Using a carbonfiber rectengular tube as a wing spar

[Merlin]

I was thinking of using Carbon fiber as spar for my project because its light and strong, sadly its also pretty **** pricy.
i found some for a decent price on Alibaba but i don't know how reliable it is, it could be fake and i'll end up crashing like a bird poop on a windshield.

could you adise me?
thank you.

 

[Norman]

Pretty pricey? Have you compared it to aircraft grade spruce? The ratio changes from day to day but on a strength to price basis carbon fiber became cheaper than spruce several years ago.

 

[wsimpso1]

Merlin,

Several issues. The shear loads and bending moments start out at the tips at about zero, and then rapidly build up so that at the root, you have huge bending moments and modest shear loads. And because airplanes are meant to fly, weight is the enemy - so you really ought to tailor the ability of the spar to carry shear and bending moments. You use carbon fiber to make light parts...

To carry big bending moments in carbon fiber at low weight, you need enough unidirectional carbon fiber at the top and bottom of the spar, and then reduce the amount of unidirectional fiber as you proceed from root end to tip end. To carry the significant shear at the root, the web that goes between the caps should be at +/-45 degrees and be enough to carry the root shear, then be reduced as you proceed towards the tip.

Now let's look at the rectangular tube. Maybe it has +/-45 degree laminate for a shear web, but if it has enough area for shear loads at the root, it will have big excess even a third of the way to the tip. Heavy. The bending resistance will be modest compared to needs, so you will have to add unidirectional carbon fiber to the top and bottom surfaces, and tailor it to be capable of carrying the bending everywhere. So this will now be too heavy because of excess material over much of its length for carrying shear, and it will be somewhat deeper at the root than at the tip. I guess you can make your wing shape fit over the spar, but the glue line putting the two together will be impressively heavy. Ugh.

This is why almost everyone who wants a carbon spar figures out their desired wing, figures out the acceptable spar profile at numerous positions along the wing, then makes a mold or fixtures to make a spar with highly tailored caps and web.

For your carbon fiber tube to work, you would have to get lucky on depth, width, and wall thickness. And then it will be about triple the weight it could be if tailored plus whatever high amount of adhesive to connect it to the wing skins. Not a good way to strength at min weight.

Billski

 

[Merlin]

Merlin,

Several issues. The shear loads and bending moments start out at the tips at about zero, and then rapidly build up so that at the root, you have huge bending moments and modest shear loads. And because airplanes are meant to fly, weight is the enemy - so you really ought to tailor the ability of the spar to carry shear and bending moments. You use carbon fiber to make light parts...

To carry big bending moments in carbon fiber at low weight, you need enough unidirectional carbon fiber at the top and bottom of the spar, and then reduce the amount of unidirectional fiber as you proceed from root end to tip end. To carry the significant shear at the root, the web that goes between the caps should be at +/-45 degrees and be enough to carry the root shear, then be reduced as you proceed towards the tip.

Now let's look at the rectangular tube. Maybe it has +/-45 degree laminate for a shear web, but if it has enough area for shear loads at the root, it will have big excess even a third of the way to the tip. Heavy. The bending resistance will be modest compared to needs, so you will have to add unidirectional carbon fiber to the top and bottom surfaces, and tailor it to be capable of carrying the bending everywhere. So this will now be too heavy because of excess material over much of its length for carrying shear, and it will be somewhat deeper at the root than at the tip. I guess you can make your wing shape fit over the spar, but the glue line putting the two together will be impressively heavy. Ugh.

This is why almost everyone who wants a carbon spar figures out their desired wing, figures out the acceptable spar profile at numerous positions along the wing, then makes a mold or fixtures to make a spar with highly tailored caps and web.

For your carbon fiber tube to work, you would have to get lucky on depth, width, and wall thickness. And then it will be about triple the weight it could be if tailored plus whatever high amount of adhesive to connect it to the wing skins. Not a good way to strength at min weight.

Billski

i forgot to specify that i'll be using strut, the bending will be virtually inexistant

 

[Merlin]

Pretty pricey? Have you compared it to aircraft grade spruce? The ratio changes from day to day but on a strength to price basis carbon fiber became cheaper than spruce several years ago.

I was mainly wondering if Alibaba carbon fiber was reliable

 

[wsimpso1]

i forgot to specify that i'll be using strut, the bending will be virtually inexistant

Not using a cantilever wing, but a strut braced one. Correct? Bending is still present in strut braced wings, it is just lower than in cantilever wings. The moments are identical from tip to the where the brace attaches. Put the brace at the right place and the max bending moment between brace and root fitting is as large as the bending at the brace, but of opposite sense. And then the strut braced wing gives big compression loads on the spar between brace and root that also has to be handled. The spar is still a spar, just lighter on the inner section. While the weight penalty for using a straight section is not a severe, it must still be analyzed and specified.

You are still unlikely to find a suitable COTS tube that both does the job and is reasonable on weight.

