Jeff Fokker

To you electric guru's

I have designed and built (90% there) a 1/18th scale Lockheed Constellation. (Attached photo).
I am planning on using (4) AXI 2820-10 motors swinging 9-3/4" german 3-blabe scale props.

Problems;

Weight and balance, I must put the batteries as far forward to keep the plane as light as possible. If I do this, I can keep the plane at 12-1/2 pounds flying weight. Is it OK that the longest wires to the speed controller (outboard motors) are 30" long? The speed controllers will be 2" behind the motors for all four.

What size mah Lipos will I need to run all four at 1-2 to 3/4 throttle for at least 8 minutes?
What discharge rate do you suggest?
Do you suggest 3S or 4S?
Can I run 2 motors off of one pair of 13 gauge wires?
I am planning on a separate NiMH battery pack to run the flight controls. That pack will be in the nose (for balance as well)

Thank you

jmohn

Not sure what the watts for those Axi motors are but you should shoot for about 100-120 watts per pound, more if you want to fly faster (non-scale). The wire lenght won't be a problem. You can always go large on the wire size if needed to help too. You should be fine with #14 , #13 or #12 for that length.

Go as large as you can on the ESCs and make sure you have a flight pack for the receiver and it's not running off an ESC. If one burns out at least you can fly the plane in still.


Jeff

Jeff Fokker

Jeff
Spec of the motors;
Can use 3S or 4S Lipos
RPM/Volt 1200
Max Efficiency 20-30Amps (76%)
Current Capacity 42Amps/60s (Don't know what the 60s mean)

Jeff Fokker

I think I did it right.
Used a calculator and with a 4S (14.8V), @ 20Amps, it would be 296 watts or 1184 watts total
With a 3S (11.1V), 30Amps, it would be 333 watts or 1332 watts total.

Did I figure that correctly?

Dr Kiwi

With 30" between battery and ESC you may well have a serious issue with induced voltage spikes unless you add capacitors in-line. (search for "battery wire length/versus ESC to motor" on RC Groups... it is a common question, with knowledgeable answers given).

Here is something I stole ages ago!

ESC wire length
I, personally, can tell you nothing about your query, BUT I can refer you to a well-written article which covers the topic well. [I give my Geology students a standard spiel -" I don't profess to know all the answers, but my training as a research scientist has given me the ability to find them out from those who DO know!"

In the April 2003 volume of "Quiet and Electric Flyer" Gordon Whitehead has an article entitled "No Smoking! - keep your ESC healthy" [p.62-66]. The gist of it is that due to the probability of high voltage spikes in long leads from battery to ESC ["add 3ft and you risk instant smoke"], one should avoid lengthening the battery to ESC wires (if that is unavoidable one can fit a "voltage spike suppressor" [1000uF low ESR electrolytic capacitor] across the in-terminals of the ESC). However, particularly in the case of multimotor models, one can lengthen the wires between ESC(s) and motor(s) to one's heart's content.

I take no credit (nor blame) for any of the science involved! - but Mr Whitehead seems to know what he is talking about.

Cheers, Phil

If the max. amps are 42A for 60 seconds I wouldn't want to draw more than about 35A. At 100W/lb you need 1250W so that shouldn't be a problem on 3s. What you must find out is what the amp draw actually is for those motors on 3s with those props.

blvdbuzzard

That is a good looking plane. Lots and lots of work there.


Jeff, this "Current Capacity 42Amps/60s (Don't know what the 60s mean) " The 60 seconds is how long it can handle the 42 amps before it gets to hot.

At 12 1/2 pounds, you will need around 100 to 125 watts per pound. So 12.5 X 100 = 1250 watts, 12.5 X 125 = 1562.5 watts, more can be better.

Looking at the motor specs you gave, "Max Efficiency 20-30Amps" If you run a 3s battery, and the prop pulls say 25amps, you can multiply 11.1v (3s) the amps, just guessing here 25, = 277 watts. That is for a single motor. So you have (4) motors. That is 277 X 4 = 1108 watts. That is a little low to me, but I can be wrong. Really need to know how the prop works with the motor to get better numbers. I just used a few figures from your post to see what they would add up to. You may not be able to fly with the scale props.


Here is a good site to try out the power setups.

http://www.s4a.ch/eflight/motorcalc_e.htm

I would suggest getting a watt meter to try different props and batteries to get the correct setup. Here is a low cost watt meter that works pretty well.

http://www.hobbypartz.com/aeo-p0-watt-meter.html

Your mileage may vary

Buzz.

jmohn

I think if you upsize the wire to #12 you should be ok with the length. I have the same setup (larger) on two twins with longer runs and haven't had any issues. I up-sized the wires and ESC so they could handle the extra amps and it seems to be fine.



Jeff

Jeff Fokker

Forgot to mention that I will be using only 2 battery packs which means I will be running 2 motors for each pack. That means I will need High discharge rate batteries, correct?

Dr Kiwi

You will need something capable of delivering 70A or so for each pair of motors.....you can either go with high C rate (40C 2500 ....but flight time will be very short) or high capacity, lower C-rate (20C 5000).... or multiples thereof!

Jeff Fokker

First of all, I want to thank all of you for your help so far. It has helped me to design better where each component must go.
My concerns are still weight and balance. To keep from adding unnessesary nose weight, I am putting the batteries in the fuse just behind the cockpit and the speed controllers will be just aft of them.

I will be making the wires from the batteries to the speed controllers short (10" or less).
Because this plane is a 4 motor with a 7 foot wing span, the outboard motors are way out there. Can I make the wires that go to those motors 33" to 36" from the controller to the motors and be OK?

Thanks again.