RunRyder - Carbon vs. Aluminium

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CAD - Engineering - Technical > Carbon vs. Aluminium

TechSpec
Heliman
Location: Finland

Many of the modern helis seems to use carbon as a frame material. What is the reason?, prize, stiffness, weight, crashproof....???

Also many parts are still plastic, even in high prize copters like Synergy and Avant, prize MUST be the reason, not the quality, right?

12-26-2006 Over year old.

GimbalFan
Elite Veteran
Location: Copter County, Nv

My understanding on this is that CF is used for its light weight, plastics for their low cost when stiffness is not needed as much. Aluminum frames however will result in less opportunities for RF and EM interference.

op-thwõp-thwõp-thwõp-thwõp-thwõp-thwõp-thwõp-thwõp-thwõp-thwõp-thwõp-thwõp-t

12-31-2006 Over year old.

FullTHR
Heliman
Location: US

crash result matters!

When you crash
plastic: break or OK
carbon: break or OK
aluminum: bend or rip

A bent Al frame is nearly impossible to bring it back to the exact original state.

01-13-2007 Over year old.

AceBird
Elite Veteran
Location: Utica, NY USA

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A bent Al frame is nearly impossible to bring it back to the exact original state.

Maybe not exact original but certainly good enough.

Crash …

Repaired …

A lot has to do with your experience with sheetmetal.

The disadvantage of plastic and Carbon (which is a plastic) is hidden damage. You don’t always see what is broken because the fractures mate right together. The aluminum and stainless steel combination that I made does well in a crash.

Ace
What could be more fun?

01-16-2007 Over year old.

rover
Senior Heliman
Location: Brandon,FL

Quote
The disadvantage of plastic and Carbon (which is a plastic) is hidden damage. You don’t always see what is broken because the fractures mate right together. The aluminum and stainless steel combination that I made does well in a crash.

Carbon Fiber is not just a plastic, it is a fiber re-enforced material. It has strength axis as do all reinforced materials. Also the material properties of a plastic vs Carbon fiber are different. If you look at a stress strain curve as well as looking at the material properties after the material exits the linear-elastic range you can determine that the materials fail differently. Also if you look at the slope of the linear elastic region (AKA the moduli of Elasticity) they are considerably different. Carbon Fibers were built to increase strenght while limiting deflection during loading. In the plastic Stress-Strain curve, when the plastic enters the 'plastic region' there is a period of increased stress with increased strain before failure whereas carbon fiber does not have this same reaction to stress.

As for Carbon VS Aluminum. here is my opinion.

Carbon is a better material for helicopters because of its unrelenting material properties. Aluminum is subject to age hardening as well as work hardening as well as fatigue. All of these processes (except fatigue) can result in brittleness of the material Though these process can be good for strenght, they reduce ductility. Working in the linear elastic range for helis is very important as any application relying on material strenght and dimension. Fatigue is another concern. Aluminum is an endurance limit as well as most plastics, and most materials however, the endurance limit of carbon is considerably higher than aluminum.

Rover
Mechanical Engineer

Rover
Mechanical Engineer

01-17-2007 Over year old.

HOMEPAGE

DS 8717
rrProfessor
Location: Here wishing i was somewhere else

Aluminum frames are prone to cracking due to vibration,a crack in an aluminum frame will cause glitches with PPM and lockout with PCM.
In my opinion carbon fiber is better for frames than aluminum.DOUG

01-17-2007 Over year old.

AceBird
Elite Veteran
Location: Utica, NY USA

Quote
Carbon Fiber is not just a plastic, it is a fiber re-enforced material. It has strength axis as do all reinforced materials.

Hi Rover,

Wouldn’t you say that glass filled, or carbon filled injection molded parts are fiber re-enforced materials? But their strength axis is like any other material, more dependant on the geometry of the part. Glass filled or carbon filled injection molded parts are the primary components of plastic parts in R/C helicopters and they have one huge advantage over any plate material and that is geometry. They can be molded into a shape where the most material is used at the greatest stress point. The disadvantage of molded parts is cost for low quantities and second to that is the inability to change the design easily, which also means higher cost. However for high volumes, molded parts are the best of both worlds.

I have heard your argument before and it kinda makes me chuckle. How do you poo poo aluminum when most of the WORLDS aircraft are made from it? Fatigue is a very simple problem to overcome. Just increase the cross section of a member so the stress becomes low and then it’s gone forever.

