How Airplanes Work

How Airplanes Work

There is an old joke to the effect that the propeller is a fan in the front of an airplane used to blow air across the pilot, and keep him cool: If you don’t believe that, just watch how fast the pilot starts sweating when the propeller stops turning.

While this isn’t a very accurate description of how a plane works, it does have one thing right: the key factor keeping an airplane in the air is airflow. But it’s not airflow over the pilot. Rather it’s airflow over the wings. So, how does air flowing over a wing generate lift?

To explain, we have to look at a cross section of a wing. If you notice you’ll see that in general a wing’s cross section looks a bit like a slice of watermelon turned upside down. The bottom of the wing is flat, and the top bulges in a curve. This is the key to generating lift. Now, as air passes around the wing the air molecules passing across the lower surface of the wing have a shorter distance to travel than those passing over the curved upper surface. That is the molecules above the wing travel a greater distance over the wing than the molecules below the wing. This is the basis for generating lift.

Bernoulli’s principle states that in fluid flow an increase in velocity results in a decrease of pressure. This is a description of what happens around the airfoil of a plane’s wing: as the molecules above the wing speed up to travel the greater distance over the wing the pressure above the wing decreases compared to that below the wing. This difference in pressure results in a force being applied to the lower surface of the wing, which lifts the wing, and plane, in flight.

To direct the plane while it is in the air the pilot uses the yoke to control three sets of flaps on the wings and the elevators and the rudder. To raise the plane, the elevators are used: These are the flaps at the smaller set of wings at the tail of the aircraft on a traditionally laid out plane. When the pilot wants to have the plane go up, the elevators are tilted up into the airflow, which pushes the tail down, and torques the plane so that the nose goes up. When the pilot wants to dive, he causes the elevators to push down, with the opposite effect.

The most prominent control surface that most people see is the rudder. This is one of the ways that the pilot can make the plane turn from side to side. When the rudder turns into the air stream passing it is hit by air, and that impulse transfers a torque on the plane, turning the nose of the plane in the direction that the rudder has been turned. Normally, the rudder isn’t used to turn the plane except in sharper turns.

For the most part gradual turns are performed by the use of flaps on the main wings: a set of flaps on one wing will be tilted up into the air flow, and the on the opposite wing, tilted down. This will have the effect of rolling the plane on its long axis, and turning it as well. This is the control that provides most of the yaw control for the airplane.

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