Shear webbing grain direction. Does it matter?

[Bigflyer88]

Hi all am in the process of putting some wings together and am wondering.. Does it matter which direction the grain ru s on the webbing sections on the spar between the ribs? Is one way stronger than the other or does it not matter? Many thanks Mike.

[Nigel R]

Hi bigflyer

Short answer - Grain must be vertical.

They are worthless if grain is parallel to spar

[Bigflyer88]

Awesome thanks very much Nigel I prob would have gone the other way if I hadn't given it some thought out of interest why is vertical stronger?

[Andy Stephenson]

Big flyer,

The primary force on a lifting wing is bending where the top of the wing is in compression and the bottom in tension. Most spar materials are better in tension than compression. The compression side is prone to buckling but the shear web will transfer some of this buckling stress to the bottom spar. If the grain is horizontal this will only serve to split the wood along the grain where as vertical grain won't split until the force is much greater.

A.

Edited By Andy Stephenson on 09/05/2020 16:00:57

[David Davis]

I have heard of some people building the spar webs with the grain running at 45 degrees. Right to left on one bay, left to right on the next. I've never tried it myself. Sounds a little OCD to me.

[Andy Stephenson]

A 45 degree shear web would be good to resist twisting on a wing but worse for bending.

A.

[Simon Chaddock]

Technically a shear web needs to resist a shearing force. Controlling compression buckling is a secodary function.

A material with a 'grain' is not ideal to resit shear unless it is in the form of a ply where the effect of the grain structure is negated. Placing the grain of the ply at +/-45 degrees would resist the shear forces better but a homogeneous material would be ideal.

Note the ply grain direction suggested here.

**LINK**

[Bigflyer88]

So the plan calls for 1/16 balsa webbs is it worth using 1\16 ply instead or would this extra weight be more of a problem? Or am I overthinking it lol. Its a 65inch ultimate biplane BTW.

[Geoff S]

1/16" balsa with the grain vertical is the usual way and perfectly adequate - in fact more than adequate. There's no point adding unnecessary weight by using heavy ply.

Geoff

[Bob Cotsford]

Posted by Simon Chaddock on 09/05/2020 20:23:28:

Technically a shear web needs to resist a shearing force. Controlling compression buckling is a secodary function.

A material with a 'grain' is not ideal to resit shear unless it is in the form of a ply where the effect of the grain structure is negated. Placing the grain of the ply at +/-45 degrees would resist the shear forces better but a homogeneous material would be ideal.

Note the ply grain direction suggested here.

**LINK**

Interesting thoughts Simon. Are we using it as a shear web though? Most wing failures that I've witnessed have been caused by the upper spar buckling in, possibly accentuated by the covering preventing it buckling outward. Shear would indicate the spars moving sideways relative to one another but I can't recall ever seeing evidence of that. Certainly there is shear stress present but it always seems to be primarily a failure of the spars alone, often only the upper one. I think we are using webs in compression, ie too form a full I or more commonly [ beam supporting the upper spar run.

[Andy Stephenson]

Since the shear web is always applied to one side of the spars usually on the rear side, this is likely to cause a small amount of twisting force on the spars so it is like that they do indeed move sideways relative to each other during a buckling failure.

I have in the past designed wings where the shear web is sandwiched between two spars, just because I thought it was a good idea and of course this makes the ideal "I" beam.

A.

Edited By Andy Stephenson on 09/05/2020 23:30:28

[Mike T]

Variations on a theme. I've sometimes built wings with the shear webs alternating front and back of the spars, from bay to bay. Benefit is probably marginal, but I like to tell myself it helps with torsional stiffness as well as shear loads...

Top tip - assuming the rib spacing is equal, I trim the sheet from which the webs are to be cut to the width of the webs. Each web can then be sliced off as required and working from root to tip you add alternating ribs and webs to the spars as you work out towards the tip. A perfect tight fit all along the span, but you may end up with a wing fractionally longer or shorter than the drawing

 

Edited By Mike T on 10/05/2020 00:01:21

[Simon Chaddock]

Bob

I suspect where along the span a spar fails is more significant than exactly how it fails. With all things being equal wood will likely fail in compression simply because it is stronger in tension.

I suspect that for the majority of wings the actual failure point is the result of its simplicity of construction. Few builders would consider following the level of detail that would result from a full stress analysis.

As an example, how many 'plank' wings have a constant section wing spar tip to tip although the stresses resulting from the aerodynamic loads decline to virtually zero at the tip.

[Peter Miller]

I use the standard system of 1/4" sq spars top and bottom with 1/16" webs with the grain vertical.

My Dancing Girl had a G meter fitted and a Wankel engine. 52 odd inches span for.32 engines or a Wankel.

I did a terminal velocity dive and hauled in full up elevator. The model was unharmed.

It pulled 24.8G which meant that the model weighed 95 lbs briefly

I think that one can safely say that the system is quite strong enogh.

[Bigflyer88]

Thanks guys for the replies not only have you put my mind at rest but you know I think I may have learned some things. I have gone with the 1/16 vertical grain on this one.

[John T]

Many moons ago one of the magazines had an article where they actually built some mock-up spars with the grain running in different directions. The conclusion was that there was virtually no difference in load before the spar failed.

Having said that, I still use webbing with a vertical grain because it just feels 'right'