High-speed low-pressure turbine - MTU Aero Engines

"We like to use it regularly in the company," says Roland Lederer. "We use the programme not only for the development of new product technologies, but also for production and maintenance." One example he cites is the new production hall for turbine discs in Munich, which is one of the most modern in the world: It contains manufacturing technologies that were developed as part of the LuFo programme.

To understand his delight, you have to take a look at the engines of today's commercial aircraft. In many aircraft, the centrepiece is a gas turbine that generates the thrust. A fan, which looks like an oversized paddle wheel, sucks air into the engine at the front end. Some of it is compressed and burned together with the fuel in the combustion chamber, thus driving the turbines. The exhaust gases then shoot out at high speed at the rear end of the engine. This is called core flow in specialist circles. Another part of the air, however, flows past the outside of the combustion chamber. This is the bypass flow.

In modern commercial aeroplanes today, the core power hardly contributes to the thrust. It is mainly used to drive the low-pressure turbine. This in turn causes the large fan to rotate so that it can draw in as much air as possible and direct it into the bypass flow. The bypass, on the other hand, generates most of the thrust due to the large moving air masses. The secret of efficient drives therefore lies in the bypass flow. "To put it simply, you can say that the efficiency of aircraft gas turbines improves with larger bypass ratios," says Roland Lederer. But it's not that simple. This is because the small low-pressure turbine should run as quickly as possible in order to achieve high performance with good efficiency. However, the large fan must not rotate too high, as the very high blade tip speeds would otherwise lead to high aerodynamic losses and high noise emissions. The dilemma here is that the fan and low-pressure turbine sit on the same shaft. Until now, the solution has been for the low-pressure turbine to yield to the lower speeds of the fan. This usually results in long, heavy low-pressure turbines with six stages or more.

Decoupling for more efficiency

"Together with our partner, we took a different approach," he says. "We have decoupled the fan and low-pressure turbine speed using a gearbox." The idea already existed in the 1980s, he says. But in view of the low fuel prices at the time, it was not pursued any further. However, in times when sustainability is at the top of the list of aviation stakeholders, things are different. Now the decoupled low-pressure turbine can fully utilise its advantages. "Not only does it rotate three times faster than the fan, it also only needs three stages," summarises the aerospace engineer. "This significantly reduces its weight, increases its efficiency, reduces the engine's fuel consumption and also makes it quieter."

What sounds so simple, however, presented the MTU developers with several challenges. As is so often the case, the devil is in the detail. In this case, it was the centrifugal forces that a faster-running low-pressure turbine entailed. "The material, the mechanics, the gap dimensions - everything had to be adapted," he explains. "And of course we had to prove the suitability of our developments in various demonstrations." A key moment for him was the first test of the turbine on the height test bench at the University of Stuttgart. Under the various conditions that can be simulated there, the MTU experts had to prove the performance and improved efficiency of the development. This was achieved with flying colours and ultimately paid off. "With the high-speed low-pressure turbine and part of the high-pressure compressor, MTU now contributes around 18 per cent to the geared turbofan of the A320neo," says a delighted Roland Lederer. "And the LuFo programme has played a major part in this."