Moth Spar Cap - Alternative Method - RC Groups
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Mar 14, 2006, 08:59 PM
Registered User
jtoro's Avatar

Moth Spar Cap - Alternative Method

I was not sure where to put this so hopefully this is not a redundant post.

I have built a couple of NCFM kits and have been very pleased with their product. Additionally I would like to thank NCFM for their kind help via phone.

The NCFM spar cap installation with PU glue was not to my liking (why cut up a perfectly nice piece of balsa?). It seemed that a better solution existed for allowing excess foaming PU glue to escape from under the spar caps during the curing process. The method they have described cuts the cap into segments between which the foam escapes. This foam must then be sanded flush with the cap. The following images outline a method that I have used to vent foam pressure and arrive at a continuous spar cap. I have found that this continuous balsa surface allows for firm uninterrupted bonding of the covering at this point on the wing. Hopefully this is of some use to other NCFM Moth / Halfpipe addicts.

NCFM thank you again for your fine aircraft.

Kind regards,

Joe Toro
Last edited by jtoro; Mar 14, 2006 at 09:04 PM.
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Mar 14, 2006, 09:10 PM
Registered User
Daemon's Avatar
I guess I haven't been following their latest build instructions because
that all seems unnecessary (their mods and yours).

I really don't quite understand why NCFM keeps having so many problems
with PU, because I simply have had zero problems with it. The only thing
I do slightly differently is to round out the bottom of the spar cap so
that it fits snugly to the spar. Maybe that helps more because there isn't
such a high volume of PU between cap and whatever I'm capping.

I drool PU into the bottom of the slot, smear it out thin, mist it with
water lightly. Place spar in slot. Drool PU on to of the spar and smear
onto the sides. Mist lightly place caps on spar (full length, no cuts,
no slots), and then put weights on top. Let cure overnight. Done.
If I lived in humid San Diego (where NCFM is), I'd use less or no water.

Maybe the problem NCFM has is they insist on adding water directly to the PU.

Last edited by Daemon; Mar 14, 2006 at 09:21 PM.
Mar 14, 2006, 09:19 PM
just add beer......
malachite's Avatar
Originally Posted by Daemon
If I lived in humid San Diego (where NCFM is), I'd use less or no water.
Having used PU glue in AZ I always wondered the same thing. Then I moved to SoCal and now I understand. Climate plays a big roll in the foaming action of PU no matter how much water you spray on.

Mar 14, 2006, 09:22 PM
Registered User
Daemon's Avatar
Originally Posted by malachite
Having used PU glue in AZ I always wondered the same thing. Then I moved to SoCal and now I understand. Climate plays a big roll in the foaming action of PU no matter how much water you spray on.
And that may be true, but NCFM pre-mixes water with the PU. My take
is, they should skip that step entirely. They're creating a problem
and then forced to create a complicated solution to the problem.

Mar 14, 2006, 09:51 PM
Low AltiDude
PGR's Avatar
Like Ian, I haven't had any problems with PU when gluing in spars. I make a stiff little squeegee out of plastic that fits the spar channel snugly and put a little chamfer or bevel on the two corners. Then I'll spray the channel with a mist of water and drag the squeegee through it to remove all droplets. Then I coat the channel with PU and drag the squeegee through it several more times until there's nothing left but a thin film of PU on the floor and walls of the channel and a little 1/16-3/32" bead of PU in each corner. Then I lightly spritz the spar and cap with water, shove them into place, and weight everything down for an hour or two.

I should add that I make my own balsa spar caps by running the proper sized stock through my router table rigged with the right sized round-nosed bit. This mills a half-round recess in the spar cap that perfectly fits the spar. I also wipe a thin film of PU into this recess before I shove the cap into the channel.

Done this way, you don't get a bunch of PU foam oozing out and trying to push your spar out of the channel. It foams, but only enough to fill all the voids between the channel, spar, and cap.

The trick to this is a channel which fits the spar and cap snugly and not using too much PU. Every person who has watched me do this thinks I'm not using enough glue so if I'm doing it for someone else, I'll do a sample with some scrap at the same time. When the PU sets up, I let them do destructive testing and they all come to the same conclusion:

It was plenty of glue.

