The Holy Grail-- a 6.5 m (21.3 ft) carbon fiber UAV wing, complete with outer skins, spar caps, six webs and integral trailing edge, made in a single stage.
The long road toward building a self-powered, gliding aircraft that can remain in the air indefinitely has reached a new milestone. The first, uniquely designed carbon fiber wing has emerged from Lehigh's Composites Lab. The 6.5 m (21.3 ft) wing was made in a single molding process, complete with wing planks, spar caps to carry bending moment and provide bending stiffness, six internal webs to carry shear loads and a trailing edge ready to accommodate wing flaps and ailerons.
That’s not trivial. The wing is made for an unmanned aircraft designed to fly at high altitude and be powered by jetstreams. The long-term goal of the project, led by Lehigh engineer Joachim Grenestedt and computer scientist John Spletzer, is perpetual flight.
To keep an autonomous aircraft aloft for years at a time requires not just a source of power in the sky—in this case jetstreams —but a very unique aircraft. It needs to have very low drag and long slender wings, as on manned gliders, but it also needs to be able to fly very fast. The wings must be very stiff to avoid flutter and divergence (two phenomena of flight which become very important at high speeds) and very strong to be able to perform hard, high-speed turns. The present wing was designed to not fail before the aircraft sees 20 G in the turns. Your average roller coast tops out at around 3.5Gs.
Mechanical engineer Joachim Grenestedt and computer scientist John Spletzer seem to have met that challenge, with a carbon fiber wing built as a single piece. No nuts and bolts. No glue joints. No weak points. No trimming or fitting of parts. No complex alignment jigs. The entire wing is made from thin layers of carbon fibers (0.15mm thick) configured into complex geometric shapes and placed layer by layer in molds digitally designed and CNC machined at Lehigh. The resulting wing has stronger-than-steel performance.
The researchers use carbon fiber reinforced epoxy, similar to the material Grenestedt used to build the Numerette
, the largest craft yet constructed with a steel/composite hybrid hull. The advanced material makes the wing very strong, stiff but still flexible, and extremely light. Steel is too heavy. Aluminum sheets wouldn’t work, since the material’s surface imperfections don’t allow for the geometrically perfect airfoil shape needed for low drag. While carbon fiber is expensive, it has such excellent properties that not too much is needed and it therefore becomes cost effective.
Grenestedt and a team of students fabricated the prototype this month and are now testing whether it stands up to the extreme forces it will see in flight. In the accompanying time-lapse video
, students build the prototype in about two weeks.
The top wing skin is layered into the mold, followed by the placement of the six internal webs of carbon fibers, geometrically aligned by using flexible tubing and Styrofoam as placeholders to form the structure. Then the lower half of the wing skin is layered up in another mold-half. The two mold-halves are then closed and the entire wing “cooked” in a 250-degree oven. After cure the two mold-halves are separated and there is the finished wing.
If successful, the final aircraft could alter the way we monitor weather and conduct surveillance. The multi-year project is funded by the National Science Foundation and Lehigh University.