The Airplane - Jay Spenser [27]
The second area where metal was used was in those places that had to be fireproof, such as engine firewalls and cowlings. A firewall is the metal bulkhead that prevents engine-compartment fires from invading the airframe, and a cowling is a streamlined cover for the engine. Here aluminum filled the bill perfectly. Not as strong as steel but much lighter, it also provided brackets, clamps, and other non-structural components.
Finally, flight’s inventors used metal in the form of steel wire. Used internally for diagonal cross-bracing, steel wires augmented the fuselage truss structure, significantly increasing its strength and rigidity while adding very little weight. Used externally, these “flying” wires served to rig (properly align) and brace (reinforce) the airplane’s wings, horizontal and vertical tails, and landing gear relative to the fuselage.
Steel wires were thus the ligaments to the airplane’s wooden bones. Often fitted with turnbuckles that allowed their tension to be changed, they were how the airplane’s rigging—and thus its flight characteristics—were fine-tuned.
Of course, early airplane builders also found other uses for steel wires, such as connecting the cockpit controls to the airplane systems they actuated. But more about that later—the focus here is on structure.
Airplane builders drew many lessons from the natural world. For example, anatomical studies had revealed that bird bones are often hollow to save weight. But nature’s cleverness did not end there; within these hollow bones, scientists observed cross-bracing spurs that, like reinforcing beams, offset most of the weakening that otherwise would have resulted from the deleted bone mass.
Following a similar path to weight savings, airplane builders learned to cut circles out of wood and metal in places where it would not compromise the part’s overall strength. Lightening holes, internal bracing struts, and other engineering practices suggested by nature remain standard features of aerospace design to this day.
The bones and ligaments of the airplane provided its strength. Around this structure, airplane builders often applied an enveloping skin that—as is the case with birds, humans, and other vertebrates—protected but did not support. For aviation, this non-load-bearing skin generally was doped fabric.
Muslin, a cotton cloth with a fine weave, was the aviator’s material of choice, although linen and other natural fabrics were also used. After being applied to the airframe, the cloth was brushed with aircraft dope, a varnish that draws the fabric taut as it dries to produce a hard, durable surface.
If you are ever at a small airport and you come across an old fabric-covered airplane—for example, a Piper Cub, Aeronca Champion, or Citabria—give its doped fabric a light tap with your knuckle. The thump will elicit a shivering thrum. It sounds if the entire airplane is a drumhead. Now look down this airplane’s fabric fuselage or wings. You will plainly see the fuselage longerons (fore-and-aft framing members), wing ribs, and other internal bones encased by this taut skin. It’s very different from the all-metal jetliners we routinely board.
The very earliest airplanes used relatively little fabric. At the slow speeds they operated, it was not required on the fuselage, where it merely added weight and hindered internal access for modifications or repair. For this reason, the earliest flying machines often appeared to the world like the dinosaur skeletons displayed by museums.
In contrast, fabric was required on the airplane’s wings, tail, and control surfaces, where it guided the passage of air to provide lift, stability, and control. But even here, where it had an aerodynamic role to play, the airplane’s skin never shouldered the structural loads of flight—that