Aircraft Motion
Physics of Aircraft
Lift
Drag
Weight and Thrust
Secondary Controls
Stability
Straight and Level
Climbing
Descending
Turning
Aircraft Design Features
The Stall
Practice Exam

Control Surface Balancing

Many training aircraft have cruise speeds around 100 knots (185kph, 115mph) or more. The force required to deflect a control surface into a 100 knot wind can be significant – and it only gets worse as aircraft get faster!

Many aircraft incorporate control surface balancing to reduce the control forces and assist the pilot in flying the aircraft.

Aerodynamic Balancing

Aerodynamic balancing uses a horn balance positioned forward of the control surface hinge such that the horn deflects into the airflow in the opposite direction to the majority of the control surface.

This creates a force that assists the pilot in deflecting the large control surface in flight.

Flutter

Flutter is an unstable oscillation of the airframe that occurs most commonly on control surfaces. It typically starts when a control surface deflects due to a gust of wind or other outside disturbance. If the control surface were deflected into the airflow below the wing, the airflow exerts a force pushing it back the other direction. The inertia of the control surface causes it to overshoot the neutral position and deflect into the airflow above the wing.

This can set up an unstable oscillation, with the control surface continually deflecting up and down. Flutter can lead to vibrations, control difficulty and structural failure.

Video courtesy of NASA, Stillman Fires Collection, Prelinger Archives

Mass Balancing

Flutter is most common when the centre of gravity of a control surface is positioned too far rearwards. To prevent this, aircraft designers often incorporate a weight near the front of the control surface which moves the centre of gravity forwards and closer to the hinge. This is known as a Mass Balance.

Mass balances are often hidden inside the front of control surface horns or the wedge of frise ailerons.

Flutter can also be overcome by increasing the rigidity of the control surface structure, meaning a stronger initial force would be required from the airflow to initiate flutter.

Flutter may be overcome by increasing the rigidity of the control surface structure and fitting a mass balance

Anti-Balance Tab

Some aircraft are fitted with a stabilator: a combined horizontal stabiliser (the horizontal portion of the tail) and elevator. When the pilot moves the control column fore and aft, the whole stabilator rotates.

Since the stabilator is much bigger than an elevator, the force created is stronger – meaning a smaller movement is needed to have the same pitching effect on the aircraft.

While this can be more efficient (less drag is created), it can also create a problem of over-controlling. The pilot’s inputs will have a larger effect so it is easier for the pilot to accidently over pitch the aircraft.

To counteract this, an anti-balance tab is often fitted to the trailing edge of the stabilator. The anti-balance tab moves in the same direction as the stabilator, creating a force that makes deflecting the stabilator harder.