Project: Dynamically soaring high-altitude aircraft
School: Lehigh University
Location: Bethlehem, Pa.
Director: Joachim Grenestedt
Researchers in Lehigh University’s Composites Laboratory are developing an unmanned aircraft designed to fly indefinitely at high altitudes powered by jet streams. That’s a significant undertaking. “It’s high-hanging fruit, but fruit that somebody should pick,” says Joachim Grenestedt, a mechanical engineering professor. “It’s a really cool concept.”
Perhaps even more noteworthy is how Lehigh engineers are manufacturing the glider, called the JetStreamer. They fabricated the 6.5-meter carbon fiber wing in a single molding process, complete with wing planks, spar caps, six internal webs to carry shear loads and a trailing edge to accommodate wing flaps and ailerons. “As far as I’m concerned, this is the holy grail of composites – building a wing in one piece,” says Grenestedt.
Grenestedt and John Spletzer, associate professor of computer science and engineering, lead the project, which began nearly four years ago with a small seed grant from Lehigh. The two professors, along with a co-principal investigator from Penn State University, were awarded more than $1 million in 2011 to fund the multi-year project. So far they have fabricated five wings in the Composites Lab and conducted structural tests to ensure the wings can withstand 20 G loads. To put that in perspective, the average roller coaster tops out at approximately 3.5 Gs.
Carbon fiber is the ideal material for the glider for several reasons, including strength, stiffness and the geometry of the aircraft. To remain airborne, the JetStreamer requires very low drag and long, slim wings. The wings need to be rigid to prevent flutter, divergence and aileron reversal – three phenomena of flight that are critical at high speeds. Wing strength is important for the glider to perform hard, fast turns.
“The biggest benefit of composites is probably the manufacturing process,” says Grenestedt. “Making the glider out of, say, aluminum would be very difficult because of the material’s inability to conform to compound curvature, surface imperfections, etc. Laying carbon is fairly easy.” Researchers at the university streamline the process further by cutting female molds. “A lot of places cut a male plug, make the female mold and then build inside it,” says Grenestedt. “We get rid of one step by cutting the female mold directly.”
Plies of 0.3 millimeter unidirectional carbon fiber prepreg are cut to shape using templates. The first wing skin plies are laid on the top and bottom molds, then vacuum debulked. The process is repeated, followed by spar cap plies and the last skin plies. Next the team prepares internal vacuum bags and Styrofoam placeholders. The internal web plies are laid on the placeholders and debulked. Aluminum inserts are placed into the mold to create the trailing edge profile. Final internal bags and a peel ply are added before the top and bottom mold are sealed together and cured in an oven at 194 F for more than 24 hours. The finished wing weighs only 7 kilograms.
Creating the wing in a single molding process has many advantages. There are no glue joints or weak points. It eliminates trimming, grinding surfaces, fitting parts and aligning parts using complex jigs. “If you can get rid of all those steps and just have molds, when you pop open the mold you have your finished part,” says Grenestedt. “You save a lot of time, weight and mess.”
The team has also built ailerons, flaps and a motor pod for the glider. (While the unmanned aircraft will be powered by jet streams, a motor will be used for the initial testing.) Next, it will construct the rudder and elevator. Grenestedt hopes the JetStreamer will be ready for flight by the end of the year.
Researchers at Lehigh aren’t focused on possible applications just yet. “If you asked the Wright brothers their ultimate goal, they probably wouldn’t have said supersonic flight,” says Grenestedt. However, one application where he sees potential is for monitoring weather. The unmanned glider could move in and out of the jet stream, track it and perform local measurements.