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position of the system via a rotary encoder or other position
transducer. This is indeed how car wheels are balanced. It’s
worth looking at some videos on YouTube of how it works.
Dynamic balancing can take care of those “I only feel the
vibration at 65-75 MPH” problems.
Dynamic balancing can be a great benefit to model
aircraft balancing, but it is also a complex technique that
most hobbyists do not pursue. There are some trying to make
a full dynamic balancer; others are using a cell phone as the
accelerometer or attaching a laser pointer to the motor
mounting arm and experimentally balancing the system to
produce the smallest amount of wobble. At some point in the
future, we may explore dynamic balancing, but the next
technique is cheap, easily performed, and often does a good
enough job even for videography.
Static balancing involves removing the propeller from the
system and putting it into a balancing frame. This frame
holds the propeller, centered around an axle that is
suspended on very low-friction bearings. Often, knife edge
rollers or magnetic bearings are used. If one side of the
propeller is heavier than the other, the heavy blade will settle
at the bottom of the assembly. By iterating — either adding
weight or removing weight from the light or heavy blades,
respectively — the propeller can be balanced surprisingly well.
The static balancing method is much simpler than
dynamic balancing, and is also much less dangerous as it
does not involve spinning the propeller at high speeds on the
bench top. The equipment to do this is also very economical.
While I’ve heard of people balancing an axle between two
coffee mugs, I would recommend buying a commercially
Bro 499 Tru-Spin balancer (Figure 11). I’ve used this to
balance several sets of multi-rotor propellers and haven’t had
Static Balancing Procedure
To static balance your propellers, you should mount the
propeller on the axle that comes with your balancing kit
(Figure 12). You can experiment with balancing the propeller
with and without the bore-reducing bushing inserted.
However, I have not noticed a large difference in my
experimentation. Make sure that the balancing apparatus is
level on your bench (Figure 13) and that the air in the
workspace is still. A light breeze blowing through the shop is
nice, but will provide unending headaches when balancing
propellers. Place the axle with the propeller on the balancing
stand and wait for it to settle. It is very likely that one blade
will settle to the bottom of the stand (Figure 14).
To balance the propeller, we’ve got two options. We can
add weight to the light side that is sticking up or we can
subtract weight from the heavy side that sinks to the bottom.
If you want to add weight to the light side (arguably the
easier way to balance a propeller), you can place a small
Figure 13: A simple bubble level can be used to make sure
your propeller balancer is leveled on the workbench. These
are available at the hardware store for about $3 and are
always handy to keep on hand.
Figure 12: Propellers are
clamped and centered on
the axle by two aluminum
cones. The moveable cone
is held firmly against the
propeller bore by a spring
that is held in compression
by a tight fitting piece of
Figure 11: The
prop balancer is
reliable way to