Written by: Maneth Perera

Many commercial airliners generally go at speeds significantly below the speed of sound. Meanwhile, many fighter jets and other military craft fly at speeds significantly above the speed of sound. However, very few aircraft choose to go near the speed of sound, and for good reason. The technological limitations of our aircraft are made clear by the problems with flight near the speed of sound, and overcoming these challenges will require us to rethink how we fly in the first place.

Fundamentals of Flight

Thus, we must ask: how do aircraft fly in the first place? Here, we’ll consider the most common aircraft: planes with wings and either jet engines or propellers (a method to generate a thrust force). Planes have four fundamental forces acting on them: lift, drag, thrust, and weight. Weight is just the force of gravity pointing downwards, while lift is the upward force generated by the wings of the plane. Similarly, thrust is the forward force that pushes the plane through the sky, while drag is the backwards force also generated by the wings of the plane. The wings of the plane create a combined force that points up and back, resisting both the weight and thrust of the plane.

A major part of plane design is to balance each force pair (lift = weight, drag = thrust), allowing the plane to smoothly sail through the sky at constant velocity. Following from this, a major part of wing design is to maximize lift and minimize drag. More lift means you can make a heavier plane and thus take more cargo with you (leading to higher efficiency). Less drag means less thrust is needed to take off, land, and fly level, which means less fuel is used (also leading to higher efficiency). The usage of planes to transport cargo and passengers is highly dependent on how cost-effective and efficient these planes are, making the lift/drag ratio of the wings and other aspects of flight extremely important.

Mach Number Classifications

Many of these cargo planes fly at subsonic speeds. The speed of an aircraft is measured by the Mach number, the ratio between the speed of the aircraft and the local speed of sound for the air around it (dependent on density, pressure, temperature, and other factors). An aircraft flying at Mach 1 travels the speed of sound, leading to the classifications that a subsonic speed is below Mach 0.8, a transonic speed is Mach 0.8 to Mach 1.2, and a supersonic speed is above Mach 1.2. These classifications are necessary because we distinguish between incompressible airflow at speeds below Mach 0.3, where common airliners usually fly, and compressible airflow at speeds above that. Incompressible flow means that air doesn’t change density when put under pressure, while compressible flow means that it does, causing changes in its characteristics as a fluid that aircraft travel through.

Challenges to Transonic Flight

If we could go faster, we would be able to move cargo around quicker, therefore increasing efficiency. So why haven’t aerospace engineers done so with commercial flying? The problem lies with the consequences of compressible air flow and transonic flight. For many of our plane design calculations, assuming incompressible flow is very helpful, but, as we move past the Mach 0.3 point, the errors in our calculations increase more and more. Thus, at transonic flight from Mach 0.8 to Mach 1.2, we must use complex fluid mechanics models that factor in the local compressibility of the air flow instead of more simple models. This makes transonic plane design take significantly longer, as quick theoretical calculations can’t be performed and either more advanced software or experimental data is needed.

Figure 1

A diagram of the Shift-Wing model being applied to a transonic aircraft to determine the efficiency of a certain wing design.

Source: Flyingmag

A compounding factor is the creation of shock waves near the speed of sound. The propagation of pressure waves (the movement of air caused by the plane flying through it) starts to fold onto itself when the plane flies near the speed of sound. The upcoming air is not warned of the incoming plane due to the aircraft traveling faster than its pressure waves (when usually the air can separate in time), leading to the plane smashing through stationary air instead of facing smooth airflow. This generates a shock wave, which leads to a sonic boom (an extremely loud, 110-decibel sound that can disrupt civilians on the ground) and a shock stall. The shock stall can lead to a huge spike in drag, sometimes up to ten times as much, violently disrupting airflow. The plane starts buffeting as the pilot partially loses control over his aircraft.

Figure 2

Left: A diagram of the different flows that form around an airfoil in transonic flight; Right: A Schlieren photograph that uses a special apparatus to depict different air pressures.

