Clean skies: Nasa research for green future aircraft and plane design

Nasa morphing wing for cleaner greener skies Morphing wing technology by Nasa for green and clean skies
Nasa targets some ambitious reductions in aircraft emissions and has piloted some seemingly bizarre to outright amazing concepts. In all cases, materials innovation remains key.

The automotive industry tends to grab the headlines at the moment, at least when it comes to emissions. However, putting the VW scandal to one side, massive improvements have been made as the 2020 fleet reduction in CO2 targets loom ever closer. Credit where credit is due. Auto engineers have already delivered massive tailpipe emission reductions, with further efficiency improvements destined to come. So, could similar ambitious targets be applied to aircraft manufacturers?

Though unofficial, there are some that believe obligatory targets will motivate change in the same way as it has for cars makers. Like automotive, the aviation sector is set for massive expansion and more flights. But, it is in an unsustainable environmental position.

Figures vary, but one potential target that seems to be gathering pace, at least with industry pundits, is a 75% reduction in engine emissions by 2050. Though that seems a long way out, given the longer design cycles, production runs and corresponding engine developments of aircraft, it would certainly be suitably ambitious.

Though tough, the industry would not have to start from scratch. Nasa has a history of developing novel and unusual technologies, and its blue skies approach to aviation has recently taken on a green hue.

It has published a number of studies and results from its ongoing Environmentally Responsible Aviation (ERA) project. Like automotive, much of the reduction will come from the use of innovative materials used to lighten aircraft.

First, is the work involving stitching together large sections of lightweight carbon fibre to create a damage tolerant structure known as PRSEUS (Pultruded Rod Stitched Efficient Unitized Structure).

The technique hopes to offer a distinct advantage: stop cracks forming and spreading. However, in terms of fuel reduction, another motivator is that stitching does away with rivets and fasteners. This could save weight and also help realise unique aircraft geometries such as the blended wing body.

Nasa’s Virginia based Langley Research Center put the theory to the test and made up a large composite structure that consisted of 11 panels stitched together. The part measured 9.1m long and 4.3m high. It was installed at Langley’s Combined Loads Test System (COLTS), a large hydraulically powered test fixture that essentially serves as an aircraft torture chamber, where parts are bent and twisted to breaking point. Nasa technicians even intentionally damaged critical parts of the PRSEUS composite structure to see if cracks and damage grew under stress.

The tests followed a step-by-step evolution in which first, many panels were manufactured and tested using the PRSEUS technique, and then a cube structure was assembled and tested. At each step, the idea had to prove itself and provide a number of lessons about the material’s behaviour and how best to manufacture increasingly larger structures.

“We’re still compiling all the data we gathered during the tests, but just visually you can see instances where we intentionally damaged the structure and the damage stopped where it was supposed to,” says Dawn Jegley, a senior aerospace engineer at Nasa’s Langley Research Center. “And then having covered all the tests we intended to, we did some bonus tests to find out what the ultimate limits of the structure were by exposing it to levels you would never see in flight, and even as it finally failed it still worked like a charm.”

The structure eventually suffered a tear but demonstrated it could sustain the loads and forces it would experience in flight, including those in the most extreme cases of violent turbulence.

A bug’s life

Nasa has a long history of looking at the stranger / more alternative end of aircraft research, and this part of the ERA project was no exception. It involved bugs, specifically stopping them from reducing aircraft efficiency. Bug residue has been a long standing challenge for the aviation community. Insect debris builds up on the leading edge of wings, which creates drag and increases fuel consumption. So, Nasa is looking at various types of non-stick coatings to reduce the build up. However, before Nasa engineers could develop and test non-stick coatings, the researchers had to study bug chemistry.

“We learned when an insect hits [an aircraft at high velocity] its body ruptures and the blood starts undergoing some chemical changes to make it stickier,” says Mia Siochi, a senior materials scientist also at Langley. “That’s basically the survival mechanism for the bug.”

