The Great Metal Tube in the Sky

Composite Bird

My window seat next to the wing of the Boeing 777 that would carry me across the Arctic Sea to China was anything but desired. As soon as I made myself comfortable and opened up the window shade, the massive expanse of METAL that defined the wing made it clear that I would see nothing else during the 13 hour flight. But as soon as the landing gear separated from the ground, I experienced something much more profound than the terrain I would not be seeing. As physics took over and lift was achieved, the great metallic wing began to bend and creak under the weight of the air it was using to gain altitude.

Photo by R. H. Holley

Photo by R. H. Holley

Like a bird flapping its wings

This is a perfect case of photos not doing justice to the magnificence of their subject. Nevertheless, I think it begins to illustrate it. Anyway, in observing this feat of engineering, I was struck with shear admiration for the Triple Seven and its hard working, gravity defying airframe. My admiration turned into questioning; how the hell do they do it? Is it all made out of metal? What material is able to withstand such forces without being compromised? Is it the best we have?

The first two questions are related. The efficiency of wing design is achieved through the design of its shape in cross-section. This shape is known as the airfoil. As air moves across its form, it generates aerodynamic forces: lift and drag. The shape of the airfoil determines the efficiency of lift relative to drag. Lift is the force perpendicular to the direction of motion, and drag is the force parallel to it. For the wing to produce lift, it is maneuvered into an angle of attack that turns and deflects the oncoming air downwards and changes its direction. The air then exerts an equal force upon, but in the opposite direction of, the wing. This creates lower air pressure and downward force upon the wings upper surface, while higher air pressures and upward forces create the energy that carries us off the ground and into the sky. The Triple Seven uses what’s called a supercritical airfoil, the shape of which has a smaller curve along its upper surface that delays the point that drag occurs, allowing for increased fuel efficiency at higher speeds. The aerodynamic and structural efficiency of these airfoil shapes increase as our understanding and mastery over materials progresses. The flex you see above, and that I observed and felt, is generated in the structure of that airfoil and the materials used to create it.

Aerodynamic Forces

Airfoils

To the best of my knowledge, the wing of the Triple Seven is made with aluminum and some variation of composite material, which are, well, a combination of materials (so, it’s not just a clever name). Composites differ from metals in that they are composed of two or more materials of differing physical and/or chemical properties that retain their individuality in the finished product. Like reinforced concrete (which is itself a composite of concrete and steel), materials are used together to strengthen the whole. The strengths of one material (concrete is better in compression) are shared with another (steel works better in tension) to create superhero like strength.

Found at Photobucket
by artistsean

Though we know the 777 uses composites in the structure of its wing, the specific material recipe is understandably proprietary. But if history is any indicator, it most likely consists of titanium, graphite and some sort of plastic polymer held together with epoxy and sandwiched between two layers of aluminum skin.

Found at Navyaviation

The use of composite material in aircraft design is nothing new; planes have utilized fiberglass for quite some time. Composites add strength while decreasing weight. And they have only gotten stronger and lighter with the development of advanced composites using material such as graphite, boron and Kevlar. So then, can it be done better? It seems the answer is yes. Though we can certainly continue to push the bounds of composites strength and efficiency, it seems that as of now, better means a higher percentage of composites to other material. In comparison to the Boeing 777, the Boeing 787 illustrates this well. Along with aluminum, steel and fiberglass, the 787 wings are predominately made from composite materials. And so is the rest of the aircraft. Compared to the airframe of the Boeing 777 (11% total composites) and its predecessor the Boeing 747 (1% total composites), the 787 has an astonishing 50% total composites in the design of its airframe.

If we believe that a higher percentage of total is an accurate measure of our success and progress in the development of better, stronger and more efficient aircraft, then the progression from 747 to 777 to 787 is a good indicator. But I think we can still do better. I think that there are still stronger and lighter composites to be discovered. I mean, I don’t believe that once we have a 100% composite aircraft in the sky that we will be done. We’re human, and we are driven by success and a desire to do better. And I will continue to follow our aerospace engineers into the future, and look to them to uncover something new for architects to play with.