Sparrho insight: What stops a jet engine from shredding up?

A first-time flier might feel the need to bless a 250-tonne aircraft with lucky coins, but the science is what’s keeping passengers from harm.

A Russian orthodox priest blesses the jet engines of a fighter plane

In 10 seconds? Ice, birds, and superstitious passengers are not the worst threats to jet engines. Materials called single crystal superalloys are now being used to stop the ‘creepy’ phenomenon of engine components changing shape under stress. (Check the science here)

Wasn’t there some story about a coin? Good memory. A passenger threw a coin into a jet engine for good luck last week, prompting a five-hour delay as engineers conducted a full re-inspection.

An airport worker is showing coins thrown at a jet engine at Shanghai International Airport. Source: Chinese Police

What would happen if that plane took off with the coin in its engine? The coin would have damaged the tips of the fan blades, leading to imbalance, and even severe vibration during the flight. Although modern planes can fly with one engine, it certainly wouldn’t have been an enjoyable journey.

What about that ‘creepy’ phenomenon mentioned above? Apart from foreign objects, plane engines are also subject to ‘creep’, which refers to a a gradual shape shifting of components like blades, fan discs and compressor parts due to the extreme heat inside heavily used jet engines.

Sounds dangerous! Indeed, if parts are not replaced, engines can seize up to disastrous results. Material scientists are developing single crystal superalloys, which have a higher resistance to creep due to their lack of grain boundaries. (See an example here)

There are grains in metals? Scientists use the word ‘grain’ to describe small crystals within structures with more than one crystal.
Such poly-crystallic materials are full of grain boundaries that can slip and slide, which leads to creep. (Read more here and see example of grain boundaries in the image)

Grain boundaries on a poly-crystal metal

No boundaries means no slip? Precisely so. Single crystal superalloys have a single grain and are therefore highly creep resistant, perfect for turbine components in the aerospace industry. (Learn more here)

Dead chickens and ice cannonballs: the extreme tests jet engines undergo before entering service

Manufacturers test their engines under extreme conditions to ensure that most of the time they can handle foreign objects that are sucked in.

‘Chicken gun’ test on a turbine engine

One particular test consists of firing dead chickens at running turbine engines, using ‘chicken guns’, to check that any component failure is contained within the housing of the engine. For example in case of a ‘blade-off’, detached parts must not escape the engine, as they could rip through the plane’s fuselage and endanger life.

Testers also spray water and shoot massive lumps of ice into running engines to mimic rainy and icy conditions.

This research was curated by Mark Docherty, Sparrho Hero and PhD student at the University of Glasgow, specialising in the thermo-mechanical behaviour of nickel-based superalloys.

Psst, Mark distilled 9 research papers to save you 311.2 min

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