Author: Bob Aberle
Edition: Model Aviation - 2007/11
Page Numbers: 61,62,64
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Frequently Asked Questions

Bob Aberle | [email protected]

Operating multiple brushless electric motors

This is the 44th column in which I try to give you the best possible answers to questions you have written or e‑mailed to me. Each question is given a sequential number for future reference.

Several questions and answers have been posted to the AMA website. Unfortunately that posting stopped in August 2005 and will not continue until that section of the site is revamped. If you can't find the subject matter you are interested in there, drop me an e‑mail or letter and I will try to assist you.

Q320: "I'm planning to build a scale electric powered model that has twin motors. My choice of motors are brushless outrunners. Can I place the two motors in series and use one common ESC and a single battery pack?"

A320: My best advice is not to do it. For quite a few years I've had multimotor (as many as four) powered aircraft using brushed units hooked up in a series link, operated from one common ESC and a single battery pack. With modern brushless motors, that advantage is lost.

Although some modelers have gotten two brushless motors to work off one ESC, the chances of doing that reliably are slim. Brushless motors require a dedicated ESC for each motor. If you have a twin‑motor aircraft you need two brushless ESCs; if you use four motors, you need four ESCs. Using multiple brushless motors can get expensive, is more complicated, and will weigh more compared to using brushed motors.

I decided to design and construct a fantasy‑scale electric twin of the Burnelli CBY‑3 Loadmaster lifting‑body transport. I now use only brushless motors, so I asked the people at Castle Creations for advice about twin brushless‑motor applications. Joe Ford of Castle Creations wrote an excellent two‑page article about brushless multimotor power systems; you can find it on the Castle Creations website (see Sources).

Key points and recommendations from that write‑up and my experience:

  • Use a separate ESC for each brushless motor.
  • Use identical ESCs — ideally from the same production run — to avoid differences caused by product revisions.
  • With Castle Creations Phoenix ESCs you may leave the BEC connected on each ESC. Multiple BECs connected in parallel add to the total current handling capability and allow more servos to be operated without a separate RC system battery.
  • Use only one battery pack as the primary supply. If you need more capacity, parallel two battery packs of the same capacity, but ensure there is only one positive and one negative feed from the battery pack(s) to the ESCs. I made a "Y" connector that combined both ESC battery leads into a single connector to my single battery pack.
  • Use the Castle Link programming device (or equivalent) with a PC to precisely program the recommended settings into each Phoenix ESC.

The Castle Creations article tells the full story and is a must‑read for anyone planning multimotor brushless installations.

Q321: "I understand the company that first came out with an altimeter device (Winged Shadow Systems: 'How High') now has another on‑board monitoring accessory that provides in‑flight airspeed readings. Have you tried this new product? How difficult is it to install in an existing model?"

A321: Winged Shadow Systems recently began advertising the How Fast airspeed measuring device. It's a tiny unit (roughly the same size as the How High), weighs less than 3 grams, and measures airspeeds from 15 to 500 mph with 1 mph resolution.

The How Fast consists of a PC board with the electronics and a pressure switch. An LED protrudes from one end of the PC board and must be visible from the exterior of the aircraft; the LED flash rate is how you measure airspeed. The kit includes two 4‑inch lengths of 1/8‑inch‑diameter aluminum tubing for the static and pitot probes. These probes must be mounted out on the model’s wing, beyond the propeller arc, and are connected back to the PC board with clear flexible tubing. The instructions cover locating, mounting, and connecting the probes.

A cable from the PC board plugs into any auxiliary receiver channel. For non‑RC models (CL or FF), you can power the How Fast with a small battery pack of 3.2–12.0 volts.

During flight the How Fast records the highest speed attained; the reading is stored as long as the unit remains powered. You read the airspeed after landing by counting the LED flashes or by using the companion product See How. Place the See How over the LED and it counts the flashes, providing a digital readout of the airspeed.

Installation is straightforward and might be easier if done during construction, but it's still simple for existing aircraft, ARFs, and RTFs. Current price (at the time of writing) is $44.90 plus postage. See the Winged Shadow Systems manual for full instructions.

Q322: “One of my electric models uses a 6‑cell Li‑Poly pack (nominal 22 V). I had to use a separate battery pack for my RC system because the BEC inside my ESC couldn’t handle anything above 12 V. I forgot to charge that extra pack and crashed when the RC system ran out of power. Is there any way to prevent this?”

A322: In recent years several add‑on BECs have been developed to eliminate the need for a separate RC battery pack. One example is the U‑BEC sold by Hobby Lobby International. Newer ESCs with switching regulators can also provide a regulated 5 V (or other voltages) to operate your RC system from the main battery pack; switching circuits can accept much higher input voltages than linear regulators.

Examples:

  • Jeti SPIN ESCs (and similar products from other manufacturers) include switching BEC functionality.
  • Castle Creations makes the CC BEC, an 11‑gram device selling for about $24.95. It handles battery voltages up to 6S Li‑Poly (≈25 V) and can supply up to 10 A — enough for most receivers and servo loads. The CC BEC is programmable via the Castle PH‑Link and a PC; you can set the regulated output voltage from 4.8 to 9.0 V in 0.1‑V increments (default 5.5 V). This allows you to fine‑tune servo speed, but check receiver and servo limits before increasing voltage substantially. Visit manufacturers’ websites for details.

Q323: “I recently visited the AMA website and discovered a new format. The information seems the same, but it isn’t obvious where some things are located. For example, I wanted to order plans and couldn’t find the ‘Plan Service.’ Can you help?”

A323: A new AMA website format appeared in June 2007. The old site had a Plans Service box on the home page; in the new layout you must click “Shop AMA” (top or bottom of the home page). That brings up six items, one of which is “Plans Service.” Clicking that takes you to the Plans Service section to purchase aircraft plans.

I understand a search program will be added so all my frequently asked questions and answers can be more easily located by category and subcategory. If you have suggestions or comments about the new format, I would like to hear from you.

MA

Sources:

Transcribed from original scans by AI. Minor OCR errors may remain.