Air Resistance is a Real "Drag"

In last month's issue, we explained how the wings of an aircraft produce lift. For a wing to provide lift for takeoff, air must pass over the wing. The force required to push an aircraft through the air is called thrust. Thrust is created by the jet engines and must overcome the retarding force known as drag.

There are two main categories of drag -- induced and parasite. Induced drag is that drag which is attributed to the production of lift. The magnitude of induced drag decreases as the airspeed increases. Water skiers experience this phenomenon when being pulled by a boat. Starting at rest, and mostly submerged, the skier feels a massive amount of drag while being pulled through the water. As the speed increases, the skier skims over the surface of the water and the drag is significantly reduced. Because of the high speed at which jet transport aircraft cruise, induced drag makes up less than one-quarter of the total drag.

Parasite drag is made up of three components. Skin friction is the resistance air molecules encounter as they flow over the aircraft's surface. Notice the smooth seams of the aircraft's skin; this design minimizes the skin friction. Form drag, also called profile drag, is caused by the air pressure in front of the aircraft. Fast moving aircraft are designed to be sleek to minimize the frontal area and the form drag. The third component is interference drag. If the aircraft's components disrupt the air's smooth, or laminar, flow, the cost is paid in interference drag. The landing gear is raised into the body of the aircraft after takeoff to decrease this drag. Passengers can feel the airflow smooth when the landing gear is retracted.

The thin air at high altitudes reduces parasite drag. Flying at 40,000 feet, the air has one-fourth the density as at sea level. If jet transport aircraft could fly as high as space shuttles, they could shut down the engines and coast, since no drag exists if there is no air around the craft.

Induced drag decreases and parasite drag increases with airspeed. Total drag is the sum of the induced and parasite drag components. The slowest speed an aircraft can fly is known as the stall speed. Total drag is very high at the stall speed because of the induced component. As the airspeed increases, the drag decreases to a minimum point. As the airspeed continues to increase above this point, the parasite drag causes the total drag to increase.

Aircraft designers are making airliners more and more efficient. Reducing the total drag of an aircraft allows it to travel faster and use less fuel. Aeronautical engineers know that air resistance is a real drag!

Return to Feature Articles