Wing joiner question

[chlee]

Hi all,
Another quick newbie question for you all.
I don't quite understand how a joiner in a composite two-piece wing works. I've watched Phil Barnes/Bill Haymaker's video and have some experience bagging one-piece HLG wings. However, I've never inspected a Laser, Fusion, Image, Mantis, etc. up close, so I'm not sure how to go about this sort of wing.

Assume that the wing has carbon tube spars and an angled steel joiner rod. Does the joiner go through the fuselage as in diagram 1, or does it directly join the wings as in diagram 2? (I suspect both methods are used...)

If a steel rod is bent, it probably has a curve/radius at the bend rather than a sharp corner. Would this curve not prevent the joiner from completely fitting into the wings in diagram 2?

Finally, if I were to use, say, a NSP Laser 2M fuselage, which method would I use?

Thanks in advance,
- Chung

[Ollie]

The NSP Laser fuselage uses method one.

In either case the inside diameter of the carbon tube spar will be greater than the diameter of the bent joiner bar.The bar can be fitted with four disks to build out its diameter to match inside diameter of the carbon tube spar. The disks can be positioned to avoid the bend or the holes in two of the center disks can be oversized to allow for the bend in the rod and the gap caused by oversizing the hole backfilled with reinforced epoxy. The second illustration requires that the wings be bolted to the fuselage. The carbon tube spars should be reinforced wher the disks bear on the carbon tube spar to prevent the prying action of the joiner bar from splitting the carbon tube spar. Plywood ribs set into the cores before bagging are one solution to reinforcing the carbon tube spar.

Since the diameter of the carbon tube spar is so much bigger than the steel joiner bar, it is often not even necessary to use a bent joiner. A straight joiner can be matched to a straight brass or aluminum tube and the metal tube can be skewed in the carbon tube spar to allow for dihedral. The space between the outside of the metal tube and the inside of the carbon tube spar can be filled with reinforced epoxy.

[chlee]

Ollie,

Thanks as always for the clear description. It hadn't occurred to me that the joiner could be thinner than the carbon tube.

If you don't mind me pressing you further:
I understand how your methods would work in, say, a 3M wing with a 1/2" spar tube and 3/8" steel joiner. But I still run into some issues with a 2M wing because of my desired dihedral and available spar/rod sizes.

I was specifically thinking of using a 3/8" ID wrapped carbon tube for a 2M wing with 6-7 degrees dihedral (being an inexperienced pilot). First, I'm assuming that a steel rod would be difficult to bend 12-14 degrees with ordinary tools without putting curvature into the arms; no such pre-bent rod seems to be commercially available. Second, it would be difficult to achieve the high dihedral using a straight joiner while keeping the length reasonable. (Will have to check this geometry). So, the next best way seems to be a combination of your last method with a 1/4" joiner pre-bent to -any- angle. ....I just have to find such an item!
Are my assumptions reasonable?

Another option I was considering was a 3/8" 7075 T6 aluminum from Hilaunch (the bend can be specified). I'll check this with some joiner-sizing formulae to see if it's overkill.

Thanks for the advice,
- Chung

[Ollie]

Assuming an airfoil at least eight percent thick and a minimum chord of eight inches at the outer end of the carbon spar tube, a 1/2 inch inch inside diameter carbon spar tube should fit in the thickness of the foam core very nicely. A straight, 1/4 inch diameter music wire or nitrided injector pin joiner bar will be plenty stiff and strong enough. A 9/32 od brass tube 1.78 inches long can be skewed at a seven degree angle inside the 1/2 inch diameter spar tube to receive the music wire joiner bar. The short joiner bar will have a strong prying action on the carbon spar tube. The spar tube will need to be over wrapped at each end of the joiner tube with carbon or kevlar tow to prevent splitting.The short joiner bar with its associated tubing and reinforced epoxy surround will be lighter than the long, bent rod approach below.

If you prefer to use a longer, bent 1/4 inch diameter music wire joiner in a smaller spar tube you can easily do so. The 1/4 inch music wire can be bent through 14 degrees by just putting a 36 inch long piece in a heavy duty vise and pulling horizintally, perpendicular to the jaws of the vise on the long end of the music wire. The wire will bend close to the jaws of the vise and return to straightness elsewhere. The curved part of the wire can be accomodated by simply starting the brass joiner tube about 1/4 inch from the wing root, leaving a countersunk spot in the spar tube at the root. You should still over wrap the spar tube at the ends of the brass tubes to prevent splitting but, with a longer joiner rod, less wrapping will do.

[chlee]

The wire will bend close to the jaws of the vise and return to straightness elsewhere.
Ah, I did not know this. This certainly makes things easier.

I see that the straight joiner is workable, but I will definitely have to reinforce the tube as you say to keep it from bursting.
On the other hand, if I forgo the Laser fuselage/SD7037 wing in favor of a thinner Aegea-style wing, then I'll likely have to use your last method with a 3/8" tube to fit everything. Will verify the dimensions. I see there's a decision to be made here...

You've been a tremendous help, Ollie.
Building/designing is so much fun; but I really should practice more with my wood 2M to justify all this effort. :)

- Chung

[MikeBanyai]

Check out Adrews Bookmarks

http://isoar.ca/~andrewm/bkmks/RC/Wing_Rods/index.html

HiLaunch has spar tubes and joiners

http://www.hilaunch.com/Left.htm