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Jan 10, 2021, 12:43 AM
Everything’s A Compromise
Larrikin's Avatar
Thread OP
Question

Carbon tow spar cap sizing.


G'day,

Dunno if this is the right place to ask for help ... but here I am, cap in hand.

I'm wanting to calculate the cross-section of the spar caps for a project I'm working on.
In the past, I'd have just thought of a number and doubled it ... extra weight be damned.
This time I'd like to use a lay mans method that's somewhere between an over-engineered guess and a collection of lines with jumbled greek characters and numerals that are beyond my comprehension and pay grade.

If you're aware of simple calculation for carbon tows that considers bending load and spar depth etc, I love to see it.

I've found THIS on the Charles River site.

So far I have ...

"Take 1/4 of the span in inches and multiply it by half the breaking strength of the tow line in pounds"

66" x 100lb = 6600 in-lbs . ("This is the maximum bending moment at the center of the wing in inch-pounds")

"Divide the maximum bending moment by the (top to bottom) spar depth in inches."

6600 lbs/1.25" = 5280 lbs ("This is the force at the center of the wing required of the the spar caps, in pounds.")

It is then suggested to "Find the strength of the spar cap material in pounds per square inch".

What the ... ???
Modulus, tensile, compression ...? The tow I have is normal/standard modulus (not HM)

Can you help with this number ... or do you have a better method?
I'm all ears.

TIA
David.
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Jan 10, 2021, 10:06 AM
Balsa breaks better
Thermaler's Avatar
Subscribed to your thread too David!

Info I would like for a project that has been held up for the lack of asking the question you are asking . . . ;>)

Joe

Balsa Breaks Better
Woodys Forever
Jan 10, 2021, 01:25 PM
Duane, LSF IV
Wazmo's Avatar
See Mark Drela's document describing how to calculate spar cap sizing.
Jan 10, 2021, 05:22 PM
Everything’s A Compromise
Larrikin's Avatar
Thread OP
Thanks Wazmo,

So,
plugging Dr Drela's value, "cap modulus : E = 2.0x10^7 psi (for common T-300 carbon fiber)" into the next part of the calculation,
"Divide the force in the spar caps by the strength of the spar caps to get the required cross section of the spar caps." I get ...

5280/2.0x10^7 = 0.000264
What area is that, square inches, square feet??? PSI suggests that it's inches.
Square feet makes more sense because it converts to ~25mm^2 ... or 25mm x1mm.
Add in a fudge factor of 1.5 would be 25mm x 1.5mm.

I'm beginning to feel like I should combine old and new methods by doubling the value that I calculate

Aged, confused and drowning in uncertainty.

D.
Last edited by Larrikin; Jan 10, 2021 at 05:39 PM.
Jan 10, 2021, 07:22 PM
Duane, LSF IV
Wazmo's Avatar
Spar cap area (Ac = P / sigma) should be in square inches. It looks like you've confused design cap stress (sigma = 140,000 psi) with cap modulus (E = 2.0x10^7 psi). Double check that you're using all the equations with the units specified for each.
Jan 10, 2021, 11:24 PM
Registered User
You may like this spreadsheet. It has been around quite some time, I think I was turned on to it from a thread in the past. If you don't use it for primary calculations it might at least be a sanity check.

http://www.acrodesigns.com/excel/SparStrength.xls
Jan 11, 2021, 12:45 AM
Everything’s A Compromise
Larrikin's Avatar
Thread OP
Quote:
Originally Posted by Wazmo
Spar cap area (Ac = P / sigma) should be in square inches. It looks like you've confused design cap stress (sigma = 140,000 psi) with cap modulus (E = 2.0x10^7 psi). Double check that you're using all the equations with the units specified for each.
Ok, sooooo ...

