Ptarmigan

Is it possible to wire one speed controller to two electric motors?

R101

Yes for brushed motors

No for brushless

Dave Lilley

I would add that for brushed motors, the ESC should have a total amp rating higher than the two motors combined, at their max amp rating for the prop and battery that you are planning to use. It isn't always needed, but I prefer to have a 10% to 20 % buffer.

Example: For a 40 amp motor, I will buy a 50 amp ESC. The reason is that under certain loads, a motor could pull more than its highest rating.

Since the price and weight for the higher amp (within ~25%) is often negligible, I feel that it is better to have an ESC that could handle a higher amp load.

Burrito Bandito

More info please.......

I am building the deHavilland Twin Otter by Anton Eiesel published in last year's May or June M.A.N. I originally bought these plans to put .10 glow engines in, but the structure is so light and I now just don't want to do the mods and beefing up, so I am going to go electric.

The plans call for two speed 480 motors. I seem to recall that he used the HIGH DOLLAR Phasor motors, but I was wondering if basic 480 brushed motors would work. I can provide all up weight info and wing loading estimates from the article, but in the meantime, are there ESC's out there that would handle both 480's smoothly with mirrored throttle response? Do I just get two motors and esc's and use a Y harness for the throttle channel?

I know nothing about electrics, but want to learn more.

Tell me what you need to know to help me, and I will get the spec's.

Gonna love this plane!

Thanks,
John

Dave Lilley

If you have two brushed motors, you can use one speed control. The output positive and output negative from the ESC are each split into two wires (Y). One positive and one negative then go to each motor. This is a parallel setup.

As for which motors would work with your plane, I would need more info. Do you have a link to an article or site that shows the model and its specs?

Is the following the specs for your otter?

Wingspan: 65 Inches
Chord: 7 Inches
Wing Area: 455 Sq. in.
Weight: 4-5 lbs
Wingloading: 23 oz per sq. ft.


Do you want a geared or direct drive system?

Ptarmigan

Dave Lilley, I plan to use either two to four Speed 400 to 700 motors likely. I may either have direct drive or geared.

Dave Lilley

I will run a few different options through MotoCalc tomorrow, but off the top of my head, a geared 400 or 480 would be a good, cheap option. A more powerful and more efficient option would be to use a brushless system, but then you would need two controllers, and of course, it would cost a bit more. Do you plan to use NiMH or LiPoly cells?

FYI, the brushed ESC and motors would provide for smoother and better sync'd motor operation.

As for the price, a small brushless motor that would replace the 480 size motor would cost about $75 each, but considering that they are more efficient, generally last long (since there is no brush wear), and can be purchased with higher turns that make gearboxes unnecessary, they are a good option. The down side is you really don’t want to use them on a seaplane, since they are much more expensive to replace if you plane does an imitation of a submarine. (Many cheap brushed motors can survive in the water fine, as long as they are rinsed well, dried quickly, and lubed after.)

Anything above a speed 480 would be too heavy in my opinion.

What size prop does the glow version use?

Dave

Ptarmigan

I would use NiMH battery. I actually plan to design my own airplane, with a wingspan between 60 to 80 inches out of either all foam or with some balsa wood. Doesn't a speed 480 or greater motor have more power in terms of how much it can lift? I know they are heavier of course.

Burrito Bandito

Dave,
Please find listed below the specs, per Anton and the article. Looks like you already figured out the model though....

Model: de Havilland Twin Otter

Type: sport-scale twin electric

Scale: 1/12

Wingspan: 65 in.

Length: 50 in.

Weight: 3.5 to 4.5 lb.

Wing area: 455 sq. in.

Wing loading: 17.7 to 22.8 oz./sq. ft.

Power req’d: 2 Speed 480 motors, or 2 .10 glow engines

Power used: 2 Kyosho 7.2V Speed 480s w/2 Maxx 2.5:1 gearboxes, a Schulze 35e ESC and an 8-cell, 2000mAh battery

Prop used: Master Airscrew 10x6 electric

Radio req’d: 4-channel (elevator, rudder, aileron, throttle)

Flight duration: 4 to 6 min.

Dave Lilley

You have to be careful when talking about electric motors and power. For example, bigger does not always mean better, and power does not always mean thrust. As you probably know, you can spin a small prop at high RPM, and it still may not fly your plane. Brushed direct drive brushless motors are often good at turning a small prop at high RPM, but that does not provide enough usable thrust. Often, a larger prop, even if it is turning fewer RPM, is needed. Therefore, sometimes gearing a smaller brushed motor is the better choice than using a large direct drive brushed motor.

I have brushed motors that can turn an 11” prop at a reasonable speed when geared, but only a 7” prop when set up for direct drive. A brushed motor that can turn the same 11” prop will be much heavier and pull more amps. If the plane could even hold the larger brushed direct drive motor, it is safe to assume the plane would need higher capacity batteries to compensate for the higher amp load and to bring the flight times back to an acceptable level. Higher capacity batteries often (but not always) means more weight, and this could also increase the weight and relative wing loading. If your not careful, you could quickly end up with a plane that flies marginally or not at all.

