Variable-sweep wings, often called swing wings, let an aircraft change the angle of its wings during flight. With the wings spread out, the aircraft has a larger effective wing area and better lift at lower speeds, which helps during takeoff, landing, and slow maneuvering. With the wings swept back, the aircraft reduces drag and delays problems caused by high-speed airflow, which helps at transonic and supersonic speeds.
This design matters because it lets one aircraft balance the needs of both slow flight and fast flight.
Key Facts
- Lift equation: L = 0.5 rho v^2 S CL
- Drag equation: D = 0.5 rho v^2 S CD
- Wings forward: larger effective span and area help produce lift at lower speeds.
- Wings swept back: reduced wave drag helps at high subsonic and supersonic speeds.
- Sweep angle is the angle between the wing and a line perpendicular to the fuselage.
- Variable-sweep aircraft trade mechanical simplicity for a wider useful speed range.
Vocabulary
- Variable-sweep wing
- A wing design that can rotate in flight to change its sweep angle for different speed conditions.
- Sweep angle
- The angle at which a wing is slanted backward or forward compared with a straight wing position.
- Lift
- The upward aerodynamic force that supports an aircraft in flight.
- Drag
- The aerodynamic force that opposes an aircraft's motion through the air.
- Wave drag
- Extra drag that forms near transonic and supersonic speeds because of shock waves in the airflow.
Common Mistakes to Avoid
- Thinking swept-back wings always produce more lift. Sweeping the wings back usually reduces low-speed lift, so aircraft spread the wings for takeoff and landing.
- Ignoring wing area in the lift equation. Lift depends on L = 0.5 rho v^2 S CL, so changes in effective area and lift coefficient matter along with speed.
- Assuming variable-sweep wings are free performance improvements. The pivot structure, actuators, and moving parts add weight, cost, and maintenance needs.
- Confusing sweep angle with angle of attack. Sweep angle is the wing's backward slant from above, while angle of attack is the tilt of the wing relative to the incoming airflow.
Practice Questions
- 1 An aircraft has rho = 1.2 kg/m^3, speed v = 70 m/s, wing area S = 45 m^2, and CL = 1.4 with wings spread. Calculate the lift using L = 0.5 rho v^2 S CL.
- 2 At the same altitude, an aircraft increases speed from 80 m/s to 160 m/s while S and CL stay constant. By what factor does lift increase according to L = 0.5 rho v^2 S CL?
- 3 Explain why a variable-sweep aircraft uses wings spread forward during takeoff but sweeps them back during high-speed flight.