Composites vs aluminum

HomeBuiltAirplanes.com

Help Support HomeBuiltAirplanes.com:

Little Scrapper

Well-Known Member
HBA Supporter
Log Member
Joined
Jan 3, 2014
Messages
5,894
Location
Wisconsin
In my quest for composite knowledge I'm struggling to figure out something. Even at Airventure in Oshkosh I couldn't get a straight answer to a question I had, it's like witchcraft or something. Here's my question.

"What is the carbon fiber weight and thickness equivalent to aluminum for the use of a cowl."

Turns out, there's quite a few answers to this. I left Oshkosh confused but I do understand there's variables to this.

I saw one lay-up that was 2 outside layers of C.F. that sandwiched two layers of regular glass. It seemed heavier than aluminum and incredibly stiff.

My gut feeling tells me aluminum for a cowl will always be lighter than composites, is this true?

Does a secret formula exist to create a cowl from C.F. that is lighter than aluminum and can withstand the heat from the engine?

I was with my buddy when he fitted his RV9a cowl, it was a heavy dreadful beast. Some cowls are thick and heavy and some are thin and light. Why? What's going in here?
 

pictsidhe

Well-Known Member
Joined
Jul 15, 2014
Messages
8,812
Location
North Carolina
A sheet of 1/16 woven carbon will be about as stiff as a sheet of 1/16 aluminium, but only 60% of the weight. The carbon will be about twice as strong aas 2024. These numbers assume affordable grades of carbon...
Carbon takes a lot of heat, the resin determines a composite's temperature resistance.
 

cheapracer

Well-Known Member
Log Member
Joined
Sep 8, 2013
Messages
6,294
Location
Australian
Carbon Fibre will weigh 60% of an equivalent piece of aluminum.

It will be as as strong or stronger depending on layups, resin ration etc.

However, that's a direct comparison, in the real world it's actually lighter again as you don't need a lot of the hardware to assemble aluminium structures if it's molded to shape.
 

TFF

Well-Known Member
Joined
Apr 28, 2010
Messages
14,345
Location
Memphis, TN
The shape being made and how easy it is to make plays a part. An aluminum nose bowl is awfully pretty, but you will either have a lot of time beating metal with the appropriate skill or buy one that is stamped with big tooling. A composite nose bowl once the mold is made, duplicates are easy to pop out. How heavy hast to do with damage tolerance. Hit a bird in a day to day airplane and you want damage resistance. If you were racing at Reno, you will trade it all as long as it does not crash you for light weight. Where you stand is always a function of what you want out of it. Making molds are time consuming. If you want something special or are going to make a run, its worth it. If you are making something that can be a simple wrap of aluminum, it better be worth the effort.
 

Little Scrapper

Well-Known Member
HBA Supporter
Log Member
Joined
Jan 3, 2014
Messages
5,894
Location
Wisconsin
Thanks for the replies.

I guess I was looking for more numbers than anything. 1 C.F. & 1 Kevlar? 2 C.F. & 1 S glass sandwiched?

As I indicated in the first post, there's all sorts of people doing it different. My gut feeling tells me most of them don't do actual math on this so they just wing it so to speak.

Someone on here must have done some math and come up with a formula that's an acceptable replacement with a decent safety margin.

I'm digging in to this topic, it's interesting. I'd like to do some composite pieces on my Cassutt but I don't want to guess, I'd like to do the math.

Can anyone help me with the math?
 

Pops

Well-Known Member
HBA Supporter
Log Member
Joined
Jan 1, 2013
Messages
8,912
Location
USA.
Local man has a company that does VI and makes carbon fiber parts for the Tailwind on the side. He made a set of doors for the Bearhawk and saved 18 lbs on the airplane if I remember correctly. The doors built to plans are steel frame covered with .025 alum. That is right and left front doors and the 2 side cargo doors.
 

pictsidhe

Well-Known Member
Joined
Jul 15, 2014
Messages
8,812
Location
North Carolina
If you want ultimate stiffness to weight, use a light core. Glass is not light, but it is cheap, hence its common use as a core. Kevlar is not as stiff as carbon, but it is slightly lighter and once you've thickened the part to gain stiffness, it may about balance out. It is incredibly tough. A kevlar core, carbon skinned composite would stay together if broken. As TFF pointed out, the effort is in the mould. That may influence you to start on the light side for non vital parts and replace if necessary.

Some composite comparisons here
 

Little Scrapper

Well-Known Member
HBA Supporter
Log Member
Joined
Jan 3, 2014
Messages
5,894
Location
Wisconsin
Thanks.

