FLUG REVUE August 2004: Boeing 7E7 progresses

BOEING 7E7: “MORE EFFICIENT THAN THE A380”

By Sebastian Steinke

Walt Gillette, Vice President and Airplane Manager of the 7E7 programme, took the occasion of a visit by FLUG REVUE to his office to announce spectacular fuel consumption records for Boeing's latest twin-jet: “The 7E7 will use less fuel per passenger than an A380.” Boeing has set itself a target of reducing the 7E7's fuel consumption by 20 percent compared with comparable types around today, such as the 767. “Boeing has always taken the lead and been the first when it comes to innovation.”

7E7 windtunnel tests

But why has the manufacturer waited almost 15 years to finally launch a completely new civil aircraft programme? Gillette: “The problem was that we had to wait for new technology. It will only be available for incorporation into an aircraft in 2008. Besides, it is the airlines who demand these innovations, not us.”

Firstly, he explains, in addition to engines without bleed air, there are new materials available today which are needed to reduce fuel consumption, while at the same time it is now possible to use numeric flow simulation (finite element/computational fluid dynamics) to work out the truly optimal aerodynamics. “We want to avoid local areas with supersonic airflow, which only cause drag and noise.”

According to Walt Gillette, about eight percent of the twenty percent of the fuel economy is due to the new engines, whose fan diameter is larger relative to the core than on the previous generation of engines. A further three percent improvement is contributed by each of the following features: the advanced aerodynamics, the more efficient systems with less weight, the lower overall structural weight and the optimal sizing, that is, the made-to-measure basic design which, in the case of the 7E7, began with a clean sheet of paper.

Gillette explains that the new generation of engines, when fitted to the A330, which was not optimised for this right from the start, produces only a six to seven percent improvement in fuel consumption, compared with when they are installed on the 7E7. On the other hand, due to this major advance in technology, it would be well worthwhile using the 7E7 engines on a possible future version of the 747.

On the 7E7, for the first time, engines from two different manufacturers will be fitted on the same pylon. It will even be possible to change the engine model at a later date: all that will be necessary will be to change the engine monitoring software. This innovation should be especially attractive to leasing companies and the used aircraft market.

The experienced designer is surprisingly sceptical about aluminium, the traditional aircraft material. “Aluminium has become a relatively expensive material, for which you have to pay 'boutique prices'. The majority of world production is destined for the consumer goods industry, for example, for drink cans or ladders. The proportion of aluminium accounted for by aerospace consumers is dwindling. If you need special alloys, then for the most part you have to finance these developments from your own pocket. That is becoming expensive. On the other hand, carbon fibre offers far more industrial applications, and this fact is exerting downward pressure on prices.” According to Gillette, Boeing would have needed eight different aluminium alloys for the 7E7, for which precise data was provided to the metal factories. Only one type met the specification.

Gillette suspects that a similar “boutique technology” applies to bleed air systems: “Only the aerospace industry uses bleed air, everyone else uses electricity. It is time to move away from expensive high-pressure air technology. To control the voltage in the 7E7, for example, we are using parts similar to those used on the French TGV high-speed train,” says Gillette, explaining his preference for bulk-produced products. For the pressurising system and probably the majority of the anti-icing, the 7E7 will probably use a lighter electrical system. The auxiliary power unit (APU) in the tail will have a modular design so that it can later be exchanged for a fuel cell, as it does not have to generate any bleed air any more. The 7E7 has three 5,000 psi hydraulic systems.

Although the composite fuselage of the 7E7 promises fuel consumption advantages due to its low weight, many potential customers are likely to have reservations as to how to maintain it. In the rough airline daily routine, ground vehicles, passenger steps and baggage carts are always running into parked aircraft and leave behind dents, scratches and sometimes even tears in today's aluminium structures which are not always harmless. Whereas dents in aluminium components can be routinely removed, polished, strengthened or replaced with riveted patches, composite materials, with their layered structure, are relatively difficult to examine for damage and complicated to repair.

Walt Gillette dismisses these arguments. “We could equip the entire aircraft with a neuronal glass fibre network of sensors that report any damage. But we probably won't need that because the fuselage is simply so strong. For example, the fuselage skin around the doorways is 2.54cm thick. If someone knocks into the shell at this point, it can survive for as long as the aircraft. We have developed two repair methods using patches. The first one takes an hour to install and lasts to the next C check, while the second one takes two hours and lasts for a full aircraft lifetime.”

