Mass and aerodynamic balance

Mass and Aerodynamic Balance

Mass balance and aerodynamic balance are design measures on control surfaces serving two main purposes:

1. Flutter prevention (mass balance).
2. Reduction of control forces for the pilot (aerodynamic balance).

Mass balance

Purpose

Flutter is a self-excited oscillation of the control surface that can occur at high speed. Mechanism:

1. Control surface oscillates randomly at some frequency.
2. Mass centre of the control surface aft of the hinge line → oscillation amplifies.
3. Resonance with wing structure → catastrophic oscillation in seconds → structural destruction.

Prevention

Counterweight forward of the hinge line:

• Mass centre of control surface shifted to or forward of the hinge line.
• Result: no more self-excitation → no flutter.

Implementation

• Lead weight at the forward end of the rudder, ahead of the hinge line.
• Horn balance (extension forward of hinge line): part of control surface extends in front of pivot → mass centre forward.

POH significance

Pilot must not remove or modify counterweights — loss of mass balance → flutter risk at Vne.

Aerodynamic balance

Purpose

Reduction of control force (hinge moment) the pilot must apply at the yoke.

Mechanism

Part of the control surface forward of the hinge line:

• Flow on this leading edge produces aerodynamic force opposite to the deflecting trailing edge.
• This force assists pilot deflection → less force required.

Implementations

1. Horn balance

Projecting "horns" at control-surface leading edge (forward of hinge):

• Example: ailerons with horn arrangement at inboard or outboard end.
• Effect: lift force forward of hinge → assists deflection.

2. Shielded horn balance

Horn partially shielded by main profile:

• At small deflection shielded — little effect.
• At large deflection exposed — strong effect.
• → Force curve more linear for pilot.

3. Internal balance

Internal balance: control surface has internal chamber with diaphragm that on deflection creates pressure difference → force forward of hinge.

• Examples: some airliners.

4. Frise aileron (see lesson "Adverse Yaw")

Frise ailerons integrate aerodynamic balance.

5. Servo tab (balance tab)

Small moveable tab at the end of the control surface that moves opposite to the control surface on deflection:

• On pitch-up: servo tab down → up-lift on tab → moment assists pilot deflection.
• Benefit: dramatic force reduction, especially in large aircraft.

Anti-servo tab (compared)

Anti-servo tab: tab moves with the control surface (not against).

• Effect: increases force → control "feels reliable".
• Used in stabilator designs for control stability.

Combined — example C172

Cessna 172 elevator:

• Mass balance: counterweights internally at the forward elevator area.
• Aerodynamic balance: light horn at outer elevator end.
• Trim tab: at inner elevator end.

CS-23 requirements

CS-23 Subpart D Design and Construction requires:

• Flutter demonstration at Vne + margins.
• Control-force limits for pilot effort (yoke and pedal forces).
• Structural integrity under all load conditions.

Maintenance consequences

• Inspection of counterweights not pre-flight mandatory but important in maintenance procedures.
• Repainting can shift counterweight position → re-balance required.
• Repairs on control surfaces must not disturb mass balance — strict quality control.

Practical PPL awareness

• Pilot operates controls with low force — thanks to aerodynamic balance.
• Mass balance is invisible but critical — POH notes not to be ignored.
• If control force is unusual after maintenance → contact maintenance immediately.