The work is related to what is often described as the ‘holy grail’ of aircraft design: the development of a laminar flow wing. Laminar flow itself is not so much the problem, it is the masses of fixtures, rivets, seams, and panels needed for access and movement of flaps, which interrupts airflow, producing small pockets of turbulence and creating drag.

“Laminar aircraft wings are designed to be aerodynamically efficient,” says Siochi. “Surprisingly, all you need are little bugs that trip the flow and you lose part of this benefit. If you have bugs accumulating, it causes the airflow to trip from being smooth and laminar, to turbulent, which causes additional drag.”

Engineers at Langley developed and tested more than 200 coating formulations in a small wind tunnel, then selected five for flight testing. Nasa and Boeing engineers spent two weeks in Shreveport, Louisiana, chosen in part because of its significant bug population. There, the team tested the non-stick wing coatings to assess how capable they were at combating insect remains from sticking to the leading edge of the right wing of an aircraft. As most insects fly relatively close to the ground, to test the coatings a Boeing 757 made 15 flights from the Shreveport Regional Airport, which included several takeoffs and landings.

“One of the five coating / surface combinations showed promising results,” says Fay Collier, an ERA project manager. “Early data indicated one coating had about a 40% reduction in bug count and residue compared to a control surface mounted next to it.”

The materials scientists turned to nature for inspiration, specifically lotus leaves, to create the right combination of chemicals and surface roughness in the test coatings.

“When you look at a lotus leaf under a microscope the reason water doesn’t stick to it is because it has these rough features that are pointy,” adds Siochi. “When liquid sits on the microscopically-rough leaf surface, the surface tension keeps it from spreading out, so it rolls off. We’re trying to use that principle in combination with chemistry to prevent bugs from sticking.”

The Shreveport flights followed another set of Nasa tests, this time in Seattle. An aircraft’s vertical tail is a significant source of drag at cruising speed. To move away from the classic tailplane design, Nasa is looking at the potential of using 31 small blowing actuators on the rear surfaces. Known as the Active Flow Control Enhanced Vertical Tail Flight Experiment. It features tiny jets installed on a full size vertical tail to blow air and provide side force stability. If successful, it could lead to smaller vertical tails or their redundancy altogether.

However, perhaps the most interesting and potentially game changing technology is the morphing wing that has been developed. Nasa researchers, working with the US Air Force Research Laboratory (AFRL) and FlexSys, successfully completed initial flight tests of its shape changing wing technology (main picture).

The tests carried out in California saw 22 research flights during the past six months with what is being called an experimental Adaptive Compliant Trailing Edge (ACTE) flight control surface. Flap angles were adjusted from -2° up to 30°.

ACTE technology, which can be retrofitted to existing aeroplane wings or integrated into entirely new airframes, enables engineers to reduce the structural weight of a wing and to aerodynamically tailor the wings to improve fuel economy. Although the flexible ACTE flaps were designed to morph throughout the entire range of motion, each test was conducted at a single fixed setting in order to collect incremental data with minimum risk.

The ACTE project involves replacement of both of a Gulfstream G-III’s conventional 19 foot aluminium flaps with the advanced, shape-changing flaps that form continuous bendable surfaces. The flexible flaps are made of composite materials to a patented design from FlexSys.

Using a flexible material to generate the continuous bendable surface, the flap does not have any gaps, known as slots, within the wing surface. The two triangular gaps between the flaps and the fixed wing are bridged by flexible material.

There is no doubt the Environmentally Responsible Aviation (ERA) project will be referenced in coming years to improve the fuel efficiency of aircraft. The big question, beyond Nasa and the aviation industry at large, is whether the European Union and US authorities will implement legally obliging emission reduction targets in the same way as they have done for the automotive industry?

Nasa claim a 75% reduction is possible, and provide a good grounding in some of the technologies that could help the industry make a flying start. However, without clear targets in place, and sanctions if they are not met, the really compelling motivation is simply not there.

Justin Cunningham

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