# has anyone used fiberglass laminates for gusset material

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#### Aerowerx

##### Well-Known Member
Some simple calculations will show the tensile stiffness and strength of the plywood gussets normally used, ...
That reminded me....

Several years ago I started a thread, asking if anyone knew how to size gussets. Someone started to look into it, but then the topic seemed to die. And some people scoffed at my question as being a waste of time.

So how DO you know how big and thick to make your gussets?

As I see it, something between toilet paper soaked in T-88 (cheap and readily available) and 3/4 inch plywood (plenty strong) should work.

#### wsimpso1

##### Super Moderator
Staff member
Log Member
So how DO you know how big and thick to make your gussets?

As I see it, something between toilet paper soaked in T-88 (cheap and readily available) and 3/4 inch plywood (plenty strong) should work.
LOL. I suspect a rather empirical approach has been used in the past. I am sure that analytical approaches exist... Sounds like a dive into the aero structures books in order. The modern way of checking them is to FEA the thing and iterate.

I was proposing reverse engineering of a known good plywood gusset in composites, using their profile shape, and picking the thickness to mimic stiffness and strength. That starts with the assumption that the base part is adequate... I went further to propose that the strength of the composite part be at minimum 4/3 of the strength of the plywood part in order to preserve appropriate FOS on the composite part.

Billski

HBA Supporter

BJC

#### BBerson

##### Light Plane Philosopher
HBA Supporter
The gusset is sized to transfer the tension or compression of the verticals. And may be larger if the designer also wants a shorter unsupported span for the cap strips in compression.

#### Hot Wings

##### Grumpy Cynic
HBA Supporter
Log Member
The gusset is sized to transfer the tension or compression of the verticals.
I'd have probably phrased that to say:
The gusset is sized to help transfer tension and compression by adding glue area and additional load path.

It also makes a difference how the load is applied. In the example above if the fabric were to be glued without rib stitching the top center gusset on the nearest rib might not be up to the job due to grain direction of the plywood.
If rib stitched the load on the vertical is always compression.
If the fabric is glued to the top and bottom cap, with no rib stitching, then it could be in tension.

#### Aerowerx

##### Well-Known Member
The cemetery is full of "TLAR" engineering!

#### BBerson

##### Light Plane Philosopher
HBA Supporter
Not sure what you mean by additional load path. Generally, the truss node is where the force is transferred and focused at one point.

#### Hot Wings

##### Grumpy Cynic
HBA Supporter
Log Member
IF the vertical is in tension with no gusset all you have is the glue area of the butt joint. Add the gusset and it now participates in transferring the load to the cap strip, or the other way around. Without the gusset the limiting factor is probably the glue joint, which is weaker than the wood truss member due to limited area. With a proper gusset the limit is again defied by the truss member.

Same idea applies the diagonal members, but in shear.

Side note: One of the reasons I find the Aeronca paper gussets so interesting is that they are isotropic. Grain direction doesn't matter.

#### BBerson

##### Light Plane Philosopher
HBA Supporter
Yes, the gusset increases the area (when I said "the gusset is sized...)
And yes, more importantly the gusset puts the glue in shear, much preferred from a simple butt with the glue in tension.