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

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

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

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