Winged Wonders

Theoretically, a wing of infinite span is one way to prevent the two airflows meeting and creating wingtip vortices, however, that’s not exactly feasible. Increasing the span would lead to potential structural issues and an increase in another form of drag; parasite drag. What is feasible and employed in various styles on an ever-increasing range of aeroplanes is the design feature known as the winglet. While winglets cannot eliminate wingtip vortices, they can significantly reduce them by directing the two airflows such that the union is more subtle and at a distance away from the lift-producing surface of the wing. Additionally, the winglets effectively increase the span of the wing and consequently the aspect ratio; that is the ratio of the wing’s length to its breadth, further reducing the strength of the vortices. Combined, these features significantly reduce the drag, thus enhancing lift and reducing the ever-crucial fuel burn figures.

Still, the benefits must be weighed up against the increased weight and the increase in parasitic drag created by the wingtip additions. Furthermore, the inward load created by the winglet and the increased lift at the wingtip increase the bending moment upon the wing, so structural issues must also be considered in the design stage. However, the opportunity to gain the increased performance and efficiency offered by winglets is proving to well and truly outweigh the costs involved in countering their limitations. The winglet has arrived.

An Upward Tend.

Whether the upturned wingtips of a soaring bird can claim credit for the inspiration of winglets cannot be said with certainty, however, the development of the device even pre-dates powered flight to 1897 when English engineer Frederick Lanchester patented ‘wing end-plates’ as a means of combating drag at low speeds. The Wright Brothers experimented with various devices and William Somerville registered his design for functioning ‘winglets’ in 1910. Twenty years later Vincent Burnelli released his concept of ‘end plating wingtips’, while the 1950s saw the emergence of the more subtle ‘Hoerner Tip’ which is commonplace on light aircraft today.

The 1970s saw Burt Rutan’s VariEze emerge with significant winglets which also served as the vertical stabilisers on his futuristic composite-built canard. In the bizjet world, Lear were the first to incorporate winglets when they dispensed with the tip-tanks and modified their “Longhorn” wing with the aerodynamic device, resulting in the Lear 28. Further modifying the wing to recoup lost fuel capacity, the re-named Lear 29 boasted improved fuel efficiency of around 15 per cent of which nearly half was attributable to the winglets.

However, it was the oil crisis of 1973 that provided the catalyst for the development of the large scale winglets that dominate the landscape today. Dr. Richard Whitcomb, an aeronautical engineer at the NASA Langley Dryden Research Centre, predicted that winglets could lead to significant fuel savings. In conjunction with the USAF, a modified KC-135 tanker was flown through a series of trials to find the ideal design for jets cruising at transonic speeds. These tests exceeded the wind tunnel predictions and equated to a fuel saving of more than 6 per cent.

The data was out and in a world that craved for fuel efficiency, winglets were set to venture forth from the wind tunnel and test aircraft and onto the wings of aircraft great and small. And yet as their development continued, even more was learnt about the aerodynamic devices whose roots dated back to the nineteenth century.

Horses for Courses.

The increasing prevalence of winglets resulted in a closer examination of the benefits they had to offer. As early as the first modified Lear jets, a degree of increased directional stability was observed, while increased take-off performance and even extended engine life emerged as positive outcomes stemming from the incorporation of winglet technology. While the most substantial element, fuel efficiency was no longer the sole advantage of winglets. Increasingly different designs emerged beneath the umbrella of the primary concept and the terms ‘fence’, ‘blended’, ‘raked’ and even ‘Sharklet’ began to emerge.

Boeing’s first foray into winglet technology was seen on the 747-400. Standing six feet tall, the winglets were simple and typical of the first generation but still resulted in a three per cent fuel saving over the comparable 747-400D which did not possess winglets. Even so, these early airline winglets met the wing at a fairly rudimentary junction which could generate there own turbulent airflow in the form of interference drag. Hence, it was soon recognised that the current design was actually negating some of the winglets benefits and work on a more subtle transition from wing to winglet took place. The subsequent design was the ‘blended’ winglet.