You may notice that I keep mentioning brushed motors in my examples. There are many options in higher turn brushless motors that have enough torque to turn large props at reasonable amp loading. They often are fairly efficient and very easy to setup, since they do not need gearboxes. They can sometimes be lighter. If you don’t mind setting up a gearbox, there are even more options in geared brushless motors. (Gearboxes are not that hard to set up.) They are usually more efficient than the direct drive higher turn brushless motors, and they nearly always more efficient than the geared brushed motors.

Gearing any motor also decreases the relative amp draw when compared to a direct drive motor that can turn the same prop. There are two major down sides with geared motors. The first is that with increasing gear ratios, you will need larger props to get the same amount of thrust. The second downside is that higher gear ratios lower the prop speed, which limits the top speed of your plane. (If you are not mindful of this, you could end up with a plane that barely flies faster than its stall speed.) Finally, a geared motor sometimes needs more cells than a comparable direct drive motor. A thing to consider with geared motors is that there is a sweet spot. This is the point where geared motor can turn a prop that can be used on the target plane, the necessary batteries (number, type, and size) will fit in the target plane, the amps are low enough for the batteries you want to use, and the pitch speed high enough for more than just cruising.

When considering geared and direct drive motors, I find it useful to consider the following questions.

What size prop can the plane use?
(This is usually limited by the type of plane (sport, sailplane, scale, slowflyer, etc) and how much clearance there is for the prop.) I will often target the prop size used in a comparable glow version of the plane, or to a known plane of relative size and type. If those comparisons aren’t available, I try to target the prop size to the relative scale of the plane and the scale speed that I want for the plane.)

What is the target weight of the plane?
(I try to target the final flying weight under a comparable glow version of the plane. With NiMH cells, this is rarely possible, but with the new LiPoly cells, it is finally becoming a reality.)

Does your plane need higher speed, higher thrust, or both?
(For fast planes, I go with a direct drive motor. For scale or slower flying planes, I often use with a geared motor. If you need both, it often costs more money.)

How much room is available for the power system? (motor or motor/gearbox)
Some geared systems are long. I have planes that could not use a particular brushless system because there was not enough room in the plane. The same can be true for large direct drive motors, which often have more girth.

Do you want to use a more efficient brushless motor or a less expensive brushed motor?

I hope I have not overwhelmed you with options, but I just wanted to point out some of the differences and some of the things you may need to consider. Even for very experienced electric modelers, it is often it is very helpful to use a software program such as ElectriCalc or MotoCalc to help determine motor, gear drive, prop, and battery choices. They are not perfect, but they can let you test a large number of power systems to get a relative idea of what may work.

If you answer the questions I posted, I can run the numbers and provide a few choices.

Dave Lilley

John,

I recommend using one controller, with each output lead from the ESC wired in a Y. I would recommend that you consider using one of the new 3300 NiMH packs that are available. They would give you 40% more flying time at the same weight. If you want even more flying time, look at LiPoly. I know they are new and a little more intimidating, but I can honestly say that if I were starting fresh and didn’t have a large investment in NiMH and NiCad, I would mostly use LiPoly. I believe they are the future of electrics.

Dave

Burrito Bandito

Dave,

Thanks for the thorough information. I must admit, that I am a bit confused, but between you and William Robinson, I am getting there. To answer some of your questions:

1. Prop Size - the designer/builder used 10x6 Master Airscrew electric props, so there is a strating point.
2. The target weight of the plane is 3.5 to 4.5 lbs. I think I am doing well in keeping the frame lite, but I suppose that the equipment (batts. etc.) will be where the weight comes from.
3. Speed vs. Thrust - I plan to fly this plane in a scale like manner for relaxed fun. I suspect that thrust will be the more important issue for me.
4. Room for power system - the nacelles are pretty roomy for an electric motor. I can take measurements from the plans tonight. If you are talking about room for power as in where batteries can go, well....let's just say a lot! This is a stick built fuselage, and it lends itself to a very roomy interior with location flexibility.
5. Since this is my first electric and first twin, I would like to go the inexpensive route i.e. brushed motors. I think I will like it though. Electrics will give me the flexibility to fly in alternate locations when time does not allow me to get to the field.

If you need more perspective, please go to www.beldonemodels.com There are some photo's there of the Otter, both finished and in the bones. You seemed to know the model I was talking about before I posted the spec's though. He also talks there about what he used in the model. I think I told you everything there already, but you may see something there that I missed.

Thanks,
John

**This is not an ad for Beldone**

Dave Lilley

I think I know somebody who converted that plane to electric power. I will write him and asked which system he used.

Dave

Burrito Bandito

Dave,
Hope I didn't confuse you, but the plane was designed for electric. I was going to convert it to glow, but in the final analysis, decided to go the electric route. Hope that clears things up.

John

Dave Lilley

Ah.... oops