What is drag in the context of aircraft flight?

In the context of aircraft flight, drag is defined as the force that tends to hold an aircraft back. It is a critical aerodynamic force that acts in the opposite direction to an aircraft's thrust, which is the force propelling it forward. Drag arises from the interaction of the aircraft with the air as it moves through the atmosphere, and it is influenced by several factors, including the shape of the aircraft, its speed, the density of the air, and the surface roughness.

Understanding drag is essential for pilots and aviation personnel, as it affects fuel efficiency, speed, and overall performance of the aircraft. Reducing drag can lead to more efficient flight operations, and various design considerations, such as streamlined shapes and minimized surface irregularities, are implemented to achieve this.