Modern Sculpture at Williams

Williams International's "power curve" display of its engines, from the 770-pound-thrust FJ22 to the 3,000-pound FJ44-3, is an education in the fine art of fan design. Not only do the fans of the different engines differ in size, but their blades display a wide variety of shapes depending on their size and when they were developed.

The complex shape of the blade reflects the fact that the blade is not doing the same job, in the same environment, at any two points along its span. Airspeed is higher at the root than at the tip, so the designers sweep the blade. The FJ44-3 blade is swept forward at the root and backward toward the tip, giving it a scythe-like shape. The FJ22 blade is simpler, with forward sweep all the way.

The inner part of the blade has a little extra twist and is rather stouter than the tip. It carries the stresses of the whole blade, and acts as a supercharger for the core rather than as a propulsor. It has another important job: a meat-chopper for any birds that get in the way of the engine. The new Williams fans have done a pretty good number on test chickens; remarks one engineer: "Parts is parts." The goal is to ensure that avian subassemblies that make it through the fan are too small to damage the smaller and more delicate blades in the core.

The FJ44-3 fan displays an extra twist at the tip-possibly a result of better understanding of the airflows at the tip-to-case boundary. Blade design has also been affected, generation by generation, by manufacturing technology. New machine tools make it possible to mill ever more complex shapes from solid forgings.

The final figures of merit in fan design are efficiency-which translates into more thrust from the same core power-and the ability to push more air through a fan of given diameter, which reduces the weight of the engine and the drag of the cowl. The 3,000-pound FJ44-3-not that much larger than the original 1,900-pound FJ44-1-shows how far the technology has come in little more than a decade.

By Bill Sweetman