Angle of attack vs Angle of incidence - the complete guide
Understanding Angle of Attack and Angle of Incidence in Aircraft
When studying the aerodynamics of an aircraft, two critical angles often come into play: the angle of attack and the angle of incidence. Although these terms are sometimes used interchangeably in casual conversation, they represent distinct concepts that are fundamental to flight dynamics.
1. Angle of Attack vs. Angle of Incidence
Angle of Attack (AoA):
The angle of attack is defined as the angle between the chord line of the wing and the relative airflow (or flight path). In simpler terms, it’s the angle at which the wing meets the oncoming air.
The angle of attack is a dynamic parameter that changes continuously during flight, depending on the aircraft’s maneuvers, speed, and altitude. For instance, during takeoff or when climbing, the angle of attack increases, while it decreases during descent or cruising.
The angle of attack directly impacts the amount of lift generated by the wing. Exceeding a critical angle of attack leads to a stall, where the airflow over the wing separates, drastically reducing lift.
Angle of Incidence:
The angle of incidence, on the other hand, is the fixed angle between the wing’s chord line and the aircraft’s longitudinal axis (or fuselage reference line). This angle is set during the aircraft’s design and construction and does not change during flight.
The angle of incidence determines the basic lift characteristics of the wing at different phases of flight. It ensures that the wing generates sufficient lift even at low speeds (e.g., during takeoff or landing).
Why Are They Different?
While the angle of attack is a variable parameter influenced by flight conditions, the angle of incidence is a structural feature of the aircraft.
During flight, the angle of attack changes to adapt to the required lift and drag forces, but the angle of incidence remains constant because it’s built into the airframe.
2. Why Does the Wing’s Angle Change Towards Its Tip?
Aircraft wings often feature a design characteristic known as washout, where the angle of incidence decreases progressively from the wing root to the wingtip. This twist serves several important purposes:
a) Prevention of Wingtip Stalls:
During high-angle-of-attack situations, such as takeoff or landing, the wing root’s higher angle of incidence ensures that it stalls before the wingtips. This is crucial because if the wingtips were to stall first, the aircraft would lose a significant portion of its roll control (as the ailerons are located near the wingtips).
By keeping the wingtips operational even when the root is stalling, the pilot retains better control over the aircraft.
b) Improved Lift Distribution:
The washout helps distribute lift more evenly across the span of the wing, reducing structural stress and improving aerodynamic efficiency.
c) Reduction of Induced Drag:
By decreasing the angle of incidence at the wingtips, the design minimizes the strength of wingtip vortices—the spiraling airflows that form at the wingtip due to pressure differences. This, in turn, reduces induced drag and improves fuel efficiency.
d) Stability and Handling:
The gradual change in angle contributes to smoother flight dynamics and improved stability, particularly during slow-speed operations.
Conclusion
The angle of attack and angle of incidence are two key concepts that play distinct yet complementary roles in aircraft performance. While the angle of attack is a variable factor directly controlled by the pilot’s actions, the angle of incidence is a fixed design parameter that ensures optimal lift characteristics. Additionally, the deliberate change in the wing’s angle of incidence towards its tip (washout) enhances safety, efficiency, and control. Together, these design elements showcase the intricate balance of engineering that makes flight possible.