Swept-wing aircraft are longitudinally stable in the same way that fixed-wing aircraft are: by placing the center of lift behind the center of gravity, and having an additional downforce created by the tail. This creates a “two-legged” stability:
The primary stability problem with swept-wing aircraft, that straight-wing aircraft do not suffer, is pitch-up. Because the wing is swept aftwards, and because a stall progresses from the wing tip to the wing root, the part of the wing still generating lift as a wing stalls moves further forward:
This means the center of lift moved forward too, pulling the nose up. A force that pulls the nose up as an aircraft slows and approaches the stall is obviously destabilizing, as it will cause the aircraft to stall even more. This pitch-up tendency was first made famous in the F-100 Super Saber, where it was nicknamed the Sabre dance. More than a few young pilots were sadly killed by the aircraft’s tendency to violently pitch the nose up just as the aircraft approached the stall.
Modern swept-wing aircraft control pitch-up by using wingtip devices, such as wing fences or slats, to control the span wise flow of air at low speeds and increase the stall margin, as well as angle of attack protections such as stick shakers/pushers to prevent high angles of attack from developing.
In addition to these longitudinal stability problems, swept-wing aircraft also have a lateral and spiral stability problem called Dutch roll. Dutch roll is caused by a self-sustaining interplay between the swept wing and the aircraft body. If the aircraft yaws, the upwind wing presents a greater wingspan to the wind, creating more lift:
The additional lift generation also produces additional drag, which pulls the wing back and the nose into the wind. The upwind wing, generating more lift, also rises, causing the aircraft to roll downwind.
The aircraft is now turning downwind, which changes the direction of oncoming wind, and the process now repeats itself in reverse. Dutch roll can continue indefinitely if not corrected:
Modern aircraft reduce the tendency of Dutch roll to develop by using yaw dampers, which use the rudder to reduce or eliminate sideslip when it happens.
Author – Tim Morgan