The mold discussion is for a different thread I suppose. For this thread I'm trying to nail down and understand what combinations people are using and maybe some math or reasoning behind it.

This thread is similar to what I found at Airventure, some theories and discussion but no actual combinations from people who built and tested it. I mean that in a nice way, I'm looking for more technical information.

So, besides molds, doors etc, what combination works for say a turtle deck where I could easily use aluminum? Maybe let's just start with that?
 

TFF

Well-Known Member
Joined
Apr 28, 2010
Messages
14,345
Location
Memphis, TN
I think you are asking what becomes the monthly question that new posters always ask, " I want to make an RV but in composite; how thick do I make the parts?" The answer is almost always, you design in one or the other, or dont design composites with "metal thinking." Making structural parts is one thing, making fairings is another. The math is way too complex for me to design a turtle deck except figuring out how many yards of material and some rule of how much resin to make one layer, two layer, three layer of X weight material vs how much a piece of aluminum would weigh. Because you are trying to make something that is only there to resist airstream pressure and not hold a wing together, you would have to decide if one layer will keep its shape, two, three, or add a core and make the weight and shape. Other considerations could be something like making a removable deck for maintenance, so can it keep its shape when removed and attach points that will be used often. In your situation, you know what's coming.
 

Little Scrapper

Well-Known Member
HBA Supporter
Log Member
Joined
Jan 3, 2014
Messages
5,894
Location
Wisconsin
I guess I would disagree. Decades of people building I'm assuming we'd learn something. I just need to find people who have. I'm not an engineer so that's how guys like me slowly get answers.

And I really don't think I need to decide if it will hold it's shape, I'm hoping mathematics will. At some point if experienced builders haven't done it then the math should work it out.

Aluminum must have some math behind it. Per a given thickness and grade there's gotta be some math behind its stiffness etc. I'm thinking there's gotta be some math behind combining a certain thickness of carbon fiber with a backing of day regular glass.

Now, I'm plumber not a mathematician but I'm still betting I could figure some of this out.

Anyhow here have personal experience calculating and figuring this stuff out?
 

gtae07

Well-Known Member
Joined
Dec 13, 2012
Messages
2,090
Location
Savannah, Georgia
I was with my buddy when he fitted his RV9a cowl, it was a heavy dreadful beast. Some cowls are thick and heavy and some are thin and light. Why? What's going in here?
If I were to guess, having handled an RV-6 cowl plenty of times...

The RV cowl is fiberglass with a thin hexagonal core material in the middle portion. It isn't very thick, partly because you need clearance around the engine and partly probably because of some of the curvature and how much the core can handle.

The edges and the entire nose section around the inlets and spinner has no core and is just solid laminate. I assume this is partially for handling and impact, and partially for the need to add hinge halves or other fastening methods to secure it to the airframe. If it has the intake scoop for a vertical induction, that part is also solid laminate with no core. Again, I assume partially for impact and handling and partially because the curvature gets so tight I don't know how you'd get a core in there.

And that's just the raw kit form of the cowl.

You can figure that the cowl is going to have multiple layers of pinhole-filling primer applied (or even thinned resin squeegied on!) to fill pinholes and seal the interior side against fluid drips, dirt, grime, etc. Maybe a layer of reflective heat shield, too, plus the above-mentioned camlocks or hinge halves. Add filler applied heavy-handed and sanded down a few times, maybe. Add a couple layers of paint, and a big slot right up the middle of the bottom on a nosewheel airplane.

That's how an RV cowl gets so heavy.



Many (most?) RVs are not going to be great examples of composites work. RV builders often dread the composites portions and "brute force" the work into shape; many went with a metal airplane to try and avoid the composites work. Simple fairings and such wind up really thick and heavy because they're hand-applied open wet layups, typically with high resin fractions and lots of filler applied. It may have a nice finish in the end, but underneath all the paint and filler it might be pretty ugly. I've learned a lot about composites from this forum and visiting our composites shop at work; I intend to apply those lessons to my RV when I get to that point.
 

gtae07

Well-Known Member
Joined
Dec 13, 2012
Messages
2,090
Location
Savannah, Georgia
LS, you really can't just say "X thickness of one material is equivalent to Y thickness of another" and build your structure that way. There are certain situations where you can get away with that with different metals, but even then there's a lot more to it than just tensile strength.

