Dihedral Brace or Not?

[HELModels]

I am wondering how to best attach wing halves to center section?

I could use CA and glue them together as is, but I am wondering if it is better to also put a dihedral brace?

Should 5minute epoxy be used instead of CA at such critical joint?

I used 2 ribs at each wing root and angled the innermost rib so when dihedral was propped up at tips the rib would be vertical and would meet the center section rib flush. That works but I am wondering if a dihedral brace is necessary. This is a totally scratch effort, so no plans to refer to.

The span is 32" and chord 4.25" and if I ever finish it should be a parkflyer with 4-6 oz. gross.

Let me make it clear that what I mean by dihedral brace is piece with the dihedral angle which would connect the center to the wing halves and be like a shear web(runs perpendicular to and attached to top and bottom spars)

This might seem like terminal minutia, but it is an issue which might mean the difference between a wing that snaps like a toothpick at the slightest yank on the elevator or a wing that can handle some abuse.

Any help is appreciated.

http://lister3.home.mindspring.com/Wing.jpg

[jim davison]

Generally a wing of this size should not need a brace. A thin coat of 5 min. will do just fine.

[banktoturn]

ElectroStorch,

If it were my wing, I would want a dihedral brace, or equivalent. What you want is to securely connect the upper and lower spar members across those joints. A plywood brace is one traditional way to do that. A nicely glued butt joint alone, which is what you will have if you proceed as planned, is less than I would want. A strip of fiberglass or carbon fiber across the joints might be a nice addition, and if you're going to do that, you might just as well wrap the joint all the way around, not just at the spars.

Good luck,

banktoturn

[Dereck]

I'd go with the dihedral brace, though a bandage would work as well on a tiddler like this.

if you do use a brace, 1/16" or even 1/32" ply would do fine at this size and weight. Taper the brace off rather than have it end abruptly at a wing rib. It only needs to go out to the first wing rib on each panel. It has to connect all the relevant spars as you rightly assume, but it doesn't matter whether it's ahead or behind the spars. To avoid it poking though to the covering, make it the same depth as the distance between the outer faces of the spars.

Assuming a really good fit, wood glue rather than epoxy will do fine here, and save a tad of weight.

D

[HELModels]

Thanks All!!

save a tad of weight.

Too late, I proceeded before reading this reply. I found some scrap 1/32 basswood and made a slot right behind the spars. The brace spans the enitre centersection on out to 3/4 distance to 2nd rib. I followed most of the suggestions except for glue - 5 min epoxy.

Now, the question is do I drill a bunch of lightening holes in the sheeting and the ribs?

I realize this should have been done when the ribs were cut, but all the ribs have 0.5 oz. fiberglass reinforcement. I know, extra weight right there, but also extra strength.

Drill from the fiberglass side to avoid tearing the ribs?

[Ollie]

First you strengthen the ribs with fiberglass, then you propose to weaken them with holes. Where is the logic in this? Keep it simple and start over by picking the balsa rib material thickness and density to match the strength requirement without resorting to extra work that is self defeating.

[BMatthews]

I'm with Ollie 100% on this one. Not trying to be mean but your extravagent use of wood overall totally goes against the concept of a super light parkflyer. And then you add fiberglass to ribs that are overstrength in the first place? With the structure shown in the pics I'd be very surprised if the model isn't overweight even before you add the motor and radio gear. It's great that you're designing your own stuff but it doesn't hurt to closely study other folks plans and pictures for some guidance.

I suggest you do a www.google.com search for "free flight plans download" and study some of the methods for designing and building light wings. Even some of the ads for built up parkflyers at the online catalogs at Hobby Lobby and others will show you pictures of some better options for wing design. I know it sounds cute but the idea is to build lots of air into your design and only use enough of the "other stuff" to hold a proper shape.

Spar joiners do not need to be full depth. Just adding some 1 1/2 by 1/8 strips to the front and back of each spar joint would do the job. End grain but joints, even with epoxy, only provide a small fraction of a face grain joint. The idea is to use the dihedral "braces" or my little load transfer strips to ensure the spars use face grain joints to transfer the loads. Consider that the end grain butt joint is not doing more than probably 20% of what the spar can support. So that means that if the glue joint fails then the whole load is suddenly in that little bit of basswood. Fortunetley the joints at the leading edge and trailing edge as well as all the sheeting will be assisting the spars so it may be that there is no need to worry.

[HELModels]

Where is the logic in this?

First, I cut a bunch of ribs out of 1/16 balsa and they tore and snapped at the slightest touch. Fed up, I put some 6 0z. glass cloth on a balsa sheet, library pasted copies on top and cut the ribs out. This resulted in ribs I could use to hammer the wing together with 10d nails if I wanted to. Weight? About as much per rib as if I had used 3/32 balsa, which I did not have on hand.

Airfoil fidelity? it has it thanks to the LE TE sheeting.

Sometimes a little extra seemingly unnecessary work will make up for a lack of materials or tools.

no need to worry

Who's worrying?

Again, thanks to all who have offered Constructive criticism and suggestions.

[HELModels]

start over

No, I decided to salvage what I had. That meant cutting away the sheeting until I had about 1/2" sheeting on LE and 1/4" sheeting on the trailing edge. Yes, I did remove all remnants of the photocopied profiles.

I will consider this wing a learning experience and a rough draft.

As you can see in the new pic, the wing is covered with some unknown material which was about 30 years old and has about half the thickness of monokote. It came from the last model I built and covered.

You can also see that the leading edge has quasi cap strips which resulted from cutting away sheeting.
Does this small amount of material add any strength?

The next wing will not have this time consuming feature if it is not necessary.

The tail is next.

I welcome comments.



http://lister3.home.mindspring.com/Storch.jpg