05 February 2016, © Leeham Co: In recent Corners, we looked into technologies which have made the new breed of airliners more efficient.
We’ve talked about how new engines can raise efficiency by about 15% and how aerodynamic improvements, like more efficient split winglets, can add another 1%-2% over single blade winglets. We have also looked into modern ways to manufacture the more resilient and lighter composites structures that designers want to use to increase aircraft efficiency.
There is one area which we have not covered: the aircraft’s systems and how these can be made more efficient. An improved system architecture can add the efficiency improvement of a split winglet. So let’s have a look at the trends in aircraft systems.
We start this week with power distribution.
Power to those that have a need
A civil airliner has three ways to deliver the power that the aircraft’s different systems need. It can be hydraulic, electrical or bleed air based systems that take power from the engines and route it around the aircraft. It is important how the power gets to its users. Why is easy to understand when one observes what happens to the aircraft’s engines efficiency when power gets zapped to feed the aircraft’s system.
At normal cruise conditions, the engine fuel consumption is raised by around 5% because of the power needed to drive things like movables, lightning and air condition. A 5% efficiency loss is major and any potential for improvements in how power is distributed and consumed will be welcome.
Let’s go through the different ways and see what gets done.
Each engine has one or two hydraulic pumps which supply power to the aircraft’s hydraulic systems (an aircraft normally has two or three independent hydraulic systems, and sometimes four, for safety reasons). The hydraulic systems have recently raised pressures from 3000 PSI to 5000 PSI to reduce pumping losses (less fluid needs to be moved, so less frictional loss) and increase power density. The high power density makes hydraulics suitable to power the aircraft’s movable control surfaces (stabiliser, rudder, ailerons, spoilers), landing gear and brakes with comparatively small and light servos.
Further improvements in efficiency is more to do with simplifying the way system redundancy is achieved than fundamental changes in how the hydraulics system works on aircraft. More and more electrically driven extra pumps or servos remove the need for redundant extra hydraulic systems for the aircraft (on top of the normal two). It thereby lowers the aircraft’s weight.
Another system with high power density is bleed air from the engine’s compressors. This bleed air gets cooled and regulated to 200-250°C and 275kPa/40 PSI in the engine pylon’s pre-cooler. It is then routed to users around the aircraft, Figure 1.
While bleed air is a simple way to get compressed air to the aircraft’s air conditioning system or heat to de-ice the wings, it requires cooling away energy in the pylon pre-cooler and it means routing 250°C air around the aircraft. Any leaks can destroy electrical cabling or even weaken aircraft structures. Therefore leak detection systems are required and different shut-off or rerouting valves need to be operated by the pilots should a leak occur.
More electrical system
Boeing’s 787 was the first aircraft project to say “there is a better way of doing this; we replace the bleed power with electrical power.” It is simpler and more efficient, Figure 2.
Boeing claims that the more electrical distribution of power is up to 3% more efficient than a mixed bleed and electrical system. Aircraft designers who have stayed with the classical mixed system say the gains are less but don’t dispute there are gains.
The reasons they haven’t jumped on the more electrical bandwagon is because there are problems before there are gains. Every major change in an architecture has a learning curve and the reliability problems seen during initial operation of the 787 has in no small part been caused by the system changes caused by a more electrical architecture.
Electrically driven air conditioning compressors have failed and the massive power electronics required have given trouble. The power electronics is needed because classical aircraft electrical systems had the main power, 115V 400Hz AC, created by constant speed Integrated Drive Generators (IDG). These delivered 400Hz power regardless of engine RPM.
Such IDGs are expensive, heavy and require maintenance. Therefore, modern aircraft, including the 787, bolt AC generators straight to the engines gearbox and out comes varying frequency 235V power.
Systems that don’t care about frequency consume this power directly, like de-ice mats or heaters in galleys. Others that need the stable 400Hz get that supplied via solid state power converters that make both 115V 400Hz and 28V DC from the AC power. For classical mixed architectures with AC generators, these converters handle reasonable power levels, with each engine generator delivering around 150 KVA and part of this is converted.
The generators of a 787 delivers up to seven times that, or 1,000KVA, and the conversion electronics are now housed in water-cooled large racks. These new conversion racks have created problems and many user systems that were air powered but now changed to electrical power have added their initial problems.
Evolution in aircraft systems goes in steps. There is always an advantage to stay with a known system and be second with a new system; one does not have to go through the learnings the first mover endures. The more electrical architecture of the 787 has created a major part of the problems the aircraft has endured. A well-known example is the Lithium Ion batteries.
Where other programs could change to the older NiCad batteries, Boeing was forced to make the Lithium Ion batteries work. The more electrical architecture changed the aircraft’s brakes from hydraulic brakes to electrically operated ones. The safe stopping of an aircraft with no engines running then requires the type of power curve that only a Lithium Ion battery could deliver (within a reasonable size).
The more efficient more electrical architecture has had its gremlins. But there is no doubt it is the way to go. An efficiency gain of just 1% makes it interesting and Boeing claims more. It is just a matter for other aircraft projects to pick the time when they think the technology is ripe and then get off the fence.