It is not enough to add all the form and the skin-friction drag values of parts that where separately tested to obtain the total parasite drag. Interference drag is obtained from testing parts assembled rather testing them separately. As this test is conducted, the wake of one part may affect the drag of the another. This effect may be favorable as well as unfavorable. Since the airplane is constantly moving the air is forced up at the wing tips.
Angle of attack is greater than angle of attack .
The original explanation of lift and drag assumed an ideal airflow. Induced drag results from imperfection in the airflow caused by lift. Two theories offered here to explain the induced drag. As explained earlier, the pressure below an airplane wing is higher than the pressure above it. As a result, there is a constant tendency of air to flow from bottom to top.