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From the Rogue Eagles R.C. Club, Medford, Oregon

How to Convert a Gas Model to All Electric

by Melvin S. Harder

1. Determine weight from catalogue. In the case of the Ultra Stick 120, the published weight is nine to 11 pounds. An electric conversion will weigh about 10 pounds, total flying weight.

2. Determine desired performance. I selected unlimited aerobatic performance and 3-D; 150- to 200-watts-per-pound. 746 watts=one horsepower.

3. Motor selection. Ten pounds x 150 to 200 watts=1,500 to 2,000 max watts capability motor. I selected a Hacker C50-7XL, which has a max watts capability of 1,700 watts (again, watts=power). Select your motor based on cost, quality, and personal preference; my buddy has one. There’s a lot to choose from.

4. Use the software program Moto Calc (motocalc.com).

     a. Enter in: wingspan, wing area, weight, wing loading, and

         wing shape.

     b. Enter in: motor constant, no-load current, resistance,

         weight, brushless, and out runner or in runner.

     c. Enter in: gear ratio, propeller diameter (from motor

         recommendation range), and propeller pitch (from motor

         recommendation range).

     d. You get from the Moto Calc calculated spread sheet: stall

         level, level flight speed, top speed, thrust, and max amps.

5. The speed of the propeller is regulated by the ESC, which regulates the frequency of the max amps—low speed is less frequent, high speed is more frequent. Pulse technology.

6. ESC. From the max amps listed in Moto Calc, plus a little head room, select an ESC. Moto Calc told me that my max amps was 107.5 so I selected a Castle Creations Phoenix 125 (max capability of 125 amps). Again, ESC selection depends on cost, quality, and personal preference. There is a lot to choose from.

7. Use Moto Calc for a battery recommendation starting point. Battery capacity is the battery’s C rating multiplied by amps (5,000 mAh=5 amps). For me, 25C x 5 amps=125 amps drawn capacity. 125 amp>107.5 amps.

8. The battery tray is formed using 1/8-inch plywood and some ¼ x ¼-inch guide rails on both sides of the tray. I used a Velcro strap to hold the battery in place.

9. For an electric airplane, the CG is constant; no gas is used causing the CG to move backwards during flight. Battery placement is used to balance the model.

10. Safety

     a. Mount servo switch and motor arming plug well behind the

         propeller—best so you can stand behind the wing, well

         away from the propeller.

     b. Keep in mind that electric motors have 100% torque at 1

         rpm. Gas motors build up to that point.

     c. Starting sequence

              i. Put throttle in middle position

              ii. Turn transmitter on

              iii. Turn servo switch on

              iv. Check control surfaces

              v. Plug motor arming plug (motor will make a tweaking

                 sound)

When you are standing at the pilot station, move the throttle back to the idle position, which will arm the motor (motor will make another tweaking sound). You are ready to fly. Q

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