You may not be aware of this interesting aeronautical fact: A modern jet engine is actually the world's largest Chicken-o-matic. Yes, that's right. As part of the testing process required by the FAA to ensure that aircraft engines can withstand certain types of bird strikes, chickens are fired from a cannon directly into an engine's inlet. Given that the turbine blades are spinning at around 15,000 rpm, these birds are pretty much cooked, so to speak.
Although PETA hasn't actually complained or anything, jet engine manufacturers would do practically anything to avoid these so-called bird ingestion tests. The cleanup is really messy and they're getting tired of chicken soup for lunch.
Just kidding. The real reason engine manufacturers don't like to do physical testing is that it costs big bucks. "Given that impact dynamics vary with velocity and that a bird can strike an engine anywhere, it could take years of testing to analyze half a dozen different blade designs," explains Joe Metrisin, a senior structural engineer at Florida Turbine Technologies.
So Joe-like many of his industry counterparts-has figured out a way to reduce the need for live testing by using simulation software to accurately predict the structural integrity of engine fan blades. ANSYS/LS-Dyna is his FEA package of choice to model bird strikes. He and co-worker Brian Potter describe their analysis work in an excellent and illuminating paper (click here ).
An animated view of a model bird strike.
Joe uses the simulation results to optimize blade geometry before building a single prototype. The upshot? In less time and lower cost, engineers are designing engines that meet FAA bird strike requirements.
One of the more critical aspects of the analysis is modeling the bird's material characteristics, which change with velocity. At really high speeds, the properties of a viscous fluid seem to work best-a fact that Dan Aykroyd could fully appreciate!