Delta wings are triangular wings often used on fast military aircraft, supersonic jets, and some experimental aircraft. Their swept leading edges reduce drag at high speed and provide a strong structure with room for fuel and landing gear. At low speed or during sharp maneuvers, a delta wing often flies at a high angle of attack, where ordinary wings may begin to stall.
Delta wings can still make useful lift because of a special flow pattern called vortex lift.
At high angle of attack, air separates from the sharp leading edges of the delta wing and rolls into two spinning vortices above the wing. These vortices create low pressure over the upper surface, which adds lift beyond the lift from normal airflow deflection. The vortices can stay stable over a wide range of angles, allowing controlled flight during steep approaches, takeoff, and tight turns.
If the angle becomes too large, the vortices can break down, causing loss of lift, buffeting, and reduced control.
Key Facts
- Angle of attack is the angle between the wing chord line and the oncoming airflow.
- Lift increases when pressure above the wing is lower than pressure below the wing.
- For many wings at small angles, L = 1/2 rho v^2 S C_L.
- Delta wings create leading-edge vortices when flow separates from sharp swept leading edges.
- Vortex lift adds to conventional lift by lowering pressure over the top of the wing.
- At very high angle of attack, vortex breakdown can reduce lift and make control difficult.
Vocabulary
- Delta wing
- A triangular wing planform with highly swept leading edges, often used for high-speed aircraft.
- Angle of attack
- The angle between a wing's chord line and the direction of the incoming airflow.
- Leading-edge vortex
- A spinning flow structure that forms above a sharp swept leading edge when air separates and rolls up.
- Vortex lift
- Additional lift produced by low pressure inside strong vortices above a wing.
- Vortex breakdown
- The loss of a stable vortex structure, often causing a sudden change in lift and aircraft behavior.
Common Mistakes to Avoid
- Treating vortex lift as the same as normal wing lift. Vortex lift comes from separated spinning flow above the wing, not only from smooth attached airflow.
- Assuming separation always means stall. On a delta wing, controlled leading-edge separation can form stable vortices that increase lift.
- Ignoring airspeed in lift calculations. Since L = 1/2 rho v^2 S C_L, doubling speed increases the dynamic pressure by a factor of four.
- Thinking a delta wing is best for every flight condition. Delta wings are excellent at high speed and high angle of attack, but they can have high drag and need higher landing speeds.
Practice Questions
- 1 A delta wing has area S = 40 m^2, air density rho = 1.2 kg/m^3, speed v = 90 m/s, and lift coefficient C_L = 0.9. Use L = 1/2 rho v^2 S C_L to calculate the lift.
- 2 During a high angle of attack maneuver, vortex lift raises C_L from 0.8 to 1.2 for the same aircraft, speed, and air density. By what percent does lift increase?
- 3 Explain why a sharp leading edge can help a delta wing at high angle of attack, even though sharp edges often cause flow separation.