Reducing control forces
Horn balance
Geared tab (or Balance tab)
Move in opposite direction to control
surface
Making turning easier
Frise ailerons
Differential ailerons
(Nimbus 3 uses spoilers as well)
Providing feel
Springs in the control circuit
No drag but force not related to speed
Anti-balance tabs
Moves in same direction as control
surface
Proportional force but extra drag
Avoiding flutter
Mass balancing