Hershey bar wing vs composite wing - how much drag?

DR wrote:
Hi,

I don't see why a composite should be heavier:

For carbon composite, the Young's modulus is ~70GPa for a density of 1.3
g/cm3. Al has the same Young's modulus but twice the density (2.7
g/cm3). For glass the strength is about half but again the weight is
halved too -so it's not a gain over Al. I think the composites excel in
their lack of rivets and joining pieces tho...

Cheers MC


If strength were the only issue, you'd be right on.

But there is also the question of stiffness.

Composite structures tend to get strong enough long before they
get stiff enough.

Then there is the "margin of safety".
Metal and wood wings are designed to a 50% MS.
Composites tend to go to 100% extra.
That alone means more weight.

Richard

BobR wrote:


From wha I have read in the past, the major reason for lack of weight
reduction in composite structures results from differences in the
design standards. The design standard for metal wings is based on a
1.5 times specification. Thus, a wing rated for 3g's is designed for
4.5 g's. The standard used for composite wings has been set at 2
times specification. The composite wing rated for 3g's is designed
for 6g's and as a result any weight savings is lost to the extra
strength. The difference in the standards was ment to compensate for
perceived quality variations in composite contstruction techniques.


The main reason for the 2x standard has to do with the fiber alignment
(or rather misalignment) of the laminations in the spar. Since this is
the single heaviest, and most important component of the wing, its
construction is critical. Unfortunately, with traditional wet layup
techniques, perfect alignment of the fibers in the spar is not possible,
thus decreasing its strength. The obvious solution recommended in the
books is to increase the design over design to compensate.

Not too long ago, I saw that someone had solved this problem by using
small diameter, precured carbon-fiber rods as the core material for the
spar. This solves the disadvantages of the traditional techniques.

"Evan Carew" wrote in message
t...
BobR wrote:


From wha I have read in the past, the major reason for lack of weight
reduction in composite structures results from differences in the
design standards. The design standard for metal wings is based on a
1.5 times specification. Thus, a wing rated for 3g's is designed for
4.5 g's. The standard used for composite wings has been set at 2
times specification. The composite wing rated for 3g's is designed
for 6g's and as a result any weight savings is lost to the extra
strength. The difference in the standards was ment to compensate for
perceived quality variations in composite contstruction techniques.


The main reason for the 2x standard has to do with the fiber alignment
(or rather misalignment) of the laminations in the spar. Since this is
the single heaviest, and most important component of the wing, its
construction is critical. Unfortunately, with traditional wet layup
techniques, perfect alignment of the fibers in the spar is not possible,
thus decreasing its strength. The obvious solution recommended in the
books is to increase the design over design to compensate.

Not too long ago, I saw that someone had solved this problem by using
small diameter, precured carbon-fiber rods as the core material for the
spar. This solves the disadvantages of the traditional techniques.

Jim Marske has been involved in sailplane construction for many years. I
believe he was one of the first to use carbon rods in the spar caps. Check
out his website for more information: http://marskeaircraft.com/

Aluminum wings can be "profiled" with performance results close to a
composite wing. (http://tinyurl.com/2r8b7d) The time involved is such a
project is normally 400+ hours.

Wayne
HP-14 N990 with profiled aluminum wings
http://www.soaridaho.com/Schreder/N990_Near_Arco.jpg

"cavelamb himself" wrote ...
At these speeds I suspect surface condition is a small part of the
overall drag.

However!

If the new wing were a couple hundred pounds lighter, then you'd
see some inprovement in speed.

It takes power to stay aloft.

The heavier the plane, the more power is required just to stay up.


Richard,

That's not really true for a light airplane. The only place weight shows up
in the drag equation, and thus the power equation, is in the induced drag
term.
But,because the wing on a light airplane is relatively large, the induced
drag at cruise is small. Cruise induced drag is lift coeffients squared
divided Pi e Aspect Ratio. Light airplanes cruise at small lift coeffients
of around 0.1 to 0.2. It can be shown that they will fly the farthest on a
pound of fuel at L/D max. Lift coeffients around 0.6 to 0.8. So, an
increase in airframe weight doesn't increase the cruise power requirements
very much.

Of course, an light airplane could be designed to fly at L/D max but the
wing would be tiny and you'd pay for it on the slow speed end. With a
single engine and relatively inexperienced pilots, it would be a handful at
slow speeds. Both the BD-5 and the Questar venture are examples of under
winged airplanes that have poor engine out safety records.

Where weight does show up is in climb performance. One of the things that
make an airplane "fun" is how well it climbs. You don't spend much time
there in a cross country flight, but a large high aspect ratio wing with
lots of power will give the pilot the feeling that the airplane is a good
flying airplane.

One of the problems I've had in the past is how much should a designer try
to protect a future user of a product? I've decided that a minimalist wing
is a bad design in the light plane market.

Rich

DR wrote:
Hi,

I don't see why a composite should be heavier:

For carbon composite, the Young's modulus is ~70GPa for a density of 1.3
g/cm3. Al has the same Young's modulus but twice the density (2.7
g/cm3). For glass the strength is about half but again the weight is
halved too -so it's not a gain over Al. I think the composites excel in
their lack of rivets and joining pieces tho...

Last I knew, Young's modulus was a measure of stiffness, not strength.

Matt