For MRO Network Daily, Chris Kjelgaard exclusively reveals another important first to be added to the CFM International LEAP engine programme’s long list of technological and production achievements.

Already one of the most successful commercial jet-engine programmes ever, before it has even entered service, the CFM International LEAP programme incorporates several new materials and production technologies which represent important ‘firsts’ for the civil aero engine industry.

Exclusively, MRO Network Daily can reveal another important technology ‘first’ not identified previously for the LEAP programme.

In an interview and e-mail statement, Gareth Richards, GE Aviation’s LEAP programme director, confirms to us that, for the first time in any commercial engine, the LEAP modulates the amounts of cooling air flowing to the internal air passages inside its high-pressure turbine (HPT) blades.

“We modulate the flow going to the internal blade passages of the HPT,” confirms Richards. “This is for performance gain, by having the capability to reduce flow during low-power phases [of flight] such as cruise.”

Richards explains that, during low-power phases such as cruise flight, the temperature of the exhaust gas flowing through the HPT stages after exiting the LEAP’s TAPS II advanced combustor is lower than when the engine is required to produce high levels of thrust, such as during take-off and climb.

As a result, the engine’s HPT blades don’t need as much cooling air to flow through their internal air passages during cruise to provide sufficient resistance to the exhaust gas temperature (EGT) as they do when the engine is producing high levels of thrust.

In modern commercial engines with high core air-compression ratios and advanced combustors which thoroughly mix core air and fuel into a homogenous vapour inside the fuel injector nozzles – so that combustion is complete, extremely efficient and produces low levels of emissions of noxious gases – EGT can reach 3,000 degrees Fahrenheit at maximum thrust.

Today’s commercial turbofan engines bleed relatively cool air (which nevertheless measures several hundred degrees Fahrenheit) from the engine’s high pressure compressor (HPC) stages to cool an engine’s HPT blades via the blades’ internal air passages.

Passing through the HPT blades’ internal passages and then flowing out through dozens of tiny holes on the surface of each blade, this cooling air creates a thin boundary layer of air above each blade to protect it from the intensely hot temperature of the exhaust gas flow.

Together with advanced thermal barrier coatings applied to the surface of each HPT blade, this cooling air prevents the blades from eroding and even melting in the exhaust gas flowing from the combustor.

But, vital though the HPT blade cooling air is, drawing it off from the HPC reduces the amount of core airflow through the engine’s combustor and turbine stages. This reduces the engine’s overall thermal efficiency and thus also the engine’s overall fuel efficiency.

By modulating the amounts of cooling air going to the HPT blades’ internal passages during different phases of flight, so that each blade receives less cooling air when it doesn’t need it, an engine’s fuel efficiency can be improved.

However, until the LEAP, all commercial jet engines have directed the same amounts of cooling air to their HPT blades during low-power phases of flight as during high-thrust phases of flight.

CFM has solved this challenge in the LEAP, in developing a new generation of full-authority digital engine control (FADEC) software, which it calls FADEC 4.

According to Richards, modulation of HPT blade cooling air in the LEAP is performed by the FADEC software running each LEAP engine’s dual-channel FADEC 4 engine control unit (ECU).

Each ECU is a powerful computer which analyses hundreds of operating parameters, many times per second. It then uses this data to adjust electronically a myriad of engine operating parameters, to optimise engine performance at all times.

Richards also reveals that, in addition to the LEAP FADEC 4’s ability to modulate HPT blade cooling via the blades’ internal air passages, its software also allows two other types of air-cooling modulation of the LEAP’s turbine stages.

“We also separately modulate the cooling to the outside of the HPT case to control [blade-tip] clearance of the HPT; and we separately modulate cooling to the outside of the LPT (low-pressure turbine) case to control LPT [blade-tip] clearance,” says Richards. “All three [systems] are independent and all are controlled by the FADEC.”

Other new-generation engines – such as the Rolls-Royce Trent XWB – also use modulated air-cooling of turbine cases.

These techniques involve modulated cooling – through a series of manifolds – of the turbine casings surrounding the HPT and LPT (or intermediate pressure turbine, in the Trent XWB) in order to maintain optimal clearances between the blade tips and the turbine casings during each phase of flight.

Additionally, these turbine casing modulated-cooling techniques are also designed to reduce the amount of MRO required to repair or replace worn turbine blade-tips.

These techniques work almost in an opposite way to the modulation of the HPT-blade internal air cooling employed in the LEAP.

Reducing cooling through the manifolds while the aircraft is manoeuvring on the ground allows the turbine casing to expand, providing what one Rolls-Royce executive describes as “reasonable clearance” between the turbine-blade tips and casing so that the tips do not wear.

In flight, increased amounts of cooling air are sent to the turbine casings, contracting the casings and reducing the clearance between them and the turbine-blade tips, particularly during cruise.

This helps optimise the efficiency of the engine in all phases of flight. It does so by minimising the amounts of air able to flow between the blade tips and the casing, which is air lost in terms of doing productive work.

CFM’s LEAP engine is also known for two other technological design ‘firsts’. One is its all-carbon-fibre fan, made from a carbon-fibre hub and 18 highly three-dimensional carbon-fibre blades.

No commercial aero engine as small as the LEAP (which is regarded as mid-size) has had a carbon-fibre fan until now, because no carbon-fibre fan blade could be made of that relatively small size that was strong enough to withstand the enormous stresses upon it during periods of maximum rotation, such as during take-off.

However, CFM developed a new carbon-fibre fan blade whose design involves weaving individual carbon-fibre strands on gigantic Jacquard looms into a complex, three-dimensional laminate and infusing epoxy resin into the structure by means of a proprietary transfer moulding technique.

This creates what Richards describes as a “fifth-generation” carbon-fibre fan blade. Each individual blade contains 7 kilometers (4.35 miles) of carbon fibre. After being cured in an autoclave, the finished blade is so strong that an entire Airbus A350 XWB could be suspended from it without the blade breaking, according to Richards.

In another technological ‘first’ for a commercial-aircraft engine, CFM has employed ceramic matrix composite (CMC) material in the LEAP.

The first-stage HPT shroud in every LEAP engine is made of a CMC material consisting of silicon carbide fibres within a silicon carbide-and-graphite matrix. (The material is known as SiC/SiC.)

Each shroud is a ring of 36 tightly fitting, white-coloured CMC parts. Together, these form a ring round the inside of the HPT casing outside the circumference of the first HPT rotating stage. This creates a snug fit between the tips of the HPT blades and the shroud, preventing exhaust gas from leaking round the blades beyond their tips and thus increasing the stage’s efficiency.

Two other LEAP ‘firsts’ – both involving new production technologies – are worth mentioning.

On its LEAP assembly lines in France, Safran Aircraft Engines uses laser projectors at each assembly step to project holograms on to the engine of the parts or assemblies required at each location. This shows mechanics where and in what orientation each part should be installed.

Also, instead of having humans inspect each assembled LEAP to assess its production quality, Safran Aircraft Engines is using camera-bearing robots to compare every area of each engine with reference images the robot’s database holds of the corresponding area on a fully approved engine.

 

The robot photographs any non-standard areas it sees, highlights these areas within red lines on the image and then sends the image to a human inspector for further action.