When a hawk folds its wings, it plummets to the earth in a controlled high-speed dive. Not exactly the kind of scenario we'd imagine being ideal for modern aircraft. But in flight wing folding can have advantages and NASA wants to make it happen.
The idea here isn't to create controlled dives, of course, but to increase the efficiency and capabilities of aircraft. The space agency calls this the Spanwise Adaptive Wing (SAW). So far, articulating wings on airplanes have largely been for parking purposes, to allow planes to take up less space on an aircraft carrier's decks or fit into smaller hangars. A few very large aircraft have articulating wings to allow them to taxi between infrastructure at an airport. What NASA is working on is very different from all of this.
Of course, the National Aeronautics and Space Administration isn't new to aircraft design experimentation. They've been working on a lot of similar things for years. The Environmentally Responsible Aviation project, for example, has been working on bendable wings. Another project under the SAW umbrella is using these wings to reduce or eliminate the vertical tail fin on a plane. This would have obvious environmental benefits by greatly reducing drag and aircraft weight, thus reducing fuel use.
Supersonic aircraft and very heavy aircraft both require vertical stabilizers for yaw control during flight. NASA says that smaller, more precise wing articulation and control is now possible thanks to advancements in the actuators required to move the wings. Most wing articulation is currently accomplished with heavy, bulky actuators that are usually hydraulic rather than electric. This has been because hydraulic actuators are the most time-tested physical movement option on aircraft today. There hasn't been much change in how they work since the 1960s.
"We are revisiting folding-wing aircraft," says NASA's Matt Moholt, "because new technologies that did not exist in the 1960s allow actuation to be put in tighter wings, in smaller volumes." This allows a much thinner wing and a thinner portion of the wing to become articulable. Namely the tips of wings, which are normally rigidly fixed.
One example of the use of this technology would be for supersonic aircraft. When flying at very high speeds, aircraft create a lot of lift, but have less yaw control as a result. Folding the ends of the aircraft's wings up or down would add stability by creating more vertical surface to augment the rear tailfin. Yet the plane would not lose critical lift during the difficult takeoff and landing stages because the wings could then be straightened to add the lift surface to flight.
Moreover, NASA says, the adjustable wing ends would be able to change position dynamically, creating mixtures of lift and yaw control as needed during different flight conditions.
NASA plans to test these ideas on the scale model PTERA (Prototype-Technology Evaluation and Research Aircraft) in the spring of 2017. That will coincide with ground-based tests of full-sized actuators capable of scale wing articulation. The objectives of this testing will be the validation of tools and the vetting of the system's theoretical vehicle control law evaluation. Analysis of airworthiness and potential fuel savings will also be undertaken.
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