Designing a wing rod connector

[shaneyee]

I am building a 100" wingspan electric flying wing. The chord is 17.5" and the wing is without taper in the middle 72" with eliiptical tips. The airfoil section is the BW050209. Followers of the "On the Wing" column in the R/C Soaring Digest will recognise that it is inspired by the R2 built by Bill Kuhlmann. However, I will be using a conventional D box using 1/16 sheeting and spruce spars. The spar caps are 1/2" x 1/4 at the root tapering to 1/4 x1/8 at the tip and the sparweb will be vertical grain balsa. The wing is 1.75" thick at the root.

I will build the wing in 2 sections. I want to join the wings with a wing rod but I am unsure of the size and material. The largest straight rod I can fit between the spar caps is 1" x 1/2 " This plane is my low tech project so I am avoiding carbon or boron and the other good stuff.How do I size the wing rod connector if I were to use Aluminium bar stock or maple or steel rod?

Thanks

Shane

[Ollie]

To find the bending moment at the wing joiner, subtract the maximum expected G's times times the weight distribution along the span form the lift distribution to get the bending load distribution. Then integrate the load distribution to get the spanwise shear distribution. Integrate the shear distribution. Integrate the shear distribution to get the bending moment distribution which includes the bending moment at the joiner. Use the bending moment to size the joiner.

A simpler but lrss accurate approach would be to assume the wing structural weight distribution matches the lift distribution and that the weight of the motor, battery and radio components are centered at the middle of the wing and their total weight times the maximum expected G's equals the maximum lift. Half the maximum lift times half the semi span equals the maximum bending moment at the center of the wing.

An even simpler approach is to match the joiner strength to the spar strength. The maximum bending moment that your spar can take is about 875 pound-inches. This is based on the criterion that the joiner shouldn't fail until the spar is about to fail also.

[shaneyee]

Thanks Ollie.

I'll match the joiner strength to spar strength.

How about 1"X 1/2" thick hardwood fitted between the spar caps and sandwiched between ply shear webs? The whole thing then wrapped in 2 layers of 2oz glass... The hardwood joiner will have an elliptical opening at both ends to reduce the stress riser.


Shane

[Mike Taylor]

I have a very old 10' WS Cirrus from the 70's with over 300 hours of airtime - slope and thermal soaring, with innumerable hand, bungee and winch launches.

The wings are joined to the fuselage with ~ 3 mm steel pins built into the wing and riding in hard brass tubes built into the fuselage center section. The rods are epoxied into the wing root between the spruce spars in a plywood sheared box. I forget what the pins are packed with but I believe it was balsa. This joining system seems to be all that is required. I have burst the film on the bottom of the wing winching the plane, but the wing joint is still sound.

[Ollie]

I'm sure that it will do very nicely. Depending on the length of the joiner you may require additional thread wrappings at the ends of the box to resist the prying action of the joiner on the joiner box. For example, if the bending moment is 875 pounds and the length of engagement of the joiner in the box is 3 inches, then there will be a force of 875 inch pounds divided by 3 inches equals 292 pounds of force at each end of the box trying to split the top and bottom spar caps apart.