From: firstname.lastname@example.org (Drakeal) Organization: America Online, Inc. (1-800-827-6364) Date: 21 Jun 95 02:57:01 References: 1 Followups: 1
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> I read in yesterday's New York Times that the >delivery of GE90 engines to Boeing for use in 777s >for British Airways could be delayed to due >failure of the engine in the 'bird ingestion test'. >In this test, an eight pound bird carcass is fired >from a cannon into an aircraft engine running at >full throttle. Evidently, the engine survived the >test but had unacceptable levels of vibration after- >wards. > The article went on to say that GE was >working on a redesign. I wonder what kind of design >features an engine could be fitted with that would >help to pass such a test? Presumably other engines >have passed this same test. Is anyone familar with >such features? There really isn't much you can do to shield the blades of one of today's high-bypass turbofans from birds and the like, not without killing your inlet efficiency. The trick is to make the blade strong enough to withstand the impulsive load of that incoming turkey. This is what I've heard: GE's ambitious hollow composite fan blade program was intended to put it leagues ahead of competitors Pratt & Whitney and Rolls-Royce in thrust/weight. But it ran into snags when they tried to certify it with the FAA. The agency gave them the choice of 1) 8-lb birdstrike test or 2) artifically separating a single blade at the root, whichever was tougher. GE chose (2). The FAA later found out that that was the lesser of two evils for GE and made them run (1). They lost not one but two blades at the root, and 3 more at partial span. The resulting imbalance reportedly ruined the rotor system. Boeing grounded the GE-powered 777. And there it sits. To make a blade birdstrike-worthy, you have to beef up the root of the blade such that when it is struck by a large object out at the tip, it will not initiate a big enough crack that it will fracture before you next get a chance to inspect it. The rules for how long it has to operate successfully after the 'strike vary by type of certification program, and I don't know what they were for the 777. The problem with composites is that they're difficult to analyze. The layered fibers that make them up have different strengths in different directions. This makes a crack propagation analysis really hard because as different layers tear, the load gets shifted in unpredictable ways. Metal blades, such as P&W's hollow titanium fan blades, are homogeneous; their properties are uniform throughout, so all you have to do is know the stress field. This is not to say that designing even a metal blade is simple; beefing up the root area of the airfoil decreases the aerodynamic efficiency of that area, so it's a compromise all the way. But P&W's is certified, and GE's is not; that is evidently making a difference to airline customers. No doubt composite blades are the wave of the future, but right now they are suffering definite growing pains. It's probable that a new generation of composite materials will have to come along before nonmetallic blades will become commonplace.