Struts in a bi-plane - are they needed?

[AmokCrow]

I'm going to scratch build a slow-flyer bi-plane using the premade styrofoam wings the LHS sells. Because of the (undercambered) airfoil and styrofoam structure of the wings, struts would be an annoyance. So, can I just leave the struts out or would there be aerodynamic consequenses in doing so?
The plane would be approx 180g, powered by GWS IPS. The wings are about 60 cm (24 in.) tip-to-tip and both wings (upper and lower) would be identical with the exeption of the upper wing having ailerons. I'm not sure of total wing area.
As you may have guessed already, this will be my first biplane, so I'm kind of clueless here. Any help appreciated.

Thanks
-Michael

[Ollie]

The function of struts are not aerodynamic. The struts function is structural.
With positive G lift loads, the upper wing spar is in compression, the lower wing spar is in tension, the flying wires are in tension and the struts between the wings are in compression. The team work of the wires and struts carries the shear load.

If either wing is enough in strength and stiffness (compression, tension and shear loads for lifting), without struts and wires, then forget the struts and wires.

[Sparky Paul]

It depends on how the upper wing is mounted.
If that's secure against the wing bending from side to side, struts aren't needed.
A pylon mounted wing will need a stiff pylon to rest on.

[ghoti]

In the early days biplanes were built because it allowed a giant box beam ot two wings and struts and wires to hold the thing together. That was before we knew how to make strong wings. Historically viewed, the lower wing got smaller and smaller yet serving as basis for a box beam. Then the lower wing finally dropped out when not needed for strength.

Two wings means twice the number of tip vortices and twice the induced drag. They were slow. Two wings can be much shorter than a single one, pound for pound. This reduces the radius of gyration of the craft and allowing for quick turns. (The wing mass is closer to the fuse that way.)

For the modeler, biplanes yield slower flying and rapid rotation and that can be great fun, especially for 3-D airplanes.

[Griffin]

Stuts are not always just for strength.
Almost all 3d foam models like to gain some knife edge surface area with wide struts.
-Steve

[peterangus]

AC

Without struts, there is a possibility that your wings may be strong enough in bending.

But it is very unlikely that they will be stiff enough in torsion, especially since they are undercambered.

[saucerguy]

Stress test it before taking it to the air, all I have to say, the struts may be necessary, also keep in mind, you will push the boundries of it "likely" so may need the extra support enduring the G's.

[AmokCrow]

Thanks for all the replies.

The plane was finished yesterday (finally) and I tried to fly it today.

Unfortunately it crashed almost immediately and quite oddly. You see, when I launched the plane it started immediately a slow turn to the left. The plane had no rudder, so I tried to turn it to the right with ailerons, but for some reason it just wouldn't start rolling even with full stick. I could see the ailerons move, so it wasn't a malfunction.
Out of ideas, I tried to roll a little left instead, wich it slowly did, but when I tried to correct to level again, it started to turn left while not changing its direction of flight. At this point it tip-stalled to the left and crashed hard.
The battery tore out half of the depron fuselage, so I don't think this thing will fly again.

Now, I'm totally puzzled. I've never seen somethink like this before.
If anyone can tell me what it could be that went wrong, I'd be thankful.

-Michael

[ghoti]

Wilber and Orville discovered adverse yaw during their first flights and that observation induced them to add a rudder (in front).

I would guess that happened to your biplane or perhaps a combination of that
plus incorrect motor side thrust.

Whenan airplane (with dihedrial) rolls one wing makes for greater lift-- and drag-- than the other. That drag pul;ls back on that wing so adverse yaw.

Just my best guess. Bill

[Sparky Paul]

I just completed a GWS Tiger Moth 3D... box-stock... at low rates, the adverse yaw is amazing! It's nearly unturnable.
Adding in rudder assist makes it better.
The plane is more a rudder plane than ailerons, although the roll rate is remarkable.
After the first two flights yesterday, I put in a LOT of differential aileron and coupled the rudder 20% to the ailerons.
It flies much better at low rates.

[fhhuber506771]

An example of a full scale aicraft that NEEDS its struts is the Tiger moth... (a model of which is pictured by Sparky...)

The GWS models of the Tiger moth have fairly flexible wings and need thier "interplane" struts.

There is a .30 size Tiger Moth kit by Gee Bee products which recommends you leave the interplane struts out when you fly it. They are for display only! The wings are rubber band mounted... ailerons only in the lower wing (which is correct for scale) and in a hard roll... you can see the wingtips on one side almost touch each other...
Seems kind of "flaky" until you think about it. The wing with the ailerons wants to roll faster than the wing without... and the rubber bands allow the movement. Putting in struts strong enough to limit the wings from moving would create issues in setting up the incidence... and make assembly/dissasembly a chore.... the rubber band mount and flying without the struts just works.

********

so.. the need for struts on a biplane has a few factors involved... and there's even one example where putting them in is more of a problem than a solution.

********

BTW... the full scale prototype for the Fokker Dr1 had no interplane struts... the wings were strong enough without them. The struts were put in on the production version to make some general happy. They could be shot away and the plane had no problems.

Unfortunately... there were issues with the wing covering which had it peel off at high speed... But the wing structure was not the fault.