Purdue University engineers have been using an on-site wind tunnel to conduct experiments aimed at yielding critical data for the design of the X-51A test vehicle.
According to Steven Schneider, an aerospace engineer and professor in Purdue’s School of Aeronautics and Astronautics, no other wind tunnel runs quietly while conducting experiments in airstreams traveling at Mach 6. Quiet operation, which more closely simulates actual flight, is critical for collecting data to show precisely how air flows over a vehicle’s surface in flight. NASA previously operated a tunnel capable of similar results but it is not currently in use.
To properly design vehicles that fly at hypersonic speeds, engineers need detailed information about how airflow changes from laminar, or smooth, to turbulent as it speeds over an aircraft’s surfaces.
The Purdue research is focusing on the forebody of the craft, using a 1-ft-long model for wind-tunnel testing. The findings are providing information in two areas: maintaining the turbulent flow of air into the engine’s combustor and increasing the amount of smooth airflow over the vehicle’s upper surfaces to reduce potentially damaging friction and heat.
The X-51A, powered by scramjet engines, is expected to evolve into missiles capable of flying at Mach 6, enabling them to hit mobile, time-critical targets. The project is being led by the Air Force Research Laboratory and DARPA, and the vehicle is being built by Pratt & Whitney and Boeing.
The aircraft is wedge-shaped with a scoop-like cowl on its underbelly, where air rushes into the inlet of the engine’s combustor. The air entering the inlet must be turbulent at hyper speeds or the engine could fail, causing the engine to crash. To convert the air to turbulent flow before entering the inlet, a raised strip of metal is placed near the inlet to trip the air from smooth to turbulent. Research findings will enable engineers to determine precisely where to place the trips and how far they should be placed from the aircraft’s skin.
Experiments under quiet conditions yielded more accurate findings than those conducted under noisy conditions. Data from the quiet experiments indicated that the trips should be raised twice as high. Researchers are able to switch the wind tunnel back and forth from quiet to high-noise airflow, allowing them to compare the quality of the data.
To obtain quiet flow, the throat of the Mach 6 nozzle must be polished to a near-perfect mirror finish, eliminating roughness that will trip the flow near the wall from laminar to turbulent. For the wind tunnel itself to remain quiet, it must be entirely free of particles. A single speck of sand inside the tunnel could cause turbulence, damaging the finish and ruining the quiet effect.
Friction and heat created from air flowing over the top of the vehicle increases drag and necessitates a heavier thermal protection system for the vehicle’s thin metal skin. Wind-tunnel data is being used to assess the performance of that portion of the vehicle.
Researchers used a temperature-sensitive paint to measure how hot the skin of the model gets during testing. The paint was applied to a nylon strip inserted into the model, and by shining a blue light onto the strip during testing a temperature-dependent red light is generated from the paint. The intensity of the red light signals how hot the surface is.
“Laminar airflows can have eight times less heating than turbulent ones,” Schneider said. “The results of our work can be used to help determine the heating and the skin friction of the vehicle, which is important to the design of the X-51A.”
To measure the airflow velocity and turbulence, researchers use a heated wire about one-tenth the diameter of a human hair. The higher the speed of the airflow, the more the wire is cooled and the greater the electrical current needed to maintain the wire’s hot temperature. Monitoring the changing current needed to maintain the wire’s temperature reveals the changing air speed at fluctuations of up to 250,000 times per second.
A total of 18 years of research and about $1 million from Purdue, the U.S. Air Force, and private industry have been invested to perfect the facility.
“It’s finally working and getting results that are affecting the design of these vehicles,” Schneider said.