Class 5: Understanding Percentage MAC and Center of Gravity

Introduction

• Presenter: Grant
• Series: Mass and Balance
• Focus: How percentage mean aerodynamic chord (MAC) helps understand and manage the center of gravity (C of G)

Key Concepts

Center of Gravity and Center of Pressure

• Center of Gravity (C of G):
  • Moves based on loading
  • Described in relation to a fixed reference
• Center of Pressure:
  • Relatively fixed (depends on wing design)
  • Changes with angle of attack and airflow

Mean Aerodynamic Chord (MAC)

• Definition:
  • A fictional line used as a reference for wing design
  • Average chord length of a non-uniform wing
  • Used to describe positions of C of G and center of pressure

Calculating Percentage MAC

• Procedure:
  • Determine distance from a reference point (e.g., datum)
  • Use formula: (smaller number ÷ bigger number) × 100
  • Example Calculation:
    • Leading edge MAC: -40 cm
    • Trailing edge MAC: 1.2 m
    • C of G: +25 cm
    • Calculation: C of G as percentage of MAC
    • Result: 40.63% (or approximately 41%)

Practical Application

• Using Load Sheets:
  • Range for C of G described in percentage MAC
  • Important for aircraft control and safety
• Adjusting Load for Balance:
  • Move cargo or passengers if C of G is outside range
  • Use fuel as ballast if necessary
  • Unusable fuel counted in basic empty mass

Conclusion

• MAC:
  • Represents an average wing shape
  • Fictional but essential for calculations
• Importance of C of G within Percentage MAC Range:
  • Ensures controllability
  • Prevents potential flight disasters

Summary

• Percentage MAC helps manage mass imbalance in aircraft
• Essential to keep C of G within the specified range for safe flight operation