Here is another thought exercise im interested in.
If money was no object in terms of structure materials only, what would be the best/most optimal material from which to construct an a350 sized aircraft.

Or put another way, are there realistic/optimal materials that have not been used simply because they are too expensive to be commercial?



As usual, if you cant be positive in manner, please feel free to not comment at all.

When you say optimum, manufacturers trade off optimum strength, weight, construction, maintainability, repair, and cost.

Yes, exactly, so all of those minus the cost.

If cost and labour (which is cost kind of) is no problem, then one probably would build the worlds largest autoclave, a huge monster, which would be able to manufacture the whole aircraft as one carbon fibre structure, with robot-arms laying down the carbon fibre layers above each other until after hours and hours of robotic work and baking in the autoclave the whole aircraft comes out of it. Then you just add the wings, also build in one piece and then put it all together.

About the materials, perhaps one could use NFCs instead of CFCs. For NFCs you replace the "carbon fibre" with "natural fibre", materials gained out of trash and compost. NFPs in theory should be lighter and cheaper than CFCs. But if they are more durable and as strong as CFCs remains to be seen, BMW is testing NFCs in their i-Series cars apparently. I think the airline industry, especially Boeing with the 787, is already at the edge of technology...


Just found this nearly 11 year old news article about Airbus testing NFCs: http://news.bio-based.eu/airbus-to-rese ... omponents/

CARST thank you, that is the sort of answer i was looking for. Very interesting.

I dont know why others are so aggressive and rude. What is wrong with them?

Agrajag wrote:

CARST thank you, that is the sort of answer i was looking for. Very interesting.

I dont know why others are so aggressive and rude. What is wrong with them?

Who has been aggressive and rude to you in this thread?

That Pikachu person. I had his comments removed. It seems he doesnt like people asking questions. Strange notion on this forum.

Agrajag wrote:

That Pikachu person. I had his comments removed. It seems he doesnt like people asking questions. Strange notion on this forum.

Oh he is just the resident troll. Learn to ignore him, and laugh at his insecurities.

Other than composites, what would be the very best Alloy under these parameters? Aluminium/Lithium is mentioned often but is there an every better option if money no object?

Not sure there is one "very best". As mentioned, it's all a compromise. Even excluding cost, you have to consider weight, strength, repairability, safety, workability, etc...
Among metals, you have magnesium alloys that are lighter than aluminum ones, but are also weaker and difficult to work with (and magnesium burns a bit to well). It might be possible to make a lighter frame with them, though, if that's your objective.

Thanks for that. What about a Titanium alloy? What are the pros and cons of that? Wasnt it used on the SR71 for structure and skin? Have the potential titanium alloys advanced since then? Is it suitable?

When the B787 was relatively new I asked a stewardess what she thinks of the plane. She said she loves it.
For the fuselage carbon fibre is good because lack of corrosion allows for greater humidity in the cabin.
Carbon fiber is strong, but only in direction of the fibers. Obviously there have to be several layers with different directions. Aluminium alloys are strong in all directions. AFAIK that's one reason carbon fibers are used more in wings than in fuselages.

"Despite its high initial strength-to-weight ratio, a design limitation of CFRP is its lack of a definable fatigue limit. This means, theoretically, that stress cycle failure
cannot be ruled out. While steel and many other structural metals and alloys do have estimable fatigue or endurance limits, the complex failure modes of
composites mean that the fatigue failure properties of CFRP are difficult to predict and design for. As a result, when using CFRP for critical cyclic-loading
applications, engineers may need to design in considerable strength safety margins to provide suitable component reliability over its service life."
https://en.wikipedia.org/wiki/Carbon_fi ... ed_polymer

Is the "complex failure modes of composites" the reason composites are less used in fuselages with their repeated pressurization cycles?
But isn't a wing facing repeated turbulences far more cyclical loaded? Or is one pressurization cycle more comparable to an extreme wing bending in extreme weather?


Glass Laminate Aluminum Reinforced Epoxy (GLARE) is a sandwich of several thin layers of aluminium with glass fiber reinforced plastic in between. It's a FML=Fiber Metal Laminate.
" http://www.compositesworld.com/blog/pos ... e-of-glare " discussses FML=Fiber Metal Laminate versus CFRP (Carbon Fiber Reinforced Plastic).
"... FML also beats composites on weight saving in small fuselages because the latter “carry a weight penalty, due to minimum thickness for damage tolerance.”
However this is a statement from a web page promoting composites. The main advantage of GLARE is tensile strength. It's good only for certain areas of the plane and it's expensive.