The 7E7 programme has been designed right from the start as a family of aircraft. In addition to the basic version, the 7E7-8 with transpacific range, there will also be a short-haul variant, the 7E7-3, and a stretched version, the 7E7-9. How can one optimise a family of aircraft so as to minimise fuel consumption, when the different members of the family are intended for completely different purposes? Walt Gillette: “The 7E7-8 and -9 have the 'right' wing. The wing is designed for the requirements of the 7E7-9. By comparison, the short-haul version, the 7E7-3, has a shorter wing and less range. Its winglet restores some of the efficiency, but admittedly not all of it. On the other hand, it can be parked in the small ground handling space allowed for a 767. This gate utilisation is valuable in its own right. Inside, we are reducing the material strength. This means that a lot of the 7E7-3, for example the landing gear, will be lighter. Only the rudder will be bigger, as the 7E7-3 will take off from shorter runways at a slower speed, which means that bigger steering input is necessary in the event of an engine failure. Although the larger 7E7-8 has actually been calculated for longer distances, due to the higher strength of its composite structure, if required it can also be used for wear-intensive short distances like a 7E7-3. Some Asian airlines are already doing that today with the 777.”

According to Walt Gillette, Boeing is currently developing all three versions in parallel. As soon as the basic structure of the aircraft family has been fixed, for example, the undercarriage bay is to be uniformly dimensioned for all three versions, they will then temporarily stop work on the 7E7-9, which will appear later, and initially complete only the 7E7-3 and -8, for which orders have already been received. “We are starting with the detailed design of the front and are slowly working back towards the tail. Originally, the 7E7 was planned to be narrower, with only seven seats per row. At the request of the airlines, we then significantly widened the fuselage to eight seats.” The enlarged 7E7 fuselage cross-section is only about 18 inches narrower than the 777 and only an arm's length narrower than the 747 on each side. Version 7E7-8 will attain “firm configuration” status in mid-2005, version 7E7-3 then following half a year later.

In the cockpit, the 7E7 strongly resembles its cousin, the 777, so that conversion training is required to last only three days. The most striking enhancement is the two standard head-up displays. As Gillette points out, “The instrument panel has the same dimensions as in the 777, but the new displays are a lot bigger. Despite this, they can be configured to appear just like in a 777. The 7E7's electronic yoke is similar to the familiar one in the 777. The computers will also create the same feeling as in the 777.”

Amplifying the previous 777 standard, Gillette is planning more extensive electronic protective mechanisms for the 7E7, similar to those offered by Airbus. For example, if the aircraft is flying so slowly that a stall is imminent, thrust is automatically applied. And if the Mach limit maximum operating speed (MMO) is exceeded, the elevator automatically deflects upwards, causing the aircraft to climb and hence lose some of its speed.

If the pilot wants to fly unusually tight turns which generate more than 2.5g, the artificial counter-pressure which he feels in the control horn will be doubled. However, unlike the fixed limits in Airbus aircraft, the pilot will be able to override this by applying the appropriate force. Gillette: “There have been cases of Boeings pulling 5 to 5.5g in emergency situations which were still able to land in one piece. The design adds another 50% safety margin to the maximum accelerations required by the authorities for certification purposes. We don't need any active gust load control, as the extremely strong wing structure can handle all loadings.”

The angle of roll, he adds, is electronically limited initially to 30º, but this can be overridden by exerting twice as much force. During banking, there will be no further need in the future for compensation of the elevator. The 7E7 computers automatically correct the loss of lift during banking, and the jet continues to circle until the pilot straightens up again.

A 7E7 wind tunnel visit

Boeing Field airport, to the south of the outskirts of Seattle: in an inconspicuous production building at the edge of the company grounds, Boeing's transonic wind tunnel hides away. When FLUG REVUE visited it on 25 May, a 2.13m long model of the 7E7 was already in position in the tunnel. An “oil flow visualisation test” has been scheduled for today. This entails releasing coloured oil from the model so as to reproduce the pressure gradients of a boundary layer at Mach 0.85, the cruise speed planned for the future 7E7. But before wind tunnel engineer Robert Hill starts up his 55,000hp electric motor, he has to notify the power station, as the equipment already draws 26MW from the mains supply at the “cruise”, and at full throttle this rises to as much as 42MW. Tom Cogan, Chief Project Engineer, explains that the shape of the cockpit nose on the model is already virtually the same as the final shape, whereas the empennage, wingtips and other details still have to be optimised. “We have already advanced one-third of the way.” The programme included comparing 50 to 60 wing configurations alone. There are wing sets for take-off and landing with extended flaps, leading-edge flaps and undercarriage, and models for the cruise with different loading conditions. The new composite material used on the 7E7 has the effect of changing its aeroelastics, for the wing no longer bends and twists as much as an aluminium structure. Altogether, the 7E7 engineers will need 15,000 wind tunnel hours at various stages of the development work. Because every wind tunnel has its own characteristics, Boeing is also using facilities belonging to the University of Seattle, NASA in California and QinetiQ in Farnborough in parallel. On top of this are ever better computer-aided simulations. Aerodynamics expert Cogan explains, “On the 777, we were still telling the computer the form that the pressure distribution should take. Today we enter the desired performance, and the computer tells us the pressure distribution and shape needed.”

From page 30 of FLUG REVUE 8/2004

Last updated 8 July 2004

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