Why Are Aircraft Wings Twisted?

Aircraft designs today have come a long way from where they started with the first flight in 1903. The beginning of man-made flight was characterized by many accidents and tragedies, but safety quickly became a top priority for early aerospace engineers. In the process of creating a sturdy, airworthy design, wing-twisting was discovered. Many early wing designs were ill-equipped to withstand the bending forces resulting from aerodynamic lift that keeps the aircraft aloft, and aircraft wings snapping off during changes in direction was a recurring issue. To better understand the physics behind this major obstacle of early flight, we will explore the discovery of divergence and how aircraft engineers eventually overcame the issue of wing-twisting.

The main goal of early flight was simply to create a structure that could leave the ground, and the skill of the pilot was expected to handle the challenges of being airborne. Rather than focus on the science and physics of flight, the development of aircraft was led predominantly by hobbyists and daredevils who were willing to throw themselves into the air and hope for the best. Many flight pioneers paid with their lives, such as Charles Rolls, and accidents were commonplace. Therefore, in an attempt to improve safety, the main factor that required more focus was understanding the new frontier of operating conditions. Unlike trains and cars, planes are subject to additional forces during travel. While creating lightweight designs ensured aircraft would leave the ground, it did not guarantee safety or reliability, and early aerospace engineers were presented with the conundrum of creating sturdy and airworthy designs.
Early attempts to improve safety involved increasing the thickness of many components, including the beams, frames, and plates that comprise the aircraft. Counterintuitively, this created heavier aircraft that were not only less likely to fly, but were also more likely to fail during flight due to the issue of load imbalance across the airframe. In turn, this led to the discovery of wing-twisting.
Due to the aerodynamic lift that acts on wings to keep aircraft aloft, wings are subject to a great deal of bending forces, specifically during changes in direction. For this reason, early monoplane designs like Dutch engineer Anthony Fokker’s D8 would break apart in flight. The thick spars that supported the thin fabric of the wings carried all wing-bending loads. Based on the belief that sturdier spars would lead to more durable wings, the German military demanded the thickness of all spars be increased, yet this presented the perplexing result that thicker spars create more accidents and make aircraft weaker.
Fokker decided to take matters into his own hands and discovered the phenomenon known today as “divergence.” By running many tests, Fokker found that wings would not only rise, but they would also twist as a result of aerodynamic loads, even when there appeared to be no twisting load applied. He found that the leading edge of wings were subject to an upward twisting motion, that of which increased the angle of attack and the lift created by the wings, thus, further increasing wing twisting and so on in a detrimental feedback loop.
In particular, this phenomenon could be observed in the many instances of wing snapping as pilots pulled out of dives due to the increase in lift generating the catastrophic feedback loop.
Divergence occurs when the vertical aerodynamic lift under the wing is biased toward one of the wing’s two beams. When the load carried by one beam is greater than the other, the entire wing bends and twists. Due to many factors of flight, the center of pressure is not simply situated between the front and back of the wing, but instead it is often located directly behind the front spar. As such, the designer must focus on minimizing the unequal distribution of lift force, in turn minimizing twisting. The solution is to create a more flexible wing, which Fokker accomplished by reducing the thickness of the rear spar. This also offers the benefit of keeping aircraft lightweight for flight. Today, wing design is much more complex than a piece of thin fabric spanning two spars; however, all designs account for the principle of divergence. 
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