Active flow control X-Plane uses virtual control surfaces made from air
DARPA has awarded Aurora Flight Sciences a phase 2 contract for its CRANE (Control of Revolutionary Aircraft with Novel Effectors) program, following wind tunnel testing of a small-scale X-plane that uses compressed air bursts instead of control surfaces.
Elevators, ailerons, rudders, flaps, flaperons, stabilators ... From the Wright Brothers' Flyer to the F-35, nearly every airplane ever built has used moving control surfaces to subtly change its shape during flight, altering the aerodynamic pressures and forces to adjust pitch, yaw and roll.
DARPA's CRANE program will not; its target is to build "an X-plane demonstrator that can fly without traditional moving flight controls on the exterior of the wings and tail." Why? The agency hopes to eliminate the weight and mechanical complexity involved in moving control surfaces. It wants to reduce drag for greater efficiency, it wants to run thicker wings for structural reasons and to hold more fuel, and it wants to simplify high-lift systems, improve overall performance and enable higher angles of attack.
This aircraft will have no exterior moving parts. It'll fly instead using Active Flow Control (AFC), using a series of nozzle arrays along the wings connected to a pressurized air system, capable of blowing controlled bursts of air that can directly modify the air pressure and flow around the aircraft, and mess with the boundary layers between streams of air moving at different speeds. Effectively, it's designed to create virtual control surfaces out of compressed air.
A similar AFC program underway at NATO's Science and Technology Organization had small-scale models flying some five years ago, but Aurora Flight Sciences has been working mainly in a wind tunnel to date. In phase 1 model testing, the company built a 25% scale prototype aircraft with 11 movable conventional control surfaces, as well as 14 AFC banks fed by eight individually controllable air supply channels. Over four weeks of wind tunnel testing, more than 14,000 data points were collected.
Now, as phase 2 of the project begins, Aurora has begun the detailed engineering design of a full-scale unmanned AFC test plane, with a 30-foot (9-m) wingspan, a gross weight of 7,000 lb (3,175 kg), and the capability to fly at speeds up to Mach 0.7 (537 mph, 864 km/h). This will be a modular machine, with swappable wings to test different wing shapes and sweeps, and the ability to change out entire AFC effector nozzle banks to test different designs.
Should DARPA pick up the option on phase 3 of this project, Aurora will build the thing and get it into flight testing sometime in 2025.
“Over the past several decades, the active flow control community has made significant advancements that enable the integration of active flow control technologies into advanced aircraft. We are confident about completing the design and flight test of a demonstration aircraft with AFC as the primary design consideration,” said DARPA's CRANE Program Manager Richard Wlezien. “With a modular wing section and modular AFC effectors, the CRANE X-plane has the potential to live on as a national test asset long after the CRANE program has concluded.”
“Given all that we have learned about AFC and its application to tactical aircraft in prior phases of CRANE, the next step is to prove out these learnings in flight,” adds Graham Drozeski, vice president of government programs at Aurora. “The CRANE X-plane is designed specifically to explore the effectiveness of AFC technologies at mission relevant scale and Mach numbers.”