The theory of flying, or how an object heavier than air can stay up in the air, has always been one of the great mysteries for humankind. People have yearned to fly like a bird since time immemorial. Leonardo da Vinci’s sketches are one evidence of our determination to solve that mystery. Da Vinci never rose to the skies himself (unless he did so in secret during his experiments at his own estate), but since then Sir Cayley, Otto Lilienthal and the Wright brothers and many other aviation pioneers, step by step, have made that dream come true. The airplane was invented, with its ability to take off the ground and fly based on the lifting force generated by wings.
And here is the common misconception I wish to correct: the engines do not generate the lifting force. Therefore, even in the unlikely event that all engines would fail in air, the plane would not drop like a stone from the sky. The plane will be able to glide and it remains perfectly steerable. So even the largest of passenger planes behave like a sailplane if necessary, and with a surprisingly impressive glide ratio. So “Sully” Sullenberger, after the fateful bird strike, had a fair amount of time to steer the aircraft, attempt to restart the engines and discuss with the air traffic control the best places to land, and finally prepare the plane for the relatively successful emergency landing in the Hudson River.
So, aircraft can fly because of their wings. The wings generate lifting force when moving forward with sufficient speed, in other words, when the wings meet with a sufficiently powerful air flow. The role of the engines is to create the thrust that generates that speed. With no engine power, the airspeed required to maintain the lifting force of the wings can be achieved by turning the plane into a mild descent. An example: an unpowered passenger aircraft at a cruising altitude of 10,000 metres can glide up to 150 kilometres. In fact, even during normal passenger flights, most of the approach takes place with engines on idle – that is, by gliding. Only towards the end of the approach are the engines pushed back into action to control the landing speed and slope.
The crux in the lifting force of the wing is in the angle of attack. This angle refers to the small positive angle at which the wing meets with the drag, the front slightly higher than the back of the wing. Thanks to this position, the wing diverts the airflow downwards both over and under the wing. To understand the diversion under the wing think of a shower of water that hitting a downward sloping surface turns to flow downwards. Over the wing, the airflow follows the smooth surface of the wing from the leading edge towards the trailing, diverting the airflow slightly downwards. And here’s the point, which you may be familiar from the physics class at school: all forces always have a counterforce. The diversion of airflow downwards means that a force is also generated that lifts the wing upwards.
But doesn’t that mean you could fly with an old barn door, you might ask. In theory, absolutely, if only there was a way to create sufficient airspeed and you didn’t have to worry about air drag. In practice, flying an aircraft is a constant fight between lift and drag. In order to keep a plane off the ground, the lifting force of the wing must displace the drag it creates. That is why the wings are of the slightly rounded, aerodynamic shape that disrupts the airflow as little as possible. Barn doors are not that aerodynamic and their rough, angular shape would in practice create too much drag, which would negate the lift.
If you are lucky enough to have wing seats, you can see sections in the wings that move during the flight. The flaps on the trailing end of the wing are pulled in during cruising but pushed out and downwards during climb, approach and landing. In the out position, they increase the downwards diversion of airflow and the lifting force of the entire wing, which is why the plane can fly at a slower speed. The ailerons at the tip of the winds help the aircraft turn left and right: when the aileron on one wing goes up, it goes down on the other, the lift increases and decreases respectively. The third moving sections are spoilers, which are lifted into an upright position. This diverts the airflow from the surface of the wing and reduces the lift. This may be necessary to increase the glide slope or to reduce the speed of the aircraft.
Da Vinci would certainly approve, if he knew how his early visions have now been developed to a point where billions of passengers are transported by air around the world each year. It is easy to understand why aerodynamics may have seemed such a mystery. It is after all about powerful forces that air, which is invisible to the eye, is able to generate. Some people find the idea of flying in an aircraft up in the sky on nothing but thin air unpleasant. However, it is all based on simple, proven laws of physics that are widely known and understood. The aircraft wing must be the height of achievement in the field of aerodynamics. Therefore, you can rest assured when boarding an aircraft and trust that it can fly.