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.
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..
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
I worked for a company in Utah called ACT Aerospace. Up till about
four years ago we had the contract to build the main spar to the
Cirrus SR 22. We used a female mold. It was layed up as a "C".
Then vacuum bagged and cured in an autoclave. It has been a long
time but if my memory is correct there were seven or eight layers of
pre preg carbon weave for the spar web and like 65 different layers
of uni for the spar cap top and bottom of varying length to tie the
sar all together. It was 36 ft long and weighed 88 lbs.
If you look at their web page actaero.com
you can see a white rack of about 8 or 10 spars. you can also see
a spar lying in the us tank ready to be US scanned. Not a great
pic but it may give you guys some ideas.
Kay Caldwell
Was that for the Cirrus SR 22?
Good data point.
Are the Chinese making it now?
Paul Lamar
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