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Old Oct 16, 2012, 10:30 PM
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FlyBoy20's Avatar
United Kingdom, Wales, Swansea
Joined Aug 2012
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Question
When is carbon fibre rod ..not?

I bought some 'carbon fibre rod' today in the local model shop. I thought at the time it didn't seem very stiff, not like the rod I bought years ago. So when I got home, I compared it with the old one.

It was like spaghetti in comparison!

So, what's going on, are there different grades now or what? Do I have to interrogate suppliers before buying to make sure I'm getting the Real McCoy?
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Old Oct 16, 2012, 10:46 PM
I think I'm inverted. Maybe.
acetech09's Avatar
United States, CA, Pacifica
Joined Apr 2012
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There can be a vast difference in composite rods...

(Just checkin' - they're both solid-core right? Tubes will be quite stiff compared to solid rods.... and tubes are more common than rods.)

... there shouldn't have been a huge difference - you can tell a difference between cheap and expensive carbon fiber, but it shouldn't be like spaghetti as you said.

Measure true diameter and length, and weigh it to the old one. A good-quality rod like your old one should be lighter as it uses less resin and more carbon fiber.
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Old Oct 16, 2012, 10:53 PM
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Latvia, Ventspils pilsēta, Ventspils
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There are many factors to CF ....

Wall or rod thickness

Density of the fibres

Carrier / binding resin used.

These all affect the properties of the CF you can buy.

Not everyone wants bone-stiff rod or matt ... some want it to be able to curve and form to complex shapes ... and then resin into place. Others want stiff to impart rigidity to a form. Plus the people who want something inbetween.

Personally - I prefer bamboo skewers ! but then again I'm a cheapskate and object to paying for delivery from a shop 3hrs drive away etc. For sheet applications - I use very thin balsa with thinned epoxy - creates a really tough laminated skin.
ie : HK Parkjet is well known for the elevons to be flexible ... ok for stock speed, but as soon as you start to push the speed up - they need stiffening. Many use CF rod set into them. Me ? 1/16th balsa sheet on the entire underside of the elevon set with thinned epoxy. Those elevons do NOT twist or flex at all ... surface is smooth ... no ridge or bump from CF rod.


Nigel
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Old Oct 17, 2012, 01:04 AM
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Livermore, CA
Joined Sep 2004
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Usually the black rods that seem to flex way too much, are fiberglass rods with black resin.
If you sand the end, and it doesn't stay black like CF, but looks more white, its probably fiberglass or a mixture of fiberglass and CF.
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Old Oct 17, 2012, 07:11 PM
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Originally Posted by solentlife View Post
.. I use very thin balsa with thinned epoxy - creates a really tough ..laminated skin...
Interesting technique. I'd like to try that, but what do you use to thin down epoxy?
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Old Oct 17, 2012, 08:08 PM
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Carbon fibre is for the very few professional model fliers.What the parkfly manufacters pretend is cf is just black plastic tubing.Which is good enough for us and cheap.All we need is rigid tube to hold foam wings in place, not space shuttle technology.
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Old Oct 17, 2012, 09:20 PM
222 km/hr Parkjet flyer
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Latvia, Ventspils pilsēta, Ventspils
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Quote:
Originally Posted by FlyBoy20 View Post
Interesting technique. I'd like to try that, but what do you use to thin down epoxy?
The best is to get to a shop that sells laminating Epoxy resin ... such as West Systems by Gudgeon Brothers.
Many DIY / Builders suppliers will have the thin variety ....

If you just want to thin a small amount - then Methylated Spirits will do it - but check first that material you are laminating to is not affected by the Spirit. Thinning thicker 'Glue' epoxy seriously weakens it ... so care.

Nigel
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Old Oct 18, 2012, 03:00 AM
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United States, AK, Fairbanks
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Quote:
Carbon fibre is for the very few professional model fliers.What the parkfly manufacters pretend is cf is just black plastic tubing.Which is good enough for us and cheap.All we need is rigid tube to hold foam wings in place, not space shuttle technology.
Black plastic tubing? That's interesting. Got links?

