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During last week I visited Tanagra airport (which is located few kilometers north of Athens) for the annual airshow of the Greek Air Force. I had the opportunity to watch at a close range skilled pilots perform some amazing aerobatic maneuvers like spins, loops, rolls and much more. It was like they were defying the laws of physics and the aerodynamic forces. As if their planes were able to produce the required “Lift” even when they were inverted!
But… did they? Can all conventional airplanes fly inverted? And if they can, does this mean that we can turn an airplane’s wing upside-down and still expect it to fly? If we want to answer all these questions I suppose we have to start from the basics, we have to start with the definition of “Lift”.
So, what is Lift?
Let’s say we observe an airplane which maintains a constant speed and altitude. Certain forces act on this plane during this horizontal flight. If someone tries to name them he will probably start with the most obvious, its Weight. The force that every aviator loves to hate and tries to overcome.
However, for achieving a smooth horizontal flight, another force, equal in magnitude and opposite in direction must be applied on the airplane. Otherwise, it would never be able to leave the ground. It needs a force to lift it in the air. It needs… “Lift”!
According to NASA’s official webpage:
“Lift is the force that directly opposes the Weight of an airplane and holds the airplane in the air”
So, it seems that without Lift it’s impossible to stay in the air for long, you will eventually hit the ground. Lift is that force which holds the airplanes in the sky.
But, still, we haven’t answered to our original question. How is it possible for an inverted airplane to generate Lift? Which are the mechanisms that produce this aerodynamic force?
How Lift is generated?
Well, there have been many theories that tried to explain the phenomenon which takes place on an aircraft’s wing, many of which were incorrect or oversimplified. The truth is that the generation of Lift is a complicated phenomenon and to understand it in depth requires study and broad knowledge of physics and aerodynamics. However, some basic principles are easily understood even by someone who is not familiar with the laws of aerodynamics, but if we want to be on the same page we have to start from the beginning.
So, let’s go back to 1687. An English physicist, named Isaac Newton, publishes three books with the title “Philosophiæ Naturalis Principia Mathematica”, (or more simply: Principia) which contain 3 of the most fundamental physical laws of classical mechanics. The Newton’s Laws.
According to Newton’s 2nd Law, a force can’t exist without acceleration. Therefore, in order to produce Lift, the wing of an airplane must change the air’s speed or direction… or both! To better understand this and also find which geometries produce more Lift we have to do some experiments; we have to use a Wind Tunnel!
In a wind tunnel, it’s much easier to measure the produced lift and come to safer conclusions about the selected geometry of our airplane’s wing. When we place the wing in the tunnel and turn the fans on we notice that as the air leaves the trailing edge of our airfoil is deflected by few degrees! Actually, as we increase the angle of attack, and therefore the generated lift, the deflection is even larger. It seems lift is somehow related to the air’s deflection…
To test this theory we have to examine a flat plate positioned parallel to the flow so that we get almost zero deflection at the trailing edge of our object. And… the results are as we expected. Almost zero lift is generated with this setup. But maybe it has to do with our airfoil, right? Maybe, in order to produce some lift, we have to use this special, curved geometry of an airplane’s airfoil. The thing is that if we increase the angle of attack we see that even with a flat plate… we have Lift!
According to NASA’s official webpage:
“Lift occurs when moving flow of fluid is turned by a solid object.”
With all these in mind, I suppose we can come to a safe conclusion. There is no need for a specially designed airfoil to generate some Lift. We all know that since we were children; this is how a paper plane flies! As long as we have the right angle of attack and an airflow of high speed, even a large board can make us fly!… Even an inverted wing!
So yes! An airplane with its wings upside down, and under certain circumstances, can still produce enough Lift to stay in the air. The deflection of the stream around the wing means that the stream accelerates and, according to Newton’s 2nd Law, it produces the force we all call “Lift”. This is how an airshow pilot manages to fly an airplane inverted. As long as the wing has the right angle of attack to achieve the required deflection, the airplane keeps flying.
However, new questions come to my mind as a result of this answer.
How the deflection of the stream generates Lift on the wings? Why do the wings of an aircraft have this specific geometry and why don’t we use flat plates instead? Why does the air seem to travel faster on the upper surface of the wing?
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