Load Factor

What is Load Factor?

Load factor (n) is the ratio of lift to weight (n = Lift / Weight). In level, unaccelerated flight, lift equals weight, so the load factor is 1g. During manoeuvres, such as turns or abrupt pull-ups, the aircraft must generate more lift to maintain altitude, increasing the load factor above 1g.

Positive and Negative Load Factors

Positive load factors occur when lift acts upwards (as in normal flight), while negative load factors arise when lift acts downwards (e.g., inverted flight or abrupt nose-down manoeuvres). Aircraft are certified for specific positive and negative load factor limits to ensure structural safety.

Load Factor in Turns

In a level turn, the load factor increases with bank angle. For example, at a 60° bank, the load factor reaches 2g, meaning the aircraft structure must support twice its weight. The formula for load factor in a level turn is n = 1 / cos(bank angle), making bank angle the key variable, not aircraft mass or speed.

Load Factor and Stall Speed

As load factor increases, so does stall speed. The aircraft must fly faster to avoid stalling at higher load factors because the wing reaches its critical angle of attack at a higher airspeed. For instance, at 2g, stall speed increases by roughly 41% compared to wings-level flight.

Limit and Ultimate Load Factors

The limit load factor is the maximum load the aircraft can sustain during normal operations without permanent deformation. The ultimate load factor is higher (typically 1.5 times the limit) and represents the absolute maximum the structure can withstand before failure. Exceeding the limit load factor can cause permanent damage; exceeding the ultimate load factor risks catastrophic structural failure.

Gust Load Factor and Influencing Factors

Gusts can rapidly increase load factor. The gust load factor depends on factors such as lift-curve slope, aspect ratio, wing sweep, altitude, wing loading, aircraft weight, wing area, equivalent airspeed, and the speed of the vertical gust. These parameters affect how much extra lift (and thus load factor) is generated when a gust is encountered.

Load Factor Calculation

For level turns: n = 1 / cos(bank angle). For general manoeuvres, load factor is always calculated as Lift / Weight. Remember, load factor is independent of aircraft mass for a given bank angle.