Lightening the lift

Civil aircraft designers are continually improving wing designs with natural laminar flow (NLF) sections. With such developments fuel savings of around 6% per flight are claimed to be possible. However, there is an inevitable trade-off between the smoothness of wing surfaces for cruising speeds and movable high- lift devices such as flaps and slats needed to generate lift at take-off and landing. These moving elements of modern wings unavoidably create turbulence and drag, reducing efficiency.

One high-lift device employed on the leading edge of aircraft wings is the Krueger Flap. This aerodynamic profile and its corresponding mechanism, is named after the German engineer Werner Krueger who invented it in 1943. The flap hinges down from the underside of the wing when needed to increase the critical angle of attack of the wing. When deployed it increases lift and delays the onset of a stall. The flap is then retracted during cruising flight.

One of the advantages of the Krueger flap is that that it leaves the upper wing surface smooth when retracted, and while fully deployed it can protect the wing from insect and dirt accumulation and surface erosions.

Making this assembly as light as possible has obvious advantages in terms of reduced fuel burn, but materials that can lead to more integrated designs also have huge potential.

Belgium headquartered Asco is a technology specialist supplying design, development, precision machining, processing and certified assembly of high-lift devices for the aircraft industry. Its customers include most of the major OEMs in the aircraft business alongside tier 1 and tier 2 suppliers.

In recent years the company has been working on its Deamak Project, a programme to design and build a Krueger Flap using composite materials. The aim of the project was to produce a flap from Carbon Fibre Reinforced Plastic (CFRP) that could be manufactured using out of autoclave processes.

Brieuc Spindler, Asco engineering director, describes the manufacturing as a ‘one shot' process: “It is one shot because the part produced does not need any additional assembly. It is net shaped out of the mould including complex internal cavities.

“The flap is fabricated using Resin Transfer Moulding (RTM) as the production method. In this process, dry fibre geometries are shaped and inserted into a double-sided closed mould. Once the mould is closed, it is positioned into a press for heating. The mould cavity is put under vacuum, injected with resin and the fibre-resin composite cured to obtain a one shot complex CFRP component.

“Unlike traditional autoclave processes based on prepreg materials, RTM offers the ability to produce the part faster as no large air volumes need to be heated during the production cycle. Moreover, the RTM process has significant automation potential using robots to open and close moulds etc., reducing even further the manufacturing cycle time.

“Given the predicted future aircraft production rates of approximately 60 aircraft per month, it speaks for itself that the production rate potential of a process is a very important parameter. Additionally in terms of cycle time, manufacturing a composite integrated one-shot Krueger Flap allows a drastic reduction of assembly time compared to a metallic multi-component assembly. RTM is also more environmentally friendly, as the energy consumption is lower compared to the traditional autoclave process for a similar component.”

Inspecting internally

One major issue Asco had to overcome was inspecting the composite components to the specifications required so that consistent quality could be guaranteed.

Spindler continues: “A typical problem with integrated components is the inspection because it is difficult to examine the interior of the part by traditional methods. The team succeeded in overcoming these issues by enhancing state-of-the-art NDT inspection methods. The results showed good part quality on the very first prototype we manufactured, providing us with confidence about the path we had taken.

“Regarding the geometrical quality of the component, as it is cured in a double-sided mould, the required tolerances are guaranteed as they directly depend on the mould surface quality and mould dimensions, aspects which we can strictly control.”

One of the reasons closed mould RTM is a viable option for the Krueger Flap application is the size of the part. As Spindler explains the process is not so easily applied for larger wing components.

“Given the limited size envelope of the Krueger Flap, and Asco's high-lift products in general, RTM remains competitive over other processes typically used for large skin surfaces such as resin infusion, where expensive tooling would rule out closed mould RTM manufacturing.

“The combination of the limited size of the parts, the high production rate potential, energy-efficiency and the fact that Asco prototypes have complex geometries, means that the RTM process is a baseline solution.”

Until Asco's recent developments it was generally thought that CFRP leading edge high-lift devices did not offer the same protection against bird strikes and dirt and insect build up as metal alternatives. Accumulation of dirt and insects can create significant problems on wing surfaces, creating turbulence and reducing laminar flow.

“Composite materials in general are far less suitable to absorb impact energy as they are inherently less ductile compared to metals. It was judged that Krueger Flaps, mounted on the leading edge of the aircraft and prone to potential bird strike, could not be made from composites for this reason. However, by using a suitable design concept, Asco has proven that such structures can be made from composites, consequently reducing the component weight.

“In this regard, it is not so much the material used than the specific design, its aero shape and anti-contamination surface coatings that create the enhanced protection against insect damage/build up on the Krueger Flap. Indeed part of the solution lies in the specific coating that provides improved contamination protection compared to traditional paint scheme.”

Lighter by design

Spindler says the new CFRP flap is 15-30% lighter than traditional designs either milled from a solid block of metal or assembled out of ribs and metal skins.

The new flap was recently nominated for the Best Project award as part of the Clean Sky research programme which looks to reduce CO₂ and NOX emissions with aerospace companies across Europe.

The flap will now be flight tested on a modified Airbus A340 under the Clean Sky BLADE project (Breakthrough Laminar Aircraft Demonstrator in Europe), aimed to design, manufacture, assemble and perform flight demonstrations of an aerodynamically optimised natural laminar flow wing under Airbus and Saab leadership in conjunction with major industry first tiers.

So what does this development mean for wing designs of the future?

“With our advanced Krueger Flaps, Asco is providing a high-lift device solution for the leading edge, enabling advanced slatless passive laminar wing aerodynamic designs. The Krueger Flap on top of being lighter with bird-strike/insect shielding also has integrated electrical de-icing reducing further the complex bleed air architecture, further reducing the energy consumption of the aircraft. This is essential to innovative wing technologies converging towards reduction of CO₂ and NOX emissions,” Spindler concludes.

www.asco.be