How can airplanes fly upside down?

The buoyant force is the force that keeps the aircraft in the air. The upper part of the aircraft wings is more dished than the lower part. Therefore, the air passing through the upper part of the wing moves faster than the air passing through the lower part. This speed difference causes pressure difference to occur between the upper and lower parts of the wing, and a force arises in the direction perpendicular to the air flow direction from the lower part of the wing to the upper part.

In this case, on the planes moving upside down, the buoyant force must act in the same direction as gravity because of the shape of the wings, which means that the buoyant force pushes the plane towards the ground. However, the buoyant force depends on the angle of attack as well as the shape of the wings.

The angle of attack is the angle between the veter line (the axis connecting the wing's break and trailing edges) and the flight direction. The buoyant force increases in proportion to the angle of attack up to a certain value. Above a critical attack angle value, the flow of air around the wing becomes irregular and the buoyant force suddenly drops.

While fighter jets and stunt jets flying upside down, the angle of attack can be kept at an appropriate value and enough buoyancy can be created to keep the plane in the air. In aircraft with symmetrical wing profiles, the buoyant force arises more depending on the angle of attack than the pressure difference between the upper and lower parts of the wing. Wing profiles of aerobatic aircraft are generally symmetrical.