Jet Engine > Fan Section
At the very front of the engine is the Fan Section. This is where cool air ranging from -50ºC to +50ºC is taken into the engine from outside. This temperature range isn’t the challenge in this case, but consider what else these components experience. Being the intake for the engine, fan blades and discs are exposed to foreign objects such as dust, rain, and even birds. This means the fan section needs to be corrosive resistant and impact resistant. When spinning, the centripetal force on the fans blades is typically about 100 tonnes, then you have to also consider vibrations. This means the fan section needs to be able to cope with Low and High Cycle Fatigue (LCF &HCF). Furthermore, when you look at a jet engine the fan blades are big. Big & strong generally means heavy, and heavy means a less efficient jet engine. So making the fan section lightweight is also important.
To design components that operate effectively under these conditions, two approaches are used. The first approach is to develop and refine the material from which the fan section is made from. Currently the dominant material used for the fan section is Ti 64 or Ti-6Al-4V, which derives its performance from a bimodal microstructure. This material has been in use since the 1950s and is still being researched and developed for better performance. In some some modern blades are made from composites but the majority are still titanium alloys.
The second approach to improve the fan section performance is to optimise the fan blade design, as reducing the weight of a blade can be achieved by simply using less material. This has its own challenges as the thinner a component becomes, the less structural integrity it has. It is not only strength that needs to be considered, but also creep, fatigue, corrosion, and introduction of weak points due to the design. The following animation looks at the technological design leaps made going from solid first generation blades used in the 1950s through to 4th generation swept DBSPF ‘girder’ blades. The first generation blades use snubbers that protrude from the blade to prevent it from twisting. As engines got more powerful and faster, this design meant the fan blade was too flexible so it was replaced by a hollow blade with a honeycomb structure. The hollow blades reduce weight whilst maintaining rigidity and strength. Reducing the weight further, the honeycomb was replaced by a girder structure in 3rd generation blades. The latest generation of blades have an additional ‘swept’ design stemming from a greater understanding of aerodynamics through better computing and simulation.
The fan section is not just the blade but also the disc the blades connect to. The fan disc transmits the rotations from the shaft to the blades and so has an important job which can result in stresses equivalent of up to 100 tonnes. When we think about a single flight, the engine is turned on the and fan disc experiences stress which then is removed when the engine is then turned off after landing. This sounds like a single LCF cycle and so when choosing a material we look at how it performs in LCF tests. For the disc we need a material that resists both crack initiation and propagation in equal measures. So looking in the cupboard to see what we have, materials with bimodal microstructures have good LCF properties and turns out we already have one ready to go, Ti 64 which is the same stuff we make the blades out of.
The final part of the fan section which is really the very first part, is the nose cone. The nose cone of a jet engine is is made of a composite normally glass fibre bound by resin. It has to be able to cope with dust, debris and birds. The most interesting feature of this component is the very tip of the cone. Flying through the air high in the atmosphere it is cold, and so the tip of the nose cone builds up ice. To stop this the very end of the nose cone is made of rubber as it flexes and breaks up the ice as it forms.
Just for fun…
Flying through the air means a jet engine may cross paths with other things in the air. To check if an engine can handle such an incident, a number of dead, unfrozen birds are thrown into it. The bigger the engine, the larger and more numerous the birds it has to be able to cope with. There are 4 typical bird-strike tests. The first is small birds weighing about 0.25kg. These birds don’t cause a lot of component damage but can become stuck in the engine disrupting the air flow. The second test is for a medium sized bird weighing 1.1kg, approximately dinner for 2. A total of 4 birds are fired at the same time targeting the most vulnerable radius of the fan section. Getting to the family sized bird, the third test involves a single 3.6kg bird fired once again at the weakest radius. The final test is a 2kg bird fired at the the weakest point on the whole engine. The engine must be able to continue running safely after each of these events, so next time you order the chicken on a flight, look out the window and take a moment to honor its comrades whom sacrificed themselves for your safety.