Carbon fiber has only one advantage over any other material and that is lower weight. But how much lower? Because carbon fiber is a resin (plastic) you have to add a bunch of metal washers to spread the load around every screw. This is not necessary for metal frames. So how much difference are we really talking? I am going to guess that a carbon fiber frame saves about 5 oz. So if you fly with 16 oz of fuel in the metal frame heli you will be about the same weight as 22 oz of fuel in the carbon frame heli. That’s 25 minutes vs. 35 minutes of flight time. That’s not a big deal for most people. In a nitro ship it might make a difference but not a gas machine.

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Aluminum is an endurance limit as well as most plastics, and most materials however, the endurance limit of carbon is considerably higher than aluminum.

“Endurance limit” Boy, you got me here. Where do I look up a chart for these values of said materials? And by the way it’s not just carbon it’s carbon fiber. There are bonds that must be maintained between the carbon and the resin in order for the structure to keep its properties. These broken bonds are the hidden failures of carbon fiber material. Yes, they can be seen with the use of a microscope but I haven’t heard of anyone using a microscope to check the integrity of their helicopter frame.

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Many of the modern helis seems to use carbon as a frame material. What is the reason?, prize, stiffness, weight, crashproof....???

In answering TechSpec’s original question the simple answer is a weight advantage, everything else is a disadvantage. If you need the weight advantage then go for it. Many people want it but few need it. Just like all the other bling associated with R/C helis.

Ace
What could be more fun?

01-17-2007 Over year old.

rover
Senior Heliman
Location: Brandon,FL

Quote
I have heard your argument before and it kinda makes me chuckle. How do you poo poo aluminum when most of the WORLDS aircraft are made from it? Fatigue is a very simple problem to overcome. Just increase the cross section of a member so the stress becomes low and then it’s gone forever.

It is not the stress alone that causes fatigue wear, it is the cyclical loading that causes fatigue not stress alone. Very important in aluminum.

Quote
Wouldn’t you say that glass filled, or carbon filled injection molded parts are fiber re-enforced materials?

Sure but the re-enforcing is done differently, with Carbon fiber plate stress is transfered along the strenght axis when loaded. now in a glass filled part, the fibers are randomly oriented giving a good and general re-enforcement but does not distribute evenly, under tension of course, compression is another topic.

Quote
I have heard your argument before and it kinda makes me chuckle. How do you poo poo aluminum when most of the WORLDS aircraft are made from it?

Are you aware that more than one aircraft has been downed meaining crahsed due to fatigue?

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“Endurance limit” Boy, you got me here. Where do I look up a chart for these values of said materials?

Well i actually typed the opposite of what i was thinking, Steel has an endurance limit, where as aluminum has a finite fatigue life and no endurance limit. I suggest that you search the terms 'endurance limit', 'fatige life'. Also you should note that aluminum can fail when the cyclical load introduces a stress that is below the yield strength, making the design of aluminum parts very important and defining a service life for such parts. Aircraft have a service life.

THis is a quote from my text book used in materials.
"
The significance of the fatigue limit is that if the material is loaded below this stress, then it will not fail, regardless of the number of times it is loaded. Material such as aluminum, copper and magnesium do not show a fatigue limit, therefor they will fail at any stress and number of cycles. Other important terms are fatigue strength and fatigue life. The stress at which failure occurs for a given number of cycles is the fatigue strength. The number of cycles required for a material to fail at a certain stress in fatigue life."

I was weighing the benefits of carbon fiber and aluminum as asked, You dont have fatigue with carbon, cyclical loading below the yield strength is not a problem with aluminum it is, and btw aircraft are subject to cyclical loading.

If you want to know more about materials and their applications, i suggest that you go for a good mechanical engineering education. I do recommend this because if you knew, fatigue doesn't go away, you either get it back in weight, or you deal with a product lifetime.

Rover
Mechanical Engineer

01-18-2007 Over year old.

HOMEPAGE

jester4
Veteran
Location: Brampton, Ontario (Flyin' outta Mississauga)

Quote
I do recommend this because if you knew, fatigue doesn't go away, you either get it back in weight, or you deal with a product lifetime.

Would the product lifetime in aluminum be shorter than the useful life of an R/C heli?

01-18-2007 Over year old.

