Test flights are getting underway of a Boeing 757 with an actively blown vertical tail and new wing-leading-edge sections, which could pave the way for the wider use of natural laminar and active flow control technology in future airliner designs.

The aircraft, which first flew on March 17 from Boeing Field near Seattle, is the company’s third ecoDemonstrator technology testbed, following earlier campaigns with a 737-800 and 787-8. Supported by the European airline group TUI and conducted jointly with NASA as part of the agency’s Environmentally Responsible Aviation (ERA) program, the testing will focus primarily on two methods of protecting wing leading edges from the laminar flow-destroying effect of residue left by insect strikes, as well as the performance of the active flow control (AFC) tail.

Increasing the use of natural laminar flow (NLF) on an aircraft wing has the potential to improve fuel burn by as much as 15%, while AFC technology could lead to a 17% reduction in tail size, which would reduce drag and weight, cutting as much as another 2% in fuel burn. Preserving NLF is difficult, however, as even small contaminants from insect remains will trip the flow from laminar to turbulent, destroying the performance benefit.

For the laminar flow experiment, the second and third leading-edge slats sections on the right wing have been modified with a total of eight accretion insect-mitigation (AIM) panels. These are treated with different types of “bug-phobic” coatings designed to keep the surface free of contaminants and will be tested “by flying in environments where we can find lots of bugs,” says 757 ecoDemonstrator program manager Doug Christensen. “We have spent a lot of time with entomologists and done research around the country about where was the right place to go during this period. It turns out the best opportunity to find the bugs is in Shreveport, Louisiana,” he adds.

As insect life is rarely a factor above 10,000 ft., the 757 will conduct much of this phase of the program with low-level flying around the circuit in Shreveport. High-definition cameras mounted in the forward cabin will view the test section through optically clear windows provided by NASA and will record the volume and size of the insects as they strike the surface. “That was part of the study we did with the entomologists. The researchers wanted a variety of sizes and distribution of bugs. It was that detailed,” Christensen says. Test planners listed all the airports capable of handling a 757 around the U.S., then matched this with information from entomologists about insect breeding and migration patterns to determine the best location for testing.

The corresponding section on the left wing is modified with a 22-ft.-span glove section supporting a variable-camber Krueger flap which will be deployed during landing and takeoff to cover the leading edge. The glove protrudes just ahead of the leading edge and is positioned with a sweep angle slightly lower than the 757’s regular 25-deg. sweep (as measured at 25% of the chord). Although Krueger flaps have been tried before as insect-mitigation screens, previous designs caused additional drag. However, the newer design being tested is variable-camber and designed to retract as seamlessly as possible into the lower wing surface. The effect on laminar flow and the transition zone to turbulence will be monitored and recorded by pressure sensors on the wing surfaces and by infrared and high-resolution cameras mounted on a trapeze located on the crown of the fuselage.

The tail tests, which also form part of NASA’s ERA effort, will evaluate the ability of active flow jets mounted on one side of the tail to increase rudder effectiveness. Vertical tails are sized to cope with the asymmetric forces resulting from loss of an engine on takeoff, so Boeing and NASA believe that increasing rudder effectiveness 20% will enable substantial reductions in size, weight and drag. “Even if you can reduce the tail size by 10-15%, then that’s still a significant benefit,” Christensen says.

The AFC, mounted on the right side of the fin only for the tests, comprises 31 passive sweep jet actuators positioned ahead of the rudder leading edge. This is fewer than the 37 deployed in previous NASA wind tunnel tests in which “they found the ones on the tip were less effective,” he adds. Pressurized air is supplied from the auxiliary power unit and is pre-cooled from its 380-deg. bleed exit temperature by running it through an externally mounted heat exchanger beneath the aft fuselage. The heat exchanger is taken from the 757’s original environmental control system and plumbed into a duct system that runs along both the front and rear spars of the stabilizer to ensure an even supply to the actuators.

Each actuator is a solid-state unit with no moving parts and an internal feedback loop that causes an air jet to sweep back and forth across an arc. The action reenergizes the separated flow, reattaching it to the rudder even at higher deflection angles. “We will do a set of runs with and without the system running,” says Christensen. “The rudder itself is instrumented, so we can see the rudder hinge moments and how much force is being generated. We will also have a chase aircraft looking at flow cones that are fixed to the rudder to see how much of the flow will be attached.” 

To maximize the asymmetric forces on the rudder, the aircraft is equipped with a Pratt & Whitney PW2037 on one wing and a more powerful PW2040 on the other.

Flight tests are expected to run through July. Following the completion of the test program, the 757 will be broken up using special recycling methods under an agreement with the Aircraft Fleet Recycling Association.