Air Transport World

Airframes 30% lighter in the 1990s; that's the promise of lightweight fiber-resin composites and aluminum-lithium, but improved skills in non-destructive testing are needed.

Dusseldorf--Sessions about lightweight substitutes for conventional aluminum alloys in aircraft structural applications attracted considerable attention at the Airmec '85 conference earlier this year. And with good reason.

Carbon fiber reinforced plastic composites (and newly developed aluminum-lithium alloys) are lighter than conventional aluminum alloys of equivalent strength and promise a considerable reduction in airframe structural weight. Lighter weight means lower fuel consumption and improved performance. With composites, weight saving could be as much as 30% with a corresponding reduction in fuel consumption of about 15% in aircraft delivered in the mid-1990s, NASA estimates. With aluminum-lithium, the weight saving could be 15%.

The case for composites is not only that they are lighter than aluminum but they lend themselves to fabrication processes than can greatly reduce the number of parts in a built-up structure, and therefore can reduce assembly costs. They also are corrosion-free. The case for aluminum-lithium is that it can be machined and fabricated basically the same as conventional aluminum alloys and requires no great redesign. The drawback to aluminum-lithium is that it is expensive, so its future rides largely on the importance of its lighter weight.

Successful experience

Successful experience with composites in military and commercial aircraft has pretty well assured their more extensive use in structural applications in future airliners. Much has been learned about the design, fabrication and behavior of these materials. There have also been advances in understanding the mechanisms of failure and the development of non-destructive techniques for detecting and evaluating damage. But there is a way to go before the body of knowledge about the airworthiness of structural composites matches that which has been accumulated about aluminum.

The oil crisis in 1973 provided a major push, led by the National Aeronautics and Space Administration, to develop composites for aircraft structural use. Since that time, refinements in formulations, and production and fabrication techniques have produced a steady improvement in properties. Engineers have developed design guidelines, especially for critical areas such as joints and cutouts, although there is much yet to be done. And extensive static test and flight experience has produced a growing confidence in the suitability of composites for all types of commercial aircraft structures. Their suitability for secondary, relatively unstressed structures is well established by now. The next big step is to prove them out in primary structures.

Hard to produce

Aluminum-lithium is not as far along the development path. It is tricky to produce the alloy--lithium is a very reactive metal and the molten alloy must be handled with great care to prevent explosions. Only in the last couple of years have production difficulties been mastered and plates, sheets and extrusions become available for evaluation by the industry.

NASA's composites program

NASA's program was begun in 1976 under the Aircraft Energy Efficient (ACEE) Composites Office. The program's objectives are to develop the know-how for predictable designs and low-cost fabrication, to accumulate enough test and manufacturing experience to accurately predict durability for product warranty purposes and costs for product pricing, and to assure safety for FAA certification, and maintainability for acceptance by the airlines. Under NASA's sponsorship, the work has been contracted out to the major U.S. transport manufacturers. …

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