Page 136 - Build Your Own Quadcopter_ Power Up Your Designs with the Parallax Elev-8

Chapter 5: Quadcopter Propulsors 115

have been relatively unchanged since the days of the Wright brothers. Modern propellers are
about 80% efficient, which is about the same as the Wrights experienced. Propeller efficiency
is defined as:
propulsive power output thrust + axial velocity
H = =
shaft power input resistance torque + angular velocity

Basically, efficiency is about how much effective power is produced versus how much is
input. Input, in our case, is the rotating outrunner motor, and output is the resultant thrust
that is created. Efficiently operating propellers depend on an optimal angle of attack (AOA),
which is mainly a function of how fast the propeller is rotating and how much power is being
generated. In complex airplanes, the propeller AOA is adjustable; however, in our case, the
propellers have a fixed pitch, or AOA. The AOA is set by the manufacturer in a compromise
setting that tries to optimize efficiency for the expected operating range. There is really
nothing you can do to improve propeller efficiency other than to try different models that
have been designed to closely match your requirements. I think that the propellers provided
in the Elev-8 kit are adequate to do what is needed. They are also relatively inexpensive,
which is a key consideration, since you will need to replace them after flight mishaps.
Selecting a properly sized propeller is an important activity and a topic that has some
degree of anecdotal qualities associated with it. Everyone has an opinion on this topic so I
will offer some general guidelines:

• Thrust is proportional to the propeller surface area; therefore, larger propellers
mean more thrust. When compared to the small-diameter propellers, the large ones
also need more power in order to spin and achieve the same angular velocity.
• Conversely, small-diameter propellers need to spin faster to create the equivalent
thrust of a large-diameter propeller.
• Quadcopters tend to hover more than to fly in straight lines. A smaller pitch, or
AOA, is better suited for hovering than for aerobatic operations.
• Propellers should always be balanced to reduce vibrations. Investing in a quality
propeller balancer is a good idea.
• Carbon-fiber propellers are stronger than and vibrate less than plastic propellers.
Unfortunately, they cost quite a bit more than the plastic. Hold off on this investment
until you improve your flying skills and lessen the amount of propeller damage.

Table 5.5 shows some standard propeller sizes used in quadcopters to aid you in selecting
a propeller. It is very important to determine what functions you want your quadcopter to

Model Diameter Pitch Description
APC 1047 10 4.7 Popular size for midsized quadcopters. This is the
type used in the Elev-8
RPP 1045 10 4.5 Another popular size for midsized quadcopters
EPP 1245 12 4.5 Large propeller suitable for larger quadcopters
EPP 0938 9 3.8 Small propeller suitable for small quadcopters
EPP 0845 8 4.5 Small propeller often used on small quadcopters

Table 5.5 Standard Quadcopter Propellers