MadSci Network: Physics

Re: At what height is air resistance no longer negligable?

Date: Sun Aug 1 14:38:24 1999
Posted By: Madhu Siddalingaiah, Physicist, author, consultant
Area of science: Physics
ID: 931790317.Ph

Hi Ellie.

Air is a fluid, similar to water in many ways. Fluids can be described by a number of characteristics. Two major characteristics are density and viscosity. Density basically measures how heavy a fluid is for a given volume. Air is much less dense than water and Helium is even less dense than air. This is why a Helium filled balloon rises. Viscosity measures the thickness of a fluid. Motor oil is much thicker than water, so it is more viscous. Some compounds that appear solid are actually fluids based on strict definitions. Glass is actually a very, very viscous fluid! Fluid dynamics is a field of study that describes the nature of fluids and behavior of objects within a fluid. Fluid dynamics is used heavily by Aerospace Engineers, but chemists and physicists use it as well in their work.

Now back to your question. When an object moves through a fluid, energy is needed to move the fluid out of the way. This energy appears as resistance and actually heats the fluid. Dense, viscous fluids like water require lots of energy to move. For this reason, conventional ships take enormous amounts of power to move through water. Even nuclear powered vessels do not exceed about 40 miles per hour. The resistance to motion is proportional to the frontal area of an object, the density of the fluid, the square of the velocity, and a factor known as the drag coefficient. The drag coefficient is a number that depends on the exact shape of the object.

Moving a fluid out of the way is only one part of the resistance. Objects moving in a fluid experience viscous drag and often create turbulence. Viscous drag is a characteristic of the type of fluid and the surface of an object. In a simplified way, viscous drag is a result of a rough surface, but is actually a molecular phenomenon. Turbulence is caused by sharp corners or trailing edges. Tubulence gives rise to vortecies which are like tiny cyclones or whirpools. The surfaces of modern aircraft are made as smooth as possible to minimize turbulence. Turbulence is a complex phenomenon which is diffucult to completely eliminate. It is said that only God truly understands turbulence.

Air resistance can be estimated using simple equations, but a wind tunnel is needed to measure resistance accurately. A wind tunnel is a carefully designed structure in which large fans force air at a specific velocity through a test section. Objects are placed in the test section in order to measure factors such as drag coefficient and lift coefficient. Based on this data, the motion of freely falling object can be readily determined. Other factors, such as air density and temperature, can be plugged in to estimate behavior at high altitudes.

All of this calculation and simulation must be tested under real conditions as subtle effects are often difficult to predict. During the development of the Boeing 777, the nose landing gear doors would shudder violently as they were buffeted by turbulent air during takeoff. This effect was not observed in the test cases and was only discovered during initial flight testing. The engineers quickly fixed the problem by installing baffles in the gear cavity to eliminate the buffeting.

Years ago when I was a student at the University of Maryland, I did some work on the design of a sail boat rudder for the St. Francis 12 Meter Challenge, more commonly known as the Americas Cup challenge. A scale mode of the hull of the ship was placed in the University wind tunnel for testing. I think team America won the challenge that year, maybe my work helped them win!

I hope I answered your questions fully. If not, please send me email at

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