787 special: Electric dream

Boeing’s more-electric initiative is one of the major design features of the 787, offering increased efficiency, better control and reduced systems weight

The more-electric system on the 787 really is more electric, generating around twice the amount of electricity of a particularly power-hungry aircraft such as the E-767 airborne warning and control system. “It’s probably the biggest change to the systems architecture of any aircraft. We’re talking about a variable-frequency higher voltage system capable of 1.45mW, or enough for around 400 homes,” says 787 systems director Mike Sinnett.

“There’s a lot of talk in the industry about whether we’re overextending on this, but frankly the engine doesn’t care whether its power is extracted electrically or pneumatically. We hear from the ‘equivalent horsepower’ school that a pneumatic engine is just as effective as an electric engine. But that’s the whole point; our aircraft does not draw as much horsepower off the engine in cruise, so it doesn’t burn as much fuel. If you look at the extraction profile you see the amount of power you pull off [the engine] is just what you need, so the engine isn’t working any harder than it needs to. We’re only getting what we need, and we only use what we generate.”

The bulk of the power comes from two 250kVA generators mounted to each engine, which also act as starter units. “That’s half a megawatt in each engine,” says Sinnett. “The units are designed to be activated with one or both, and with both we start quicker.” The engine power is augmented by two 225kVA generators attached to the APU. As well as being used to start the engines, electrical power replaces the traditional pneumatic system and drives the environmental and cooling systems, moves the undercarriage legs up and down, controls the brakes and runs the anti-icing system. Engine cowl anti-ice and nacelle heating will still be provided by a small amount of engine bleed air, some of which will also be used to help maintain operational stability within the engine itself.

Reduced weight

Boeing believes that using electrical power is more efficient than engine-generated bleed air and the related pneumatic system, and expects the new architecture to extract as much as 35% less power from the engines. Conventional pneumatic systems generally develop more power than is needed, causing excess bleed air to be dumped overboard. The ducting used to pass the pressurised air around the aircraft has to employ check valves and pre-coolers, and is itself made of titanium, which adds hundreds of kilograms in weight to the aircraft, says Sinnett.

The electric system is also inherently easier to monitor and control, and produces only enough power as needed. The power, which comes off the generators at variable frequencies, is conditioned in the electronics bay before being distributed to the appropriate systems.

Hamilton Sundstrand has responsibility for the bulk of the 787’s electrical system, with three of the four main work packages including the electrical power generation and start system, the remote power distribution system and, in partnership with Zodiac, the primary power distribution system. The remaining package, the power conversion system, is the responsibility of Thales.

In mid-July, Hamilton Sundstrand formally inaugurated its Airplane Power System Integration Facility (APSIF) at Rockford, Illinois, which is being used as a key part of the Boeing 787 test and development programme. In previous new airliner projects, a facility like the APSIF would have been found only at the airframe manufacturer level. Its location at the site of a major supplier is a prime example of how Boeing has federated responsibility for 787 development around the industry.

The APSIF is being used to verify the performance of several major systems for the more-electric aircraft, many of them provided by Hamilton Sundstrand. The company is providing the 787’s environmental control system, electric power generation and start system, remote power distribution system, auxiliary power unit (APU), primary power distribution system, high-voltage DC equipment racks, emergency power system, nitrogen generation system and electric pump subsystem.

“There’s virtually no system we haven’t done before, but here we are doing them all in the same aircraft,” says Hamilton Sundstrand aerospace systems president Tim Morris. “It’s a pretty big package of equipment to do all at once, but already we’re seeing the benefits of integration. When we find problems we can work those between the systems pretty quickly and trade-off to find solutions,” he adds.

Early production

After starting up, the APSIF first demonstrated the APU and main engine start sequence, before switching over to produce power to drive the aircraft’s systems. “We’re running the lab all the time and adding more equipment to it every day. We have an APU and a control room for the laboratory, which we link to those in Seattle,” says Morris, who adds that two large dynamometers act as surrogate engines.

The lab has already uncovered issues that have resulted in changes. The design of the power feeder lines, for example, was altered after supply and electromagnetic interference issues were uncovered due to the impedence and twist of the existing cables. “I think that’s why Boeing thought the APSIF was such a good idea,” he says.

“We are starting to deliver hardware this month [September] and we have a schedule of hundreds of pieces,” says Morris, who adds that the focus is on supporting the ramp-up to meet Boeing’s aggressive initial production rate. The company has to date guaranteed sufficient parts for the first seven shipsets (the seventh being the first aircraft to be delivered to a customer), and additional early production aircraft beyond.

For emergency power Hamilton Sundstrand is supplying the ram-air turbine (RAT), which will drop out of its belly faring housing in the event of engine failure. The RAT has been tested in Calspan’s transonic wind tunnel in Buffalo, New York “up to the limits and beyond”, says Morris. ■

Source: Flight International