is that as the battery packs get older they may discharge at different rates, causing one motor to be weaker than the other. This creates more torque on one side of the aircraft, and in some cases this could roll the plane over in a death spin, especially if one battery quits at slower speeds.

The other connection option is to use a Y-connector to join both motors to one larger pack of the required size to feed both motors. The advantage here is that when the battery is running low or is weaker, both motors will reflect that in their performance, maintaining equal torque on both sides of the aircraft at all times. The disadvantage is a little more complicated wiring setup. On a four-motor plane, such as a B-17, use a Y-connector to join the inboard motors to one battery pack and the outboard motors to another battery pack. That way even if one battery totally fails, you will still have motors on both sides of the plane working.

 

Electric Basics, Electrics, Featured News, John Reid

About the author

West Coast senior editor About me: I’ve been involved with RC aircraft since high school and have flown just about everything. I started my RC career with scratch-building, but now like many pilots I rely on ARFs to get me in the air. My main focus is on pylon racing, aerobats, combat and scale warbirds.

3 Responses to “Setting Up Twin Electrics”

  1. John Finley says:

    I have an old Pica Deullist 240 kit that i am finally getting started on. I had originally planned on 2 x .45 glow, but with the improvements in electrics since I bought it, I am now looking to electric. I have no experience with electric, so it is a very ambitious project. Your article is helpful, but I need to learn a lot more before powering the plane. From what I see, the build is normal, but most of the changes need to be made at the addition of the motors, correct?

  2. YnotMe2 says:

    I am a Offshore Electrician and for those who may not understand series, parallel or series/parallel .. Maybe you could but a wiring diagram in the article. DC voltage is as dangerous as AC voltage. Don’t forget battires are stored energy. Be safe and have fun.

  3. Gary Chaplin says:

    Hi John,
    I have a Beaufighter scratch built in gestation, ⅛ scale, wing span approximately 87 inches, wing area 7.86 ft², frame weight (no engines or undercarriage) 12 lbs. I originally intended to power it with a pair of .60 two strokes however I am now investigating the practicality of using electric power. I can see the advantages of electric and having flown twin 2 and 4 strokes in the past and experienced the difficulties and frustrations therein I am interested in the alternative.
    I have been searching the forums for relevant information on twin electric set ups for a model this size and have so far been unsuccessful. Your article is the most informative I have found.

    I am particularly interested in:-
    1. Motor/prop choice. My experience with twins is that two .40 four strokes do not provide equivalent power to one .80 four stroke (I built two almost identical sport models, a twin and a single, the single .80 would climb vertically to around 500 feet, the twin .40 would quickly run out of speed and fall over at around 150 feet. Does that experience hold true for electric? What I have found so far leads me to believe that I would need around 1200 – 1600 watts (depending on finished weight) so would two 600 – 800 watt motors do the job?
    2. I would like to run scale size (or near to ) propeller which would be around 19 inch however none of the threads that I have followed so far really explain in terms that make sense to me the motor/battery requirements for swinging a large prop. My experience with electric motors is that the larger the load the more current they will consume up to the limit of current available or until the smoke comes out. Manufacturers charts that I have found for .60 or .80 electric motors do not give information for props this size. Is there some attribute of outrunner motors that will let them labour at low revs with a big prop without overheating?
    3. What would be the alternatives for battery power? Single large capacity battery supplying both motors in parallel, twin battery/ESC fully independent set up, twin batteries with an equalisation strap or something else? What are the pro’s and con’s? Your article answers some of the questions but I’d love to know more.
    4. Can two ESC’s be controlled from a single (throttle) channel via a Y lead or would it be advantageous to slave another channel to the throttle and have the two motor/ESC’s completely separate?
    5. Is there a way to adjust (balance) the out put of a twin set up or is the quality of modern motors/ESC’s such that this is unnecessary?

    There is plenty of space in the nacelles of the Beau for motors and batteries, there would be good airflow and the positioning of the weightier bits well forward is attractive in a snub nosed design so that’s the way I’m leaning at the moment,.
    Any advice welcome.
    Regards,
    Gary

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