As an aircraft gets close to the speed of sound, airflow over parts of the wing can accidentally go supersonic before the rest of the plane does. That creates shockwaves, drag spikes (called “wave drag”), and a whole menu of stability headaches. Sweeping the wings delays all of that. Here’s the trick: when you tilt a wing backward, the airflow “sees” a smaller component of its speed hitting the wing head-on. Aerodynamicists call this the effective normal Mach number. Lower that number, and you push those shockwaves farther up the speed range — meaning smoother handling, less drag, and better fuel efficiency near cruising speeds. Swept wings also help create a more gradual pressure distribution, which keeps airflow attached longer and reduces the violent buffet you’d get from early shock formation. It’s why commercial jets cruise comfortably at Mach 0.78–0.85 instead of fighting physics every minute. It’s not all upside: swept wings reduce low-speed lift and tend to stall from the ti...