Mar 14, 2006, 10:51 PM
Registered User
jtoro's Avatar

No Slots


I am interested in your comment. I tried the technique you described on one of two test caps. Neither had slots holes gaps etc. Both spars were solid (no shaping of side touching spar). One used glue with your mist technique the other was glue mixed with water. Both bulged the foam around the spar. As you mention perhaps it is the volume of glue. I will try your idea of mating the underside of the cap to the spar.

I do not see the glue as problematic. The comment was directed at a method to arrive at a continuous spar cap. Something cleaner and more elegant. It seems that your technique eliminates the need for slots (saving time) and fits the cap to the spar (adding time). Maybe no slots and no shaping with your same glue technique is something to try...but I was not aware that this topic had already been beaten to death

My apologies if this was not placed in the correct location. Perhaps you can direct me to the appropriate location.


Last edited by jtoro; Mar 14, 2006 at 10:58 PM.
Mar 15, 2006, 12:04 AM
Just Toss It !!!
MATIN's Avatar
At the risk of beating this to death, with my apologies to Derek and others:

I, too, like the full spar cap and scoop out the middle (round) to reduce the amount of PU.

But more importantly I do two things that I think most people overlook: First, I don't let the PU push the spar cap out. Second, I make the clean up of the excess glue easy.

1. To counter the foaming PU that oozes out (humid air or not), I tape the sides of the channel (and the ends), with two rows of 2" masking tape (not the blue kind but the beige colored that's more tacky) on both side to protect the surface of the wing (4" is as wide as PU has ever oozed out on me).

2. To counter the force of the foaming PU, I always put a sheet of Saran Wrap the top of the wing and the top beds on top of that, and then I weigh the whole thing down with a stack of magazines about 4" high, from wingtip to wingtip. No glue in existance can push the spar cap up under that kind of load. As a side benefit (and maybe more importantly) the wing, sandwiched between the top and bottom beds, turns out true, without bends or twists.

3. After the glue has cured, I will have some ooze-out, but by peeling the masking tape back, I get rid of it without having to sand it off. This is actually rather easy due to the foamy nature of the glue and the fact that by putting pressure on top, the ooze-out is really thin (as thin as your spar cap stands proud of the top surface).

It should also be noted that with my new & improved spackling technique, the glue ooze out is actually desirable. Unless you don't feel like sanding.

And we all know what you get if you don't feel like sanding: A Bumpy Finish!

(Dang! I just did this last night but didn't take pictures!)
Mar 15, 2006, 12:06 AM
Registered User
Greetings, I/J/M/P!

All good info. There are many different techniques to use, with differing amounts of goo, and all will produce varying results. I should probably update the site/manuals to get rid of the over-ominous warnings, but I still have a few dozen manuals left, and am determined to use them up!

For the upcoming "bigger pipe", we're pre-PU gluing all of the new spars in, 2 full wings at a time (with cross weighting), and have not had any issues in a year or so. The only problems we really ever ran into, were at the initial stages, when using a full balsa spar cap. In a stock config, the full cap just created too much back pressure for the glue, and did, in fact, force the spar out of the channel on a few occasions, with 6-8lbs of weight per wing. These days, we much prefer to use three 3" strips of balsa capping per side (tip, root, mid), with a 2lb weight over each cap, and let the glue dome over the channel. It is immensely easier to sand/shape PU glue to the airfoil than it is a balsa cap, and stronger, actually, given the extra volume and wicking of the glue. I can't imagine ever going back to epoxy now, given the great reduction in outboard weight, much less the radically improved torsional rigidity of wing when using PU glue.

The technique listed at the bottom of our FAQ page is what we are presently using, but there are many ways that work beautifully. I'm just very happy to see the swing towards PU vs. epoxy on any plane, as the overall performance does increase substantially.

Overall, Gorilla Glue is very easy for us to work with now, having spent the time to get a routine down. I just don't recommend anyone using it for the first time on a wing, without becoming familiar with its expansion and forces on a scrap, in their own ambient environment (temp and humidity do play a factor). Routinely, I will add blips of PU mixed with water, after the fact, over any portion of the spar that doesn't appear to be expanding as much as the rest... painless.

The diameter of the Gorilla glue we bead into the channel is about that of a chopstick; it is then mixed onto the channel walls with a flux brush intermittently dipped in water, for the flow factor. Same thing is done on top of the spar, once it is layed in the channel, and prior to positioning the little caps/weights.

As anyone who has done it can attest, mask the bottom of the wing/channel area with masking tape, and use wax paper or celophane, to separate the wing from the beds (or risk the inability to do so once the glue has set).