5280/140000 = 0.0377in^2 ... which coincidentally converts to ~25mm^2.
Who'd have thunk it?
Jan 11, 2021, 12:48 AM
Everything’s A Compromise
Larrikin's Avatar
Thread OP
Quote:
Originally Posted by CGordon
You may like this spreadsheet. It has been around quite some time, I think I was turned on to it from a thread in the past. If you don't use it for primary calculations it might at least be a sanity check.

http://www.acrodesigns.com/excel/SparStrength.xls
Thanks CG,
Have you used this yourself?

From the Spreadsheet (thanks again) I have the following numbers ... but I don't know what to do with them.

3300 in-lbs = Moment @ Centerline.
12600 psi = Maximum Bending Stress @ Centerline.

For this, I used a conservative 6G load number. Yes, it's for a scale model of an Open Class glider and I agree if you're thinking that 6G is extreme.
But have you seen how quickly things can go wrong on the tow?
This is for the I beam spar where web width "b" equals the width of the spar cap "b". (** Lots of bonding area).

Where to from here?

D.
Jan 13, 2021, 10:42 PM
Entropy is happening!
Jim.Thompson's Avatar

Good subject!


Dave,

Thanks for putting up this thread at this time.
I currently have such a decision right in front of me as you/we/I write.

However, I have a small fact to remind you of:

You and I and most of the Western world are working in metric units of measurement now; surely you must have already noticed, right?

The resort to "Olde Worlde" imperial (former British) units is regressive and on a person level, phases me out completely. Who are you trying to appease?
Don't bother answering that; we already know!......................

From this site here: https://www.worldatlas.com/articles/...al-system.html

Who Uses The Imperial System Today?

Now, this might surprise you, but there are only 3 countries in the world that are still officially using the imperial system: The United States of America, Myanmar, and Liberia.

Which is a very short list indeed!

I rest my case..................

Respectfully yours etc.

Jim.
Last edited by Jim.Thompson; Jan 14, 2021 at 03:46 PM.
Jan 14, 2021, 05:04 AM
Balsa breaks better
Thermaler's Avatar
Graduated high school in 1976 with the knowledge of conversion from Imperial to Metric, it was coming they said. So is Jesus.

I have patience . . .

For those that didn't learn it in high school . . . ;>)

https://www.calculator.net/conversion-calculator.html

Joe


Balsa Breaks Better
Woodys Forever
Jan 21, 2021, 07:20 AM
Registered User
The germans mostly use Christian Barons spreadsheet. His website is not up anymore, but I have attached the file. Change extension from .txt, to .xls.

Lars
Jan 22, 2021, 05:57 AM
Everything’s A Compromise
Larrikin's Avatar
Thread OP
Quote:
Originally Posted by LN-JET
The germans mostly use Christian Barons spreadsheet. His website is not up anymore, but I have attached the file. Change extension from .txt, to .xls.

Lars
Thanks so much, Lars.
I haven’t seen Christian Baron’s name for a very long time.
I’ll crunch some numbers and see how it compares to the fumbled attempts so far.
D.
Last edited by Larrikin; Feb 22, 2021 at 08:17 PM.
Feb 12, 2021, 03:08 PM
Registered User
Quote:
Originally Posted by CGordon
You may like this spreadsheet. It has been around quite some time, I think I was turned on to it from a thread in the past. If you don't use it for primary calculations it might at least be a sanity check.

http://www.acrodesigns.com/excel/SparStrength.xls
Really nice spread sheet. But I need to find the bending strength of CF tow mixed with 6000 PSI epoxy.

What do others use for that value?

Tom f.
Feb 12, 2021, 03:38 PM
Entropy is happening!
Jim.Thompson's Avatar
Can anyone get that text file of Christians to open?
I cannot get it to open in Linux Mint 19.3

Jim.
Feb 12, 2021, 03:42 PM
Registered User
Quote:
Originally Posted by Jim.Thompson
Can anyone get that text file of Christians to open?
I cannot get it to open in Linux Mint 19.3

Jim.
I renamed the extension to .xls and it opens fine in Excel.
It is quite comprehensive and worth getting opened.


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