The 49th International Paris Air Show, which took place June 20-26 at its usual venue (Le Bourget), provided irrefutable evidence that even in the midst of a less-than-sparkling global economy the aerospace industry is once again putting on its party hats.
People’s exhibit number one, as they say in court, was the record number of new aircraft orders placed. Airbus and Boeing together signed over $25 billion in airliner orders the very first day of the show and Airbus by itself chalked up an estimated $73 billion in sales during the run of the show—mostly orders for a new, re-engined version of its Airbus 320 family of airliners. Boeing, for its part, booked about $22 billion in commercial aircraft deals in Paris for its new 787 and 747-8 airliners (both of which also made a first fly-in visit to the show) and its existing 777 and 767 models.
In terms of technological developments the Paris show found aircraft manufacturers taking a page from the playbook of their automobile-making brethren in seeking weight and size reductions as an antidote to increasing fuel costs. In aviation that starts with replacing, where possible, existing aluminum skin and structures with composite materials.
As for internal systems aerospace designers are focusing on better use of power consumption, higher electronic network speed and more bandwidth to handle the burgeoning amount of digital data modern aircraft systems produce. Making the task harder is the fact that all of this has to happen with improved reliability because aircraft are expected to stay in flight much longer today that they were designed for only a couple of decades ago.
The Paris show also stirred more discussion about using electrical power for extracting and distributing the non-propulsive energy used in an airplane, an ongoing goal the industry refers to as the “More-Electric Aircraft,” abbreviated MEA. Among other objectives MEA aims to remove the need for on-engine bleed air and hydraulic power generation, substituting the use of electrical and electronic systems to trim operating and maintenance costs as well as cutting the emission of pollutant gases from the aircraft.
To properly explain how this is being done first requires a little background.
Today’s civil aviation transports are propelled by high bypass ratio turbine jet engines. Bypass ratio is defined as the ratio of the mass flow rate of the stream passing outside the engine core divided by that of the stream flowing through the core.
Non-propulsive aircraft systems are typically driven by a combination of different secondary power types such as hydraulic, pneumatic, electrical and mechanical power, all extracted from the aircraft’s main engines in different ways. For a traditional jet turbofan producing about 40MW of thrust high pressure air is “bled” from the engine--bleed air is compressed air taken from within the engine, after the compressor stages and before the fuel is injected in the burners—and in this example provides some 1.2MW of pneumatic power. A gearbox driven hydraulic pump adds 240kW of hydraulic power, engine fuel and oil pumps account for about 100kW of mechanical power (mechanical power is obtained from the engine by a drive shaft and distributed to a gearbox to power various pumps) and gearbox-driven generators provide another 200kW of electrical power.
Electrical power, pneumatic and hydraulic power are distributed throughout the aircraft for driving subsystems such as flight control surface actuators, landing gear extension and retraction, brakes, in-cabin systems such as lighting and galleys and, in the case of military aircraft, weapon systems.
In a traditional commercial aircraft architecture, the engines provide the majority of secondary airplane system power needs in pneumatic form from a bleeding air compressor. The newer More Electric Aircraft, such as the soon-to-enter commercial service Boeing 787, uses electrical systems to replace most of the pneumatic systems now found on airliners. In the 787 electrical system power will be used to run avionics and provide icing protection, cabin pressurization and air conditioning, flight control actuation, landing gear power and energy for cabin systems (lights, inflight entertainment) among other ancillary aircraft systems use.
The “no bleed air” architecture of the Boeing 787 offers operators a number of benefits, including:
· Many electrical systems are inherently more efficient than their conventional pneumatic or hydraulic counterparts. For instance, losses in electrical cabling are lower than those in hydraulic or pneumatic piping. Electrical systems also are lower in weight than comparable pneumatic systems and offer reduced lifecycle costs.
· Electrical systems can be designed to provide a needed function on demand and only on demand. Currently central hydraulic lines are kept energized during the entire flight. However, some of the larger users of hydraulic flow, such as landing gear and secondary flight controls, require this power for only a short time. Electrical systems can be switched on and off as needed, thus conserving power.
· Pneumatic systems that divert high-speed air from the engines rob conventional airplanes of some thrust and increase the engine's fuel consumption. With all of the high-speed air produced by the engines now all going to thrust the airplane's engines can produce thrust more efficiently. According to Boeing the 787’s no bleed air systems architecture will result in a predicted fuel savings of about 3 percent.
On Boeing’s 787 a pair of large generators on each engine extract a total of 500 kilovolt-amps (kva) per engine. In all the 787 has six starter/generators—(two 250-kva. units on each engine and two 225-kva. units on the Auxiliary Power Unit) for a total generating capacity 1,450 kva.
The primary generators are directly connected to engine gearboxes and operate at a variable frequency (360 to 800 Hertz) proportional to the engine speed. The six-pole generators make the maximum 800 Hz. at 16,000 rpm and weigh about 200 lb. each.
The 787 uses an electrical system that is a hybrid voltage system consisting of different voltage types: 235 volts alternating current (VAC), 115 VAC, 28 volts direct current (VDC), and ±270 VDC. The 115 VAC and 28 VDC voltage types are traditional, while the 235 VAC and the ±270 VDC voltage types are the consequence of the no-bleed electrical architecture that results in a greatly expanded electrical system generating twice as much electricity as previous Boeing airplane models.
The 787’s 115 and 235 VAC is supplied with frequency varying from 360-800 Hz. For components that need the traditional constant frequency power, a small amount of 115 VAC at 400 Hz. is created. There will also be 28 VDC for avionics.
Brakes, ice protection, engine start and environmental control systems will all be electrical on the 787. In a major departure from previous aeronautical design practice the aircraft’s cabin will be pressurized by electric motors, too, and not by bleed air. Boeing claims that electric compressors are better suited for the cabin than engine bleed air.
The 787 also utilizes an electro-thermal ice protection scheme, in which several heating blankets are bonded to the interior of the protected slat leading edges. The heating blankets may then be energized simultaneously for anti-icing protection or sequentially for de-icing protection to heat the wing leading edge. Boeing says that this method is significantly more efficient than the traditional system because no excess energy is exhausted. As a result, the required ice protection power usage is approximately half that of pneumatic systems.
