Photo: 123RF
A Super Jumbo Airbus A380
Why do airplanes not just fall out of the sky due to gravity?
Gebo Pol wants to know how an iron bird, such as the Airbus A380 weighing about 560 tonnes, is able to stay in the air and not fall out of the sky due to gravity.
Well, we have to thank the inventors, Orville and Wilber Wright for that. They have shown that Lord Kelvin’s statement, “heavier-than-air flying machines are impossible” wrong about 120 years ago.
A plane’s ability to stay in the air is attributed to the shape of its wings which is similar to a curved elongated tear drop. The upper surface is more curved than at the bottom. When air flows across this wing, the pressure at the top of the wing is lower than that at the bottom.
When that upward force is greater than gravity pulling the plane toward the earth, the result is a wing with a lifting force.
This is Bernoulli’s theorem which explains lift as a consequence of the curved upper surface of the wings but this theorem cannot explain how a fighter jet plane can fly inverted.
For this, there is another theory based on Newton’s third law of motion of action and reaction. It states that a wing keeps an airplane up by pushing the air down.
You can test this theory by an ordinary experience when you stick your hand out of a moving car, say at around 100 mph, and tilt it upward, the air is deflected downward, and your hand rises. This is lift.
Now having got airborne, can a plane drop out of the sky?
The answer is no as long as the plane is above the stalling speed. A plane will stall if it flies too slowly. This will arise when the airflow over the top of the wings separates and the wings lose lift. That is when the plane can literally fall out off the sky.
It could take several thousand feet to recover from a stall and pick up speed to start flying again. If the plane is already too low to the ground it would crash.
Even if all engines are failed at a higher altitude, a plane will not fall out of the sky as long as the speed is above the stalling speed. As such, it will not be able to maintain height but would gradually glide down to Earth.
A typical airliner will glide about 105 miles from 40,000 feet with the loss of all engines. Air Transat Airbus A330 holds the record for the longest passenger aircraft to have glided without engines for nearly 75 miles and landed safely at an airport on the Azores in the Atlantic Ocean with 306 occupants on board.
However, a plane can drop off the sky especially if it enters into a jet upset and its speed drops below the stalling margin.
On 1st June 2009, Air France Flight 447 from Rio de Janeiro in Brazil to Paris crashed because the pilots had lost control of the plane when its air speed sensor was frozen. The plane went out of control and it stalled due to the speed sensor failure. 228 precious lives were lost.
Moral of the story – a pilot must never allow a plane to drop below its stalling speed.
View a video on ‘How do planes stay in the air?’ here