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Airlines: Today and Tomorrow

THERE IS a feeling in aviation that a change is needed. The established way of configuring commercial aircraft – indeed, really the only way known to do so – with a tubular fuselage, swept wings and tail, and under-wing engines (previously also tail-mounted ones) has been delivering essentially just incremental performance improvements, and seems likely to continue doing so unless there is a breakthrough in engines, materials or structures. But a new configuration brings its own set of challenges. Much of what is commonly accepted will need to be re-thought. Still, a great deal of experimentation and development is underway – the question is: how will it manifest in the next generation of aircraft? This all hinges around the element of LIFT

The essence of flight is lift, and the essence of the aircraft is the wing. Unfortunately, the wings of in-service aircraft produce far less lift than they are theoretically capable of. The key is the layer of air flowing just adjacent to the wing surface, the boundary layer. This is required to be laminar, however, in practice it is very easily disturbed and becomes turbulent.

Laminar flow is characterised as being achieved by passive means through the design of the surface, which results in natural laminar flow (NLF), or by active means (active flow control, AFC), or suction through the surface or the blowing of air over it or, finally, by a combination of both, called hybrid laminar flow control (HLFC).

What must be considered a firstgeneration laminar-flow control system is entering service aboard the Boeing 787-9 airliner, it is of the HLFC type, and fitted on the vertical and horizontal tails. It appears (not much has yet been revealed) to use suction through small doors on the inboard lower surface of each tailplane and on each side of the fin to remove the boundary layer, however the technology will no longer be introduced on the 777X now under development, and it seems no longer also on the 787-10 follow-on to the 787-9.

airliners

The MIT D8, showing the fuselage-top contouring to entrain boundary layer air into the engines. Source: NASA / MIT / Aurora Flight Sciences

Even while its HLFC system was entering service, Boeing had gone back to studying NLF. In 2015 it kitted and flew an ex-airline Model 757 which it called the ecoDemonstrator with a Krueger flap (a type of leading edge device which folds into the wing underside) on the port wing to protect the leading edge from insects (particulate matter on the wing surface would disturb flow laminarity, and insects are a big culprit), thereafter the US National Aeronautics and Space Administration (NASA) tested different non-stick coatings on the starboard wing to reduce the residue on the leading edge from insect strikes.

An extensive NLF flight demonstration is planned by the European Union (EU), an Airbus A340, called the Breakthrough Laminar Demonstrator in Europe (BLADE), is to start test-flying in September 2017 (NLF ground demonstrations have already been conducted) and continue over a period of 123 hours, with an interim non-flying six months for analysis of the initial results and adjustment where necessary for the final flights.

The aircraft will have two different NLF wing-section cover types, a continuous surface on the port wing, and one with a joint on the starboard (the equipment is in the process of being fitted to the aircraft).

Notably, the demonstration is part of the Clean Sky 2 programme, which seeks to demonstrate technologies which are close to a production level. HLFC is being studied in another EU programme, Active Flow - Loads and Noise Control on Next-Generation Wing (AFLoNext). AfloNext is designing and manufacturing a HLFC system for the vertical tail of an airliner (an Airbus A320 is being used, a test aircraft of the German Aerospace Centre (DLR) with a flight demonstration (Clean Sky 2) planned for the second quarter of 2017 (the structure is currently being designed).

The system is also being incorporated into a wing leading edge section for a ground-based demonstration. The test sections (tail and wing) have a perforated surface, through which suction is applied (this surface type contrasts with the door-type of Boeing’s 787-9).

Flow control over non-planar surfaces or planar ones where there may be a separation such as between a primary wing or tail surface and an aerofoil, would be of the active (AFC) type. An example of such a separation is that between the fin and rudder, and on this area of the EcoDemonstrator Boeing had installed an AFC system, it was an air-blowing system and had 31 jet actuators arranged in a line along the starboard side of the tail, parallel to and just before the separation with the rudder. An example of a non-planar surface is the area around the leading edge slat, the structure of which, as engine nacelles become bigger (as bypass ratios increase - as they are expected to), will become increasingly affected - as will, in turn, the smoothness of the flow in the region.

Another is the outer wing, where flow separation due to deployment of the outboard leading-edge slat could affect the winglet.

AfloNext is studying AFC on the junction between the wing and engine pylon at low speeds and high angles of attack (take-off and landing), and at the outer wing, with large-scale wind-tunnel tests planned at Russia’s Central Aerohydrodynamic Institute (TsAGI) in the last quarter of this year.

AERO STRUCTURES

As alluded to, a gap in an aerodynamic surface reduces the possibility of having a laminar air flow over it. US company, FlexSys, has developed a flap which functions without separating from the wing, called FlexFoil.

It has been evaluated by NASA (which calls it the Adaptive Compliant Trailing Edge) in a sixmonth series of 22 flights during 2014-2015 on a Gulfstream III business-jet-based test aircraft.

Both of the Gulfstream’s conventional flaps were replaced with FlexFoil ones. The FlexFoil flap morphs – changes its shape – but this was not examined in the flights, each of which was flown with them at a fixed setting between -2 and +30 degrees. The company says there are no moving parts in the morphing mechanism and that the flaps are lighter and require less power to operate than conventional ones.

It estimates that the Flexfoil flaps can improve fuel consumption by 3,5% as a retrofit, and by 12% as part of a new-build aircraft. FlexSys has established a joint venture with winglet specialist, Aviation Partners, to commercialise the technology for flight controls, leading and trailing edge devices, anti-icing and active winglets, for retrofit and new production.

airliners

An advanced modern winglet, that of the Boeing 737 MAX. Source: The Boeing Company

Morphing structures were also studied in a recently completed EU programme. Smart Intelligent Aircraft Structures (Saritsu) produced an adaptive (morphing) and gapless droop nose (a wing leading-edge device), which it wind-tunnel tested at Tsagi. One challenge found was that the outer skin must be flexible yet sufficiently stiff............................ For the FULL ARTICLE please subscribe to our digital edition.

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