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        Discussion The lazy method - Carbon Spar Caps

#1 TOSSPilot Feb 08, 2013 06:13 PM

The lazy method - Carbon Spar Caps
 
I am wanting to cap my spars in the center section with carbon and have found the numerous postings that discuss the process. The plane has a relatively small spar cap (3/8x1/8) and I noticed that I can just a replacement piece of carbon with the same size as the bass spar caps, therefore avoiding the gluing and fitting process.

Two main questions -

1. Why not just replace the bass spar cap witth the carbon one vs. laminating the carbon tape? Too much added weight? Cost?
2. Does a nostalgia class plane get disqualified using a solid carbon spar cap?

I am about to order the carbon and just wanted some feedback on this idea before I push the order button.

Thank you for any input.

Craig

#2 LVsoaring Feb 08, 2013 08:08 PM

1) I would think cost would certainly be a factor...

2) Good question..... I don't know but am interested to find out.

#3 1967250s Feb 08, 2013 10:39 PM

Does it really need a stronger spar cap? What plane is it? Wing dimensions? Skin/covering?
What will be it's uses?

#4 TOSSPilot Feb 08, 2013 11:17 PM

It is the Mirage. The wing strength is probably decent in stock form and I know this would be an over kill but I do plan on flying it in open class competition and would like the extra launch/winch ability without worry.

I do plan on wrapping the joiner boxes with kevlar thread. Not sure if I am creating a major stress point at the wing joiner box going from carbon spar back to bass wood in the tip panels?

The cost is about $25 vs. $8 for the tape but not really a big concern if it is easier than laminating carbon tape and sanding down the wood for the correct fit.

I have not seen too many post about just replacing the entire spar cap with a pure carbon cap.

Thank you again for the thoughts

.

Craig

#5 TOSSPilot Feb 08, 2013 11:22 PM

On the CST website, this is the product.

Carbon Fiber Rectangle, 0.125" x 0.375" x 48"
3.2 mm x 9.5 mm x 1.2 m, ??? grams T726L4 $12.95

#6 ieyasu Feb 09, 2013 02:36 AM

In case it's not too late, here are some numbers. Carbon fiber is something like 20-30x stronger than spruce or basswood for an equal size, but it's also heavier. If you use solid 1/8x3/8 carbon caps, I'd bet everything else in the wing will break before those caps. Some quick math shows each 48" cap would weigh about 2 ounces, while a wood cap of the same size would be more like 0.6~0.7 ounce.

If this page is still authoritative, you just might squeak by with a solid carbon spar, depending on the CD.

#7 Thermaler Feb 09, 2013 03:04 AM

Quote:

Originally Posted by ieyasu (Post 24079868)
If this page is still authoritative, you just might squeak by with a solid carbon spar, depending on the CD.

Those are the original rules not the current rules.

Joe

thermalrider.com

#8 John Walter Feb 09, 2013 09:44 AM

The Miarage is a great plane. The key to its design is the balance among all is components.

A solid CF spar will weigh too much. Without changing the rest of the spar, you would simlpy tear off the tips before the main spar saw any substantial load. Also, since there is no balsa sheeting, the wing would flutter long before the main spar loads became an issue.

.007 CF on the top spar, or .014 top and .007 bottom would give more than plenty of strength.

#9 LVsoaring Feb 09, 2013 10:01 AM

1 Attachment(s)
Quote:

Originally Posted by TOSSPilot (Post 24079066)
It is the Mirage. The wing strength is probably decent in stock form and I know this would be an over kill but I do plan on flying it in open class competition and would like the extra launch/winch ability without worry.

I do plan on wrapping the joiner boxes with kevlar thread. Not sure if I am creating a major stress point at the wing joiner box going from carbon spar back to bass wood in the tip panels?

The cost is about $25 vs. $8 for the tape but not really a big concern if it is easier than laminating carbon tape and sanding down the wood for the correct fit.

I have not seen too many post about just replacing the entire spar cap with a pure carbon cap.

Thank you again for the thoughts

.

Craig

Quote:

Originally Posted by John Walter (Post 24081609)
The Miarage is a great plane. The key to its design is the balance among all is components.

