why dont we build the spar first? - RC Groups
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May 02, 2011, 08:55 AM
cq cq de kr4is
flexsphincter's Avatar

why dont we build the spar first?

what im wondering is, as long as i build a straight spar, and do all to make it winchable/strong, then lay it on the plans, cut the ribs to fit to leading edge, and, cut to fit to trailing edge , throwing away the center/spar notch in rib it seems like it would be an easier /stronger build, what am i not seeing. does the "notch"in the rib for the spar mean any thing? ive got a 2mtr wing on the bench to be built and was thinking of going against the "conventional" build.
if i did the spar solo i cud put webbing continuous threw spar and shear webbing also instead of cutting to lenght between ribs.. ive searched the forums and not found the answer.
just asking
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May 02, 2011, 09:05 AM
Registered User
I've seem few models built like that.
The first time I saw it I thought that the wing might break at one of those points alongside the spar (either in the front or the back of it) but the planking of the D box and the cap strips coming from the back to the top of the spar do a great job holding the whole thing together and the spars are rock solid having uninterrupted shear webs.
Just my $0.02
May 02, 2011, 09:59 AM
Registered User
I have been building the spar first and adding ribs later for the last five years. https://www.rcgroups.com/forums/showthread.php?t=699593 shows how I build this way. I have crashed models built this way twice and the ribs break before the glue joint at the spar. I use epoxy to glue the rib to the spar and the sheeting extends behind the spar far enough to anchor the aft rib to the sheeting. The most difficult part of the build is to be sure that the nose and tail sections of the airfoil maintain alignment. The flat bottom airfoil used makes this easier but I have also cut a foam bed to the bottom airfoil contour for other airfoils. The last photo in the build thread shows the crash that destroyed the center wing panel. The model was rebuilt with a new center wing panel and flown again. https://www.rcgroups.com/forums/showthread.php?t=969978
May 02, 2011, 11:20 AM
Registered User
I did this on a model a long time ago. I cut 80 front ribs and 80 back ribs out of 1/32" wood (I think) so that the covering wouldn't sag much even without a d-box. Had tab on the bottom of front ribs to hold them up, which I got rid of afterwards. This was constant chord or it would have been even more of a PIA. Flew well but was ugly.

If you don't get the two rib halves perfectly aligned in the vertical plane, your glider will fly strangely.
May 02, 2011, 07:42 PM
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R2R's Avatar
The Airtronics Legend is built that way.
May 02, 2011, 09:16 PM
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John Walter's Avatar
The orginal Airtronics Grand Esprit was designed to build the spar first, then add leading and trailing ribs.
May 02, 2011, 09:48 PM
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MS_in_NY's Avatar
Shear webbing is aligned with the grain going top-to-bottom of spar. This is done for deliberate structural reasons. There would be no advantage to a continuous piece of end-grain shear web even if you could find one, and handle it without breaking it.

May 03, 2011, 02:41 PM
Hot Dawg Glider Pilot
schrederman's Avatar
If you look at Mark Drela's write up about the Bubble Dancer, it's explained in detail. The ribs, butted up to the spar, are only as strong as the butt jount and the materials adjacent to the joint. Having the ribs continue through the spar is stronger. I have built wings both ways, and I believe the continuous ribs make a stiffer wing in torsion, which is why I stopped building a continuous spar and using butt joints for the ribs.

May 04, 2011, 11:19 PM
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rogerflies's Avatar
I built the spars first on my Saggitta XC. You just need to be really careful in positioning the front parts of the ribs to the spar.
May 05, 2011, 03:48 PM
AMA 353531
rdeis's Avatar
Originally Posted by schrederman
...The ribs, butted up to the spar, are only as strong as the butt jount and the materials adjacent to the joint. Having the ribs continue through the spar is stronger. ...
I would think you'd want to cap the ribs with something that runs the full chord rather than relying on a butt joint.
May 05, 2011, 06:46 PM
Registered User
Originally Posted by rdeis
I would think you'd want to cap the ribs with something that runs the full chord rather than relying on a butt joint.
I use epoxy for the butt joint at the spar and have never had a failure of the glue joint. The rib always breaks elswhere. The top sheeting extends 1 inch aft of the spar as suggested by Mark Drela and the bottom sheeting extends at least 1/4 inch aft of the spar. I have destroyed two wings built this way and the rib never failed at the spar.
May 05, 2011, 08:55 PM
cq cq de kr4is
flexsphincter's Avatar
tu for the posts i will build next wing spar first ,,,because i can
May 07, 2011, 10:33 PM
Proud member of LISF and ESL
I watched a BD spar being built with alternating shearwebs and ribs and thought it was a very good and very strong design.

I am working on a Bird of Time now and it uses the same approach.

The shearweb's job is to keep the top and bottom spar at a constant distance from each other. I don't see this being compromised by having the ribs in the mix. This would be especially true when you wrap the spar with carbon, kevlar or unwaxed dental floss.

I also feel a rib that extends through the spar gives you better alignment.

Of course this from a guy who definately does not consider himself an accomplished builder.
May 08, 2011, 05:37 AM
Registered User
As long as you can wrap the fishbones, I guess you're right about no penalty. Haven't attempted that my self yet.

However, in addition to keeping the spars aligned at the correct difference, the shear web also carries the shear loads. Surprise surprise. ;-p

(thought experiment: Imagine that you've built the spar as normal, with lots of extra wrapping, except that between the caps and the shear, the glue never set. What happens when you push up on the tip?)
May 09, 2011, 05:12 AM
Proud member of LISF and ESL
I see your point on the shear webs.

As for wrapping, for a competition glider like the bubble dancer, wrapping would seem highly recommended. But for sport flying it is probably not required.

My Legend, recently sold, has carbon capped spars that were never wrapped. The plane is probably at least 15 years old and the carbon cap is fine, under the sheeting.

If I was building a plane where building the fish bone in order to wrap the spar was either inconvenient or impossible, I would still not hesitate to carbon cap the spar. Just make sure you get a good epoxy bond between the carbon and the spar.

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