Why Do High-Speed Aircraft Have Swept Wings?

A swept wing is the most common planform for transonic and supersonic jet aircraft. Swept back wings are by far the most common, but forward and variable sweep wings are also in use. You may have noticed an aircraft with swept wings and found yourself wondering why they look that way. The purpose of swept wings is to reduce drag, therefore allowing an aircraft to fly faster. However, how they do this is not as simple to explain. Essentially, sweeping the wings makes the aircraft feel as if it is flying slower. In turn, this delays the effect of supersonic airflow, therefore delaying wave drag.

A basic aspect of lift is that air accelerates as it travels over the top of a wing. This means, if an aircraft is moving at Mach 0.8, near the speed of sound, the air flowing over the wing could reach a speed of Mach 1, resulting in supersonic flow. The speed at which air flowing over the wing reaches Mach 1 is known as the aircraft’s critical Mach number.

The problem with this is that airflow doesn’t stay at supersonic speeds forever. Rather, the airflow speeds up to exceed Mach and subsequently slows down, reverting back to a subsonic speed. When the air slows back down to below Mach 1, it results in a shock wave. Air flowing along the wing creates pressure waves that move at the speed of sound. This means the pressure waves are unable to move forward through the supersonic airflow, and instead build up into a massive pressure - a shock wave. Shock waves generate significant drag. The air flowing over the wing crosses a massive pressure threshold that eliminates the energy from the airflow, resulting in drag. The air can also lose so much energy that it separates from the wing entirely, producing a specific type of drag called wave drag.

So how does wing sweep prevent drag? By reducing the amount of acceleration of drag, wing sweep delays the start of supersonic flow. On a straight wing aircraft, the entirety of the airflow over the wing travels parallel to the aircraft's chord line (the imaginary line between the wing’s leading and trailing edges). On a swept wing, only a portion of the air flows parallel to the chord line, with the other portion flowing perpendicular (spanwise flow). As only airflow moving parallel to the chord line will accelerate, swept wings reduce the amount of acceleration and therefore delay the critical Mach number, allowing the aircraft to fly at a higher Mach number before it begins to create wave drag.

Despite their benefits, swept wings are not without their drawbacks. As stated, by reducing the airflow parallel to the chord line, you decrease the amount of lift the wing creates. This is not a problem at high speeds, where only a small angle of attack is needed to create lift. At slow speeds, however, the wing sweep can force the aircraft into a high angle of attack and put it at danger of stalling. The stall pattern can also be affected by sweeping the wings. As you near the wingtip, the amount of spanwise flow increases, decreasing the wingtip’s effective airspeed. This can cause the wingtip to stall prior to the wing root, meaning you will lose control of the aileron when the stall begins. This is countered by placing flow fences atop the wings to prevent spanwise flow from compounding.

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