why isn't there composite tube fuselage design?

.......... :-)) wrote:
Bonding titanium to carbon tubes does not sound like a homebuilable
technique to me.


The joints would not need to be titanium but could be made from carbon
fiber as well. Bonding of carbon fiber to carbon fiber would present
few problems. The majority of tubular frames I have seen are designed
so that the stress on the frame remains in compression.

You are right the joints will always be the problem. However it is
interesting to study the way in which items such as bicycle frames
and yacht
steering wheels are made in one piece from tubular sections in carbon
.... I
won't say any more on this. You can find plenty of info on the web.


That would be the obvious method if you are constructing the frame from
raw materials instead of existing carbon fiber tubes.

There are fouth major problems/issues that I can see.

First .... if you layup a tube from carbon you cannot put all of the
fibres
along the tube axis. Some will need to wrap around. As carbon fibre
is
highly orthotropic (i.e. different material properties in different
directions) then the impressive properties of the carbon fibre along
the
fiber axis will not be attained in the completed tube. Don't get me
wrong
the carbon will still be lighter and stiffer for a given weight but
probably
not by as much as you would expect.


The solution to this was on display at Oshkosh about three years ago.
Ever see the Chineese finger lock? They were using the save weave
technique to form the carbon fiber tube, wetting out after forming to
desired size and curing. The result was the optimum strength in all
directions.

Second ... the properties of carbon composites are strongly dependant
on
moisture and temperature conditions. For example the compressive
strength of
Fibercotes E-765/T700 24K uni prepreg is 147 ksi at room temperature
/ dry
conditions. At elevated temperature and equilibrium humidity
conditions in a
tropical environment the compressive strength drops to 88 ksi ! Not
really
any better than steel. Compressive moduli and any other resin
dominated
properties (such as shear) will exhibit similar behavior. Yout need
to
design for these low strengths at the extreme environmental
conditions in
any composite structure and thus at room temperature you will end up
with
larger than required margins and hence more weight.


Those properties can be programmed to just about any desired result.
The accepted standard for most composite design is 2X vs 1.5X for steel
equilivent design. The issue will always be a design problem, and the
best would be to design to the best properties of the materials being
used.

Third .. composites suffer from microcracking and other issues which
will
limit the laminate strains to approx 4500 micro strain at ultimate
load (a
rough number ... complicated issue not room to explain here). What
this
means in plain english is that you can only use approximately 1/3 of
the
potential strength of the material if you want a structure with long
life.


A property not unique to composites.

Fourth ... composites have no ductility like metals and I would hate
to be
sitting in a fuselage made of carbon tubes if it hit anything. The
tubes
would fracture and splinter and the pilot would be in all sorts of
trouble.
To control this behaviour you would need to add another tougher
composite
material such as Kevlar to the laminate (common practise in fwd
fuselages
for gliders to improve crashworthiness).



Composites have proven to be very crash worthy in practice but I agree,
that the carbon fiber tube frame would not provide the same protection
of a tube steel frame. It goes back to designing to the qualities and
strength of the material.