ARNOLD AIR FORCE BASE, Tenn. -- Recently, Arnold Engineering Development Complex test personnel had a chance to return to the basics of scientific research, if hypersonic flight research is ever basic.
Led by Dr. Michael Spottswood, with the Aerospace Systems Directorate of the Air Force Research Laboratory, a small research group came to the von Kármán Gas Dynamics Facility, or VKF, at Arnold Air Force Base to study the behavior of flight-weight structures in near hypersonic airflow over long durations.
VKF is a continuous-flow, hypersonic wind tunnel most often used to study the aerodynamics of aerospace vehicles using scaled models.
“In its early days of existence, a significant portion of the wind tunnel testing by AEDC was focused on basic research because the facilities offered unprecedented capabilities to investigate aerodynamic and aerothermodynamic phenomena to help understand not only vehicle response, but also gain insight on facility operational characteristics,” said John Hopf, an aerospace engineer with the 716th Test Squadron.
“About 40 years ago, wind tunnel testing began primarily leaning toward validating flight vehicle performance characteristics and building aerodynamic math models used to refine flight control algorithms. Science experiments in the wind tunnel became increasingly infrequent, usually consisting solely of a small amount of additional air-on time trying out new test techniques and/or instrumentation at the end of an existing RDT&E [research and development test and evaluation] test program.”
The AFRL research group is gathering data to build the base of knowledge needed to further hypersonic vehicle development.
“We go all over the world, really, studying the behavior of high-speed aircraft fuselage structures,” Spottswood said. “And we have been on a goal to study them in as representative an environment as possible.”
After experiments in the facilities at their home station of Wright-Patterson Air Force Base in Ohio, and at facilities in Germany, their experiments culminate with testing in VKF.
“What this facility affords us as a structures group is the ability to study large-scale, representative flight-weight structure in a continuous flow extreme environment,” Spottswood said.
Anything Mach 5 or faster is considered hypersonic. Traveling at such speeds generates extreme amounts of heat.
Spottswood compared behavior of the fuselage to that of an empty pan on a stovetop. Just as the pan will expand and deform from the heat, the fuselage of hypersonic aircraft can deform as heat builds on the surface causing expansion. That deformation can alter the airflow over the vehicle, which alters where the heat builds up, that new change in temperature can cause what is referred to as a “snap-through” or a shift from concave to convex or vice-versa.
Understanding these effects should both improve designs and the efficiency at which a flight-ready design can be attained.
The data collected is shared with partners in government, industry and academia to refine computational models.
“Many AEDC personnel are unaware of how significant of a role they play each day just doing their job to support day-to-day operations,” Hopf said. “While the job seemingly culminates in blowing air through a tube, the test cell, the significance of reliably accomplishing that task at the extreme pressures, temperatures and velocities required to operate various facilities at AEDC cannot be overstated.”