I am working on the design of the carbon fiber six inch diameter
tube spar.

The spar is the heaviest part of the aircraft structure so it pays
to make it out of expensive carbon. The skins of course can be glass
or carbon depending how much money you want to spend.


Paul Lamar

Paul, I am sure you are aware of the method that Paul McCready used
to make the carbon fiber spars for the Gossemer Albatross:

He used Aluminum irrigation tubing (~3-4" D) and laid the carbon
fiber up on the tubing, then dissolved the Aluminum out with KOH.
Bill Schertz

I was called in as an aerodynamic consultant on the one that
eventually won the Kremer prize by Jack Lambie round about 1976 or
1977. My girl friend Rhonda and I drove up to the valley to have a
look. I was surprise to see a propeller about 4 or 5 feet in
diameter made from two model airplane wings and wire braced. I
suggested a larger diameter prop not wire braced and further geared
down. They did that and went on to win the $50,000 prize :-)

I was not aware he built the spar that way. I was thinking of using
a metal or plastic tube covered with several layers of garbage bag
or other thin plastic material cut in long strips. After the epoxy
is cured the spar can be slipped off the tube. I have done that many
times on shorter composite tubes. Sometimes the strips can just be
pulled out from between the surfaces. That gives enough clearance to
remove the spar from the tube.

How's the Kiss Cruiser doing? Do you like living up there in the US
bush? :-)

Paul Lamar


Paul you can also use braided carbon fiber. You get it in a flat
role with different diameters. This way the "braided fibers" are in a
more or less 45 bias in the weave to the span of the spar. Do 2
layers then dremel cut out of the male plug ( split the carbon pipe)
to get off the male plug. Then do several layers braided over the
"first split mold" to the different stations according to the bending
loads and specs Vacuum bag each layer as you progress and scuff
between layers to get nice finished surface between layers

Le Roux Breytenbach ZA


One needs some unidirectional fibers on top and bottom of the tube
to take the high bending stresses. These should be layered with the
thickest layers near the center of the wing.

Paul Lamar


Spar caps can use pultruded carbonfibre rods
http://www.marskeaircraft.com/carbon-rod.html <http://www.marskeaircraft.com/carbon-rod.html>

Carbon Rod - Marske Aircraft Corporation

www.marskeaircraft.com <http://www.marskeaircraft.com> <http://www.marskeaircraft.com Learn more
about Graphlite Carbon Rod, Pricing, and how place an order with
Marske Aircraft Corp




Le Roux Breytenbach


Hi Paul, Round tube spars are usually an inefficient use of
materials for a spar! The only bits taking the loads are the upper
and lower one eights. Why not a box spar or an I beam? Ray Tolhurst

Aw so that is assuming the wall thickness is constant around the
circumference :-)

The I beam is hard to layup. The round tube is the cheapest male
mold.

However I am leaning toward a tapered box tube sections made from
.032 thick  5052 ten feet long then tig welded together to make a 20
foot long male mold. The welded joints are then sanded.

It is hard to find 20 foot look press brakes and 20 foot long sheets
of aluminum.

If you seal the small end with a tig welded cap. The aluminum male
mold will pop out of the cured spar when water pressure is applied
to the small end.

Patent pending :-)

BTW Another reason for rectangular or square tube spar is it
transmits the torsional load from the ribs into the spar better. It
is a simple matter to layup angle reinforcement layers to do this as
they are linear and not curved. In fact one may find pultruded angles
that can be cut in short pieces..

http://dragonplate.com/ecart/categories.asp?cID=34 <http://dragonplate.com/ecart/categories.asp?cID=34>

Check out Pitching Moment  Characteristic in Von Doenhoff page 179.
Also page 214 for Fowler flaps. I am looking for a C sub L of over
2. My C182 is fantastic in that regard.

BTW I am thinking of offering plans if you have Solid Works or
Rhino.

Paul Lamar


Paul


I do have an idea to built  by using a big pvc pipe which the inside
diameter meet your requirement

Cut it in half and install over a long steel angle or steel channel
to keep it straight,

Wax it inside and  lay down the carbon fiber and trim each side with
a knife before full cure,

Sand each inside edges and rejoin both sides with several carbon
fiber ribbon

By placing it with a long rod and attaching the wet ribbon who have
prewet over polyethylene plastic strip.

When in place  use a special  made curved roll as same radius of the
inside tube and attached to the long rod ,

Before removing the  polyethylene strip  to prevent the roller to
stick on pre wet ribbon,run it several time over the wet ribbon to
make sure it lay down perfectly and bubbles removed.

Let it cure and when done remove the plastic  polyethylene strip
over the jointing ribbons

You will have a perfect outer diameter and strong spar to built your
wing the spar .


JEAN PRUDHOMME

Hi Paul

Have you considered fiber reinforced tubes for the spar

These are relatively cheap and light weight

I happened to have a tube in the shop 110 mm OD, 3.2 mm wall as
shown in picture It weighs 4.9 lbs for a 4 ft section or about 1.25
per linear foot

In spite of the thin wall this tube is tough and stiff (maybe not a
desirable feature but I am not an expert). I would estimate this
would easily carry a ton of side pressure mid span. Unfortunately I
don't have a strain gage or would run some tests with the engine
hoist I don't remember what I paid but it was pretty cheap maybe $25
per foot ballpark

Also, years ago I was doing some experiments while trying to make
the worlds lightest bike frame and the process I came up with is
applicable for making wing ribs. Each rib would be approx 200 grams
for a 4 ft wide rib section and strong as heck. Scales linearly. Not
sure how competitive weight-wise with foam ribs. Can you let me
know?

