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

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?

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

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¬

On Thu, 29 Apr 2004 16:35:41 -0400, Todd Pattist
wrote:

"John C" wrote:

The author asserts that:
" The ratio of empty to gross weight is one of the most telling measures of
structural efficiency.
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

They don't conflict. The author acknowledges that lighter
weight is possible with a composite if you use advanced
composite techniques. The 7E7 uses such techniques, but
they are expensive. Modern racing gliders and small
composite aircraft are not built using those techniques, and
they don't use composites to get weight reduction. They use
composites to get smooth curves, low drag and high speed for
about the same weight.

The author's "structural efficiency" formula ignores drag,
and that's the main concern for those using composites.
Todd Pattist

I agree with Todd; further, the ratio comparison likely is significant
only within similar types of aircraft. I suspect the advantages of
one construction technique over another change rather significantly
from ultralight to glider to SEL to MEL to heavy.

Have we any experts around to comment?

On Fri, 30 Apr 2004 15:29:22 -0000, "karel adams"
wrote:

Does this imply that a slow & sturdy aircraft does not
profit as much from the composite advantages
and hence can better be built from aluminium?
And that likewise a sleek fast tourer better be composite?

KA (learning slowly)

That sounds pretty right Karel. Real world laminar flow did not
really begin to happen until the advent of super smooth composite
airplanes. Laminar flow isn't something an airplane that does not
cruise faster than 130 kts or so needs.

The P-51 Mustang is well known as one of the first fighters to make
use of a laminar flow wing. Many ascribe it's long range and high
speed to the wing design. In fact it very likely did (then and now)
not achieve much laminar flow for several reasons. It was discovered
that even slight imperfections in the wing caused the laminar flow to
trip to turbulent. Dents, scuff marks from ground crew, patches, butt
joints in the aluminum sheeting all caused the laminar flow to trip to
turbulent. In addition, it was found that the area within the
propwash was not laminar. The Mustang had a mighty big prop that
washed about a quarter to a third of each wing.

So achieving laminar flow isn't easy. Getting attached laminar flow
is one of the big reason (as I understand it) why Burt Rutan designed
the rear engined EZ series of airplanes.

Corky Scott

So achieving laminar flow isn't easy. Getting attached laminar flow
is one of the big reason .....

This brings up a question Ive had..

A laminar flow wing is better than a non one.....

At what speeds does the advantage become significant? Or at what speeds does it
really pay to opt for a laminar wing?

And... is a laminar wing that happens to be dirty etc and not working in a
laminar fashion STILL better than its non laminar from the start counterpart
wing at the same speed?

take care

Blll

"BllFs6" wrote in message
...
So achieving laminar flow isn't easy. Getting attached laminar flow
is one of the big reason .....

This brings up a question Ive had..

A laminar flow wing is better than a non one.....

At what speeds does the advantage become significant? Or at what speeds
does it
really pay to opt for a laminar wing?

Laminar flow is easier to achieve at high Reynolds numbers and the Reynolds
number increases with speed. See:
http://www.efunda.com/formulae/smc_f...c_reynolds.cfm

Therefore the answer is that a laminar wing pays off at all speeds but is
most effective in the "drag bucket" of the airfoil in question - that is
within a range of AOA where extensive laminar flow is achieved. This range
almost always extends below the AOA used for cruise flight and almost up to
the stalling AOA.

And... is a laminar wing that happens to be dirty etc and not working in a
laminar fashion STILL better than its non laminar from the start
counterpart
wing at the same speed?

All wings have some laminar flow and none have all laminar flow. The more
you have, the better. Wing sections designed to have a large amount of
laminar flow (Laminar airfoils) are always better. All airfoils are
degraded to some degree by surface roughness. Even laminar airfoils that
are very sensitive to surface roughness will be better than one not designed
for extensive laminar flow.

It's better to think of airfoils as better or worse and not to group them
into laminar and non-laminar. Since WWII, almost all new airfoils have been
designed with the goal of achieving as much laminar flow as possible when
used in the intended application.

Bill Daniels

karel adams wrote:

schreef in bericht
...
On Fri, 30 Apr 2004 15:29:22 -0000, "karel adams"
wrote:

Does this imply that a slow & sturdy aircraft does not
profit as much from the composite advantages
and hence can better be built from aluminium?
And that likewise a sleek fast tourer better be composite?

KA (learning slowly)

That sounds pretty right Karel. Real world laminar flow did not
really begin to happen until the advent of super smooth composite
airplanes. Laminar flow isn't something an airplane that does not
cruise faster than 130 kts or so needs.
(...)
So achieving laminar flow isn't easy. Getting attached laminar flow
is one of the big reason (as I understand it) why Burt Rutan designed
the rear engined EZ series of airplanes.

OK. If one wanted an easy-to-fly tourer, cruising at 120 kts or so,
wouldn't it be a nice compromise to build the wings in composite
and the fuselage in aluminium?
Are there homebuild desgins like this?

Karel


Corky Scott

Like the Glass Star? Steel tube frame, composite fuselage skins,
and stressed skin aluminum wing.

Richard