Plane Wings, Pressure Differences, and Thunderstorm Downbursts

Airplane wings have a particular shape. They are designed this way so they can lift the plane off the ground. The forces responsible for lifting the plane off the ground are produced by the wind flow over the wing. You can demonstrate this by shaping a piece of cardboard like a wing, securing it to a table and blowing air over it (See below). The cardboard will lift off the table!

Bernoulii's Principle (named after the Swiss scientist Daniel Bernoulli) states that when a fluid in steady flow speeds up, the pressure lowers and where the fluid slows down the pressure is high. As the air flows over the wing it speeds up as it travels over the top of the wing, lowering the pressure causing a pressure difference between the bottom and top of the wing, resulting in an upward force called the lift. There is also a drag force acting to slow-down the moving wing. The net force, if the wing is designed correctly, lifts the wing and the plane off the ground. The key to designing wings is reducing the drag while increasing the lift.

The shape of an airplane wing generates a low pressure above the wing surface as air flows over the wing. The low pressure produces the lift that enables the plane to fly. Air must flow quickly over the wing to generate lift. Airflow is a combination of the wind and the aircraft engines. For the plane to maintain its altitude the lift must balance the weight of the airplane. The motion over the wings is a relative motion. With a headwind the engines do not need to provide as much forward movement to lift the plane. So, a rapid shift of wind direction from a head wind to a tail wind can be very dangerous. To see this consider a plane on take-off with a microburst at the end of the runway.

As the aircraft flies into a microburst it encounters a strong headwind, can therefore cut-back on the engines, and still meet the minimum relative speed required for take off. Suddenly, while the plane is in the air, the wind shifts and the plane unexpectedly encounter a tail wind. The relative speed of the aircraft may no longer be strong enough to maintain its altitude.

Knowing that wind-shifts exist in the vicinity of runways is very important to safe flying conditions. The Federal Aviation Administration (FAA) requires large airports to operate microburst detection systems. These systems provide enough advance warning of a microburst-induced wind shear to allow pilots enough time to take corrective actions.