V-n Diagram

What is the V-n Diagram?

The V-n diagram (or V-g diagram) is a critical aviation chart that plots load factor (vertical axis) against airspeed (horizontal axis). It visually represents the flight envelope—showing where the aircraft can safely operate without risking a stall or structural failure.

Key Features Explained

• Stall Boundaries: The curved lines on the diagram indicate the maximum positive and negative load factors before the aircraft stalls at each speed. The lower boundary is the 1g stall speed (VS), while the upper and lower curves show accelerated stall speeds as load factor increases.
• Structural Limits: Horizontal lines mark the maximum positive and negative load factors the aircraft structure can withstand. Exceeding these risks permanent damage or failure.
• Critical Speeds:
  • VA (Design Manoeuvring Speed): Highest speed for full, abrupt control deflection without structural harm.
  • VB (Design Gust Speed): Maximum speed at which the aircraft can safely encounter a specified gust.
  • VC (Design Cruise Speed): Maximum speed for normal operations in turbulence.
  • VD (Design Dive Speed): Absolute structural speed limit, beyond which flight is unsafe.
  • VNE (Never-Exceed Speed): Operational speed limit for pilots, usually at or below VD.
• Gust Envelope: When gust effects are considered, the diagram is overlaid with gust lines, showing how sudden wind changes can increase load factor even at constant speeds.

Relationships and Calculations

• VA and VS: VA is calculated as VS multiplied by the square root of the maximum load factor (VA = VS × √n_max).
• Envelope Boundaries: The rightmost edge of the diagram is set by VD, not VC or VMO.

Operational Use

Pilots use the V-n diagram to avoid exceeding structural or aerodynamic limits during manoeuvres, turbulence, or gust encounters. The diagram is unique to each aircraft type and configuration, and actual values are found in the POH/FM.