I've been following this discussion topic for a while; this is my first E post.  I think I'm a con-vert!

My first foray in to electric C/L was converting my Lite Ringmaster OSFP20 (27oz) into electric.  I originally started with an AXI 2808/26 soon to find out that it was not enough power.  The long story short, after many prop and motor changes using cheap Chinese  knock-off motors from eBay, I finally settled on a 2814/16 KV1050 which increased my flying weight from 27oz to 31oz because of 4s battery weight.  But even with the additional weight the mode now l flies with authority instead of barely going through the  OTS pattern.  It actually turns better on corner that it did with the OS 20FP.

Cobra 2814/16 KV: 1050 3.84oz.
Prop: APC E 10 X 5
Turnigy 4s 14.8V 1800mah
ESC: Castle Pheonix 35
Lines: 012 X 56
Timer: E-flite (you can easily set RPM and Time prior to flight)

Now, I am trying to convert my existing  .60 size stunt ship to E.  It is a modified Magnum with a built-up wing that is slightly thicker that a Sig Magnum.  It has a Saito 62 4-stroke. Even with that, it still is tail heavy so there is about 2ozs  in the nose to balance  it.  It is 70oz oz 720 sq in.  That put it at about 14lb/sq in.  The major problem is that it is tail heavy    I could probably trim a few inches off the rudder  and /or easily loose a few more ounces removing the solid rudder with a built-up  rudder.




Play around with just weights on an Excel spreadsheet, here is what I have so far.

With Electric Components   Grams   Weight (oz.)      Current Nitro Version   Weight (oz.)
Magnum  (no motor)                              47.00                                              47.00
Cutout motor mount and lightened tail      -3.00         
Cobra 3526/12 KV:650                253            8.92                  Saito 65 prop tank        20.30
Prop & hardware (Est.)                100            3.53                  Nose Weight               3.00
Castle Ice 50, Xtron timer        24.5            0.86         
 30c Lipo 3000mAh 5s               355           12.52         
                                     Total              69.84                               Total         70.30


1.   Yes it looks like I have 23oz’s to work with for motor, battery and  mounting hardware.  The problem is that it needs to be very far forward to equal the current nitro setup.  It looks like I might have to remove 3-4 oz of weight in the tail.  That might not be a problem because the rudder is huge.

2.   I am also not sure about battery 5s or 6s.  I could probably get away with 5s if I use a 13 X 6 prop. For a 13 X 4 prop, I need a 6s?

3.   In order to calculate the mah  required,  you must know the inflight current draw.  Cobra motor has very detail prop chart with give maximum amp and rpm for a given prop.  I know that you must allow for headroom  so your power setting should only be 70-80% of maximum power.  How to you calculate the amps if you only  know the current at maximum.  Do you just multiply by the same percentage?    Also, isn’t the inflight current draw different than  static current draw>

Heman  Lee

heres a quick and relatively accurate way to work out your required capacity

Total Watts = weight /16 *112
Average current = total watts/battery voltage
mAh Used - flying time/3600x (average current x1000)
Recommended capacity = mAh used/7.5*10

That is my question?  You do not know the average current draw, unless you take a real time plot of it during flight.  I have not even purchase the motor yet?

The spec for the motor is base on the max amps which remains constant.
Max Continuous Current ....................................... 52 Amps
Max Continuous Power on 4S Li-Po ...................... 770 Watts
Max Continuous Power on 5S Li-Po ...................... 960 Watts
Max Continuous Power on 6S Li-Po .................... 1150 Watts

Prop Spec
13 X 4E  40.13amp @22.2v @ 11,392 = 891 watts

So, using this prop do I just take 75 to 80% of the maximum and get
(80% x 32.1 amps @9600 )?

Therefore my battery usage would be:
((2.6ah) x ( .8 ) x (60)) / (32amps) = 6 minutes of flight time

Mike Palko electic article just use 35amp.  How did he come up with the 35amps?

Heman -

lets use an example - if your plane weighs 70oz then here is what you end up with

total watts = 70/16*112 = 490
average current = 490/18.6 (for a 5s pack) = 26.3
MAH used = 6minutes/3600 x2.63*1000 = 2630
Recommended capacity = 2630/7.5*10 = 3500mh

pretty simple

Heman,
Mike's point is well taken: do a survey of similar airplanes in the "list your setup" sticky thread above.
Yes it will take a while, and at 70 oz I would probably recommend a 5S 3000 or 3300 and a motor with a Kv between 700 and 750 that weighs between 7 and 9 ounces.
I use a figure of roughly 0.6 watt-hours per ounce of all-up weight to finish the pattern, then divide by 14.4 or 18 volts to get capacity used.The battery should be one-third larger than that number. The numbers are similar to what Wynn suggested.
Kepp us up to date on what you are doing ...
    Dean P.