I would like to add some questions which stray from your "A350 size" question, but refer to optimum material:
" http://www.pvcfittingsonline.com/resour ... gth-chart/ " has a discussion about PVC pipe diameters and it's characteristics.
Apparently with increasing pipe diameter the tensile strength increases, but bursting strength (caused by water pressure) decreases.
Is "bursting strength"/ fatigue by repeated pressurization or tensile strength the limiting factor in today's Aluminium/ Lithium fuselage designs? From the DC 8 I assume it's fatigue, but does somebody know for sure?
A321: fuselage 3.95 m outer width, length 44.51m. Length/ diameter = 11.2..
DC 8/61-63: fuselage 3.73 m outer width, length 57.1 m. Length/ diameter = 15.3.. . Very long.


I assume sooner or later somebody will find a way to mass produce CFRP parts.
(Which makes me wonder if Bombardier's C-Series was a bet that by the time of introduction mass production of CFRP would be much cheaper.
In which case both Airbus and Boeing were brave to bet that this won't happen.)
Considering that increased humidity decreases strain on the body: How much more expensive would be a narrowbody with carbon fiber fuselage with today's technology?

No problem. Im all for learning new stuff, especially about materials. I find it fascinating.

Agrajag wrote:

Thanks for that. What about a Titanium alloy? What are the pros and cons of that? Wasnt it used on the SR71 for structure and skin? Have the potential titanium alloys advanced since then? Is it suitable?

Titanium has the strength of steel with half the weight. It can also tolerate very high temperatures, even higher than steel. On the SR71 it was used for it's heat resistance, as friction heat from continuous >Mach 3 flight was to much for aluminium to handle. And steel was much to heavy (the Mach 3 capable Mig-25 used steel, witch made it very heavy, compromising range, among other things).

On the other hand it still is quite a bit heavier than aluminium (~double), and difficult to work with. Granted you don't need as much of it, being stronger, but i think an all titanium frame should still be heavier. Not much point in making an airliner frame primarily out of it. I believe its used in bits and pieces on the frames, where higher strength is needed (bolts, some struts, landing gear, etc), and it's used a bit more heavily on the engines (rolls royce makes the entire fans out of it), but that's it.
Titanium has some interesting properties, but is not some magical material. It has it's uses, like any other.

You should google the images of "what different engineers think how an airplane should be made" there are lots of different good ones.

Anyways, you could in theory make one giant plate, that somehow had grain directions beneficial to hoop loading for the fuselage portion but beneficial for the wing in other ways. Then start cutting away everything that... isn't an airplane.

Of course that comes with its own problems. No crack stoppers for instance.

If safety is not a concern, would an inflatable airframe be lighter than anything else?

Try rowing an inflatable raft and get back to us.

GF

I recall reading about research into materials that will change their shape when subjected to an electrical impulse, and then return to their original shape when the impulse is taken away. The idea for aircraft would be a wing or flight control that can assume various shapes according to the needs of flight. Don't remember what type of materials this was called but, it would obviously need to be conductive and very pliable with a high degree of durability.

I presently work for FedEx as a structures/composite technician. We still use the aluminum alloys, stainless steel, and titanium. We also work with carbon fiber (CFRP) and fiberglass (GFRP). The problem with metals is that when they are in complex shapes, it is difficult, if not impossible, to repair or fabricate them at the typical hangar structures shop unless you have very expensive equipment to do so. Many require you to fabricate them out of annealed alloy and then heat treat to a certain temper, a time consuming process. Composites, as they are today, are almost as bad in that they are chemical reactions demanding specific environments to be done correctly and they usually are not fast repairs. Because of this, I think the next big thing will be some type of material that can be used to fabricate parts or repairs where the time will be fast, it can be done easily at almost any location or temperature, and will nor require a bunch of expensive equipment. I recently saw a video of a new epoxy resin made in Austria that can be used in composite manufacturing or repairs that uses a single touch of intense light that then begins a chain reaction throughout the whole composite stack-up and requires no heat blankets or thermocouples and is set and cured in seconds. Amazing!!

Another area of interest is additive manufacturing. This is sometimes called 3-D printing. The companies that are into this are already using carbon fiber and ceramic matrix materials formed into unbelievable shapes and complex forms that surpass CNC machining capabilities.

As for what would be the ideal material for aviation, I can't say, and suspect that there will not be any one best solution but maybe many.

Gregory