Billski

 

[Rik-]

T

Merlin,

Several issues. The shear loads and bending moments start out at the tips at about zero, and then rapidly build up so that at the root, you have huge bending moments and modest shear loads. And because airplanes are meant to fly, weight is the enemy - so you really ought to tailor the ability of the spar to carry shear and bending moments. You use carbon fiber to make light parts...

To carry big bending moments in carbon fiber at low weight, you need enough unidirectional carbon fiber at the top and bottom of the spar, and then reduce the amount of unidirectional fiber as you proceed from root end to tip end. To carry the significant shear at the root, the web that goes between the caps should be at +/-45 degrees and be enough to carry the root shear, then be reduced as you proceed towards the tip.

Now let's look at the rectangular tube. Maybe it has +/-45 degree laminate for a shear web, but if it has enough area for shear loads at the root, it will have big excess even a third of the way to the tip. Heavy. The bending resistance will be modest compared to needs, so you will have to add unidirectional carbon fiber to the top and bottom surfaces, and tailor it to be capable of carrying the bending everywhere. So this will now be too heavy because of excess material over much of its length for carrying shear, and it will be somewhat deeper at the root than at the tip. I guess you can make your wing shape fit over the spar, but the glue line putting the two together will be impressively heavy. Ugh.

This is why almost everyone who wants a carbon spar figures out their desired wing, figures out the acceptable spar profile at numerous positions along the wing, then makes a mold or fixtures to make a spar with highly tailored caps and web.

For your carbon fiber tube to work, you would have to get lucky on depth, width, and wall thickness. And then it will be about triple the weight it could be if tailored plus whatever high amount of adhesive to connect it to the wing skins. Not a good way to strength at min weight.

Billski

The Quickie Q200's use a Carbon Fiber Spar tube in them. It is a tapered tube as there is no need for as much strength at the ends of the canard as there is at the root as your suggesting but this idea is nothing new as the Q's were what? 1990ish.

 

[thjakits]

i forgot to specify that i'll be using strut, the bending will be virtually inexistant

You still have bending!! You ALWAYS have bending - the strut is just a different way to approach the overall wing design which distribute the loads (ALL spar carried loads!) to 2 pick-up points instead of 1....

Everything said about spar activity and suggested approach to a tailored spar stay valid! Just the spar will have a different tappering layout and different sizing compared to a cantilever...

Alibaba carbonfibre: you CERTAINLY want a material certification and a complete data-sheet!

Carbonfibre comes in as many forms as glassfibre and varies in quality just as much!

Just ordering from Alibaba because it's a good price is a near certain way to disaster!

Carbonfibre is AWESOME, but a serious ***** to actually get ANY benefit!!
You need to EXACTLY calculate your requirements (including safety factors, which are usualy higher for carbonfibre for it's severe failure mode - no warning by yielding - it just snaps, so you need to have a lot more safety factor to compensate), then EXACTLY calculate with the fibre at hand (which you cannot if you do not have the full data-sheet), then you need to design the exact layout of the fibre and the expoy you want to use (again complete data-sheet needed!).
The ONLY data-point you do not need at this stage is "price".....

The weight savings possible with carbonfibre come - usually - with a VERY high price, because of the exact design neccessary to realise the possible weight savings....
Frankly - I BET you would be able to build lighter and cheaper with NONE-carbonfibre!
(Are you equipped with vacuum? ...and a oven big enough for your spar?)

If you buy a ready made rectangular tube - you are already sub optimal as it is not optimized....

Ask Space-X about carbonfibre ....and why they switched to stainless!!!

You mentioned struts - maybe your ride is similar to a Zenith? Check the various build videos and have look at the wings and especially the spars!!

 

[Merlin]

You still have bending!! You ALWAYS have bending - the strut is just a different way to approach the overall wing design which distribute the loads (ALL spar carried loads!) to 2 pick-up points instead of 1....

Everything said about spar activity and suggested approach to a tailored spar stay valid! Just the spar will have a different tappering layout and different sizing compared to a cantilever...

Alibaba carbonfibre: you CERTAINLY want a material certification and a complete data-sheet!

Carbonfibre comes in as many forms as glassfibre and varies in quality just as much!

Just ordering from Alibaba because it's a good price is a near certain way to disaster!

Carbonfibre is AWESOME, but a serious ***** to actually get ANY benefit!!
You need to EXACTLY calculate your requirements (including safety factors, which are usualy higher for carbonfibre for it's severe failure mode - no warning by yielding - it just snaps, so you need to have a lot more safety factor to compensate), then EXACTLY calculate with the fibre at hand (which you cannot if you do not have the full data-sheet), then you need to design the exact layout of the fibre and the expoy you want to use (again complete data-sheet needed!).
The ONLY data-point you do not need at this stage is "price".....

The weight savings possible with carbonfibre come - usually - with a VERY high price, because of the exact design neccessary to realise the possible weight savings....
Frankly - I BET you would be able to build lighter and cheaper with NONE-carbonfibre!
(Are you equipped with vacuum? ...and a oven big enough for your spar?)

If you buy a ready made rectangular tube - you are already sub optimal as it is not optimized....