Originally fitted to the Gulfstream II, but today most easily recognised as the distinguishing feature of the Boeing 737NG aircraft, the blended winglets provide a transition region which gradually converts the horizontal wing into its vertical accomplice. Replacing the previous angular junction further reduces the drag and increases the efficiency of the wing. The 737 winglets are substantial, standing over eight feet tall and constructed from carbon fibre composite skins over a Nomex honeycomb stabilising core, while the leading and trailing edges retain good old fashioned aluminium. Lightweight and distinctive, the winglets have equated to an increased range of five per cent while also significantly reducing drag on take-off and landing. Particularly on take-off, this increased efficiency equates to increased performance, or when the performance isn’t required, the ability to de-rate the engine thrust. This de-rating of thrust ultimately leads to less noise, lower fuel burn and reduced engine wear and emissions. The winglets can also enhance performance in the climb and cruise phases. Interestingly, the towering winglets of the 737NG result in increased directional stability, but also a reduction in its crosswind limit. This stems from the minute increase in rudder input required to counter the increased stability in the landing phase compared to a non-wingleted aircraft. Since there needs to be a controllability margin for gust response and the like, the maximum demonstrated crosswind value has reduced by a knot or two to preserve the margin.

The blended winglet has been so successful that it is now offered as a retro-fit option on the Boeing 757 and 767 by the blended winglet specialists, Aviation Partners Inc (API). Boasting up to 5 per cent improvement in fuel efficiency the modification has been taken up by airlines around the world including American Airlines, Hawaiian Airlines and regionally most notably, Air New Zealand. The Kiwi carrier has fitted the 3.4 metre high blended winglets to their 767-300ER fleet and aside from the impressive statistics; the classic airliner looks downright attractive with her new additions. However, while modifying a proven wing in such a manner may offer benefits, there is also the need to review the structural integrity of the wing design in light of the increased bending moments that will result. As such, you can’t necessarily retro-fit winglets to every aeroplane available.

While Boeing ventured down the blended winglet path, Airbus adopted a winglet style that became equally recognisable as their signature; the wingtip fence. Arrow-shaped and extending both above and below the wingtip, the core purpose of the ‘fence’ remains the same. Pioneered on the A300 and A310 airliner programmes, Airbus has continued with their distinctive fences through their entire range, right up to the patriarch of the fleet, the A380. Like all winglets, the wingtip fences aim to reduce induced drag and wingtip vortices with the pay-off coming in the form of increased performance and fuel efficiency. However, it would seem that blended winglets have the edge in efficiency and savings, so in 2009 Airbus unveiled the ‘Sharklet’. Sharklets are effectively blended winglets with a catchy name, boasting the same benefits and similar fuel savings in the realm of 3.5 per cent. Offered on new production A320 aircraft, the Sharklets will also feature on the new A320neo variant.

Beyond the blended winglet an even more recent player has emerged in the world of wingtip technology; the raked wingtip. The design involves an increased sweep on the outer wing section rather than a substantial vertical winglet surface; the goals of the raked wingtip are nevertheless very similar. The chosen device for both the Boeing 777-300ER, 747-8, 787 and Airbus A350, the raked wingtip can also be found on the new maritime variant of the Boeing 737, the P-8A Poseidon.

The raked wingtip promises to offer even great efficiencies with initial Boeing and NASA testing suggesting a further two per cent in drag reduction over their blended winglet contemporaries. Like the blended winglet, the raked wingtip increases the apparent aspect ratio of the wing to reduce the vortices, however, it does physically increase the wingspan. As such, like the other devices, it creates an increased bending moment on the wing. These structural issues and increase in span needs to be assessed for each type, particularly as wingtip clearance can be limiting at airports predominantly designed for domestic operations. For this reason, the proposed Boeing 787-3 had intended to utilise a shorter wing and blended winglets.

Future Wings.

The prevalence of winglets has grown rapidly over recent years. Whether saving fuel and emissions on a global scale or squeezing a little more performance from a light aircraft, the gains in efficiency have proven their worth. Winglets are rapidly becoming as commonplace a design feature in the manner that fairings emerged on fixed undercarriages decades ago. Yet, even as the savings mount and the statistics impress, the pursuit of the perfect wingtip device continues.

Aviation Partners have been working with the Spiroid Winglet which sees the wingtip effectively forming an elongated loop on the outer section of the wing. Aiming to halve the induced drag of wingtip vortices and reduce fuel consumption in the cruise by 6-10 percent, a Gulfstream II test aircraft reportedly recorded fuel savings greater the 10 per cent. The future may already be here.

Whether the future airliners wingtips are turned up, swept back or twisted into a loop, aviations life-long battle with drag is set to continue. And while it is a battle fought on a number of fronts, the evolution of wingtip devices has seen tremendous ground already made. To date they have produced increased aircraft performance, enhanced environmental outcomes and even smoother, safer airspace for aeroplanes following in their wake. And while there are still challenges ahead, a great deal has already been achieved by these winged wonders.