Frequently Asked Questions - 2007/11

Operating multiple brushless electric motors
? Frequently Asked Questions Bob Aberle | [email protected]
Also included in this column:
• Measure and record your
model’s airspeed
• Electric motor timer for FF
applications
• Programmable add-on BEC
• New AMA Web-site format
Mock-up of twin-motor system connected to RC system
that operates ailerons and elevator. A separate ESC is used
with each brushless motor. Both BEC circuits remain
connected. One battery pack supplies power.
To connect battery leads from the two ESCs Bob
fashioned his own adapter cable. The Deans four-pin
male connector plugs into the battery pack.
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 Web site. 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.
When it comes to the modern brushless
motors, that advantage is lost. Although
some modelers have gotten two brushless
motors to work off of one ESC, the chances
of doing that in a reliable manner are slim.
Brushless motors require a dedicated
ESC for each power plant, so if you have a
twin-motor aircraft you will 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 as
opposed to using brushed motors.
I have decided to design and construct a
fantasy-scale electric-powered twin of the
Burnelli CBY-3 Loadmaster lifting body
transport. I now use only brushless motors,
so I asked the people at Castle Creations if
they had any advice for twin brushlessmotor
applications.
Better than that, I learned that Joe Ford
of Castle Creations has written an excellent
two-page article about brushless multimotor
power systems. You can find it at the
Bob designed this
fantasy-scale electric twin
version of the Burnelli
CBY-3 Loadmaster. It
spans 36 inches and
weighs 19 ounces.
November 2007 61
11sig2.QXD 9/21/07 9:27 AM Page 61
The companion See How reads the How
Fast’s LED flashes and then provides the
exact airspeed (in this case 47.9 mph) on
a digital LCD.
Castle Creations’ CC BEC provides a BEC function when using up to 25 input volts. It
can also supply up to a 10-amp load. It eliminates the need for a second battery to
operate the RC system.
The How Fast components in Bob’s Nostalgia RC Impulse. He removed just enough
covering to add the circuit board, two probe tubes, and interconnecting clear plastic
tubing. Then he re-covered this part of the wing.
Winged Shadow Systems’ How Fast mounts inside your model. It consists of a circuit
board, two aluminum tubes, and clear plastic tubing weighing less than 3 grams.
address listed at the end of the column.
The first part of Joe’s write-up explains
why you need a separate ESC for each
brushless motor. I also learned that to do the
job right you need identical ESCs. They
should even be from the same production
run, in case the product was revised during a
period of time.
With the Castle Creations Phoenix line
of ESCs you can leave the BEC connected
on each ESC. There is no need to remove a
power lead on all but one ESC.
In fact, by having more than one BEC
connected, you are adding to the currenthandling
capability. That means you will be
able to operate more servos without an extra
RC system battery.
Another recommendation is to use only
one battery pack. If you are short on
capacity you can parallel two battery packs
with the same capacity, but ultimately there
is only one positive and one negative feed
from the battery pack to the ESCs. I made a
“Y” connector that combined both ESC
battery leads into a single connector going
to my single battery pack.
The Castle Creations article tells the
entire story and is a must-read. You can
even learn how to precisely program
recommended settings into both Phoenix
ESCs using the Castle Link programming
device along with a PC.
By the way, my Burnelli transport will be
the subject of a future construction article.
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?”
62 MODEL AVIATION
11sig2.QXD 9/21/07 9:30 AM Page 62
A321: Winged Shadow Systems recently
began advertising the new addition to its line
of aircraft accessory items: the How Fast
airspeed measuring device.
This tiny electronic unit (which is
roughly the same size as the How High)
goes onboard your model. It weighs less
than 3 grams and can measure airspeeds
from 15 to 500 mph with a resolution of 1
mph.
The How Fast consists of a PC board
containing all the electronics along with a
pressure switch. An LED exits one end of
the PC board, and when installed inside the
aircraft it must be visible from the exterior.
The flashing of that LED is how you
measure the airspeed.
In addition to the PC board you receive
two 4-inch lengths of 1/8-inch-diameter
aluminum tubes, which are designated as the
static and pitot probes. They must be
mounted out on your model’s wing, beyond
the propeller arc. Each probe is connected
back to the PC board via clear plastic
flexible tubing. The instructions tell you
how to locate, mount, and connect the
probes.
