# Carbon cases and not only ...

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#### LiederB

##### New Member
My greetings to everyone! Here is my process. If you are interested, ask questions...

#### Victor Bravo

##### Well-Known Member
HBA Supporter
Thank you for posting this.

But also, thank goodness for Youtube's "mute" feature

There are several steps in the process that are shown repetitively, and yet several other steps that are not shown or shown adequately. For example, during the first laminations, it is not shown if or when you remove the white colored backing sheets from the carbon. It is not clear when those sheets are in place and when they are removed.

Also, how is the carbon held onto the backing sheets, such that it stays in place but the backing can be removed without disturbing the weave?

#### LiederB

##### New Member
Thank you for posting this.

But also, thank goodness for Youtube's "mute" feature

There are several steps in the process that are shown repetitively, and yet several other steps that are not shown or shown adequately. For example, during the first laminations, it is not shown if or when you remove the white colored backing sheets from the carbon. It is not clear when those sheets are in place and when they are removed.

Also, how is the carbon held onto the backing sheets, such that it stays in place but the backing can be removed without disturbing the weave?
After moistening the carbon fiber with resin, the fabric does not disintegrate ...
White Scotch Tape Backing Cut

#### D Hillberg

##### Well-Known Member
Rather do aluminum or steel = End of parts life beer money

Plastics and synthetic you have to buy a hazmat drum = No beer money

#### LiederB

##### New Member

This part weighs 2.17 lbs with the wheel. It can be installed on Kitfox7.

#### proppastie

##### Well-Known Member
Log Member
How did you decide on how thick to make the parts

#### Lendo

##### Well-Known Member
Having looked at a Carbon Main Landing gear legs, helped by Billski who did the calculations, I assume it was calculated then trialed and tested.
George

#### proppastie

##### Well-Known Member
Log Member
calculated
so what was the load calculated for (G loading you used)....and do you understand the assumptions or formula used to determine the thickness.....for example the OP tail spring looks like "black steel or black aluminum"...that being usually meant to mean the same thickness and dimensions except made out of Carbon Fiber.....certainly that is less weight but could the part be even thinner and weight even less had some form of calculations been used to determine a new thickness. Oh and is the existing carbon part stiffer less stiff than needed or than the original part.

This is not a criticism ....it is a beautiful part and thank you for posting......however I see it as an opportunity using a simple case to learn some elementary stress analysis for Carbon....should the OP or Lendo care to comment. ....would be looking for Loading.....allowable for the material.....along with special calculation methods used for CF..........one can not get much more simple than a cantilever beam loaded at the tip as the tail spring represents. Theory with general equations often is much harder than a real world example with real numbers......One reason I like Bruhn because he has mounted many real world examples.

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#### wsimpso1

##### Super Moderator
Staff member
Log Member
I know nothing of how this tail wheel assembly was designed or analyzed and the maker did not say on the video. I will say that it looks like a "black aluminum" design. The vagaries of a competing metal design appear to have been carried into the graphite fiber part. That may help with marketing...

I can tell you that if the builder merely duplicated dimensions of an aluminum fork, it is WAY overbuilt while still lighter than the base aluminum part.

As for the spring, if it is unidirectional carbon and duplicates a steel tailwheel spring in cross section and length, it will have about a 30% lower spring rate than steel, and be about as strong as a heat treated 4340 spring, so it will suck up more energy than the steel one, which is good because the graphite one will likely be turfed when its landing energy limit is exceeded while the steel one will bend but still be on the airplane.

Once one has the unidirectional part of the spring designed, any highly stressed unidirectional composite is subject to edge delamination due to shear. This is usually countered by putting wraps of woven cloth around the whole thing, with fibers at +/- 45, which slightly stiffens the spring, and may modestly lower the max load the spring can carry.

Then there is how to attach it to the airplane. If clamped, the clamping method has to avoid any corners bearing on the spring surfaces - a crowned shape designed to have gradual reduction in contact stresses tapering to no contact at max spring load works. Local thickening of the spring may also work, but will require a length increase to get back to a low enough spring rate.

The bolts through the spring drive greatly reduced loading and thus bulk at the wheel end of the spring. Bolts through the forward end will drive the design to an even bigger spring.

All of this raises the spring rate and reduces the max landing energy before broken parts occur, but may still make it comparable to the competition for energy absorption while saving some weight.

I do know that if this engineer was building it, I would have started with the worst landing case for the airplanes intended (FAR Part 23 and lots of STOL video out there to analyze in order to extract worst case) then tailor the fork and spring to the anticipated loads and landing energy to be absorbed. More weight would have been saved, and it might even be sturdier in the real world if graphite can be removed from a place where it is not doing any good and put someplace where it helps more. Composites really do allow this sort of optimization.

Billski

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#### proppastie

##### Well-Known Member
Log Member
if it is unidirectional carbon
from the video it appears to be BID woven ? ......

#### wsimpso1

##### Super Moderator
Staff member
Log Member
The fork is bidirectional carbon at 0-90. Making it strong enough and still saving weight is child's play. The manufacturer skipped what they must have considered proprietary parts of the process, then showed us the entire assembly including the tailwheel spring, which probably has more opportunity for weight save than the fork. I spent text on the spring, which I feel they should have too.

#### proppastie

##### Well-Known Member
Log Member
what would the raw materials for say the fork as shown in video might cost?.......off the top of your head ......no reason to research it because it will change in the future

#### wsimpso1

##### Super Moderator
Staff member
Log Member
Carbon fiber cloth, when you can find it, is about $50 a pound. Epoxy starts at about$12 a pound and goes up from there. Tooling and man hours looked like the expensive part of the assembly.

#### LiederB

##### New Member
Hello! Merry Christmas and Happy New Year to everyone.
Thanks for the good response to my post.