Thread: Cessna C337 SID
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Unread 08-20-09, 06:26 PM
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Ernie Martin Ernie Martin is offline
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The two links contained in one of the above messages were very informative.

The one important element that may be pertinent to the upcoming meetings and where we have critical data to contribute is cabin weight.

This is somewhat simplified, but in the first of the two links, Mike Busch makes the case that Cessna's analysis seems to have assumed large cabin loads and low fuel loads in the stress analysis, yielding fatigue lifes which are much shorter than if it had assumed low cabin loads and high fuel loads*. I believe that 337s are mostly operated with low cabin loads (often with 1 or 2 occupants) and often on long-endurance surveillance missions (with large fuel loads). I know that I never use my 5th and 6th seats. We must try to get Cessna to consider this, which will result in much longer safe life if the 337 wing structure and stress distribution bears some similarities to the 400 series.

Ernie

* "The tensile stress on the critical wing main spar elements is a function of cabin load (i.e., zero-fuel weight), not of gross weight. In fact, fuel weight (which is outboard of the “ hot” region) actually reduces the stress. It turns out that fatigue life is exquisitely sensitive to changes in tensile stress—a small reduction in stress can result a big increase in safe life" (abreviated excerpt). Yet Cessna proposed the same safe life for the 401 models (which came with seating for 6) and for the 402 models (which came with higher-density seating of up to 11 passengers). And Cessna did this despite the fact that "there are six known instances of 402s with cracked spars, but no known instances of 401s with cracks."
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