balance. Vibration will likely cause the prop to move until one
wall of the bore is touching the motor shaft. By this point,
your airframe will be vibrating violently and there will likely be
damage to it and possibly to you. It is also unwise to drill out
a propeller bore without proper machining equipment for the
same reason.
Direction
Propellers are sold in clockwise
(CW) and counter-clockwise (CCW)
versions; also referred to as right
and left handed propellers. For a
quad rotor, you need two of each.
The direction of rotation is defined
when looking down from above at
the top of the propeller. Most
manufacturers mark the propellers
with a letter on the casting, but you
can also look at the direction that
the leading edge of the propeller
points (Figure 9). Make sure that
you have propellers of the correct
direction on motors rotating the
appropriate direction!
If your quad will not lift off,
but rather tries to flip over in one
direction or vibrates with no life, it
is very likely that the propeller
and/or motor direction
configuration is incorrect.
Material
Model aircraft propellers are
commonly made out of wood, plastic, or carbon fiber.
Wooden propellers are common on model airplanes, but I
haven’t seen them used on multi-rotors. Wood is a pretty
heavy material, so it adds weight to the vehicle and is much
harder to speed up or slow down compared to a similarly
sized plastic or carbon fiber propeller.
Plastic propellers are the most common, and what we’ve
used on our home-built quadcopter and also the Parallax
ELEV- 8. Plastic propellers are lightweight, economical to
manufacture and buy, and easy to balance. I’ve flown plastic
props on most everything I’ve owned — airplanes and quads
alike. In a crash, you can almost bet on the brittle plastic
failing and needing a new propeller. The bending and
fracture of the propeller does absorb some of the energy of
impact though, so it can save your
motor sometimes.
Carbon fiber propellers are very
expensive compared to their plastic
counterparts, but some pilots are
devoted fans. Carbon fiber is
phenomenally light meaning that
larger propellers can still have quick
spin up/down times. The propellers
are also outstandingly strong which
is good in most cases, but also
means that more force can be
transferred to the motor and
airframe during a crash. They are
also much more damaging if they
encounter objects while in flight.
Remember the video of a carbon-fiber propeller interacting with a
human simulant (https://
youtu.be/QQoTQZcwZWE)?
Why Balance
Propellers?
Have you ever noticed that
while driving your car, there was a
vibration in the steering or that you
could feel through the seat? The
48 SERVO 12.2016
Figure 7: The bore of the propeller
must be matched to the drive
motor to avoid any off-center
operation. In this photo, the
propeller size, pitch, and direction
are visible in the casting as well.
Figure 6: Notice how the angle of the propeller
changes slightly from the interior to the tip. The
angle must be reduced at the tip for the
propeller to maintain a fixed pitch, and is called
“washout” in the aviation community.
Figure 5: The tangential velocity of the propeller must
linearly increase with the distance from the center. At the
tip of a 10” diameter propeller, the speed can approach
300 MPH! At these speeds, inertia can be a real concern
and make the propellers very dangerous.
