shaping the spar cap surface

[jpolenek]

A while ago, there was a discussion in which people described how they overfilled their spar cap layup with the 3" tape and then sanded it back down after cure. This was done in order to get a better blend with the surrounding foam and produce a smoother (i.e. less bumpy) surface for the skin layup.

 

What's the best way to sand down the cured spar cap so that the correct airfoil shape is achieved? How is this done so it doesn't take an unreasonable amount of time&effort, but is not so aggressive as to risk damaging the surrounding foam or otherwise altering its shape?

 

(If this was covered in the previous discussion, could somebody point me to it?)

 

Thanks.

[TMann]

The Idea is to cut your tapes at alternating angles as the get shorter and shorter. After the cure, these trasition poits are what you are sanding to minimize the stairstep effect.

 

Cut a profile template and check often to ensure you don't overfill the spar caps.

[rickh]

What's the best way to sand down the cured spar cap so that the correct airfoil shape is achieved?

With sandpaper? (sorry, couldn't resist)

 

I use a 3"x11" piece of MDF for a sanding block a lot. One side is 60 grit, the other is 100 grit. Scotch 77 glues them on.

 

If you can avoid sanding the spar caps at all.... It sands like granite. Set the dams up correctly, check them again.

 

Attached pix is the end of one cap before the final wraps. Look close you can see some whitish dips where the bevel cut of the last tape ended (I peel-plied). You can also see where the sander (above) was used at the edges of the cap. Sands like granite.

 

I recall Bernard Siu has a dialog about using a wing root template to gauge/adjust the height of his main spar as he was building it. http://www.maddyhome.com/canardpages/pages/bsiu/index.html

 

Rick

[jpolenek]

TMann,

What you're describing sounds like it could be done by hand. I seem to recall some talk about a belt sander.

 

Joe Polenek

[rickh]

Suppose the brave could use a belt sander. me? Cluck cluck

 

As I remember, the bottom cap dams need to be several pieces. The top dams are straight, but the finished cap area will have a slight curve (front to back) to blend in with the curve of the wing airfoil.

 

And make sure you have plenty of spar tape on hand!

 

Rick

[Wayne Hicks]

Trying to shape the cured spar caps (fore and aft) is a waste of effort that will lead to damaging the cores. What I did fnd that worked was to lightly drag the canard F or G template (I forget which) spanwise along the canard as the spar cap trough is filling. After the last tape is wetted out, keep dragging the template to keep the spar cap to shape. Drag every 10 minutes or so until the epoxy starts to kick off.

 

We did this on Clark Canedy's canard. It needed very little filler over the spar caps.

[Kent Ashton]

they overfilled their spar cap layup with the 3" tape and then sanded it back down after cure

Sanding UNI - ugh. Belt-sanding UNI -- double ugh.

 

I've built six wings and/or canards and the Cozy plans method works fine. As the spar trough fills up, use a good template (I like formica) to check how the profile is looking. Cut the UNI on an aggressive angle to fill a low side. As you finish it up, squeegee a bit of curvature into the profile.

 

There will likely be a few high spots that'll need leveled with a sanding block but on the whole, the cap will not need much sanding. And a bit more filler over low spots will not hurt a thing.

[TMann]

Trying to shape the cured spar caps (fore and aft) is a waste of effort .....

Yes, pretty much. The way the plans tapes are set up you end up chasing the strands around trying to comb them out and maintain a nice alignment. I used a notched spreader. In retrospect, the spreader was made to distribute them against a flat straight surface, not the slightly domed trough.

 

My experience using the Carbon Fiber tapes was a much more uniform surface (West 703 tapes.) West also makes a glass version of the same (713) which if it could demenstrate the same specifications would be a nice alternative.

 

The strength of the spar cap is the result of continous strands from end to end. Sanding will result in cut fibers. Don't know if that's a big deal seeing as to have to sand, you would have to have the requied amount already. Too much leads to extra weight.

[Marc Zeitlin]

The strength of the spar cap is the result of continous strands from end to end.

Actually, while having continuous fibers from end to end is a good thing, and certainly makes manufacturing easier, it's not at all the reason for the strength/stiffness of the cap (or any other layup).

 

What's important is that all the fibers are pointing in the same direction, and that they're very close to one another, and overlap. The epoxy will transfer the loads in shear from one fiber to another, as long as they're pointed in the same direction - a continuous fiber from one end to the other is not necessary for full strength.

 

All that said, the easiest way to get full strength is to have continuous fibers.

[jpolenek]

As it turned out, I over-filled the canard bottom spar cap a bit - overall about 1/32", and a few 1/16" bumps. I guess I was afraid of underfilling it and compromising strength because of insufficient UND, but went too far the other way. For some reason, using the template to control the layup height, as suggested by others, didn't work that well for me.

 

After trying to hand sand it with a block and messing around with all the clogged sandpaper, I realized how much effort it took to make miniscule progress. Bumps come out pretty fast, but it really slows down when trying to reduce an entire surface by hand.

 

So I went out on a limb and picked up a small belt sander... and it worked GREAT! I started off with a 50 grit belt, and then 80 grit. Finally, I switched to hand sanding with a 80 grit block followed by a 120.

 

The method I used was to check the profile against the template every inch or so along the entire length of the spar cap, and mark the high spots with a Sharpie. The markings were then sanded away, and the whole process of checking and marking started over again. This was repeated until there were no more high spots to mark. (Also, lots of vacuuming between sanding passes was needed to prevent blockage of sandpaper/belt.)

 

I have to say that I couldn't have asked for better results. For future spar cap lay-ups, I will try to use the template more effectively to control the spar cap height and avoid so much sanding, but it's nice to know there's a backup plan that seems to work.

 

Joe Polenek

[steve]

The strength of the spar cap is the result of continuous strands from end to end. Sanding will result in cut fibers. Don't know if that's a big deal seeing as to have to sand, you would have to have the required amount already. Too much leads to extra weight.

I'll add to that...continous is NOT where it get it strength from.

If you do the JD stile of spar for the re-tracks you will need a planer to repair the mess.....ask me how i know.

do the best you can then 6.3 years later when you are micoing the strake/wing joint just knock down or add to the spot thats offensive.

[TMann]

If you do the JD stile of spar for the re-tracks you will need a planer to repair the mess.....ask me how i know.

 

I agree. The trick is to build them from the outside in. I'm putting together a page to explain how to do that. I'll post a link when I'm done.