What you're asking is kind of like "I want to make my Cassut's structure in carbon fiber; what thickness do I use?" when the real answer is, if you want to make your Cassut in composites you have to pretty much redesign the entire airplane. If you want to make an airplane that's made from one material in another material, you can't just substitute thickness A of material 1 for thickness B of material 2 and try to force the structure to be the same; it'll wind up grossly overweight because the structural features that result in an optimum structure in material 1 are not the same as those for material 2. The entire internal structure is going to be different.

A stick-built building's walls are going to look nothing like a building made with ICF. A composite airplane's internals are going to look almost nothing like a metal airplane's structure. All you do trying to apply one material to the "design philosophy" of another is wind up with something heavy and non-optimized.
 

Little Scrapper

Well-Known Member
HBA Supporter
Log Member
Joined
Jan 3, 2014
Messages
5,894
Location
Wisconsin
LS, you really can't just say "X thickness of one material is equivalent to Y thickness of another" and build your structure that way. There are certain situations where you can get away with that with different metals, but even then there's a lot more to it than just tensile strength.

What you're asking is kind of like "I want to make my Cassut's structure in carbon fiber; what thickness do I use?" when the real answer is, if you want to make your Cassut in composites you have to pretty much redesign the entire airplane. If you want to make an airplane that's made from one material in another material, you can't just substitute thickness A of material 1 for thickness B of material 2 and try to force the structure to be the same; it'll wind up grossly overweight because the structural features that result in an optimum structure in material 1 are not the same as those for material 2. The entire internal structure is going to be different.

A stick-built building's walls are going to look nothing like a building made with ICF. A composite airplane's internals are going to look almost nothing like a metal airplane's structure. All you do trying to apply one material to the "design philosophy" of another is wind up with something heavy and non-optimized.
No, no, no, no, I never said or even implied that. In fact, I clearly pointed out and used a turtle deck as an example.

The Cassutt is staying Cassutt, nothing is being redesigned. A Cassutt is a steel tube fuselage and a turtle deck is simple laying on top of the steel structure.

Instead of aluminum I'd like to use a composite.....because learning is fun.

Forum posts really get twisted in a hurry. I'm not experienced in composites but I've been around long enough to know this has successfully been done.

I think what's happening here is nobody has really done this on the forum. That's ok, but that's what's happening I think.

The same thing at Airventure. The people I talked with were teachers at a school. And like most teachers, they lack real world experience.

I think I'll just end up doing this the old fashioned way, myself. I have some customers who are engineers, I suppose that's my best starting point. Start with some math and run some tests.
 

wwalton

Well-Known Member
HBA Supporter
Log Member
Joined
Nov 9, 2006
Messages
227
Location
Hendersonville, North Carolina
I'm reluctant to comment here as I have very little real world experience with composite design. Yesterday I was watching the AR-5 tapes for the third or fourth time and Mike Arnold said something to the effect of "two layers of UNI on each side of the foam would be near minimum to avoid inadvertent damage by handling" seems like a starting point for a non structural part.
 

cheapracer

Well-Known Member
Log Member
Joined
Sep 8, 2013
Messages
6,294
Location
Australian
, or don't design composites with "metal thinking."

All you do trying to apply one material to the "design philosophy" of another is wind up with something heavy and non-optimized.

As I have built a plane that has had one material directly and successfully substituted for another, I'm wondering what actual real world experience you guys have had specific to this?

I am also heavily considering doing a parallel build of my current build directly in "Black Aluminium" (a term so fondly referred to by Autoreply where carbon fiber literally directly replaces aluminium without taking further material advantages), for a 20 - 30% weight savings with apparently an increase in strength according to the numbers, so I would love to hear your arguments against this .....
 

cheapracer

Well-Known Member
Log Member
Joined
Sep 8, 2013
Messages
6,294
Location
Australian
Yesterday I was watching the AR-5 tapes for the third or fourth time and Mike Arnold said something to the effect of "two layers of UNI on each side of the foam would be near minimum to avoid inadvertent damage by handling" seems like a starting point for a non structural part.

Thin fiberglass isn't very strong, but I can belt a piece of CF with a hammer and the greatest risk is the hammer bouncing back into your face. if I hit a piece of 6061 T6 aluminium the same thickness then major damage is done.


I'm reluctant to comment here as I have very little real world experience with composite design.
Me too, but go down to a hobby shop or some shop who makes stuff in CF, and get a small sheet or waste sample of carbon, then go get a piece of 6061 the same size and thickness and do your own tests, it's not out of your reach by any means.

You'll be amazed at what facts you'll know in a few hours.
 
Top