Aviation Week & Space Technology - 30 March-12 April 2015

AviationWeek.com/awst AVIATION WEEK & SPACE TECHNOLOGY/MARCH 30-APRIL 12, 2015 37

Guy Norris Los Angeles

Bug Smasher

Wing protection system tests could help

unlock benefi ts of laminar fl ow

B

oeing test pilots will deliber-

ately seek out insect-infested

air masses over Louisiana as

part of upcoming low-level flights of

the company’s specially modifi ed 757

ecoDemonstrator.

The flights, which form part of a

broader test campaign starting in April,

will test methods of protecting wing

leading edges from insect residue. If

this can be achieved, potentially huge

ef ciency gains may be gained by us-

ing natural laminar fl ow (NLF) in future

airliner designs (see page 44). Testing

will also focus on the performance of an

active fl ow control (AFC) vertical tail

that could lead to smaller, more aerody-

namically ef cient tails and open up op-

portunities for integrating the control

feature into new designs.

NASA, which is supporting the

testing along with the European-

based TUI Group, projects that NLF

has the potential to improve fuel burn

at an aircraft level by as much as 15%.

For the tests, the second and third

leading-edge slats sections on the

757’s right wing have been modifi ed

with a total of eight accretion insect

mitigation (AIM) panels. These are

treated with dif erent types of “bug-

phobic” coatings designed to keep the

surface free of contaminants and will

be tested “by fl ying in environments

where we can fi nd lots of bugs,” says

757 ecoDemonstrator program man-

ager 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 pe-

riod. It turns out the best opportunity

to fi nd the bugs is in Shreveport, Loui-

siana,” he adds.

As insect life is rarely a factor over

10,000 ft. the 757 will conduct much

of this phase of the program flying

around the circuit. High-definition

cameras will view the test section

through optically clear windows pro-

vided by NASA to record the volume

and size of the insects as they strike

the surface.

“That was part of the study we did

with the entomologists,” Christensen

says. “The researchers wanted a variety

of sizes and distribution of bugs.” Test

location was determined by matching

U.S. airports capable of handling a 757

with data from entomologists about in-

sect breeding and migration patterns.

While the right wing is being used

to test coatings that will prevent bugs

from sticking, the corresponding sec-

tion on the left wing is modifi ed with a

22-ft.-span glove section supporting a

variable-camber Krueger (VCK) fl ap

that will cover over the leading edge.

The glove is positioned with a slightly

lower sweep angle than the 757’s regu-

lar 25-deg. sweep (as measured at 25%

of the chord).

Although Krueger fl aps have been

tried before as insect mitigation

screens, previous designs caused ad-

ditional drag. However, the newer VCK

is designed to retract seamlessly into

the lower wing surface. The ef ect on

laminar fl ow and the transition zone

to turbulence will be monitored by

pressure sensors and by infrared and

high-resolution cameras mounted on

the crown of the fuselage.

The tail tests will evaluate the ability

of 31 active fl ow jets mounted ahead of

the rudder leading edge on one side of

the tail to increase rudder ef ectiveness.

The sweep jet actuators have no moving

parts and include an internal feedback

loop that causes an air jet to sweep back

and forth, reenergizing the separated

fl ow and reattaching it to the rudder,

even at higher defl ection angles.

Pressurized air from the auxiliary

power unit is pre-cooled from its 380-

deg. bleed exit temperature by running

it through a heat exchanger mounted

under the aft fuselage. The heat ex-

changer is taken from the 757’s origi-

nal 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.

“We will do a set of runs with and

without the system running,” says

Christensen. “The rudder itself is in-

strumented, so we can see the rudder

hinge moments and how much force

is being generated. We will also have

a chase aircraft looking at fl ow cones

that are fi xed to the rudder to see how

much of the fl ow 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. c

TECHNOLOGY AFC actuator vents

Flow visualization tufts

Rudder

APU provides

AFC air supply

AFC air supply

757 ecoDemonstrator heat exchanger

Active Flow Control (AFC) Arrangement

Air for the active fl ow-control system

is bled from the APU and cooled

by the externally mounted heat

exchanger system attached below

the tail, as pictured here in fl ight.

JOHN D. PARKER/BOEING AND AW&ST

JOE WALKER

The experimental variable-camber

Krueger section is seen midway

along the leading edge as the 757

ecoDemonstrator taxis out for its

maiden fl ight at Boeing Field on

March 17.