Happy goo!

Mar 15, 2006, 12:15 AM
Just Toss It !!!
MATIN's Avatar
Derek and I seem to be typing concurrently!

On the subject of PU and water:

Having been one of the first to mix the two for a more controlled reaction, I no longer do that with the spar.
I find that wetting the bottom and sides of the channel and then patting them dry imparts enough moisture to the mix to kick it off (maybe that's why I don't have the Big Push) In addtion, I do wet the spar itself after I sand it rough and before wetting it with PU all around. I also moisten the underside of the spar cap and smear it with PU.

And like every other procedure, the more you practice, the more you get the feel for how the material is going to behave. Don't discount experience!

Last edited by MATIN; Mar 15, 2006 at 12:21 AM.
Mar 15, 2006, 12:32 AM
Registered User
Daemon's Avatar
A chopstick's diameter bead of Gorilla Glue wetted with a brush sounds like
a *a lot* of PU and a lot of water compared to what I normally use
(and it sounds like its more than what most other people use too).

I've only had PU try to push a spar out of a wing once, and
that was when I totally forgot to put a cap on top of the spar to set
weight on. (Was a quick and dirty build of a combat PSS plane).

For weight I've got some big plastic cups filled with water (much less
heavy than 4" stack of magazines, although I've certainly got enough
around to do that too). I squeeze out a thin flat bead (tip resting
against the bottom of the channel) and then spread it out to the corners
and sides with the round end of a stir stick. Basically I'm looking for as thin
a film of PU as possible with a tick more in corners. I've snapped spars
in multiple places but never had it pull loose from the foam.

Looking forward to seeing that big Halfpipe. All my NCFM planes
are currently out of commision after much use.

Mar 15, 2006, 03:03 AM
ajroahkni's Avatar
Originally Posted by malachite
Having used PU glue in AZ I always wondered the same thing. Then I moved to SoCal and now I understand. Climate plays a big roll in the foaming action of PU no matter how much water you spray on.

There's a similar problem I have w/ PU glue. I can never get anything more than the smallest size bottle, because - even if I seal it off w/ cling or waxed paper, or a zip lock - if it's sitting for more than 2 months, it's cured, wasted.
Mar 15, 2006, 03:37 AM
Registered User
NWade's Avatar
Wow, all I did was follow the written instructions and use epoxy... Look at all the "fun" I missed; and all I got out of it was a great-flying 26-oz. wing!

Take care,

P.S. Before anyone leaps to the wrong conclusion about the weight of balsa & epoxy: I used a heavy battery pack which weighs about 1.5 times the recommended amount (overcompensating for losing some of the BB-nose-weight); so that's the likely reason it is not a 24-ounce AUW.
Mar 15, 2006, 09:32 AM
Low AltiDude
PGR's Avatar
Originally Posted by ajroahkni
There's a similar problem I have w/ PU glue. I can never get anything more than the smallest size bottle, because - even if I seal it off w/ cling or waxed paper, or a zip lock - if it's sitting for more than 2 months, it's cured, wasted.
It's simple: You're not building enough planes!

Mar 15, 2006, 10:36 AM
daedaluscan's Avatar
I put the cap on the pu and keep it inverted. I am also pretty careful about not getting any water on the tip. I use a plant sprayer to mist after applying glue. Too easy to use, I just tape the spar caps with masking tape, no problems. I use a lot less than a chopstick of glue on a spar, more like a matchstick and then rotate the spar in the channel to smear it all about.
Mar 15, 2006, 12:19 PM
Registered User
bikeman's Avatar
I mist the spar channel. Then put a thin bead of PU glue in the channel and squeegee it with a popsicle stick so there's a very thin layer of glue with about 1/16th inch fillets in the corners of the channel. I then mist the spar and wipe it with a damp cloth and lay it in. Then I'll add a TINY bead on the spar cap then put it together. Here in PA in the winter the humidity is very low so it barely foamed up around the cap. Worked perfectly. I now use PU for repairs to my M60 fuselage. I midaired a crunchy last week (NOT pretty, think exploding airplane, his not mine) and it cracked the fuse under the LE of the wing. I used a wetted popsicle stick to wet the inside of the crack, then used that same stick to force PU in the crack. I then fiber taped over the crack and the glue foamed up on the inside leaving nothing to clean up. Now that area of the fuse is rock solid, probably stronger than new. Good stuff.


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