Sunday, February 19, 2012

Nature of Efficient Design

"designs will fail if subjected to overload...that's just the nature of efficient design"

Broken Lycoming crankshaft at the fillet
Broken crankshaft at the fillet

Designs may not fail immediately;  this is the trap that entices in the consumer--the desire for more power, greater performance than designed; or a popular performance modification.

 It's delayed failure due to metal fatigue that kills. A seemingly successful patch, a popular engine modification, an antique airplane used for aerobatics, antique air tankers:
  • Aloha Airlines Flight 243
  • Japan Flight 123 520 people killed
  • N2969 Turbo Mallard Wing Breaking Off
  • China Airlines Flight CI-611  225 people kilIed.

A flight-proven repair, a proven design, long years of successful service were all present in the accidents listed above. Recently I received an inquiry from someone who was surprised that his crankshaft had broken. In talking to him he proudly listed all of his "performance" upgrades that had been done on his engine. Why was he surprised that his crankshaft broke? The only way it would NOT break is if the engineer who designed the crankshaft was inefficient in his design. 

Food for thought:  Engine horsepower output over time can be no greater than crankshaft fillet fatigue strength. Failures generally pinpoint weak links.

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