DS 8717
rrProfessor
Location: Here wishing i was somewhere else

Quote
I have heard your argument before and it kinda makes me chuckle. How do you poo poo aluminum when most of the WORLDS aircraft are made from it?

What do you suggest the build 747'S out of steel,titanium,carbonfiber,reinforced plastic.They dont have much of a choice do they.

01-18-2007 Over year old.

AceBird
Elite Veteran
Location: Utica, NY USA

Well thank you Rover, I learned something today. I have heard of the term fatigue life but never endurance limit until you mentioned it. Always good to learn something new.

Quote
Material such as aluminum, copper and magnesium do not show a fatigue limit, therefor they will fail at any stress and number of cycles.

So as you say aluminum will eventually fail if it sees enough cycles. But will it?

http://www.engr.ku.edu/~rhale/ae510/fatigue/sld002.htm

This graph ends at 10^10 number of cycles and if I extrapolate the 2014-T6 curve it would be at least 10K psi. Getting back to the real world comparison, my aluminum frame section is dead soft aluminum not a T6 temper. This lends itself to fabrication and strengthens the material at the bends while leaving the majority of the sheet metal soft. I am not sure what the alloy is. Soft aluminum limits the stress you can put on the frame before it will bend and take a set. In actual use they don’t bend and I suspect the stress is closer to 100 psi not 10K psi. So this further increases the number of cyclic loadings before failure. BTW the graph does support my statement that you can increase the cross section (lower the stress) to increase the life cycle.

Think about the aluminum casting of the cylinder on our Zenoah engines or any weed whacker, chain saw or lawn garden engine. These engines don’t get cracked cylinders over their useful life. They are certainly seeing cyclic loading. The engine casing is actually the strongest structural component of the gas helicopter. Many survive crashes that the any type of frame does not. I would say this is a case where you don’t want to get hung up on theories. It is better to look at actual applications to draw a conclusion.
.

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You dont have fatigue with carbon, cyclical loading below the yield strength is not a problem with aluminum it is, and btw aircraft are subject to cyclical loading.

I couldn’t agree more. It is my understanding that composite wings do not have as long a life cycle as aluminum wings do. I haven’t been able to find any data at all on carbon fiber sections. If you have any it would be nice to make a real comparison.

In my research, I found another interesting fact about fatigue. It starts at the surface. So in the case of a frame made of aluminum side plates you can grind off a few thousands from each surface and greatly extend the life of the frames. Try that with carbon fiber.

http://www.epi-eng.com/BAS-Fatigue.htm

Quote

WHY IS THE SURFACE SO IMPORTANT?
Fatigue failures almost always begin at the surface of a material. The reasons are that (a) the most highly-stresses fibers are located at the surface (bending fatigue) and (b) the intergranular flaws which precipitate tension failure are more frequently found at the surface.
Suppose that a particular specimen is being fatigue tested (as described above). Now suppose the fatigue test is halted after 20 to 25% of the expected life of the specimen and a small thickness of material is machined off the outer surface of the specimen, and the surface condition is restored to its original state. Now the fatigue test is resumed at the same stress level as before. The life of the part will be considerably longer than expected. If that process is repeated several times, the life of the part may be extended by several hundred percent, limited only by the available cross section of the specimen. (ref-3:8:6) That proves fatigue failures originate at the surface of a component.

Ace
What could be more fun?

01-18-2007 Over year old.

rover
Senior Heliman
Location: Brandon,FL

1.Aluminum Engines have steel or steel alloy cylinders. TO have a true aluminum cylinder, not an alloy of aluminum such as with silicone, you need to have a low friction surface as well as high strength.

Failures do begin at the surface this is attributed to the high bending stress as noted in the quote. But Remember failures happen along grain boundaries. But there is a contributing factor as well. The surface finish. a polished finish will result in a longer fatigue life.

There is also something that you did not consider about the increased cross section. It is not so much the size of the cross section as the shape. , I=bh^3 for a rectangular cross section. Another thing are the implications of a more massive cross section. weight. It comes down to service life of the part as well as a FS.

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What do you suggest the build 747'S out of steel,titanium,carbonfiber,reinforced plastic.They dont have much of a choice do they.

There are already aircraft that have composite airframes. The cost will be driven down as the production ramps due to the increased cost of metals. When the cost goes down, more people will use it. It is not currently cost prohibitive to some manufacturers

Rover
Mechanical Engineer

01-19-2007 Over year old.

