Properties

The aerodynamic forces for flight occur in a fluid. The fluid is usually either air or water, although there are other fluids. Before flight can occur the fluid must be measured to understand the forces generated by a moving object. In the Measurements section, units were introduced to help understand the qualities of a fluid. In this section, these qualities, or properties, of a fluid are defined. The units (inches, pounds, grams, meters) will be used in the following definitions. In addition, several other factors (facts or parts) are defined to help further understanding of aerodynamics. These include weight and gravity, velocity and acceleration.

Temperature

The temperature of the fluid is an important part of how the fluid behaves. Hot oil, for example, flows faster than cold oil. Warm air rises and cold air drops in a room; house designers often place heat vents at the floor level because of this. Very cold water is lighter than cool water, so it rises to the top of a lake. That's why lakes freeze from the surface down. Sound travels farther on cold days than hot days. It is crucial (important) then, to know the temperature of the fluid when computing aerodynamic quantities. As mentioned in the Measurements section, temperature has units of degrees Fahrenheit or degrees Celsius.

Pressure

The pressure of a fluid is another important consideration in aerodynamic forces. When a fluid moves over or through an object, it gives small pushes on the surface of the object. These pushes, over the entire surface, are defined as pressure. Pressure is measured as force per unit area (square inches, square meters). In metric units, pressure is measured in Newtons per square meter. In the English system, pressure is usually measured in pounds per square inch. Example: The atmosphere (air) presses on your skin at 14.7 pounds per square inch (psi).

Pressure can be powerful. A small pressure, spread over a very large area, can add up to be a very large force. Air pressure decreases as the altitude increases; pressure also decreases when the speed of the fluid (air, water) increases. When the temperature of a fluid increases, so does the pressure. The pressures on an airplane directly affects its flight capabilities!

Density

Density is a measure of how much mass (the amount of molecules) is included in a given object or volume. Another way to think about it is how tightly the molecules are packed in a volume or object. When we talk about the density of fluid (volume), we often refer to a specific volume, such as a cubic meter, a cubic foot or a slug. A slug is equal to 32.174 pounds mass.

A fluid with a lot of molecules tightly packed together has a high density; one that has fewer molecules would have a lower density. Water, for example, has a much higher density than air. A 10 gallon fish tank with water in it has much more mass in it than a 10 gallon tank with air in it. Since it has more mass, it will weigh more (more on that in a later section.) In addition, the density is used to define whether a fluid is incompressible or compressible. If the density of the fluid is fixed (constant), the fluid is incompressible; neither the mass or the volume can change. Water is an incompressible fluid. The amount of volume and mass will stay the same, even under pressure.

Gases (like air), are compressible, they will expand to fill a new volume. The mass doesn't change, but the volume increases, so the density of the gas decreases in the new volume.

An aerodynamicist must pay attention to all of the properties of a fluid (air, water) to define flow conditions. This is because all of the properties are linked together. If the pressure or the temperature of a fluid changes, its density will usually change, too. The density of air on a hot day is lower than the density of air on a cold day. At high altitudes, where the pressure is lower, the density is also lower.

Viscosity

This is one of the most difficult properties on this list to define. Viscosity is a measure of how much a fluid will resist flowing. If you spill water on an inclined board, it will run quickly down the board. However, if you spill honey on the same board, it will travel down the board much more slowly. Honey has a much higher viscosity than water. It is said that honey is a more viscous fluid than water.

When a fluid flows over a surface, it exerts a force (measured in Newtons, for example) on it. Scientists and engineers define viscosity by using units of mass/length/time. The more commonly used units are kilogram per meter second (kg/m s) for the metric system, and pounds mass per foot second (lbm/ft s) in the English system.

The resistance to flow (viscosity) is important information when designing an object (like a wing or boat hull) to move through air or water. Several math formulas are used to get the viscosity reading needed to design surfaces that will reduce aerodynamic drag.

Force

Forces have been defined as pushes or pulls on an object. To determine the units of force, scientists and engineers use Newton's second law of motion. The second law states that a force on a moving object is equal to the mass of the object times the acceleration (a measure of its motion) of the object. Various mathematical formulas are used to measure force.

An interesting point about the force is that in addition to a value and units, it also has a direction associated with it. In the figure above, the force is applied to the box to the right, therefore the motion is to the right. If the force were applied down on the top of the box, no motion would occur; since the box is already on the ground, it can't move any further. No matter how large the force was, there would be no motion. So, defining a direction for a force is very important.

Weight and Gravity

In other countries, objects are measured in terms of their mass, in grams or kilograms. In the United States, however, people use the terms for weight to also mean mass. This works okay near the earth's surface because gravity is constant, so the units of "weight and mass" stay the same. (the acceleration due to gravity is equal to 32.174 feet per second, at sea level) Because of gravity, weight is actually a force and not the true mass of an object. If an object is taken up high in the atmosphere, the force of gravity is less. Therefore, the "force" of weight is less. An object will weigh less, at high altitude, but the mass will remain the same.

Scientists must be able to separate weight and mass. Therefore, the units are: pounds mass or pounds force. Mass will not change. Pounds force will change with altitude. Acceleration of an object at high altitudes is less, due to gravity, therefore the weight of the object is less.

This is why an object on the moon weighs less than the same object on the earth. The gravitational attraction on the moon is less than that of earth, so the acceleration due to gravity is less (about 1/6th that of the earth). When an object is weighed on the moon, it will weigh about 1/6th as much as the same object on earth. Example: A 60 pound child would weigh 10 pounds on the moon!

Velocity

How fast an object moves is measured by its velocity. Velocity is calculated by dividing the distance traveled (a length) by the time it takes to travel the distance. The units of velocity are, for example, meters per second (m/s) or feet per minute (ft/min). If a person runs 10 kilometers in 1 hour, his or her velocity is 10 kilometers per hour (km/hr). If a car travels from Los Angeles, CA, to San Diego, CA , a distance of 120 miles, in 2 hours, its velocity is 60 miles per hour (mph)(120/2hrs=60 mph). One exception to these units is a term held over from sailing days, the knot. In aeronautics, the velocity of the air is often measured in knots. One knot is equal to about 1.7 feet per second (ft/s).

Rate and speed are two of the many terms used interchangeably with velocity. When engineers work with velocities, they must know the direction of the motion as well as the numerical value. They will sometimes call the numerical value the rate or speed, and then define a direction: the box was moved at a rate of 3 ft/s to the right, or the rocket traveled upwards at a speed of 120 m/s.

Acceleration

Acceleration is a measure of how the velocity of an object is changing over time. It can be found by computing the difference in velocities at first one time, then some time later, and dividing that by the difference in time. Example: A car is traveling at 60 mph at the first mile post. One mile (and one minute) later the car is traveling at 70 mph. 70 - 60=10 divided by 1/60 hr. = 600 mph (if acceleration continued at the same rate for the next 59 minutes).

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Last modified: Wed Jul 28 10:50:12 PDT 1999

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