Carbon fiber fuselage structures ideas needed...

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malipa

TU Delft AE student
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Aircraft like the 787 and the X-55 ACCA have a carbon fiber skin to decrease weight and the amount of used parts.
The X-55 lacks stringers, how can it be stiff enough without?
The 787 fuselage-mandrel has gaps for the stringers in the fuselage but why arent the vertical frames fit in te mandrel as wel?
Could the stringers and frames be replaced by a thin layer of microlattice? It is harder to repair so the skin might be strengthened...
And could the fuselage be built out of hexagonal panels with only frames at the outer edge of the panel? These edges are glued together to get one fuselage.

Anybody any other ideas?
 
malipa said:
Aircraft like the 787 and the X-55 ACCA have a carbon fiber skin to decrease weight and the amount of used parts.
The X-55 lacks stringers, how can it be stiff enough without?
Click to expand...

By using a combination of core and plydrops to simulate stringers.
 
Could you be more specific? I don't really get what you are trying to say sorry.
 
malipa said:
Could you be more specific? I don't really get what you are trying to say sorry.
Click to expand...

Skin and stringer: _______H_______
Stringerless: _______/ \______ (with core in the space and plies over the top.)
 
So... basically the stringers are integrated into the skins?
 
As sferrin said, stringers could be produced by adding local material in the build up of plies. This could be extra core, or extra strips of carbon, or any combination of the above. Here is an example of a layup with foam core between two layers of skin, with local sections of extra carbon (equal inh thickness to the removed core) where you need a stringer.

outside skin
______________
core---core---core
______________
outside skin

Also, stringers are not mandatory. They are just one way of getting the required stiffness. Several composite and several all metal aircraft have been built with fuselage sections without any stringers - because the basic fuselage shell is stiff enough. Sometimes a small increase in basic material thickness can eliminate the need for stringers, so the designer breaks even on weight (roughly), and saves on parts count. (More parts = more money).
 
Could the increased thickness hull be as heavy as the hull with stringers and frames?
 
And i you would use ies for the stringers and frames, would you need to dubble the thickness on the crossings or could you keep the same thickness?
 
Your questions are too general. There are no easy answers.

You need to start with the maximum load (or loads) the component must withstand, and an allowable stress or strain at the maximum load. If you have selected the material, this translates into the cross sectional area and/or moments of inertia you need. Then you figure out the cheapest and lightest way of getting this area and moment of inertia. Maybe this needs stringers, maybe it doesn't. Different construction shops, with different tooling and manufacturing capabilities, will give you different definitions of "cheapest". If you plan to build 1000 large commercial jets, you would be willing to spend more on tooling than someone building one or two experimental aircraft.

Weight needs to be balanced with cost and application. 1% of the empty weight of a 787 equals several paying passengers, while 1 % of the empty weight of a UAV is almost nothing. You can get almost the same weight savings as a 1 % empty weight reduction in a competition glider by telling the pilot to take their boots off. ;D

A lot of lightly loaded composite structures, like gliders, are sized by minimum number of plies needed to have enough strength to pry the part out the mould without damaging it. Sometimes this determines the cross sectional area and moment of inertia you get. Here again different shops, with different techniques and skill levels, may have different definitions of the minimum number of plies.
 

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