A full-scale 757 tail, equipped with active flow control, has demonstrated increased rudder effectiveness in wind-tunnel tests by Boeing and NASA that could lead to smaller, lower-drag vertical tails.

The four weeks of tests in the National Full-Scale Aerodynamic Facility at NASA Ames Research Center, Calif., evaluated the use of active flow control (AFC) to increase rudder sideforce on demand by delaying airflow separation over the deflected control surface.

Airliner vertical tails have their size dictated by one rare, worst-case scenario—loss of an engine on takeoff—when the rudder must generate enough sideforce to counteract the asymmetric thrust from a high-bypass engine slung under the wing.

In a family of aircraft, the tail is sized by the smallest member, where the rudder moment arm is at its shortest, and is oversized for stretched versions. NASA’s goal for the AFC project is to increase sideforce 20% on demand, and shrink the vertical tail by 17% to reduce aircraft fuel burn by 1-2%.

With funding from NASA’s Environmentally Responsible Aviation (ERA) program, Boeing took a tail from a 757 in the boneyard and refurbished and modified it for use as a wind-tunnel model, says Ed Whalen, Boeing’s research & technology program manager.

“Sweeping jet” AFC actuators were mounted on one side of the fixed stabilizer, just upstream of the rudder hinge line to blow on to the leading edge of the deflected surface. The 37 actuators were supplied with variable mass-flow pressurized air from an external source and were individually addressable so that different spacings and zones could be tested.

A key goal of the full-scale wind-tunnel test was to determine an optimum actuator distribution and mass flow for the next phase of the program, flying the AFC tail on a 757 in 2015 under Boeing’s Eco Demonstrator program, Whalen says.

Focused on the takeoff and landing phases—when generating sideforce is critical—the tests were run at 100-130 kt. Measurements included airflow tufting, surface pressures and the tunnel’s force-and-moment balance. After baseline aerodynamic data were collected, the AFC tests were conducted.

Sub-scale tests had indicated that sideforce could by increased by up to 50%. The full-scale tests showed 20-30%, “which is in the realm of what we need,” Whalen says.

“Our goal was to figure out the AFC configuration for the flight demonstration, the most efficient arrangement of actuators that meets our performance criteria,” he says. “Once we found that, we compared it with the air supply on the aircraft to balance performance with supply. We identified a couple of viable configurations and conducted broader testing of those, including safety-of-flight mitigation, so we understand the overall effect when we fly.”

Boeing previously evaluated synthetic-jet actuators, but selected sweeping jets because they scale up uniformly, Whalen says. Originally developed as logic devices for fluidic computers, and now used in windshield washers for cars, they are solid-state actuators in which an internal feedback loop causes the air jet to sweep across an arc. This increases their effectiveness in re-energizing and re-attaching separated flow over the deflected rudder.

In a practical application, there would be actuators on both sides of the tail. They would be on-demand, on/off devices that would activate on the appropriate side of the tail when the rudder deflects beyond a certain angle, to increase sideforce.

For the 2015 Eco Demonstrator flight trials, the AFC actuators on the 757 tail will be powered by air generated by the aircraft’s auxiliary power unit. Flights will be funded under Phase 2 of NASA’s ERA program.

“We are proceeding with the flight demonstration, working the contract with NASA and designing the flight AFC system,” Whalen says.