Item 1073

 OTHER: Composite - General - Tow

Database:

Information and calculations are located in FORM: Tow in the database.

Tow Size:

A comment that may be relevant to rigid blades; "There are carbon fiber suppliers that produce 50K tow sizes at lower cost/pound than the comparable 12K tow."

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Standard aerospace-grade structural carbon fiber tows are 3k, 6k and 12k.

Larger tow products are generally used in commercial, non-aerospace type applications.

Zoltek produces 48k, 160k and 320k tows.

Perhaps with the spar, deformation not strength is the limiting factor. In this case perhaps commercial tow can be used. Consider 50K prepreg carbon tow, AKZO-50K standard modulus fiber (is this carbon?). Or, possibly Toray M30S-18K intermediate modulus carbon fiber.

Tow 12K:

12K yarn has 12,000 filaments in a single stand.

Tow Count:

Carbon 12K filament tow has a cross sectional area (CSA) of 72.00 x 10-5 in2 to 75.40 x 10-5 in2 . This is an average of 0.00074 in2 [Source ~ FWCS Table 5-2]

Possible Alternatives:

50K filaments should be 50/12 x 0.00074 in2 = 0.00308 in2.

320K filaments should be 320/12 x 0.00074 in2 = 0.01973 in2.

Thoughts re Spar:

If 0.70 in2 is area of carbon thread at the blade root, at the top and at the bottom, then there are a total of 0.70 in2 / 0.00074 in2 = 946 12K strand looping around the upper studs and 946 12K strands looping around the lower studs. Note that 946 strands is 473 loops. If they loop around 8 studs then there are 118.25 loops per stud. If there are 9 tows per layer then there are 118.25 / 9 = 13.13 layers. The pin to pin gap is only sufficient for 4 (possible 5) tows, therefore there must be 3 sets of wraps per layer, for a total of 3 x 13 = 39 layers on each pin.

Consideration for Type of Wrap (Helical vs. Hoop) and Resin

Note the Fiber Volume Fractions on [Source ~ FWCS Table 5-4]. The spar and the torque tube will be Hoop.

The area of a 1" diameter circle is pi R2 = 0.7857142

 

Note:

From Aircraft Spruce:

The 12K is difficult to wet out but can be done by diligent brushing.
It appears that the 6K tow will prove most practical for homebuilding applications.

Pricing:

Form Manufacturer (in very large quantities probably):

12K carbon fiber. One manufacturer says the company will supply regular customers only, and has no fiber available for new applications. Standard-modulus fiber that sold for $7.50/lb a year ago is now selling for $10/lb to $11.50/lb,

From Aircraft Spruce:

12 K CARBON FIBERS 4LB SPOOL 12K $138.50

Drawing:

There is the drawing 1073_WORKING.dc that shows the determination of the size of the voids between the filaments.

But first note the section [Tow Count] above and the reference to [Source ~ FWCS Table 5-2]

The area of a 1" diameter circle is pi R2 = 0.78571422".

The area (void) between three 1" diameter circles, which are touching each other, is 88 * 0.02" * 0.02" squares = 0.03522". This value was derived from laying circles over a 0.02 * 0.02" grid and counting squares. See 1073_WORKING.dc

The void between circles is 0.0352 / (0.7857142 + 0.0352 = 4.2879% of area of one circle and one void. If thousands of these circles were grouped (bundled) within a larger circle there would be additional voids around the circumference of the larger circle. These additional circumferential voids will be approximately three time the area as the internal voids. It is therefore reasonable to say that under absolute conditions, the ratio of thread to epoxy would be 95:5.

Current pultruded carbon has a ratio of AROUND 70:30 carbon:epoxy

Source:

 

BUY: Composite - Tow - Carbon/Graphite - Towpreg

1370

Outside Sites:

Carbon Fiber Property Translation into Composite - A Comparison of Commercial Grade 48K Carbon Fibers Versus 12K Aerospace Fibers. http://www.zoltek.com/technical_resources/CMOOPaper.pdf ~ Have hard copy.

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Last Revised: July 19, 2005