Propeller Basics
The wall of propellers at any hobby shop is an interesting
place. There are lots of packages with specifications that
mean little to many new to the hobby. Choosing the right
propeller for your multi-rotor can make the difference
between a stable, maneuverable, and fun-to-fly machine and
a one that barely leaves the ground in a buffeting fit of noise.
Propellers, roughly as we know them, have been around
since the Archimedes’ screw. Later, the Wright brothers
realized that their airfoil research could be applied to make
more efficient propellers that used a twisted wing design. As
it turns out, their design was only around 5% less efficient
than modern aircraft propellers ( http://web.archive.org
/web/20110604093014/ http://www.memagazine.org/su
pparch/flight03/propwr/ propwr.html)! The main
characteristics of a propeller are its length, pitch, bore,
direction, and material.
Length
Length is the most self-explanatory of the propeller
characteristics. It is simply the length of the propeller from
tip-to-tip, generally specified in inches (Figure 1). When
propellers are sold, they are commonly marked as a set of
two numbers; “10x4.5” for example. The first number ( 10 in
this case) is the propeller diameter. The thrust of a propeller is
proportional to the area of the propeller in contact with the
air (and a large number of other factors).
If we look back at some geometry, we know that the
area of a circle is calculated as pr2 where r is the radius of the
circle. This means that increasing the length slightly produces
a change in the area of contact that is faster than simple
linear growth. We can see this by looking at the graph in
Figure 2. Larger propellers need to be spun slower than
smaller propellers to generate the same amount of thrust,
and are generally more efficient.
With the quadratic growth of contact area with propeller
length, you might think we should put 16” props on all of
our quads, but, as always with engineering, there is a
tradeoff. The larger propellers are also heavier than their
smaller counterparts. Anytime you start talking about
swinging masses around, the concept of inertia should come
to mind. The larger propellers with their greater mass will
take longer to change speeds than smaller propellers. If they
The
Multi-Rotor
Hobbyist
Understanding and Balancing
Propellers
By John Leeman
Post comments on this section and
find any associated files
and/or downloads at
www.servomagazine.com/
index.php/magazine/article/
December2016_MultiRotorHobbyist_
Understanding-Balancing-Propellers.
46 SERVO 12.2016
Figure 1: The length specification of a propeller indicates
the diameter. Props are commonly sold using imperial
measurement, but many manufacturers are now marking
them with metric dimensions as well.
Figure 2: The diameter of
a propeller grows linearly
with the radius, but the
area covered by the swing
of the propeller grows
with the square of the
radius. A small increase in
propeller diameter can
make a big difference in
your performance.
If you’ve ventured into a hobby shop looking
for propellers for your multi-rotor, you’ve
probably been overwhelmed by the selection
available. There are propellers of different
lengths, pitches, and materials — all of which
could fit your motor. The price difference
between seemingly similar propellers can be
large. This month, we will go over what you
need to know about propellers and how to
balance them to have the best flight
experience.