Carbon fiber is much more widely used than you seem to think. Everything from props to wing stiffening to motor cases to entire airframes can be had in CF. Many parkflyers do use black fiberglass in place of CF, but the difference in strength and weight at those levels isn't an issue.
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Old Oct 18, 2012, 04:16 AM
Tellin' it like it is!
Joined Sep 2012
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I'll tell you this straight up bro...

In the overall scheme of things, the carbon fibre spars used in the RC game are trash. Other hobbies and sports moved on from crummy pultruded rods over a decade ago. Wrapped and woven carbon is where it's at, yo!

Check out some modern archery spars and you'll start to see just how far the RC stuff is from good. Even the modest archery stuff is streets ahead.
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Old Oct 18, 2012, 04:56 AM
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Quote:
In the overall scheme of things, the carbon fibre spars used in the RC game are trash. Other hobbies and sports moved on from crummy pultruded rods over a decade ago. Wrapped and woven carbon is where it's at, yo!
Woven carbon is already used extensively in RC. Take a look at pylon racing props and airframes, or helicopter/multicopter frames, blades, and canopies.

As for spars, the people who stiffen their foamy with the biggest, beefiest bit of CF they can find are usually doing it wrong. In applications like golf clubs or bike frames there's no matrix material surrounding the carbon fiber parts, so they have to provide all their own strength and stiffness. With RC planes, you've almost always got some way to play the CF's tensile strength against the material's compression strength, or even against an opposite CF member with material in between. Rather than supporting the load by bending a single spar, you can save a lot of weight by stretching and compressing a couple of them. Thin CF flats oriented edge-on to the direction of the bending force can also be extremely effective at adding strength for like zero weight gain.
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Old Oct 18, 2012, 10:08 AM
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Originally Posted by JamesNeal View Post
I'll tell you this straight up bro...

Check out some modern archery spars and you'll start to see just how far the RC stuff is from good. Even the modest archery stuff is streets ahead.
What do you mean by archery "spars"? I've been in that game for years and never heard the word, even though I used very highly rated equipment. Just curious...

G.
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Old Oct 18, 2012, 11:49 AM
I think I'm inverted. Maybe.
acetech09's Avatar
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I think he means shafts...
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Old Oct 18, 2012, 12:17 PM
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Joined May 2003
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James, can you post some pics of your planes? Weight before and after covering would be nice too. I found some lightweight CF mat here http://www.fibreglast.com/product/1k...er-fabric-2365. Of course, that doesn't include the weight of the resin.
I've used arrow shafts for a fuselage, but never for a spar.
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Old Oct 19, 2012, 11:03 AM
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Originally Posted by acetech09 View Post
I think he means shafts...
Assuming it is arrow shafts (the remaining archery equipment, like limbs, stabilisers and sights, are made on purpose so they have specific properties for their intended purpose), there is not that much sophistication in the CF ones, except for production accuracy and consistency (geometry, weight and stiffness).

(I am not not including here the alum/carbon varieties - Easton ACE, X10, Navigator... - in my opinion only a fool would want to use such horrendously expensive materials on RC planes).

Actually, I have heard it said some years ago that RC plane and kite manufactureres were using for spars exactly the same kinds of products that were used for arrow shafts (supposedly made by the same menufacturers?), only subject to less strict specifications - which only make sense, as the requirements are vastly different.

I don't see why RC enthusiasts should be ashamed, so to say, of the materials presently used in the hobby, and much less compare them with the materials used in a totally different sport.

Not to say, of course, that improvement should not be looked for, but I think there has been a lot of that in late years (when I was youngish, as far as I can remember there was only circular flight and free flight - lots of rubber motored planes back then, balsa frames, paper covered and banana varnish).
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Old Oct 19, 2012, 11:07 AM
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What is banana varnish?
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