On 29 Apr 2004 18:32:35 -0700, (Leon McAtee)
wrote:

"John C" wrote in message ...
EMPTY WEIGHT/GROSS WEIGHT = WEIGHT EFFICIENCY RATIO

The lower the ratio, the more efficient the design."

He goes on to use this determination of strength/weight (or, structural
efficiency) to determine that composites do not offer a greater
strength/weight ration in airframe construction applications.

Just how did he do this?

But then I read about the new 7E7, which is a largely composite aircraft,
thus lighter, thus more efficient.

How do I reconcile these conflicting pieces of information?


The information is not necessarily conflicting. You can build heavy
out of any kind of material. It's just that composite planes are so
easy to build overweight compared to other materials.

Also most of the homebuilt moldless composite planes are way over
built/designed due to quality control issues. The plane must be
designed for the worst case builder and thus ends up just strong
enough when built by a poor craftsman and heavier than it could have
been if all the builders were good craftsmen. The double bite comes
when the poor craftsman uses too much resin and filler. Not only is
his weaker than it could have been, it's heavier than one built by the
good craftsmen.

When you have good quality control you can design the composite part
to tighter standards, and end up with a very efficient structure.


the problem for composite structures in the past has been their
unknown fatigue life. they are typically built to much higher safety
margins than aluminium aircraft which have been a more understood
technology.
if composites were built to the same margins as conventional aircraft
you'd see them being a lot lighter and there'd be less of a conundrum.
Stealth Pilot
Australia