Ask Space-X about carbonfibre ....and why they switched to stainless!!!

You mentioned struts - maybe your ride is similar to a Zenith? Check the various build videos and have look at the wings and especially the spars!!

Thanks, yes my build is similar to a Zenith 701 in someway but i don't want to get into revets and aluminum sheets as it will drive the cost, weight and complexity beyond acceptable margin.
for me is either wood or CF

 

[Merlin]

Thanks, yes my build is similar to a Zenith 701 in someway but i don't want to get into revets and aluminum sheets as it will drive the cost, weight and complexity beyond acceptable margin.
for me is either wood or CF.
i absolutely don't want to deal with rivets.

 

[wsimpso1]

i forgot to specify that i'll be using strut, the bending will be virtually inexistant

Here is an example of wing shear and bending diagram for a strut braced wing. Example airplane is 1320 pounds, carrying 6 g, 360 Inch span, elliptical lift distribution, root mount is at BL24, while strut is placed at BL120.6" to minimize bending moment in spar set. Note the moment at the strut and at the worst spot between strut and root have about the same bending moment, but of opposite sense - 22261 in-lb. Strut is under 5888 pounds tension, and the spar set must carry 5358 pounds compression over its roughly 96" length.

While a cantilever wing will have about 10 times as much moment at its root than the max bending moment in this strut braced wing, it is not zero and must be designed for while also carrying the compression resulting from the strut loading.

Billski

 

[stanislavz]

Thanks, yes my build is similar to a Zenith 701 in someway but i don't want to get into revets and aluminum sheets as it will drive the cost, weight and complexity beyond acceptable margin.
for me is either wood or CF

Same on my side. Try to get carbon fiber pultrusion rods, for cantilever like 701 you do need more than 100metres / 300 foot of 3x3 mm rods. Which on money basis is ~ 200 eur on this side of ocean. And it is cheaper than special aluminium profiles or tubes for similar application. Because you need only two tubes, and shipping is horrible expensive to say at least. And no one wants you sell only one-two tubes of 2024 alloy..

What kind of skin you will use ? Do you have more data of your bird ?

 

[Merlin]

Same on my side. Try to get carbon fiber pultrusion rods, for cantilever like 701 you do need more than 100metres / 300 foot of 3x3 mm rods. Which on money basis is ~ 200 eur on this side of ocean. And it is cheaper than special aluminium profiles or tubes for similar application. Because you need only two tubes, and shipping is horrible expensive to say at least. And no one wants you sell only one-two tubes of 2024 alloy..

What kind of skin you will use ? Do you have more data of your bird ?

I think i'm going to be using wood spar reinforced with unidirectional carbon fiber covered with fabric.
I will do one destructive test to make sure i'm within my safety limit.

 

[stanislavz]

Think i'm going to be using wood spar reinforced with unidirectional carbon fiber covered with fabric.
I will do one destructive test to make sure i'm within my safety limit.

Do not use ud carbon, on way round it is not cheaper and home build is not stable in its performance as it shown in books..

 

[Merlin]

Do not use ud carbon, on way round it is not cheaper and home build is not stable in its performance as it shown in books..

Could you explain please?

 

[mcrae0104]

I think i'm going to be using wood spar reinforced with unidirectional carbon fiber covered with fabric.

Why uni carbon and not carbon rods? While wood and carbon has been done (e.g. Gazaile), essentially what you end up with is the wood acting as a relatively heavy core. Do a search here and you will find a lot of discussion about wood/carbon spars.

 

[stanislavz]

Could you explain please?

Kind of ch701 in mtow/span, extra fos 2 for composites load.



One strut placed at 2 m from tip, gives you 1.42 cm2 of spar cap cross area. And this all is done not from single crystal of carbon, but from many, many separated filaments : Each carbon filament in the tow is a continuous cylinder with a diameter of 5–10 micrometers. And if you will be able to load each filament with same load - you will have all filament working as one unit. But - if you filament have some fibers at 89 degree, some are wavy, - they will not work at all. Only their neighbors will take all load. Jim Marske in his book, have tested some of ready made ud carbon spar till destruction - sad story was on broad range of permissible loads - 130 000-137000 for tensions, and 75 000 - 50 100 for compression. ~ 30% of spread for compression. And it was done by pro. If done by first time builders - big X here..

Pultrusion will give much more stable parameters...

 

[Merlin]

Why uni carbon and not carbon rods? While wood and carbon has been done (e.g. Gazaile), essentially what you end up with is the wood acting as a relatively heavy core. Do a search here and you will find a lot of discussion about wood/carbon spars.

Its mainly because i can't source reliable carbon fiber at affordable price

 

[Geraldc]

Its mainly because i can't source reliable carbon fiber at affordable price

There are products like Sika Carbodur or BASF Masterbrace for stengthening buildings that you can use.They also supply glue for stacking strips to desired thickness.

 

[Dusan]

You can buy two cf spars, build one wing, test it to failure, and if you're happy with the result, build the second wing exactly the same, and hope the second spar is similar enough to the first one