Studying Structural and Temperature Changes of Hypersonic Air Vehicles


In-atmosphere hypersonic air vehicles have been a point of interest for research since the 1960s. These operate under extreme flight conditions (> Mach 5 speeds), which introduces severe transient thermal gradients that affect the structural strength and stiffness while simultaneously inducing large out-of-plane deformations. For example, the X-15 wing skin experienced surface temperatures exceeding 900 K (Fig. 2), large deformation, dynamic snap-through, and aeroelastic behavior. The combination of these extreme conditions, often not well-understood or predicted, results in coupled aero-thermo-elastic behavior. To further complicate matters, the methods used to study and predict the behavior of flight-weight aerostructures in these environments are not validated against experimental data that exhibit the coupled behavior since the experimental datasets just do not exist.

Figure 1: Schematic of an X-15 and measured surface temperatures at Mach 5 (Courtesy of Stillwell ADB181235)

Dr. David Ehrhardt has been working with the Air Force Research Laboratory to build experimental data sets in ground test facilities where combined aero-thermal environments lead to dynamic snap-through and aeroelastic behavior using well-defined structural components. This work has shown that it is possible to accurately measure the dynamic deformation of thin aircraft structures under high-speed flow conditions while simultaneously measuring the thermal state (temperature sensitive paint or infrared thermography), pressure loading from flow conditions (pressure sensitive paint) and identifying flow conditions (Schlieren or particle interferometry velocimetry) as shown in Fig. 2.


Figure 2: Combined measurements of 3D stereo digital image correlation and Schlieren imaging with pressure sensitive paint in a Mach 2 facility on the left and infrared thermography in a Mach 6 facility on the right. (Courtesy of Brouwer 10.1016/j.jfluidstructs.2021.103429 and Riley 10.2514/1.J060408)

Important challenges remain when building experimental data sets such as these, including measurement combinations in wind-tunnel facilities of varying capabilities, capturing structural behavior under higher temperatures where flow chemistry becomes a factor. The next phase of this ongoing research will address these challenges through a series of extreme environmental experiments of aircraft-like stiffened structures in tunnels where variable Mach number is achievable and higher temperatures are achievable. The experimental results will be used to progress fundamental understanding of combined fluid-thermal-structural interactions with experimental measurement and model calibration.