This phenomenon is known as a symmetric airfoil. The shape of the airfoil is mirrored on both sides, resulting in equal lift forces on the upper and lower surfaces. As a result, the lift curve goes through the origin, indicating that at zero angle of attack, there is zero lift.
It depends on the application. For some aircraft, such as gliders, a symmetric airfoil is desirable as it provides equal lift and reduces drag. However, for most commercial airplanes, a cambered airfoil is preferred as it can generate more lift and has better stall characteristics.
The angle of attack, which is the angle between the airfoil and the direction of the oncoming airflow, affects the lift curve by shifting it up or down. As the angle of attack increases, the lift curve also increases, until it reaches the maximum lift coefficient at a certain angle of attack. After this point, the lift decreases, and the airfoil stalls.
One disadvantage of a symmetric airfoil is that it produces less lift compared to a cambered airfoil. This can result in a higher takeoff speed and longer takeoff distance for aircraft using symmetric airfoils. Additionally, symmetric airfoils are less stable and can be more difficult to control at high angles of attack.
Yes, the lift curve for a symmetric airfoil can be modified by adding flaps or slats to the airfoil. These devices change the shape of the airfoil, creating camber and increasing the maximum lift coefficient. This allows for greater lift and improved stall characteristics, making it a common modification for aircraft that require higher lift at low speeds, such as commercial airliners during landing and takeoff.