composites vs. aluminum

I know this gets talked about all the time...but I have aquestion I was
hoping someone smarter than me could help me with.

An aluminum evangelist showed me this link:
http://www.aerotalk.com/myth_02.cfm

The author asserts that:
" The ratio of empty to gross weight is one of the most telling measures of
structural efficiency.
The equations are basic:EMPTY WEIGHT + PAYLOAD = GROSS WEIGHT

Reduce empty weight by 100 lbs and the pilot can load an extra 100 lbs of
payload, fuel/ baggage/ people.

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.

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?

"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.

Think how heavy some RV's would be if the homebuilder had to roll his
own aluminum from billets...........or make his own plywood for wing
skins.....

It's all about quality control.
===================
Leon McAtee

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

You can do best with a combination.

Composites can make complex shapes and be used to make a slicker yet lighter
fuselage, but if you use a simple airfoil, it is tough to build lighter than
an aluminum wing without getting expensive.

A lot also depends on what speed the plane will be cruising at. The higher
the speed, the more important to be slick.

I would be interested to know at what speed do wing rivets really start to
hurt vs. the extra weight of a normal homebuilt composite wing. I would be
willing to take one of our more experience builder's best guess.



"John C" wrote in message
...
I know this gets talked about all the time...but I have aquestion I was
hoping someone smarter than me could help me with.

An aluminum evangelist showed me this link:
http://www.aerotalk.com/myth_02.cfm

The author asserts that:
" The ratio of empty to gross weight is one of the most telling measures
of
structural efficiency.
The equations are basic:EMPTY WEIGHT + PAYLOAD = GROSS WEIGHT

Reduce empty weight by 100 lbs and the pilot can load an extra 100 lbs of
payload, fuel/ baggage/ people.

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.

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?

John C wrote:

I know this gets talked about all the time...but I have aquestion I
was hoping someone smarter than me could help me with.



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.
You can compare the same airframe with Al alloy and carbon fiber:
Michel Colomban MC100: 202Kg empty (F-PECH)
Colomban Robin MCR01: 235kg empty (G-BYEZ)
http://www.avnet.co.uk/lts/pages/mcr1.htm

But then I read about the new 7E7, which is a largely composite
aircraft, thus lighter, thus more efficient.
Scale effect and industrial way to built, no experimental built.


How do I reconcile these conflicting pieces of information?

Basicaly, the composite carbon-epoxy is the best for strength. But, in
the case of MCR01, a carbon fiber skin for the wing, just enough for
loads is too thin face a little gravel. For durability, its need more
fiber, more epoxy, more weight. In fine, the skin is still in Al
alloy.


By
--
Gardan GY20 Minicab F-PRAZ
Philippe Vessaire Ò¿Ó¬