A cable that exits the PC board plugs
into any auxiliary, or extra, channel receiver
on your RC receiver. If you are flying a non-
RC model (CL or FF), you can substitute a
small battery pack of 3.2-12.0 volts to power
the How Fast.
A photo shows how I installed my unit
on an existing RC model. This installation
might be easier if it is done when the model
is being constructed, but it is still simple for
already built aircraft, ARFs, and RTFs.
During a flight the How Fast records the
highest speed attained. That reading is
stored as long as the battery power remains
on. You can read the airspeed upon landing
by counting the LED’s flashes or using the
companion product called the See How.
You place the See How over the LED
and it counts the flashes, providing a digital
readout of the airspeed.
You can find the complete instruction
manual for the How Fast on the Web site.
The current price is $44.90 plus postage.
Q322: “One of my electric powered aircraft
employs a 6 cell Li-Poly battery pack. At a
nominal 22 volts, I was forced to use a
separate battery pack to power my RC
system, since the BEC inside my ESC
couldn’t handle anything above 12 volts.
“Because all of my other electric
powered aircraft have single battery packs
and BECs, I forgot to charge that one
‘extra’ pack and as a result my plane
crashed when the RC system ran out of
juice. Is there any way to beat this
problem?”
A322: In the past couple years several addon
BECs have been developed and marketed
that eliminate the need for the extra (or RC)
battery pack. A device that comes to mind is
the U-BEC, which is sold by Hobby Lobby
International.
New types of ESCs have been introduced
more recently that employ a switching
circuit to provide a regulated 5 volts to
operate your RC system from the one main
battery pack. The switching circuit, unlike a
solid-state voltage regulator, can accept a
much higher voltage and still reduce it to a
usable 5-volt level to power the RC system.
One of those ESCs is the Jeti SPIN,
which is also available from Hobby Lobby.
(Other suppliers have similar products.)
The engineers at Castle Creations have
come up with their own add-on BEC: the
CC BEC. This tiny device weighs only 11
grams (0.4 ounce) and sells for $24.95. The
big surprise is that not only can it handle
battery voltages as high as 6S (Li-Poly) or
roughly 25 volts, but it can also supply an
extremely high current of up to 10 amps to
operate almost any kind and size of RC
system.
The CC BEC has another unique feature.
If you plug it into the Castle Creations PHLink
and a PC, you can select the regulated
output voltage from 4.8 to 9.0 in 0.1-volt
increments. The unit is shipped with a
default setting of 5.5 volts.
The beauty of this feature is that you can
set the exact voltage to obtain just the right
amount of servo speed. Keep in mind that
your receiver and servos have certain limits;
it is best to check with the RC
manufacturers before you increase the
voltage substantially. Visit the Web site for
more information.
Q323: “I just recently called up the AMA
website and discovered to my surprise that it
appears to have a new format. All the
information provided appears to be the
same, but in some cases it isn’t obvious
where things are now located.
“For example, I wanted to order a set of
plans and couldn’t find the ‘Plan Service.’
Can you help me out?”
A323: I knew a new Web site format was on
the way but was as surprised as anyone
when it appeared (in June 2007). As I recall,
the old format had a Plans Service box on
the home page that you could click on.
I did a little “clicking around” and
learned that you must now click on “Shop
AMA,” either at the top of the home page or
at the bottom. That takes you to six items,
one of which is “Plans Service.” Clicking on
that will take you where you want to be to
purchase sets of aircraft plans.
I’m hoping there will be an article that
introduces and explains many of the new
Web site features and those that are planned
for the future. I understand that as part of the
new format a search program will be
established so all of my frequently asked
questions and answers can be easily located
by main categories and subcategories.
If you have suggestions and/or
comments about the new format, I would
like to hear from you. MA
Sources:
Frequently Asked Questions
www.modelaircraft.org/mag/faq/index.asp
AMA Plans Service
www.modelaircraft.org/plans.aspx
Hobby Lobby International
(615) 373-1444
www.hobby-lobby.com
Winged Shadow Systems
(630) 837-6553
www.wingedshadow.com/Hfinstr.pdf
Joe Ford’s multimotor power-system article
(on pages 2 and 3):
www.castlecreations.com/media/castle_scri
be_archives/Castle%20Scribe%20CE-03-
Mar2006.pdf