Source: Mechanics of Flight (11th Edition)

Figure 3

Pressure wave sources that are stationary (a), subsonic (b), and transonic (c) illustrating the way that pressure waves collide with each other at transonic speed.

Source: Mechanics of Flight (11th Edition)

Running plane engines with tens of times more thrust to overcome this spike in drag leads to significantly higher fuel costs. It’s extremely inefficient to have commercial transonic flight or commercial supersonic flight due to the need to overcome these supersonic shockwaves. High-pressure parts of the airflow reach supersonic speeds while low-pressure parts stay subsonic, causing an uneven airflow that bounces these shockwaves around the plane. As the speed rises past Mach 1.6, we reach a supersonic regime where these shockwaves stabilize partially and the entire plane’s airflow moves at a supersonic speed, leading to more controlled flying conditions. The special airflows created by transonic flight challenge our ideas of lift and drag while creating a tough hurdle for even experienced pilots to overcome.

Figure 4

Left: Concorde airliner, world’s first supersonic airliner; Right: Boeing Sugar Volt concept aircraft with truss-braced wings.

Source: Left – Eduard Marmet; Right – The Boeing Company

Advances in Transonic Flight

However, researchers in transonic flight are working towards ways to overcome these challenges. Ever since the Concorde, the world’s first supersonic airliner, faced similar problems with commercial transonic/supersonic flight, aircraft fuel efficiency has only gotten better. Swept wings, or wings that point forward instead of backwards, are a common design for supersonic aircraft. These wings delay the onset of shockwaves and solve the problem of parts of the aircraft reaching supersonic speeds before other parts by delaying airflow for certain areas. Truss-braced wings are long, narrow wings jointly developed by NASA and Boeing. These wings generate significantly less drag than your average subsonic wing using aerodynamic trusses supporting a thin structure. However, this is at the cost of less durability and a higher likelihood of ice forming on the wings and altering the aerodynamic stability of the aircraft. Other experimental wing designs are being tested with new, more complex software, such as the Shift-Wing model by Luminary Cloud. These advances in software allow aerospace engineers to more easily test wing designs with extreme accuracy, making the transonic aircraft design process more efficient.

Figure 5

A depiction of the three different wing sweeps: aft-swept wings (Left), straight wings (Center), and forward-swept wings (Right).

Source: General Aviation Aircraft Design (Second Edition)

Conclusion

While many of our commercial airliners are stuck at slow speeds for the time being, engineers make technological progress in the field of transonic flight every day. Shock stalls, sonic booms, and fuel efficiency pose significant hurdles to transonic and supersonic flight, making supersonic jets a military-only pursuit. However, new hopes for these engineering challenges present themselves through new wing designs and further research on transonic fluid mechanics. Maybe one day we can see a revolution of sound barrier-breaking speeds in our commercial transit systems.

References

Alfred Cotterill Kermode, R H Barnard, and D R Philpott. 2007. Mechanics of Flight. 11th ed. Harlow: Pearson Education Ltd.

Bombardier. 2022. “Global 8000.” Bombardier. The Boeing Company. May 19, 2022. https://bombardier.com/en/aircraft/global-8000#bba-pdp-section-1.

Gudmundsson, Snorri. (2013) 2022. General Aviation Aircraft Design. 2nd ed. Kidlington, Oxford: Butterworth-Heinemann.

Newbacher, Brian. 2025. “New Aircraft Wing Undergoes Crucial NASA Icing Testing.” NASA. National Aeronautics and Space Administration. March 25, 2025. https://www.nasa.gov/aeronautics/new-aircraft-wing-undergoes-crucial-nasa-icing-testing/.

SimScale. 2023. “Compressible Flow vs Incompressible Flow in Fluid Mechanics.” SimScale. August 11, 2023. https://www.simscale.com/docs/simwiki/cfd-computational-fluid-dynamics/compressible-flow-vs-incompressible-flow/.