A solid CF spar will weigh too much. Without changing the rest of the spar, you would simlpy tear off the tips before the main spar saw any substantial load. Also, since there is no balsa sheeting, the wing would flutter long before the main spar loads became an issue.

.007 CF on the top spar, or .014 top and .007 bottom would give more than plenty of strength.

The bend load decreases rapidly as you move from the root to the tip, so that should not be a problem. I have attached a good article that I got somewhere, but don't remember where. Apologies to the author for not giving proper credit....

I tend to agree with JW on this. 14 top and 7 bottom should be quite sufficient as long as you don't try to give it a full-pedal launch with a killer zoom at the end.

One other thing....and very important.... be sure the pieces of your spar fit together well.... no gaps or sloppy workmanship allowed. The webs need to fit precisely between the upper & lower caps, and the ribs.

#10 markdrela Feb 09, 2013 10:23 AM

Quote:

Originally Posted by John Walter (Post 24081609)
The Miarage is a great plane. The key to its design is the balance among all is components.
A solid CF spar will weigh too much. Without changing the rest of the spar, you would simlpy tear off the tips before the main spar saw any substantial load. Also, since there is no balsa sheeting, the wing would flutter long before the main spar loads became an issue.
.007 CF on the top spar, or .014 top and .007 bottom would give more than plenty of strength.

What he said.

An even simpler way is to add "chordwise" 1/8" square basswood doublers to the top sparcap, stepped in length. The Mirage center panel is 48" long. Making the three doublers about 30", 18", 9" in length will give a nice ramp-up in the strength towards the center. I would also add a 1/8" square doubler about 9" long to the top tip sparcap.

This will double the wing's bending strength, and add maybe 8 grams.

#11 Mark Miller Feb 09, 2013 11:38 AM

The question is really do you need the carbon? There is a myth that a winch is a really good way to break your wing. As a matter of fact that is what they were designed to do. in fact you can be easier on a wing with a winch as long as you use the off part of the foot switch. I have launched HLG's with all balsa spars without breaking them. It can be done. Plenty of my customers have built their Mirages with no carbon. Maybe a bit of wrapping the joiner boxes and are still flying the models. So I would suggest that you think about what the real benefit will be with adding carbon to a Mirage wing. I'm not saying don't do it but just to think about it and asking questions here like this is a good start.

Mark Miller
Isthmus Models

#12 stewswanson Feb 09, 2013 12:13 PM

spar
 
Agree with the .014/.007 or.028/.014 for the center. I put 4mm square carbon (has a hole in the middle so it really isn't too heavy)tip caps in mine with kevlar wrap and laminate .080 x .5 carbon into the joiners which are a high failure component.
Stew

#13 TOSSPilot Feb 09, 2013 02:46 PM

Thank you for all the feedback. It helps.

Some of my take aways-

1. Overkill in strength and will unbalance the load charateristics of the wing.
2. Probably will add 2-3 onces of weight.
3. The plane may be fine with no carbon, so why go crazy in the other direction.

The advice seems to be geared towards a more moderate solution if I want to do something.

Thank you again.

Craig

#14 Neil Stainton Feb 09, 2013 05:31 PM

Quote:

Originally Posted by TOSSPilot (Post 24076567)
1. Why not just replace the bass spar cap witth the carbon one vs. laminating the carbon tape?

A spar needs a shear web to work. If you change the spar caps to carbon you will have to also increase the strength of the web in proportion. Probably just increasing the vertical grain balsa web thickness to the full width of the spar cap will be okay.

Neil.

#15 kzimmerm Feb 09, 2013 06:18 PM

Quote:

Originally Posted by Neil Stainton (Post 24085515)
A spar needs a shear web to work. If you change the spar caps to carbon you will have to also increase the strength of the web in proportion. Probably just increasing the vertical grain balsa web thickness to the full width of the spar cap will be okay.

Neil.

That is what I'm doing for a spar mod on my next woodie project. In fact it is going to much like the Maxa spar. It will comprise of 4 pieces of balsa cut on a 45 degree bias laminated together with a .014 carbon core. spar caps will be dimensional carbon wrapping with Kevlar. I figure it should be full-peddle winch safe.
Kurt


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