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Aircraft are extraordinary vehicles that employ the laws of physics to reach great heights and travel large distances. The following explains the basics of how airplanes work, including the different forces that act on a plane and the way different parts of a plane contribute to these forces.

Forces on a Plane

There are four main forces that act on an aircraft: lift, weight, thrust, and drag. Each of these forces is applied in a different direction.

Figure 1: The forces on an aircraft:
A= Lift,
B= Thrust,
C= Weight,
D= Drag
Picture from: HowStuffWorks.com

Forces on an Aircraft

As can be seen from the picture above, lift and weight work in opposite direction, as do thrust and drag. If the magnitudes of both forces in a pair are equal, the plane will not accelerate in either direction. However, if they are unbalanced, the plane's motion will no longer be constant:

  • If a plane's weight is larger than the lift, the plane will descend.
  • If the weight is smaller than the lift, the plane will ascend.
  • If the drag is larger than the thrust, the plane will slow down.
  • If the drag is smaller than the thrust, the plane will speed up.

Lift

Lift is the force that pushes an aircraft up into the air. It is the hardest of the four forces to understand intuitively. First, it is important to understand that lift is caused by an object's motion through a fluid (air is considered to be a fluid). As opposed to a force like weight, it cannot be created without an initial motion. Second, it is important to understand that lift is caused, mainly, by the shape of a plane's wings and the angle at which they are tilted.

 

Different Angles of Attack on an Airplane Wing

Figure 2: Different angles of attack of an airplane wing.
Picture from: HowStuffWorks.com

As an airplane's wing moves through the air, it forces the air molecules in its way to go either over or under the wing. However, because of the wing's shape and tilt, the path air molecules must travel over the wing is longer than the path they travel under the wing. This causes the air molecules over the wing to travel faster than those under the wing. It also causes them to spread farther apart. When molecules in a gas (air) are spread farther apart, the gas has a lower pressure. Therefore, the pressure of air over the wing is smaller than the pressure under the wing. This pressure difference is what causes lift to push the airplane up.

 

Cross Section of Airplane Wing

Figure 3: The cross-section of an airplane wing. Paths of air are drawn in blue.
Picture from: HowStuffWorks.com

Weight

Weight is the easiest of the four forces to understand because we experience it everyday. Our weight is what keeps us from floating off the ground. Weight is the force on an object caused by gravity.

Thrust

Thrust is the force on an aircraft that helps to move it forward. Propellers, jet engines, or rockets create thrust. It is opposed by drag.

Drag

Drag is a force that slows an aircraft down. Drag, like lift, is only created from relative motion between an object and a fluid (it does not matter which is moving, the object or the fluid, so long as there is a difference in their speeds). It can be thought of as a kind of aerodynamic friction between the object (plane) and fluid (air) as they rub past each other.

Parts of an Airplane

Flaps and Slats

Airplanes spend most of their flight time in cruise mode at fairly constant speeds and heights. Therefore, the shapes and angles of their wings are designed for cruise flight conditions. However, these conditions are much different than the flight conditions of takeoff and landing. During takeoff and landing, airplanes travel much slower and fly at lower altitudes. To help the airplane during takeoff and landing, flaps in the airplane's wings are shifted out and down from the back edge of the wing. In addition, slats on the front edge of the wing are extended. These two adjustments to the wings' shapes help to increase lift so that airplanes can takeoff and land properly.

Horizontal and Vertical Stabilizers

There are two small stabilizer wings on the tail of an airplane: the horizontal tail wing and the vertical tail wing. These wings also have flaps that help control the airplane's direction.

How Does an Airplane make Noise?

A large portion of aircraft noise is caused by unsteady airflow over different parts of the aircraft such as the flaps, slats, vertical tail wing and horizontal tail wing. These are vital parts of the plane that help to stabilize and increase lift. Another major source of aircraft noise is the plane's engine. This is the component of the plane that supplies thrust, the force needed to propel an airplane forward. For more detailed information on how airplane noise is produced, please see Sources of Aviation Noise.

The unsteady airflow around the aircraft is made up of pockets or jets of high and low pressure air. These so called pockets and or jets mix with each other and cause pressure waves to travel through the atmosphere which we experience as noise. Jet engines make noise not only from the moving parts and combustion occurring on the inside of the engine, but also through the hot jet of air coming out of the engine interacting with the surrounding air that the aircraft is traveling through. This creates the same effect as the unsteady airflow around the aircraft, but often contains pressure waves of much larger amplitude and therefore is perceived as louder.

Glossary Words:

altitude, angle of attack, drag, flaps, jet, lift, noise, slats, stabilizer, thrust

For definitions of words used in this section go to the NoiseQuest Glossary of Terms.

References:

Brain, Marshall and Brian Adkins. "How Airplanes Work." howstuffworks. 25 October 2006.

Viegas, Jennifer. "Surprising Factors Cause Airplane Noise." Discovery News. 15 December 2005. Discovery Channel. 25 October 2006.

"What is Drag?" NASA Glenn Research Center - The Beginner's Guide to Aeronautics. NASA. 26 October 2006.