Speed is one of the primary benefits of flying, and during the first half of the 20th century, there was a passionate effort to make planes faster. While slow, piston-engine aircraft dominated the 1930s, jet aircraft emerged by the end of World War II, and test pilot Chuck Yaeger ushered in the supersonic era when he broke the sound barrier in 1947. However, the troubled history of the Concorde (more on that in a future post) demonstrates that high-speed flight has its challenges. In this post, I aim to provide a brief introduction to some central concepts in high-speed flight.
A plane is said to be “supersonic” if it flies faster than the speed of sound, often denoted by the term “Mach 1.” At sea-level, sound travels at roughly 760 mph, but this is not constant everywhere. It turns out that the speed of sound varies according to the square root of temperature. Since temperature decreases as a plane flies higher, the speed of sound decreases in turn. Moreover, airflow traveling directly on top of the wing can be faster than the overall speed of the aircraft (a consequence of the shape and orientation of the wing’s airfoil with respect to the natural wind flow). Hence, there is also the concept of “critical mach number.” An airplane’s critical mach number is a value less than Mach 1, and indicates the speed a plane flies at when the faster-moving air directly on top of the wing surpasses the speed of sound. It is important to consider the critical mach number when designing planes because that is when an aircraft first begins to feel the unique physical effects of supersonic flight.
A primary physical consequence of breaking the sound barrier is the development of a shockwave around a plane. This shockwave is caused by a buildup of pressure disturbances, and takes on the shape of a cone that has a vertex toward the nose and extends toward the rear. The faster that plane flies, the tighter cone “compresses” around the plane. A shockwave meeting a surface of an aircraft (be that a wing or nose) results in what is called wave drag. The details of wave drag can be quite complicated, but the primary fact to note is that it is undesirable. After all, the need to overcome drag results in extra costs. Luckily, there are some design tricks to keep the primary surfaces of an airplane away from the shockwaves and therefore alleviate the effects of wave drag. One way of doing this is to sweep the wings back. Since shockwaves are shaped like cones that extend back toward the rear of a plane, wings that are sufficiently swept back can remain safely behind a shockwave. Similarly, supersonic planes can have long and pointy noses that allow just the tip to be exposed to the danger.
Somewhat ironically, aerospace engineers can actually further reduce drag by deliberately forming mini vertical shocks around a wing. As airflow passes from the front of a wing to the back, it sometimes “separates” from the surface of the wing before reaching the rear. This flow separation is detrimental because it increases what is called “pressure drag” and decreases lift. Therefore, it is always in the interest of engineers to design wings that allow the airflow around them to remain attached. When a plane flies supersonically, pointy airfoils (where an airfoil is a cross-section of a wing) can achieve this by creating vertical and horizontal shockwaves. As air passes from the front of a wing to the rear, these shockwaves turn the flow around the wing, and therefore reduce flow separation. Of course, these shock waves must also create some wave drag, but my assumption (still learning myself!) is that the reduction in pressure drag outweighs the addition of this extra wave drag.
There is much further detail to explore regarding high-speed flight, but these concepts hopefully help to explain why fast fighter jets look different from slower commercial planes. These ideas also hopefully illustrate that flying very fast creates unique problems to be tackled.