What Is Air Resistance

by Jerry Coffey on September 10, 2010

What Is Air Resistance

Atlantis Breaks Through the Clouds

What is air resistance? Air resistance, also called drag, is the forces that are in opposition to the relative motion of an object through the air. Drag forces act opposite to the oncoming flow velocity. Drag, unlike other resistive forces, depends directly on velocity. Drag is the component of the net aerodynamic force acting opposite to the direction of the movement and the forces working perpendicular are called lift. Drag is overcome by thrust. In astrodynamics, atmospheric drag is both a positive and a negative force depending on the situation. It is a drain on fuel and efficiency during lift-off and a fuel savings when a spacecraft is returning from to Earth.

Air resistance is usually calculated using the drag equation. This equation calculates the force experienced by an object moving through a fluid or gas at relatively large velocity. The result is called quadratic drag. Once the drag is calculated, you have to use a formula to calculate the power needed to overcome that drag in order propel and object. Power needs are the cube of the velocity, so if it takes 10 horsepower to go 80 kph it will take 80 horsepower to go 160 kph. As you can see a doubling of speed requires the eight times the amount of power. Knowing this is essential in calculating the amount of fuel it will take to make a journey.

There are three main types of drag in aerodynamics: lift induced, parasitic, and wave. Each affects an objects ability to stay aloft as well as the power and fuel needed to keep it there.

Lift induced(induced)drag occurs as the result of the creation of lift on a three-dimensional lifting body(wing or fuselage). It has two primary components: vortex drag and lift-induced viscous drag. The vortices derive from the turbulent mixing of air of varying pressure on the upper and lower surfaces of the body. These are needed to create lift. As the lift increases, so does the lift-induced drag. For an aircraft this means that as the angle of attack and the lift coefficient increase to the point of stall, so does the lift-induced drag.

Parasitic drag is caused by moving a solid object through a fluid. Parasitic drag is made up of multiple components including form drag and skin friction drag. In aviation, induced drag tends to be greater at lower speeds because a high angle of attack is required to maintain lift, so as speed increases this drag becomes much less, but parasitic drag increases because the fluid is flowing faster around protruding objects increasing friction. The combined overall drag curve is minimal at some airspeeds and an will be at or close to its optimal efficiency. Pilots will use this speed to maximize fuel consumption.

Wave drag (compressibility drag) is created by the presence of a body moving at high speed through a compressible fluid. In aerodynamics, wave drag consists of multiple components depending on the speed regime of the flight. In transonic flight (Mach 0.5 but less than 1.0), wave drag is the result of local supersonic flow are created. Supersonic flow occurs on bodies traveling well below the speed of sound, as the local speed of air on a body increases when it accelerates over the body so, aircraft flying at transonic speed often incur wave drag during operation. This increases as speed nears the supersonic barrier of Mach 1.0.
In supersonic flight, wave drag is the result of oblique shockwaves formed at the leading and trailing edges of the body. In highly supersonic flows bow waves will form instead. At supersonic speeds, wave drag is commonly separated into two components, supersonic lift-dependent wave drag and supersonic volume-dependent wave drag.

Hopefully, you understand the answer to ‘what is air resistance’ and its importance to calculating the amount of fuel needed for flight and space flight.

We have written many articles about air resistance for Universe Today. Here’s an article about the role of air resistance in space landing, and here’s an article about zero gravity.

If you’d like more information on NASA’s aircraft programs, check out the Beginner’s Guide to Aerodynamics, and here’s a link to the Drag Equation.

We’ve also recorded many related episodes of Astronomy Cast. Listen here, Episode 102: Gravity.

Sources:
The Physics Classroom
NASA

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