Canard top spar cap

[Zulu Yankee]

Hi guys and gals,

 

I just completed the top spar cap on the canard of my Cozy MKIV project. The plans say to fill the trough with plies of UNI spar cap tape. However, it does not say how many plies of this tape must be in the trough.

 

After 6 plies, my top spar cap was full. I looked in the archives and saw that other folks managed 9 to 11 plies in theirs.

 

The spar cap tape that I used came with the tub that I purchased. It was indicated to me that the tape that I used might be thicker than the type in use these days (from ACS). A friend sent me a sample and mine looks a little bit thicker but not by much.

 

I guess some of the reasons for this could be:

* Incorrect foam cores (not likely. We checked them against the templates).

* Not enough squeegying of each layer (possible).

* Thicker spar cap tape (possible, but not by much).

* A plot by "the man" to keep me from finishing.

 

I polled a few of the knowledgable folks and the responses ranged from "press on" to "stop and redo".

 

As a poll, how many plies did you have in your canard top spar cap? Also, could we get an indication from those with flying airplanes?

 

Any other thoughts?

 

Gerhard Ungerer

www.randombitcorp.com/cozymkiv

[Big Steve]

I did 9 on the top and 11 on the bottom STeve build on

[rickh]

After 6 plies, my top spar cap was full. I looked in the archives and saw that other folks managed 9 to 11 plies in theirs.

 

Six full length plies, or six plies at the center? Top or bottom?

 

I went downstairs, piled all my templates together (working on canard right now), did some seat-of-the-pants figuring. If I get six full length plies, I'm gifted. I expect I'll have 3-4 at the outside, and 8+ plies in the center.

 

My main spar was two more than STeve did, FWIW, assuming he means his centersection spar.

 

Rick

[Wayne Hicks]

Perhaps your spar cap troughs were not cut deep enough?

 

One way to know would be to measure the depths of the bottom spar cap troughs and compare them to the canard hotwire templates. If the depths match the templates -- the center core especially -- then it stands to reason that your top spar cap troughs are cut to the proper depths, too. If your bottom spar cap troughs are not deep enough, then perhaps your top spar caps aren't deep enough either.

 

To use only 6 plies when everyone else is using 50% more raises a red flag.

[Zulu Yankee]

There is a chance that I might have miscounted, but not by much. I think I had 3 full length, although I had to cut it 4 inches shorter on the last ply on the port side.

 

I remember putting one more ply (maybe two) a little shorter, one more ply a foot shorter both sides and then a last ply approx 4 foot wide across the center.

 

The first two plies were not squeegied as vigorously as those afterwards.

 

:sad: :mad:

[TMann]

To use only 6 plies when everyone else is using 50% more raises a red flag.

It would sure scare the cr@p out of me!

[jpolenek]

If the tape you used is thicker, then the number of plies needed to get the same strength may be different. You should try to find out the details of your tape, and those of the "correct" tape. If you assume that the strength of a lay-up at any given point is related to the amount of glass, then you could compare your 6-ply layup with somebody else's 11-ply layup in terms of the total cross-sectional area of glass. You might find that your lay-up is very comparable.

You would need to determine the number and size of filaments of each type of tape. Also, you would want to verify things like the surface finish of your tape, type of glass (S-glass versus E-glass) and hence tensile strength, etc.

There's probably more to it than that (I'm no composites expert), so if you take such a theoretical approach, a perusal of Marshall's Composite Basics would be a good start.

 

 

Joe Polenek

[Steve Innova]

Weigh your spar cap tape (I think it's measured by sq. yrd, check) and compare it to the wieght specified in plans. That will tell you if your particular tape had more or less material.

[Zulu Yankee]

How many plies did you have top and bottom, Al?

[Ron Springer]

There is a chance that I might have miscounted, but not by much. I think I had 3 full length, although I had to cut it 4 inches shorter on the last ply on the port side.

You might try this trick next time. As you go along and pull out the cross fiber for each ply, put each cross fiber in a pile and line them up. You will know the number of plies by counting the number of piles. You can't lose count that way. I think I even took a photo of my piles when I finished.

 

I didn't do that for the canard, but I did for the main spar, where it is a lot easier to lose count.

