V-n Diagram

A V-n diagram is a velocity-load-factor diagram. It places aircraft speed V on one axis and load factor n on the other, so the engineer can see where aerodynamic stall, structural limit load, maneuvering speed, high-speed limits and sometimes gust loads bound the usable envelope.

For a simplified positive maneuver boundary, the stall line follows:

until it reaches the positive limit load factor. The corresponding simplified maneuvering speed is:

Above that speed, the aircraft can reach the positive limit load factor before stall limits the lift. Below that speed, the simplified model predicts stall before the positive limit load factor is reached. Real diagrams include additional limits, assumptions and evidence.

Engineering Role

The V-n diagram is a compact way to reason about aircraft structural and aerodynamic limits. It helps connect stall speed, maneuvering speed, positive and negative load factors, design dive speed, gust loads, weight, configuration and flight-test clearance.

Structural engineers use it to check load cases and strain-survey points. Flight-dynamics engineers use it to define envelope-protection boundaries and maneuver test points. Test teams use it to plan incremental expansion without jumping directly to high-risk combinations of speed and load factor.

The diagram should not be treated as a generic picture. It is tied to a specific aircraft configuration, weight, center of gravity, altitude or equivalent-airspeed basis, load-factor limits, speed definitions and validation evidence.

Worked Example: Locating a Maneuver Point

An aircraft at a reviewed weight has:

Parameter	Value
Wings-level stall speed, V_S	89.2\ \text{kt}
Positive limit load factor, n_{limit}	3.8
Candidate test speed, V	170\ \text{kt}
Candidate pull-up load factor, n	3.6

First compute the simplified maneuvering speed:

At the candidate speed, the stall-boundary load factor is:

Compare the candidate maneuver:

and:

The point is just below both the simplified stall boundary and the positive limit load factor. The margin to the stall boundary is:

The margin to the positive limit load factor is:

Engineering comment: this is a narrow test point. A small speed error, normal-acceleration measurement error, gust increment, center-of-gravity shift, configuration difference or stall-speed uncertainty could move the point outside the intended clearance. A real expansion decision would require uncertainty allowances, structural evidence, handling-quality limits, air-data calibration and an abort criterion.

What the Diagram Shows

A V-n diagram commonly includes:

• positive and negative maneuver load-factor limits;
• positive and negative stall boundaries;
• maneuvering speed or corner speed;
• cruise, maximum operating or design dive speed references;
• gust-load lines or gust envelopes where applicable;
• test-point markers and cleared envelope regions;
• speed reference and configuration notes.

The same aircraft can have different V-n diagrams for different weights, configurations, flap settings, stores, ice states, structural repairs, flight-control modes or certification bases.

Distinction from Related Terms

A V-n diagram is not the same as maneuvering speed. Maneuvering speed is one point or speed reference on the maneuver envelope. The diagram shows the surrounding load-factor and speed boundaries.

A V-n diagram is not only a stall chart. Stall boundaries are part of it, but structural limit loads, high-speed limits and gust cases can govern other regions.

A V-n diagram is not a proof that the aircraft is safe everywhere inside a plotted outline. Aeroelasticity, fatigue, control-surface loads, actuator limits, buffet, flutter, sensor faults and operational procedures can introduce additional constraints.

Validation and Common Mistakes

A defensible V-n diagram states the aircraft configuration, weight, center of gravity, speed reference, altitude or density basis, stall-speed basis, positive and negative load-factor limits, gust assumptions, structural evidence, air-data calibration, uncertainty treatment and approved limitations.

Common mistakes include:

• using a diagram for the wrong weight or configuration;
• mixing true airspeed, equivalent airspeed, calibrated airspeed and indicated airspeed;
• treating maneuvering speed as protection against every gust or control input;
• ignoring negative load-factor limits;
• plotting nominal points without measurement uncertainty or gust allowance;
• using clean-wing stall data after ice, roughness, flap failure, stores or damage;
• clearing flight-test points without structural strain evidence, control-law evidence or abort criteria.