Is there a typical rib spacing used on light aircraft or are they
all over the map depending on specific designs

Regards,

Simon Saba

I was thinking about flaperons and decided it was a bad idea. I am
sure it takes a lot of flight time to get the handling just right.
So I decided to copy the C182 wing planform in carbon composites. It
has known good handling as I have about 3000 hours in one.

That meant a straight wing out to the ends of the fowler flaps and
tapered from there on out.

So a rectangular carbon  tube with reinforced top and bottom would
be good for the center section. I would need a tapered male mold
from there on out.

Here is my latest thinking on the fast bush plane. I lengthened the
nose and will put a baggage compartment up there. Great for
adjusting the CG with heavy loads.

It will also reduce the chance of flipping completely over if it
encounters an obstruction too large to bounce over.

How"s your project coming. I'll loan you my 3 rotor TC if you want
to test it on your dyno.




Paul Lamar


I love the big wheels on the Bush Plane looks totally cool in spite
of the drag. It will go well with a Unimog I like the new concept.
Duplex Fuselage extra surface area may be offset by the nice
unobstructed positioning of the engine / prop


Regards,

Simon Saba

Did you see the retracted landing gear version or are you using a
cell phone ? :-)

Getting mechanical info on blue Styrofoam is like pulling teeth from
a chicken. Dow want to sell it as insulation.

I think it is about 2 pounds per cubic foot with compressive
strength of 25 PSI. Epoxy does not affect it.

I tried Jean's idea of the two inch long drywall screw and it works
great. It is the long length that makes the big difference. 3 inch
would be even better. It takes at least ten pounds to pull it out I
am guessing.

You have a minimum amount  of time to screw the pre cured skin to
the one inch wide ribs. It might help if all the holes were drilled
in the skin first. Also more than one person would help as well.

Way back in the 1960'S I did some research on fiberglass race car
bodies.

4 oz glass cloth on both sides of 1/4 inch PVC foam was much stiffer
than .032 thick T6 aluminum and  much less weight. It was vacuum
bagged in a female mold. Phil Hill hit the wall with the 2F car and
it bounced back after bending a 4130 suspension A arm. I was real
durable and tough.

For air craft wing skins it would be great if you had a female mold
and vacuum bag. Screwing the thin skins down with drywall screws
into one inch thick Styrofoam ribs is far more practical.

My guess on the rib spacing is about a foot.

Do the bottom half first and then fill it with water to see if you
have any leaks.

If your planning on a wet wing fuel tank.


Paul Lamar

Paul, Make your male plug out of 2lb density Styrofoam, round or
rectangular (I beam is best by far). Lay up your carbon, fibers
oriented length ways, vacuum bag whilst curing. Hey presto ! done !
Leave the styro in, it weighs nothing. If you really insist on
removing it dissolve it out with acetone, done ! Chris. Hoskins.

That will work.

We are thinking of kitting this airplane.

Paul Lamar


Hi Paul, A cheep trick I use over foam insulation board is water
thinned Gorilla wood glue painted on and laying tyvec fabric in it.
Covered on both side it can be bent in 6 inch circles without
snapping into pieces. Super flexible. Great for forming compound
curves and radiuses over a form. A fiberglass female mold can be made
from the male foam mock up simply by spraying on a sealer/paint and
doing the glass work. Michael Nash, Virginia


Thanks Michael. I went to school in Fork Union and lived in Fairfax
for awhile. I learned to fly at Falls Church airport long since
closed. I still have some worthless land up in the Penn Mountains
north of Gettysburg :-)

Paul Lamar

Micheal Nash...

What thickness for the foam insulation? Are you referring to the pink
or blue stuff in the big box stores?

Once covered with tyvek will a heat knif or hot wire cut the tyvek
like it will the foam?

Thanks! Brad Chubb

   Brad,
   I prefer the pink as the blue has impurities. It works equally well on
   both 1/2 in thick materials. I expect it will hot wire cut but at much
   different speeds causing the foam to shrink away from the wire faster
   and causing an undersized part with a lip of tyvec. A band saw or razor
   knife does work for cutting multiple parts from a sheet. A fine grit
   belt sander and a light touch makes excellent Finnish fits.


   A firm
   pattern of wood or metal placed under the sheet and a piloted router bit
   will quickly get the job done, loud messy but very accurate. Hooking a
   shop vac hose to the router keeps the mess down. I originally made this
   composite for model airplanes that are very light, cheep and incredibly
   crash worthy. It keeps the big boys happy when the RC model bounces and
   doesn't explode pieces.
   Michael Nash

   Hi, Styrofoam near fuel is a real bad idea, Gasoline will turn it into
   slime.

   Murry Roszansky

Here's a more complete website on using pultruded carbon rod for wing spar:
http://www.ihpa.ie/carbon-dragon/index.php/home-top/articles/83-graphlite-carbon-rod

I like the above article because it shows how the rod layup schedule can be adjusted for the load.

I would think the form or mold for a box or I-beam spar doesn't need to be made from metal - it could be made from wood cut with a table saw.  Build the form on a flat tabletop.  The sides of the form could be doweled and screwed to the tabletop - unscrew them to release the formed spar.  I might consider using formica counter top laminate to make a slick finished surface.  The spar pieces would be made in a C-shaped form and then glued together afterwards.  If an I-beam is desired, glue the two webs of the Cs together back to back.  If a box is desired, glue the caps together (lapping - this is probably pickier to get right).

One thing that's true is that the shorter the wingspan, the less critical/interesting the weight savings of a carbon spar.  On a sailplane with a long wing and a thin airfoil, the spar is a massive structure.  On an RV6A, with thick section airfoil and short wings, the spar is a much smaller portion of the structure.  Wing skins can only get so light before being too delicate for a long service life - considering the normal bumps and dings an airplane gets from having people walk around it.

Regards,
Matt- Prather




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