[Stuart]

I am about to do the spar cap on the top, I was only able to get 7 plies on the bottom spar cap, that filled it right up. before I read this thread I was thinking I might have a thicker spar tape than was used before, I couldnt find it in the plans on how many layers we are suppose to have. I have also read that the top spar cap should take a little bit more material than the bottom spar cap. I know I am going to be the squeegy king on the top spar cap.

 

Any input would be greatly appriciated

[TMann]

Plans say keep adding until the trough is full (but not overfilled.)

 

Have fun!

[Kraig]

I used 8 layers on the bottom, and 9 on the top.

 

Kraig

[TMann]

If you take an accurate measurement of the trough, you should be able to come pretty close as far as getting an accurate estimate of legths and layers. I used this technique when I did mine in CF.

 

The problem with the plans material is there is no lateral stability to the material once the cross stictch is pulled. If there is any discrepancy in either the depth or the width, the amount of material reqired will be different. (hence the fill-it-up method as ezpressed in the plans.)

[jpolenek]

The problem with the plans material is there is no lateral stability to the material once the cross stictch is pulled.

True. And the process of removing the cross stitch further distorts the fibers, even if you hold down the UND to stabilize it as the plans state.

 

When I did my spar cap, I let the UND hang several inches over the edge on both ends. This allowed me to give each bundle of UND a little tug, first from one end, then the other. Looking at the spar cap while doing this, it was evident how much this tension was straightening the fibers. I did this a few times throughout the squeegeeing process for each layer of UND. For the pieces that were shorter than the full span, the technique had to be modified a little, but is was still possible to pull on the strands to straighten them.

 

Joe Polenek

[Lynn Erickson]

True. And the process of removing the cross stitch further distorts the fibers, even if you hold down the UND to stabilize it as the plans state.

 

When I did my spar cap, I let the UND hang several inches over the edge on both ends. This allowed me to give each bundle of UND a little tug, first from one end, then the other. Looking at the spar cap while doing this, it was evident how much this tension was straightening the fibers. I did this a few times throughout the squeegeeing process for each layer of UND. For the pieces that were shorter than the full span, the technique had to be modified a little, but is was still possible to pull on the strands to straighten them.

 

Joe Polenek

the best way to pull out the cross strands is to cut the strand in the middle and then pull the outboard end out from each end. this will pull the strands in the same way you are squeegeeing and will not catch and drag the fiber the wrong way

[Stuart]

I was able to get 11 layers on the top spar, so 11 on top and 7 on the bottom, I think I should be OK. I had 2 layers across the entire trough before I started shortening them. I just have the top skin left now

[steve]

the best way to pull out the cross strands is to cut the strand in the middle and then pull the outboard end out from each end. this will pull the strands in the same way you are squeegeeing and will not catch and drag the fiber the wrong way

i did that and put a pencil in a drill and the string wound up on it at a fast pace

, and that let my other hand lightly hold down the tape. gr8 where you don't have a lot of room to move.

[pizzahunter]

Hi all!

I think it should have been noted on the plans how many layers to put...

 

It might sound a bit.... crazy... But one can check spar on medical X-ray CT scanner. The canard is narrow enough to fit in it. Scan where where is no metal.

At least one can check the thickness of spar caps, and any big void will be visible. If the scanner and settings are good, individual layers can be recognized.

Settings for ~bone will work more or less good.

[Marc Zeitlin]

... one can check spar on medical X-ray CT scanner.

Have you performed this test on multi-layup composite structures, both known good construction and known bad construction, and verified that you can determine what is good and what is bad? Given my experience with NDT and composites, I'm extremely skeptical that this is a useful technique.

 

Plus, since no canard (or wing, or main spar) has ever broken in flight, in over 2500 VE, LE and COZY aircraft over the course of 40 years, I'm not sure what the purpose of performing this test would be.

 

The # of layers is not definitive because over time, the spar cap tapes have been different thicknesses. Having the correct trough depth will give the correct structure, however many layers it takes.

[pizzahunter]

Yes, I've tested both good and bad (in my case bad were very porous, ~12%) samples, as well as real parts. Yes, there are a lot of limitations. And yes, if you are NDT professional, it might sound silly for you)