At high speeds, even the smallest movement can have major consequences.
When an aircraft tilts sharply during flight, the air around it does not flow smoothly. It twists into powerful, swirling currents that can destabilize the entire vehicle. These swirling structures, known as vortices, can behave unpredictably, sometimes causing aircraft to pull to one side or rotate unexpectedly. In extreme cases, they can damage critical components such as sensors or wing flaps.
New FAMU-FSU College of Engineering research shows how different angles of flight affect the vortices that form behind cones in flight. The research, published in Journal of Aircraft, could help design more stable missiles and high-speed aircraft.
“Aircraft in flight are subject to extreme forces, and as speed and maneuvering increase, these forces only get stronger,” said study co-author Rajan Kumar, chair of the Department of Mechanical and Aerospace Engineering and director of the Florida Center for Advanced Aero-Propulsion. “This study helps to understand critical phenomena responsible for those forces so engineers can create efficient and more stable designs.”
How it works
Vortices are common, but under certain conditions, they can become catastrophic.
As the cone-shaped nose of an aircraft moves through the air, vortices form behind it. As the aircraft increases its angle of incidence, or how steeply it is tilted relative to airflow, the behavior of these vortices changes. At low angles, airflow remains balanced and predictable. Beyond a critical angle, however, vortices can become large and unstable. When this breakdown happens, air slows down sharply and may spread out into different patterns.
This shift creates uneven swirling flows, or asymmetric vortices, that generate unwanted side and rotational forces, causing the aircraft to veer off course. In high-stakes environments, particularly military operations, even a slight deviation can mean missing a target or losing control entirely.
What they found
To better understand the transition from stable to asymmetric vortices, Kumar’s team combined experimental testing with advanced computational simulations to model complex airflow and identify when and how instability develops.
Using this method, they simulated airflow over a cone-shaped object traveling just above the speed of sound at Mach 1.1 at three angles of incidence: 15, 25, and 30 degrees.
At a 15‑degree angle, the main swirl of air breaks down into a complex pattern resembling two intertwined spirals, which then split into many thin, tangled strands of swirling air.
At 25 and 30 degrees, the breakdown looks different. The swirl twists apart in a single spiral pattern, indicating even stronger instability.
As the angle of incidence increased, vortex asymmetry also increased. Airflow shifted from structured and predictable to unstable and erratic, illustrating how quickly control conditions can deteriorate in real-world flight.
Vortex breakdown
The study helps answer a long-standing question in aerospace research: Why do vortices suddenly become asymmetric?
The study showed that growing instabilities within airflow unite to create larger disruptions. As small secondary vortices form and interact with primary vortices, they merge into larger structures that disrupt the aircraft’s balance.
The research also showed that vortex behavior depends on several interacting factors, including the size of the vortices and their orientation relative to the aircraft. Together, these elements determine how much force is exerted on the vehicle and how difficult it becomes to control.
Why it matters: The future of flight
Understanding the forces at work on aircraft in flight has direct implications for how they are designed and operated. These findings help engineers define safe flight conditions by identifying when airflow remains stable and when additional control systems are needed. This is especially important for high-performance aircraft that rely on extreme maneuverability.
The research also supports new design strategies, including improved control surfaces, flow control techniques and future systems that could adjust automatically during flight.
Kumar and his team are expanding their research to explore vortex behavior at higher speeds and they are transonic investigating control methods that could allow aircraft to respond to instability in real time, potentially using advances in artificial intelligence and automated systems.
At Florida State University, this work is also shaping the next generation of engineers. Students involved in this research go on to careers in industry, government labs and defense agencies.
“Research outcomes matter, but our most important product is our students. They are the future of engineering and science,” Kumar said.
Doctoral student Jordan Wilkerson and Associate Professor Unnikrishnan Sasidharan Nair were co-authors on this study. This research was supported by the Army Research Office.
