Scorpion S-4020-12 Motor Tested

Originally published in the Oct. '08 Ampeer

The Scorpion S-4020-12 Tested
By Ken Myers

Innov8tive Designs Photo from Web site

     My flying buddy, Dave Stacer, decided that he would like to build an Electroflying Models Fusion sport plane. He'd seen mine flying for several years, and decided it was time for him to add one to his fleet. I recommended a Scorpion S-4020-12 to him based on the specifications published on the Innov8tive Designs Web site ( He picked one up at the 2008 Weak Signals Toledo R/C Expo.
     He asked if I'd like to check out the motor and verify that it is indeed the right choice.
     On August 28, 2008, I ran this motor through my typical motor testing, which includes gathering all the data necessary to enter into ( Drive Calculator.

Manufacturer's Specs (from the Web site):
Motor Wind: 12 Turn Delta
Motor Kv: 542 RPM / Volt
No-Load Current (Io) @ 21v: 1.92 Amps
Weight: 304 Grams (10.72 oz.)
Outside Diameter: 48.7 mm (1.917 in)
Shaft Diameter: 6.0 mm (0.236 in)
Body Length: 41.2 mm (1.622 in)
Overall Shaft Length: 96.0 mm (3.779 in)

Measured Specs:
Mathematical Motor Kv using drill press: 532 RPM / Volt
Drive Calculator Kv using computed Kt: 525 RPM / Volt
No-Load Current (Io) @ 13.6 v: 1.50 Amps, 7069 RPM
No-Load Current (Io) @ 20.9 v: 1.97 Amps, 10812 RPM
Motor Weight w/leads: 304 Grams (10.72 oz.) verified
"+" mount w/4 screws: 19.5g (0.69 oz.)
Prop Adapter: 22.0g (0.78 oz.)
Prop Adapter Prop Shaft Diameter: 7.874mm (0.31 in.)
Outside Diameter: 48.7 mm (1.917 in.) verified
Shaft Diameter: 6.0 mm (0.236 in.) verified
Body Length: 41.2 mm (1.622 in.) verified
Overall Shaft Length: 96.0 mm (3.779 in.) verified

Prop Weights:
APC 12x8E 24.1g (0.85 oz.)
APC 12x10E 22.8g (0.80 oz.)
APC 13x8E 27.9g (0.98 oz.)
APC 14x8.5 33.0g (1.16 oz.)

     This was one of the most amazing motor tests that I have ever conducted.
     I started the testing by weighing and measuring the motor and its components. I believe that this is the very first motor that I ever measured and weighed that EXACTLY matched what the manufacturer/supplier stated.
     I realize that I do not have "lab grade" testing equipment, but it is amazing how close my testing results matched the stated Kv and no-load (Io).
     For the load test, using various props, a BP Hobbies 70A Brushless electronic speed control (ESC) was used. It was set up as follows:
Cell Type and Number of Cells: NiMh/NiCD Auto Cell Count Š 0.8V/Cell Cutoff Voltage
Throttle Setting: Auto Throttle Range
Brake Setting: No Brake
Direction and Cutoff Type: Soft Cutoff
Timing Mode Setting: 1-deg
Pulse Width Modulation (PWM) Setting: 8Khz

     Average Results from Prop Tests using a 6S "A123" pack (Equivalent volts to an appropriately sized 5S Li-Po pack):
Note: The results do not represent the maximums obtainable. The pack was NOT recharged after the no load test and it was then used for the APC 14x8.5E test. The other three tests were done using a freshly charged pack. As usual, data for 5 runs, 5 seconds apart were collected using my Emeter to store the data and then it was averaged. APC 14x8.5E: 16.648v, 38.294 amps, 7518 RPM, 638 watts in (A freshly charged pack would have been much closer to 17v, but I just needed the numbers for Drive Calculator.)
APC 12x10E: 17.43v, 33.552 amps, 8004 RPM, 585 watts in
APC 13x8E: 17.502v, 33.006 amps, 8064 RPM, 579 watts in
APC 12x8E: 17.922v, 29.82 amps, 8358 RPM, 535 watts in

     The data was entered into Drive Calculator. Drive Calculator calculates the Kv a few different ways, but I used 1355 divided by the Kt value calculated by Drive Calculator to derive the number presented here.
     Scorpion states that the Kv is 542. Using the drill press test, I mathematically arrived at 532, which is less than a 2% difference. Drive Calculator's Kv of 525 is only about 3% different. Nice job Scorpion. You've set a very high standard for others to try and beat. All of the Scorpion measurements and weights were 100% spot on!
     Another popular way of "naming" brushless outrunners is using the outside diameter in mm followed by the length in mm and then the Kv. Therefore, when this motor is "named" using this convention it might be called a 4941-540 or maybe a 50-42-540.


     I'm using an AXI 4120/18 in my Fusion. That motor is paired with my Hyperion 50-amp OPTO ESC and either a 6S "A123" pack or 5S 4000mAh Li-Po. That power system moves the Fusion extremely well around the sky.
     My AXI 4120/18 doesn't pull quite as many amps as the Scorpion because it has a slightly lower Kv of about 515.
     I selected data from my AXI testing and Scorpion S-4020-12 testing that shows a comparison between these two motors using an APC 12x10E prop. The two points chosen allow for the slightly higher voltage, lower amp draw of the AXI.

Scorpion S-4020-12, 17.54v, 33.99 amps, 8040 RPM, 596 watts in
Drive Calculator indicates; watts out: 486, eff. 83.6%

AXI 4120/18, 17.75v, 30.8 amps, 7845 RPM, 547 watts in
Drive Calculator indicates; watts out: 447, eff. 83.1%

     These numbers indicate that these two motors, when used in the same application, are just about equal. There may be a slight edge going to the Scorpion, depending on how you want to evaluate the data.
     Dave's plane should fly great and certainly won't be lacking in power!

Dave's barebones Fusion

Dave's Fusion on the left, Ken's on the right

Update: Dave Stacer's Electro Flying Fusion Takes to the Air!

     On Saturday, October 4, at the very well attended EFO flying meeting, Dave's Fusion took to the air and flew GREAT! Ken flew the initial trim flight with Dave beaming the whole time. Since Keith was there, he took it up and flew every possible maneuver that this great sport plane is capable of. What a flight. Dave did several more flights with HIS new plane and was pleased that he persevered through the construction of this true builderÕs kit.

     It came out at exactly 80 oz., which gives this 569 plane a cubic wing loading of 10.2 oz./cu.ft., which is typically an advanced sport plane. The plane's power system includes the Scorpion S-4020-12, BP Hobbies 70-amp Opto ESC, a split pack (3x2) 6S1P "A123" 2300mAh pack and an APC 12x10E prop. That is one very winning combination.

Dave with his Fusion after the maiden

     Unlike other sites, there is NO paid advertising on this site! When you see an ad link, Ken Myers uses and recommends the product(s), supplier or manufacturer. He uses the "stuff" or supplier himself!