The biggest single "bolt-on" performance enhancer for airliners has been the blended winglet, developed jointly by Aviation Partners and Boeing in 1999. Originally designed for the Boeing Business Jet, they appeared in 2000, but their transfer to the 737-700 and -800 (and later the -300/500, 757 and 767) proved so popular that more than 1,750 blended winglet systems are in service with at least 100 airlines in more than 40 countries.
Yet the technology is relatively simple. As an aircraft flies, the disturbed air moving across the upper wing surface produces vortices at the wingtip. The blended winglet is shaped with a large radius and a smooth reduction in chord that controls the air and produces much smaller vortices at the tip. The result is reduced drag, which translates directly into improved fuel consumption of between 4% and 6%, and better aerodynamic performance from increased lift.
For a 737-700/800, the winglets are 2.5m (8.2ft) high, with a chord variation tapering from 1.2m wide at the base to 0.61m at the tip. They also increase the wingspan by 1.37m.
Built-in winglets provision
The internal structure consists of graphite struts and ribs, to which are fastened graphite with Nomex core upper and lower skins. The tip, trailing edge and interchangeable leading edges are all aluminium, as is the 16-bolt attachment point to the main wing structure.
All 737-700s from line number 1545 onwards have built-in provision for the winglets, which add 110kg (240lb). For earlier aircraft, conversion adds 150kg. For the 737-800 (from line number 778) the comparative weights are 170kg and 235kg, including 35kg of ballast. Modifications times for a -700 are typically four days for a provisioned wing and six days for a conversion, and for a -800 it takes four and eight days.
The earlier 737-300 has 2.1m-high winglets, giving a 2.2m increase in wingspan and adding 365kg. As with unprovisioned -700/800 models, conversion requires reinforcement of the upper and lower stringers, replacement of the upper and lower skin panels and replacement of the lower fasteners to prepare it for the additional weight and flexing movement of the winglets. The -300 also requires 40kg of flutter ballast per side.
Although the 737 winglets weigh about 55kg each, this is more than offset by typical fuel savings of between 4% and 6%, or an increase in range of 240km/h (130nm) for the -700/800 and 195km for the -300. Payload increases are 2,630kg for the -800, 2,315kg for the -300 and 2,000kg for the -700. Given the spiralling price of fuel, these savings mean that the return on investment (see table) is achieved in a shorter period of time.
Originally aircraft were converted by Goodrich at its Everett facility, next door to the Boeing production line, often in conjunction with other pre-delivery work such as fitting in-flight entertainment systems. The current demand means that the winglets, although specified as buyer-furnished equipment to be supplied by the customer, are now installed on the production line. As the installation is relatively simple and can usually be carried out at the same time as a heavy maintenance check, there are now about 30 maintenance facilities around the world carrying out this work.
The two major 737 customers are Ryanair and Southwest, and both carriers have their eye on cost reductions. Southwest has been an Aviation Partners Boeing customer since 2003, ordering 170 blended winglet sets for retrofit on its 737-700s, but also specifying them for future new-build aircraft. Ryanair ordered 225 sets plus 193 options in February 2005. As well as refits, all new aircraft deliveries from January 2006 have had factory-installed winglets.
Retrofitting on the 757-200
The 757-200 winglets are 2.5m high and give a 2.9m span increase, with a weight of 132kg per shipset plus ballast of 112kg. An additional 355kg comes from outer wing skin replacement, in-tank stringer reinforcement and fastener replacement. Flap leading-edge vortex generators are also installed under a service bulletin. Modification time is nine to 15 days (typically taking less than 3,800h). A modified aircraft gains 370km of range or 4,870kg extra payload.
The first winglet-equipped 757-200 flew in April 2005 on an aircraft belonging to Continental Airlines, which has ordered 41 shipsets and options for a further 47 aircraft. The US Federal Aviation Administration supplemental type certificate was awarded just over a month after the first flight, on 23 May. The customer list now includes American Airlines (with 20 orders), Icelandair (seven orders and 31 options) and three corporate 757 operators.
The 767-300ER has the largest winglets at 3.34m high, adding 3.3m to the aircraft's wingspan, and the heaviest at 243kg per shipset. The wing modifications are the same as for the 757-200, with 109kg of flutter ballast added. Conversion time is expected to take between 5,500h and 7,000h. The operating empty weight and maximum zero fuel weight of a winglet-equipped aircraft are increased by 1,070kg to maintain payload capability, plus there is a 590km range increase.
An American Airlines aircraft is being used in an eight-month flight-test certification programme, and Aviation Partners Boeing anticipates it will receive its FAA STC by November. Current customers include American (58 sets), Austrian Airlines (six) and Delta Air Lines (15).
The benefits of reduced fuel consumption and improved aerodynamic performance can be realised in a number of ways. With less fuel required for a particular route, a trade-off can be made in favour of greater payload. Alternatively, greater range can bring more destinations into reach with a smaller aircraft, increasing efficiency by better matching capacity to market demand. This could be the replacement of a larger aircraft flying at less than capacity or establishing new direct routes that could not support a larger aircraft.
Greater operational flexibility is created by having more lift available. Take-off can be made at a lower thrust setting (typically a 3% derate), using less fuel and reducing engine wear and tear and external noise. Normal thrust levels can produce a steeper climb-out, providing greater obstacle clearance and reducing noise as the aircraft climbs higher, faster.
Greater payload can also be carried, particularly from airfields at high altitudes and temperatures or with shorter runways. GOL in Brazil has ordered winglets sets for 60 737-800s and combined them with Boeing's short-field performance package to permit full payload operations from the short 1,320m and 1,940m runways at the Rio de Janeiro (Santos Dumont) and São Paulo (Congonhas) downtown airports.
American Airlines is a convert to blended winglets, having ordered them for 77 737-800s and 104 757-200s as well as the 767. It has calculated that for the 757 fleet, depending on fleet utilisation and average sector length, savings will be up to 760,000 litres (200,000USgal) of fuel per aircraft per year, while the 767s should achieve savings of 64-80 million litres and a reduction of 423,000t of CO2 emissions.
Once all three fleets are equipped, the airline reckons it will save over 160 million litres of fuel a year. This is in addition to the more than 360 million litres of fuel that it saves annually through its FuelSmart conservation programme. It has also expressed interest in winglets for the Boeing MD-80, with the same design being applicable to the Boeing 717, and Aviation Partners Boeing is considering these variants.
The company expects that more than 90% of Next Generation 737s, 80% of 767-300ERs and over 70% of 757-200s will be flying with blended winglets. As of the end of February, it estimated that the winglets have saved over 3.48 billion litres of jet fuel and that, by the end of 2014, this will have climbed to over 19 billion litres. As prices have climbed, the payback, in fuel savings alone, is anticipated at about 2.5 years, on a per aircraft basis, or more than 380,000 litres a year.
With increased interest in emissions from aircraft, the blended winglet has taken on a new significance, as reduced fuel burn translates directly in less CO2 being produced. It is now a "mean and green" product.