HOMEPAGE

ChristianM
Veteran
Location: Oslo, Norway

I am also a Mechanical engineer and would just like to state that Rover has got his facts straight.

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It is my understanding that composite wings do not have as long a life cycle as aluminum wings do. I haven’t been able to find any data at all on carbon fiber sections. If you have any it would be nice to make a real comparison.

Steel has about 100 times better fatigue life than aluminum as can be seen in the fatigue curve AceBird posted. Carbon fiber composites has roughly 100 times better fatigue life than steel (this is based on lots of experimental data) so carbon fiber composite laminates has roughly 10,000 times better fatigue life than aluminum. This is also a significant factor (aside from the weight saving) why the aircraft industry is moving towards using these composite laminates since it greatly extends the service life of the components.

The thing about composites is that they have a much more complicated design process. However, in the RC heli industry my impression is that they usually just replace an aluminum part with a geometrically identical CF part. This will result in a significantly stronger and lighter part but it could be optimized a lot more.

Christian

Burn fuel, be happy

01-21-2007 Over year old.

AceBird
Elite Veteran
Location: Utica, NY USA

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Carbon fiber composites has roughly 100 times better fatigue life than steel (this is based on lots of experimental data)

Can you point me to this data.

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the aircraft industry is moving towards using these composite laminates since it greatly extends the service life of the components.

It is my understanding that it is the opposite so I would like to see something official here also. I am not saying you are wrong, I would just like it confirmed by an official document or report.

Ace
What could be more fun?

01-21-2007 Over year old.

ChristianM
Veteran
Location: Oslo, Norway

You will generally not find fatigue data on composites because it depends on how the composite laminate is built up. The company that I work for have done extensive testing on composites and have published the first offshore standard for composite:
DNV-OS-C501: Composite Materials
DNV-RP-F202: Composite Risers

You will generally not find published fatigue data on composites since it is application specific and quite often proprietary, but it is well known in the industry that composites have superior fatigue properties compared with any metals. The major problem with composites is in the quality control. Composites are sensitive to contamination and humidity during fabrication. Also inspection of composite structures is a lot more complicated than what it is for metals and this needs to be considered during the design process. This is the biggest problem aside from cost.

I don't have report I can point you to but most of the new fighter airplanes coming out rely heavily on composites. Also the
new airbus that coming out uses primarily composites from what I have read in the media.

Christian

Burn fuel, be happy

01-21-2007 Over year old.

AceBird
Elite Veteran
Location: Utica, NY USA

Quote

The major problem with composites is in the quality control. Composites are sensitive to contamination and humidity during fabrication. Also inspection of composite structures is a lot more complicated than what it is for metals and this needs to be considered during the design process. This is the biggest problem aside from cost.

I don't have report I can point you to but most of the new fighter airplanes coming out rely heavily on composites. Also the
new airbus that coming out uses primarily composites from what I have read in the media.

Yeah, that's great when you got a bottomless pocket book. But R/C???

Do you think there is much quality control when you speak of our hobby? I wouldn't assume the manufacturing processes for hobby parts come any where near a government contract. Just like titanium parts for this hobby. How do you even know they are actually titanium? You don't get any certs with your parts. No tracability what so ever.

Ace
What could be more fun?

01-21-2007 Over year old.

jester4
Veteran
Location: Brampton, Ontario (Flyin' outta Mississauga)

This is what I asked earlier, but no-one addressed it. I don't think that the useful life of aluminum for R/C application is too short. Look at the R/C car guys, and all the different aluminum parts they use. I don't think that R/C heli's put nearly as much abuse through aluminum that cars do. I know you may say that aluminum bends in a crash, and I agree, but take the crash factor out of the equation, the life of the parts are usually not a concern.

01-21-2007 Over year old.

AceBird
Elite Veteran
Location: Utica, NY USA

Quote
This graph ends at 10^10 number of cycles and if I extrapolate the 2014-T6 curve it would be at least 10K psi. Getting back to the real world comparison,

jester4 I started to answer it here but I got off on a tangent. >10^7 number of cycles is considered infinite life. With such small stresses you are way beyond this number. So like you say, fatigue is not an issue for how these parts are used.

Ace
What could be more fun?

01-21-2007 Over year old.

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CAD - Engineering - Technical > Carbon vs. Aluminium

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