What is endurance of aircraft?

 Endurance in aviation refers to the longest duration an airplane can remain in flight without the need for refueling. It plays a crucial role in determining the capabilities and efficiency of aircraft. Achieving the best endurance requires a strategic approach, involving factors such as angle of attack, airspeed, wind effects, and specific fuel consumption. By understanding the interplay between these elements, pilots can optimize their flights for maximum endurance.

To achieve the best endurance, flying at an angle of attack that yields minimum fuel flow is essential. This means adjusting the aircraft's position to minimize the resistance encountered during flight, allowing for efficient fuel consumption. The airspeed also plays a vital role in optimizing endurance. When an airplane is running low on fuel and waiting for fog to lift at its destination, the ideal airspeed for best endurance is the speed that maximizes fuel efficiency under such conditions.

Contrary to popular belief, wind does not significantly affect endurance since fuel flow remains unaffected by wind speed. Although a headwind can reduce the range an aircraft can cover, the endurance remains the same. The amount of fuel consumed per unit of thrust determines the fuel flow, commonly known as specific fuel consumption. Reducing the fuel flow by minimizing the amount of fuel used to produce each unit of thrust is crucial for achieving the best endurance.

Increasing engine rpm within the design rpm range is one way to reduce specific fuel consumption and enhance endurance. Although higher engine rpm increases fuel flow, it also increases thrust. The increase in thrust is greater than the increase in fuel flow, resulting in an overall decrease in the specific fuel consumption ratio, which ultimately benefits endurance.

Another approach to reducing specific fuel consumption is flying at lower outside air temperatures and higher altitudes. Generally, endurance improves with an increase in altitude. However, it's important to note that exceeding the design rpm range at higher altitudes may decrease endurance.

Reducing total thrust is another important factor in optimizing endurance. In level flight, the forward-facing force of thrust is balanced by the rearward-facing force of drag. By minimizing drag, an aircraft can maintain stable flight with minimal thrust. This is achieved by flying at the minimum drag speed, which is also the speed for the best lift-to-drag ratio and maximum excess thrust. Minimizing drag contributes to reducing specific fuel consumption, enhancing overall endurance.

It's important to consider weight reduction during the flight as fuel is burned, resulting in reduced maximum endurance airspeed. The angle of attack for maximum endurance remains constant, but the airspeed at which this angle of attack is achieved depends on the weight and altitude of the aircraft.

Northrop Grumman Firebird in UAV configuration, 30h of endurance.

Source: Northrop Grumman

Understanding the intricate relationship between fuel usage, drag, altitude, and speed is key to optimizing endurance in aviation. Pilots must strive to strike a balance between fuel efficiency and performance to ensure the longest possible flight duration. By employing strategies such as minimizing fuel consumption per unit of thrust, flying at optimal airspeeds, and reducing drag, pilots can achieve remarkable endurance capabilities in their aircraft.