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What makes an Airplane fly? What makes an airplane fly? To achieve flight, airplanes must defeat gravity and their weight, and they do this through lift, to move forward, finally to overcome air resistance (called drag) through thrust. An airplane in flight is the center of a continuous tug-of-war between four forces: lift, gravitational force, thrust, and drag. Thrust is generated by the propeller and opposes drag caused by air resistance to the frontal area of the airplane. During take off, thrust must overcome drag and lift must overcome the weight before the airplane can become airborne. In level flight at constant speed, thrust exactly equals drag and lift exactly equals the weight or gravity force. For landings thrust must be reduced below the level of drag and lift below the level of the gravity force or weight. The lifting of the airplane is the application of Bernoulli 's Principle which is the heart of this topic. Lets discuss more.

Learning objectives

After completing the topic, the student will be able to:

  • Explore the properties of fluids (liquids and gases).
  • Examine and explore about atmospheric pressure, pressure in liquids.
  • Experiment and discuss about the effect of gravity on liquids.
  • Illustrate how to measure the pressure and relative density of a fluid using different sophisticated instruments.
  • Discuss and investigate about the principles of fluids and their relevant applications.
  • Discuss and relate Archimedes principle to everyday science.
  • Discuss and relate Bernoulli's principle and its applications to everyday science.
Three states of matter Three states of matter Three beakers containing ice, water and air (water vapor).
Fluid mechanics

Matter can exist in its form because of interatomic and intermolecular forces. In the case of liquids and gases the interatomic forces are weak and hence do not hold atoms in fixed positions as in solids.

The atoms that constitute a molecule of the liquid or gas remain at fixed positions relative to each other within a molecule. But the molecules change their positions freely within the liquid or gas, more so in a gas. This gives both liquids and gases the ability to flow, a property that solids do not have. Due to this property, liquids and gases are also known as fluids. Liquid takes the shape of its container because the molecules of the liquid are not confined to fixed positions as in solids but can move freely from position to position by sliding over one another. Molecules of a liquid are close together and greatly resist compressive forces. Liquids, like solids, are hard to compress, where as gasses are easier to compress.

Since constituents of fluid, such as atoms or molecules, are not held tightly by their neighbours, fluids have to be held in containers. Liquids take the shape of the container, whereas gases "expand" to occupy the entire volume of the container.

Molecules beautifully sliding over one another Molecules beautifully sliding over one another The bonding between the water molecules cause them to slip over the rocks without loosing contact with each other.
Molecules of fluid

Have you ever wondered: Why a liquid drop does not have an irregular shape? How can a light weight insect walk on water? Why does water rise in a capillary tube? Why water level is concave in shape and a mercury level is convex in shape?

Most of the behaviour of liquids comes from cohesive forces between liquid molecules and adhesive forces between the liquid and its container. The primary difference between a gas and a liquid is the distance between molecules. In a gas, the molecules are far more apart and free from the cohesive forces that dominate their motions when in the liquid and solid phases. A gas expands indefinitely and fills all space available to it. Only when the quantity of gas is very large such as in the earth's atmosphere or in a star, do gravitational forces limit the size or determine the shape of the mass of a gas.

The molecules of the fluid are constantly in motion and hence exert a force on their neighbouring surfaces. This force measured per unit area is called pressure.. Thus pressure is force per unit area. If we understand fully about pressures in liquids and gases, we will be able to appreciate all the properties fluids display.