A different way to build a wing

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wsimpso1

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I can talk about foam in a slightly different sense. I can take a chunk of blue 2 pcf foam with cross section 3-1/4x7 inches and it will stand between 2600 and 4000 inch pounds of bending moment. This average looking 63 year old can not quite break it in two with his hands. Other higher density foams are stronger, roughly in proportion with their density, but that is still close to nothing. In comparison, I have a 3-1/4 by 7 inch fiberglass channel that is designed to carry over 850,000 inch pounds of bending, 11,700 pounds of shear, and take whatever torsional deformation the wing generates. That is 300 times more strength than if we used solid foam in the same dimension, and is on the order of what we need in our airplanes.

Plastic foams do so little for strength that we can ignore it in our computations. Plastic foam allows us to make reasonably light, precisely dimensioned composite parts of incredible stiffness and strength so we include it...

Billski
 
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Sockmonkey

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Foam is generally fully encased in fibers and resin that contribute the vast majority of the stiffness and strength of the structures. The foam then moves with the rest of the structures. The foam is great for getting the shape you want for the composites that go around it, and then helping them stay in that shape. Beyond that, they contribute measurable but small load carrying capacity.

Use foam for carrying shear and bending (what spars do)by itself? Only for really small loads... Holding foam in your hand, it is only a little better than wet noodles...

Let's get a simple concept in place and then do some things with it. Axial load is easier to get, we will expand to bending and torsion further down.

If you have a foam and fiberglass structure and you load it, the loads will be distributed per the stiffness of each of the pieces. Stiffness in tension or compression is the sum of EA for any cross section. Imagine a strut of foam, wrapped in fiberglass and put in tension. If it is an ellipse 1"x3" and the fiberglass is 0.021" thick, the cross section will have 6.28" perimeter of glass, 0.132 in^2 of glass and 2.356 In^2 of foam. Your glass cloth and epoxy will have E of about 2.2Mpsi and the foam has an E of about 500 psi. EA of the glass is 290000 lb, while EA for the foam is about 1178 lb, for a total stiffness of 291000. The foam is around 0.4% of the total. Apply 1000 pounds to that strut and the glass carries 996 pounds, while foam carries 4 pounds.

We could look at bending stiffness as that combined with the inverse of length squared is what determines when this strut will buckle under compressive load. In bending, EI is the stiffness. Fiberglass I is about 0.010, foam I is about 0.137. EI of the fiberglass is about 22110 lb*in^2, while the foam is about 69 lb*in^2, for a total 22180 lb*in^2. Foam is about 0.3% of the bending stiffness. Torsion will be similar.

Billski
Yeah, I assumed the fiberglass was most of the strength, but fiberglass wings usually have a spar of something stronger yes?
 

Vigilant1

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Yeah, I assumed the fiberglass was most of the strength, but fiberglass wings usually have a spar of something stronger yes?
Solid core fiberglass wings would have fiberglass/epoxy wing skins and a spar. The spar would have caps of fiberglass/epoxy or carbon fiber/epoxy and these caps are joined by a spar web of (usually) the same material as the caps. With solid foam cores it is practical to lay up the spars caps and web directly on the foam, then apply the skin.
 

wsimpso1

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Yeah, I assumed the fiberglass was most of the strength, but fiberglass wings usually have a spar of something stronger yes?
I suspect some really large concept errors if you are asking that question... In factory made fiberglass airplanes or kits, yeah, some use graphite spar caps. Others use fiberglass. What makes you think that you need really strong material to make the spars? Why are spars special?

If you need to carry 30,000 pounds in a rod under tension, you can carry it with 3 in^2 of 10 kpsi material of 1 in^2 of 30 kpsi material or 0.40 in^2 of 75 kpsi materiel or 0.10 in^2 of 300 kpsi material. All will just barely do the job. Which one is lightest might be your next question...

There have been a bunch of airplanes made with 6061 aluminum tubes as main spars with a yield strength of about 35 kpsi, and many many airplanes with spars made of Sitka Spruce. Are they inadequate? Nope. If you build your wings in aluminum, 7075 -T6 is about as good as it gets, and yield strength is about 63-69 kpsi.

My fiberglass wing has fiberglass spars and spar caps. Lots of VariEze, LongEz, and derivatives use E-glass caps... Lots of Glasair I, II, and III used glass too.

Breaking strength of unidirectional E-glass-epoxy in the direction of the fibers is around 70 kpsi, while unidirectional graphite rods are about 300 kpsi. If strength were the only thing going on, you would need a little over four times the cross sectional area of uni glass as you do uni graphite rods. E of the uni E glass composite is about 4.4 Mpsi while graphite rods are around 24 Mpsi, so if stiffness were all that mattered, you would need a little over 5.5 times the area in glass than in graphite.

Billski
 

rv7charlie

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Given the discussion of spar webs in this thread, I found it an interesting coincidence that I got this 'recommendation' from youtube yesterday:

Not saying they're right, or that it is directly related to this discussion. Just a funny coincidence.

Charlie
 
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