Counterweights primarily serve three functions in aircraft design: balancing control surfaces, controlling propeller pitch, and balancing crankshafts in piston engines.

## Control Surfaces ##

Counterweights are used to balance control surfaces about the axis of the hinge to avoid potentially dangerous control flutter. This balance is very important. Pilots typically check for counterweight structural security in preflight where able. Mechanics must recheck control surface balance after painting, and painting control surfaces is excluded from the list of preventative maintenance items that an aircraft operator can perform without a mechanics license.

From the A&P Mechanics Airframe Handbook [AC65-15A:][1]

> It is this out-of-balance condition that can cause
a damaging flutter or buffeting of an aircraft and
therefore must he eliminated. This is best accomplished
by adding weights either inside or on the
leading edge of the tabs, ailerons, or in the proper
location on the balance panels. When this is done
properly, a balanced condition exists and can be
compared to the seesaw with a child sitting on the
short end of the plank. 

## Propellers ##

Counterweights are used in some constant speed propeller designs to increase the pitch of the propeller blades. Engine oil pressure is used to overcome the force of the counterweights to move the propeller blades back to fine pitch. In feathering designs, the counterweights assist in moving the blades to the feathered position.

From the A&P Mechanics Powerplant Handbook [AC65-12A][2]:

> Propellers, having counterweights attached to the blade clamps,
> utilize centrifugal force derived from the counterweights to increase
> the itch of the blades. The centrifugal force, due to rotation of the
> propeller tends to move the counterweights into the plane of rotation,
> thereby increasing the pitch of the blades.

<p>

> Feathering is accomplished by releasing the governor oil
> pressure, allowing the counterweights and feathering spring to feather
> the blades. This is done by pulling the governor pitch control back to
> the limit of its travel. which opens up a port in the governor
> allowing the oil from the propeller to drain back into the engine. The
> time necessary to feather depends upon the size of the oil passage
> from the propeller to the engine, and the force exerted by the spring
> and counterweights.

## Crankshafts ##

Counterweights are used in piston engines for both static and dynamic balancing.

From the A&P Mechanics Powerplant Handbook [AC65-12A][2]:

> **Crankshaft Balance**  
Excessive vibration in an engine not only results
in fatigue failure of the metal structures, but also
causes the moving parts to wear rapidly. In some
instances, excessive vibration is caused by a crankshaft
which is not balanced. Crankshafts are balanced
for static balance and dynamic balance.
A crankshaft is statically balanced when the
weight of the entire assembly of crankpins, crank
cheeks, and counterweights is balanced around the
axis of rotation. 'When testing the crankshaft for
static balance, it is placed on two knife edges. If
the shaft tends to turn toward any one position during
the test, it is out of static balance.
A crankshaft is dynamically balanced when all
the forces created by crankshaft rotation and power
impulses are balanced within themselves so that little
or no vibration is produced when the engine is operating.
To reduce vibration to a minimum during
engine operation, dynamic dampers are incorporated
on the crankshaft. A dynamic damper is merely a
pendulum which is so fastened to the crankshaft that
it is free to move in a small arc. It is incorporated
in the counterweight assembly. Some crankshafts
incorporate two or more of these assemblies, each
being attached to a different crank cheek. The
distance th~ pendulum moves and its vibrating frequency
correspond to the frequency of the power
irnpulses pf the engine. When the vibration frequency
of the crankshaft occurs, the pendulum oscillates
out of time with the crankshaft vibration, thus
reducing vibration to a minimum.

>**Dynamic Dampers**  
The construction of the dynamic damper used in
one engine consists of a movable slotted-steel counterweight
attached to the crank cheek. Two spoolshaped
steel pins extend into the slot and pass
through oversized holes in the counterweight and
crank cheek. he' difference in the diameter between
the pins and the holes provides a pendulum
effect.

Counterweights should not be confused with the related—but different—concept of Ballast, which provides a temporary or permanent change to overall aircraft center of gravity.


  [1]: https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_65-15A.pdf
  [2]: https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_65-12A.pdf