fredag 20. juli 2018

Fold vingene og spar brennstoff - AW&ST video

Folding the tips of a wing in flight can increase stability and reduce drag, NASA flight tests have shown. Now researchers plan additional flights to test control laws that actively adjust wing fold in flight to minimize drag. They are also proposing a project to test wing folding in supersonic flight.
The Spanwise Adaptive Wing (SAW) project, a rapid feasibility assessment under NASA’s Convergent Aeronautics Solutions (CAS) program, showed folding the outer sections of the wing in flight improved directional stability and control. In a new aircraft design, this would allow tail size and drag to be reduced.
  • Subscale flight tests will integrate wing fold into flight control
  • Ground tests will demonstrate F/A-18 wing folding
  • Supersonic inflight wing-fold demo proposed
The tests involved NASA’s subscale unmanned prototype-technology evaluation and research aircraft (PTERA), essentially an 11%-scale Boeing 737, with the outer 15 in. on either side of its 176-in.-span wing hinged to fold up or down by up to 75 deg. The sections were folded in flight using shape memory alloy (SMA) actuators built into the hinge lines.

“Analysis of the flights shows we did get yaw power out of the outer ailerons [when the wings were folded], although not quite as much as we hoped,” says Matt Moholt, SAW principal investigator at NASA Armstrong Flight Research Center. But the flights showed less wing fold than expected was required to be effective in improving lateral stability and control. “We only have to go to 40% to fly rudderless; we get enough yaw power,” he says. More fold means less lift, so 30-40 deg. of fold minimizes the loss of  lift

SAW went from design to an initial series of three flights in just 1.5 years. A second series of flights is planned with the PTERA this summer using new software that integrates wing position into the flight control laws. The aircraft will take off with the wings up 40 deg. to provide yaw power, then will fold them down in cruise to increase stability and reduce induced drag.
A minicontroller on the PTERA’s rudder will virtually eliminate its effect, so that yaw control is provided only by the folded wing. A peak-seeking algorithm will actively control the fold angle to minimize drag. “In flight, the metric is throttle position. The control system will look for the optimum wing position that minimizes throttle demand and maximizes efficiency from taxi-out to return,” says Moholt.

Slow response of the SMA actuators resulted in PTERA landing with its outer wing sections still partially down. Credit: NASA

The three-year SAW project involves NASA’s Armstrong and Glenn Research Centers, Boeing Research & Technology and Area-I, which developed the PTERA. In addition to the flight tests, the project is conducting ground tests of more-powerful SMA actuators using the folding outer wing of a Boeing F/A-18 fighter.
SAW is built around torsion actuators made of an alloy that, when heated electrically, “remembers” and returns to its original twisted shaped, and in doing so moves the wingtip. The PTERA uses an actuator with a single SMA tube that produces 500 in.-lb. of torque. NASA Glenn has ground-tested a 5,000 in.-lb. actuator with nested SMA tubes. This was used to fold the outer wingbox of the F/A-18 wing.
NASA and Boeing are now building a 20,000 in.-lb. actuator that will be used to fold the complete F/A-18 outer wing. This actuator has 12 larger SMA tubes that act in unison to drive a gearbox and fold the wing. “We want to get to 100,000 in.-lb., but it’s quite an incremental leap to go from 500 to 20,000,” says Moholt.

In three missions, the subscale PTERA folded its outer wing sections up and down in flight by up to 75 deg. Credit: NASA

Challenges faced by the SAW project include actuation rates, with heating and cooling of the SMA actuators on the PTERA each taking 2-3 min. These initial actuators use cartridge heaters and convection cooling. On one flight, when the PTERA had reached its fuel limit and was ready to land, the wingtips were not fully up when it touched down on the dry lake bed, says Othmane Benafan, SAW co-principal investigator at NASA Glenn. “We need rates in seconds not minutes,” he told the American Association of Aeronautics and Astronautics Aviation 2018 conference here in June.
The 20,000 in.-lb. actuator will use induction heating, with coils wrapped round the tubes rapidly heating the alloy electromagnetically. “We are looking at the active cooling methods we would need on an aircraft to meet the actuation rates, but for now we will use forced convection,” says Moholt.
Another challenge is packaging, and the need to fit the fold actuators into the thin wing sections near the tips. “The tube is compact, but everything around it adds volume: wiring, bearings, sensors, etc,” says Benafan. “We need to focus on smaller bearings and other hardware, such as wire adapters and connectors, so it can fit inside a thin wing.”

Vibration was also an issue on the PTERA flights. This caused a shift in a sensor reading that was caught by comparison with a string-potentiometer position sensor. “We need a more robust sensor suite,” says Benafan. Future work would also look at the power required by the actuators, which has not been addressed under SAW.
Researchers are also looking at a locking mechanism that would enable the actuator to move the wing, lock it in position, then relax. Another concept is back-to-back actuators driving a ratchet mechanism (like a socket wrench), alternating between one driving the wing fold while the other is cooling down.
NASA Glenn has developed the nickel-titanium-hafnium shape-memory alloy and is working to scale up the tubes to sizes never before produced. “Glenn is working with the material supplier, pouring melts and breaking records,” says Moholt. “They are working to make sure it scales, with the right crystalline structure.”
The raw SMA stock is provided to Boeing, which gun-drills the tubes and assembles them into an actuator. The 20,000 in.-lb. SAW actuator has 12 0.5-in.-dia. tubes, each with a gear at the end driving a ring gear that moves the wing. Boeing is also “training” the SMA actuators, a process that requires thousands of thermal cycles.
The SAW project ends in September. The team is proposing a follow-on project that would demonstrate SMA wing folding on a supersonic aircraft. Folding the wingtips down in supersonic flight generates compression lift from shockwaves under the wing and can dramatically reduce induced drag, says Moholt. This was used in the North American XB-70 bomber. Folding the tips down also increases lateral stability and control in supersonic flight, allowing a smaller tail.
Additionally, Moholt says, the center of pressure shifts aft when an aircraft goes supersonic. The Concorde compensated for this shift by pumping fuel from forward to rear trim tanks to maintain a center-of-gravity location that minimized drag. “By folding the wings down, we can keep the center-of-pressure position and avoid having to pump fuel around,” he says. The team is proposing building a 2,000-lb. unmanned aircraft to demonstrate supersonic SMA wing folding, but has yet to identify funding.

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