This is the setup that the author uses to test the thrust and current draw of micro motors. It
includes a test stand, a multimeter, and a scale to measure grams of thrust.
The author found some powerful new motors
in several micro quadcopters, such as the 2.5-
ohm 6mm motors in this Efly Hobby Smart X RC
Choosing the right motor
One of the questions that I’m frequently asked is: What size motor should I use for my micro
RC project? The question is not easily
answered because several factors can
affect the decision.
First you must determine what type of
aircraft you will model. Is it 3-D, scale,
or maybe a slow-flying sport model?
You also need to consider the model’s
weight, wing area, or wing loading. Then
you must decide whether to go with
a direct-drive brushed motor, geared,
or brushless motor. All of this can be
challenging for a novice or even for an
experienced modeler trying to get a
complex project to fly just right!
The easiest way to find the right
motor is to learn what others are using
in similar projects, or use the same
motor setup as an ultra-micro RTF
airplane of the approximate size and
weight of the aircraft you are modeling.
This approach may not work in some
cases, so you must do some testing to
find the right fit!
Today we have more choices than
ever in the micro motor department!
Along with all of the new micro RC
aircraft that manufacturers are releasing,
more powerful and reliable motors are
also being released. I recently tested
some 6mm brushed motors from micro
quadcopters and got great results.
To test the motors, I use a simple test
stand made out of light plywood, a small
accurate scale, and a multimeter. With
the test stand, I can easily measure the
motor’s static thrust and the current
draw by using the multimeter.
I also use the multimeter to determine
the motor’s ohms. The lower the ohms,
the more power the motor will usually
produce, but it will also draw more
current. If the current is too high, it may
exceed the amp rating of the
speed controller you
are using, or
give you brief
Several other factors affect the current
draw and motor thrust, including the
propeller size, propeller pitch, and
whether the propeller is gear-driven or
direct-drive. A great way to increase the
thrust of a brushed motor and reduce
the current draw is to use a gear drive.
A gear drive allows you to swing a larger
propeller at slower rpm.
Let’s look at an example of how I
found the right motor drive for my
1/72-scale Corsair design. My goal
was for the model to have scalelike
performance and be able to swing a
scale three-blade propeller. The model
weighs approximately 4. 2 grams with
everything installed except the motor.
I first tested the proven Plantraco
GB05 4mm motor drive unit that I
use in many of my designs. I like to
test the motors with the receiver and
battery that I plan to use in the model.
This gives me a more realistic thrust
reading than using a power supply with
a constant voltage setting.
I found that the GB05 produces 3. 5
grams of thrust with a two-blade carbon-fiber propeller, and only 2 grams with a
scale three-blade propeller—clearly not
enough power to fly the model with the
scale three-blade propeller.
I tested a 2.5-ohm 6mm motor taken
out of a micro quadcopter direct-drive
unit with a three-blade propeller. The
scale three-blade propeller was drawing
too much current, so I reduced the size
to lower it. This looked like it would
work because it was producing roughly
4 grams of thrust. With a ready-to-fly
weight of 5. 2 grams, the model flew
well, but the run time was too short
using the direct-drive setup.
I next tried a geared setup to reduce
the current draw, which allowed me to
swing the scale-size propeller. I decided
to test the 2.5-ohm 6mm motor in
a gearbox with a 5: 1 gear ratio. This
worked perfectly. The geared motor now
produces more than 5 grams of thrust
with a scale-size three-blade propeller!
Master Free Flight modeler Robert
Pajas, from the Czech Republic, sent
me some photos of his latest project:
a beautiful profile micro-RC B-737 in
86 Model Aviation SEP TEMBER 2015 www.ModelAviation.com