Biplane Struts

[jforkner]

Is there an aerodynamic reason the struts on a biplane are inboard from the wingtips? Can they be connected to the wingtips themselves?

I’m building a biplane from a couple of spare wing cores I have, and am struggling with how to connect the struts to the wings. Attaching the struts to the wingtips would make construction much easier. Of course, if doing so causes some aerodynamic problem (like with tip vortices, etc.), then I won’t do it that way.

Thanks.


Jack

[slipstick]

The wings are obviously more effectively supported with struts around the centre of the wing rather than just right at each end but there are plenty of models which do have struts at the tips. It's unlikely to do any real harm provided the wings are strong enough.

E.g. http://www.mjp.co.uk/model%20aircraft/panic/

Steve

[jforkner]

Great stuff, Steve. Thanks. Those pictures on the link you provided are exactly what I had in mind.

Jack

[noodle]

slipstick, that doesn't quite make sense. The wings are still cantilever wings, assuming that neither wing is significantly stronger than the other, and that no flying wires are used. If they both produce the same amount of lift, there should theoretically be no load carried by the struts in level flight. So technically, it shouldn't matter where the struts are located. I think the main reason for having struts on most bipes these days is to stabilize the top wing, keeping it from rocking from side to side.

[jkettu]

The whole wing system is a huge I-beam structure. The upper wing is the top spar cap (compression), lower wing is the other spar cap (tension) and flying wires and struts are the webbing.

[noodle]

I'm still not getting it. The struts provide little side-to-side stability. They might as well be hinged at the top and bottom. Picture a parallelogram with hinges at all four corners. That's essentially what you would have if the wings weren't cantilever.

P.S. I'm not counting flying wires here.

[jkettu]

P.S. I'm not counting flying wires here.

Ah, ok, in that case you are correct. But the wires *are* an essential part of the structure.

[noodle]

Well, yeah, sometimes. Most bipes I've seen don't have flying wires (i.e., Ultimate Bipe). Back in WWI Flying wires were very important though. :)

[Majortomski]

Most 1:1 bipes have the strut where the spar ends and the wing tip starts, so in essence it is at the end of the wing.

[jforkner]

My theory is the struts help keep the wings separated. I also believe the flying wires are used to help prevent lateral forces from collapsing the structure. That’s my theory and I’m stickin’ to it.

In any event, I appreciate the help and will proceed with locating my struts on the wingtips.


Jack

[Sparky Paul]

Model wings are generally structured so that struts are ornamental... but some scale planes, my Nieuport for one, needs them for structural integrity.
My Stearman can fly with or without.

[tim hooper]

The whole wing system is a huge I-beam structure. The upper wing is the top spar cap (compression), lower wing is the other spar cap (tension) and flying wires and struts are the webbing.

jkettu,

I can't grasp that. Surely the struts would only operate as webbing if they had an appreciable area parallel with the spar line of the wing? Struts by themselves (unless aided by rigging) do nothing for overall rigidity.

noodle,

Given your pair of cantilever wings, the only function of the strut must be to maintain the correct amount of separation - probably only useful if the model has linked ailerons to avoid trim changes in flight as the wings flex.

tim

[noodle]

That's basically what I was thinking, Tim. :)

[jkettu]

Tim: yes, you are absolutely right, sorry for less than clear expression. What I meant is that the combination of rigging wires and struts together work as webbing. Wires take the shear loads of the structure, being in tension themselves. Since wires can't take compressive loads, some kind of support (= struts) is needed to keep the spar caps (= wings) apart.

Reading what I just wrote, that's probably even more confusing... If so, just disregard me, we all seem to know how these things work, even if I can't explain it. Happens a lot in my native language too. ;)

[Work in Progress]

Even without wires, interplane struts may have an effect in increasing the effective torsional stiffness of the wings. Depending on how it's mounted an interplane strut converts the torsional rotation of one wing and resists it with the fore-and-aft and up-and-down strength of the other wing.

[portablevcb]

This depends on how the wings are constructed. In a lot of biplanes the lower wings do not have a spar that runs across the fuse, they are two different structures "pinned" to the fuse at their roots. In this case the struts are very structural, transferring most of the flight loads from the lower wing to the upper wing and its full length spar. Most of the flight load is transmitted to the fuse through the cabane struts.

If the lower wing has a full width spar through the fuse, then the struts may not be bearing any load at all and could even be eliminated entirely.

Be careful though, some designs are such that flying wires are required as part of the structural design. In that case the struts are load bearing structures.

Where the struts are located is a matter of economy in design. The farther out they are, the more bending moment on the spar at the point midway between the fuse and the strut (when the strut is loadbearing).

From what I can gather of your design, if you have some flat wingtips, then securing the strut to the end plates will add some rigidity to the wing structure.

Hope this isn't too confusing.

charlie