How does airfoil technology work

A 1-minute video released by the University of Cambridge sets the record straight on a much misunderstood concept — how wings lift. Only a handful will know that it is wrong. You find it taught in textbooks, explained on television and even described in aircraft manuals for pilots. In the worst case, it can lead to a fundamental misunderstanding of some of the most important principles of aerodynamics. To show that this common explanation is wrong, Babinsky filmed pulses of smoke flowing around an aerofoil the shape of a wing in cross-section.

Babinsky is quick to stress that he is far from the only aerodynamicist who is frustrated by the perpetuation of the myth: colleagues have in the past expressed their concerns in print and online. Where he hopes his video will help debunk the myth once and for all is by providing a quick and visual demonstration to show that the most commonly used explanation cannot possibly be correct. The original video, created by Babinsky a few years ago using a wind tunnel, has now been re-edited in high quality with a voice-over in which he explains the phenomenon as it happens.

One of his visions is to design a wing that will enable aircraft to fly faster and more efficiently. This work is licensed under a Creative Commons Licence. If you use this content on your site please link back to this page. Our selection of the week's biggest Cambridge research news and features sent directly to your inbox. Enter your email address, confirm you're happy to receive our emails and then select 'Subscribe'.

I wish to receive a weekly Cambridge research news summary by email. Einstein probably thought that his ideal-fluid analysis would apply equally well to real-world fluid flows.

He brought the design to aircraft manufacturer LVG Luftverkehrsgesellschaft in Berlin, which built a new flying machine around it. Contemporary scientific approaches to aircraft design are the province of computational fluid dynamics CFD simulations and the so-called Navier-Stokes equations, which take full account of the actual viscosity of real air.

Still, they do not by themselves give a physical, qualitative explanation of lift. In recent years, however, leading aerodynamicist Doug McLean has attempted to go beyond sheer mathematical formalism and come to grips with the physical cause-and-effect relations that account for lift in all of its real-life manifestations. McLean, who spent most of his professional career as an engineer at Boeing Commercial Airplanes, where he specialized in CFD code development, published his new ideas in the text Understanding Aerodynamics: Arguing from the Real Physics.

Considering that the book runs to more than pages of fairly dense technical analysis, it is surprising to see that it includes a section 7. I was never entirely happy with it. Where these clouds touch the airfoil they constitute the pressure difference that exerts lift on the airfoil. The wing pushes the air down, resulting in a downward turn of the airflow. In addition, there is an area of high pressure below the wing and a region of low pressure above.

It is as if those four components collectively bring themselves into existence, and sustain themselves, by simultaneous acts of mutual creation and causation. There seems to be a hint of magic in this synergy.

And what causes this mutual, reciprocal, dynamic interaction? McLean says no: If the wing were at rest, no part of this cluster of mutually reinforcing activity would exist.

But the fact that the wing is moving through the air, with each parcel affecting all of the others, brings these co-dependent elements into existence and sustains them throughout the flight. Soon after the publication of Understanding Aerodynamics , McLean realized that he had not fully accounted for all the elements of aerodynamic lift, because he did not explain convincingly what causes the pressures on the wing to change from ambient.

In particular, his new argument introduces a mutual interaction at the flow field level so that the nonuniform pressure field is a result of an applied force, the downward force exerted on the air by the airfoil. There are reasons that it is difficult to produce a clear, simple and satisfactory account of aerodynamic lift.

Some of the disputes regarding lift involve not the facts themselves but rather how those facts are to be interpreted, which may involve issues that are impossible to decide by experiment. Nevertheless, there are at this point only a few outstanding matters that require explanation.

Lift, as you will recall, is the result of the pressure differences between the top and bottom parts of an airfoil. We already have an acceptable explanation for what happens at the bottom part of an airfoil: the oncoming air pushes on the wing both vertically producing lift and horizontally producing drag.

The upward push exists in the form of higher pressure below the wing, and this higher pressure is a result of simple Newtonian action and reaction. Things are quite different at the top of the wing, however. A region of lower pressure exists there that is also part of the aerodynamic lifting force. We know from streamlines that the air above the wing adheres closely to the downward curvature of the airfoil. This is the physical mechanism which forces the parcels to move along the airfoil shape.

A slight partial vacuum remains to maintain the parcels in a curved path. This drawing away or pulling down of those air parcels from their neighboring parcels above is what creates the area of lower pressure atop the wing.

But another effect also accompanies this action: the higher airflow speed atop the wing. But as always, when it comes to explaining lift on a nontechnical level, another expert will have another answer.

But he is correct in everything else. The problem is that there is no quick and easy explanation. Drela himself concedes that his explanation is unsatisfactory in some ways. So where does that leave us? In effect, right where we started: with John D.

This article was originally published with the title "The Enigma of Aerodynamic Lift" in Scientific American , 2, February If the pressure gradient is too high, the pressure forces overcome the fluid's inertial forces, and the flow departs from the wing contour. Since the pressure gradient increases with an increasing angle of attack, the angle of attack should not exceed the maximum value to keep the flow following the contour.

If this angle is exceeded, however, the force keeping the plane in the air will decrease, and may even disappear altogether. Viscosity can be described as the "thickness," or, for a moving fluid, the internal friction of the fluid.

Viscosity measures the ability of the fluid to dissipate energy. A parameter of viscosity is the coefficient of viscosity, which is equal to the shear stress on a fluid layer over the speed gradient within the layer. Viscosity is essential in generating lift; it is responsible for the formation of the starting vortex, which in turn is responsible for producing the proper conditions for lift. Viscosity is responsible for the formation of the region of flow called the boundary layer. There are two types of boundary layers: Laminar Turbulent In a laminar boundary layer, the fluid molecules closest to the surface will slow down a great deal, and appear to have zero velocity because of the fluid viscosity.

In turn, these surface molecules create a drag on the particles flowing above them and slow these particles down. The effect of the surface on the movement of the fluid molecules eventually dissipates with distance from the surface. The area where these viscous effects are significant is called the boundary layer. In a turbulent boundary layer, eddies, which are larger than the molecules, form. The slower eddies close to the surface mix with the faster moving masses of air above.

As a result, the air molecules next to the wing surface in a turbulent boundary layer move faster than in a laminar boundary layer for the same flow characteristics. A turbulent boundary layer has the following properties over a laminar boundary layer: The drag is higher, and The turbulent boundary layer is not as susceptible to flow separation.

The two types of boundary layers may thus be manipulated to favor these properties. A streamline is the path that a fluid molecule follows. Every point along the streamline is parallel to the fluid velocity. Bertin, John J. Hubin, W. Perdichizzi, Richard F.

White, Frank M. Chord: Extends from leading edge to trailing edge of the wing Camber line: Points halfway between chord and upper wing surface Angle of attack: Angle between direction of airflow and the chord.

Figure 4: Starting Vortex Formation.