So, what your telling us is: all the designers of model airplanes who have designed wings with shear webs running vertical, are wrong.? Good luck with that position!

most older kits that I have built supplied pre cut balsa shear webs to run vertical.never had an in flight wing failure with  or with out them per design specs.for all practical purposes in most models in the under 10 pound class balsa should do fine.remember to build to fly not to crash so lighter is better.I suspect that there is a cost consideration also in using vertical shear webs as they would be easier to cut and lees waste from supplied sheets.If I remember correctly when I built my andrews H-ray in 1972 he had vertical shear webs between the spars but also what coud be a 2nd web running horizontal at the edge of the trailing edge sheeting in every bay

Grain goes vertical

http://www.auf.asn.au/scratchbuilder/beams.html
http://yarchive.net/air/plywood_construction.html
http://www.djaerotech.com/dj_askjd/d...shearwebs.html

The above links are just a couple of the many that come up when "shear web wings" or some combination is used in Google. I think there may well be some validity to the idea that shear webs at a 45 degree angle work better. In fact, plywood shear webs would appear to be even better to handle the shear loads(positive and negative)on a wing spar. We can allargue whether the grain goes vertical or horizontal or 45 degrees. Heck, centuriesearlier there were thosewho argued for a flator round earth. But in the end GPS only workedfor the round earth idea.

I took Statics too.  Yawnfest. </p>

Cratecruncher is correct in his statements.</p>

</p>

Rodney, it is highly unlikely the forces on the web are at 45 degrees.</p>

It has been an interesting discussion but I still think that there is some confusion about grain direction and the loads actually being carried. You need to keep in mind that wood is much stronger along the grain than across the grain. When you orient the wood so that the grain runs along the diagonal (not necessary at 45 degrees), the "shear" load is actually being carried in tension along the diagonal. You could in principle replace the shear webbing with cables or string running along the diagonals and end up with the same result as adding a shear plate. However, the last time I checked, cables cannot carry shear loads. I have enclosed a simple sketch which hopefully ilustrates the principle. The other thing to keep in mind is that the ribs on a model airplane has very little compressive strength and that is why it is better to have the grain oriented vertically. The links to full scale airplane construction techniques show that the ribs typically have vertical compression members where they meet the spars. If this is the case, maybe a plywood shear web at 45 degrees would be best. because it is crossed grained. At any rate, I think I will leave it at that and would like to thank those who posted the links as they make for fun reading.

Teo

I guess I was confused about what Rodney was actually saying.  First, I was taken aback by the cutting remarks he made towards CrateCruncher!  Then, I belived he was saying the shear web was to have it's grain run parallel to the spars.  He actually stated having the grain on diagonel and to use plywood.  I have no problem with this and agree it's likely the strongest.  If the airplane can handle the extra weight of plywood shear webs, have at 'er.  However, I think vertical balsa works well where the lightness is needed. </p>

I am amazed at times, how these threads can become so hostile!  I don't understand this. </p>

It is amazing, indeed!!

A question from Scotta generated an interesting argument that made many people ask themselves the reason of the webbing in a flexing beam.

In the middle of that Cratecruncher got offended and some got polarized.

Please Cratecruncher, do not get distant from the forums; your posts are always interesting and fun.
I did not need to see your diploma to know about your knowledge and education; the many post of you I had read before proved me those.

I believe Benjamin Franklin wrote: "Anger is never without Reason, but seldom with a good One."


Edited to correct quote

This is not a debate,and no testing needed. The shear webs are much stronger when yougo vertical with thegrain,it's that simple guys.

Bob

I've always run mine vertical per recommendation through the numerous instruction guides/building manuals that have been published over the years. They don't seem to be wrong and I've never seen my wings fail on me.

One solution, if you really want a super strong structure would be to make a balsa 3-ply web as follows: left 45 and a right 45 sandwiching a vertical layer.</p>

This is obviously overkill and weighty, but would satisfy all the above mentioned stresses.</p>

</p>

Thanks for the education!  Despite heated exchanges I think some good information was posted . </p>

I don't have the experience to be able to add much here.  I will realate some limited experience.  My first try at a lage scale had an 8" box girder beam for the spar.  The chord of the wing was 18".  I made the spar out of spruce caps and luan for the webbing.  The middle 1/3 was has a solid piece of spruce inside the void in the box girder.  The airfoil was hot wired from blue foam.  The leading edge was pine half round moulding.  There was another piece of sprce, 1/4 x 2" on the trailing edge to hang the ailerons on and attach the barn door ailerons and trailing edge. The whole works got a coating of brown paper and  PVA.  </p>

Cave man tech to be sure.  Thing is the wing loading with a ready to fly weight of #22 and 2000sq inches was respectable.  It did fly well, until the elevator departed the structure.  The point is that about he only thing that survived the impact was the wing.  I bet I could have done chin ups on the main spar.  I am thinking there might be something to this construction strategy.  Perhaps a box girder made of lite ply or laminated balsa would have merit? Use the ply and put lightening holes of increasing size toward the wing tips.  I did knock out the wing is a fraction of the time it is taking to assemble my Bridi Big Bee.  The foam composite wing came out much straighter too.  If I had chosen ligher materials,monocote and balsa, I would bet the weight would be competititve with a built up structure.  </p>

</p>

</p>

</p>

If you consider the design of trusses used in a bridge; or, the trusses used in the roof support of a building, it becomes easier to understand the correlation with placing diagonal shear webs in a wing spar assembly.   In full scale (which Rodney is referencing) the additional weight of building spars in this way can easily be absorbed.  It may be that a spar built in this way might work for smaller models, also.  However, my opinion is that vertical webbing works well enough, why go to the extra effort?

Disclaimer. The following is for balsa webs. Im not about to start making
webs of light ply. This is also meant to think about a less than perfect built
wing which... all mine are.</p>

So, Ive been thinking about this a little and I'll go along with the Free Body
Diagrams that show that the shear web grain orientation, vertical or horizontal,
probably doesnt matter much if the webs are properly fitted and adheared to the
ribs. No gaps, air tight.</p>

However,
(#1) how many of us have built wings with the webs not even contacting the ribs?
The webs are only adheared to the spars. So you basically have, what was described
as, a shear crack on either side of a rib.
Id bet my lunch that in cases like that, the vertical grain orientation (normal to
the spar) is the only way to go. Id surmise that the compressive and tensile strength
of the webs become even more critical so that the web does not buckle under flight loads.</p>

With no rib in contact with the webs, a horizontal grain, has no glue joints to keep
the web from buckling under load.</p>

(#2) also a wing in flight under dynamic loads arent really comparable to a static truss.
The FBD's given dont consider the twist or torque imposed on a wing during flight (lift),
nor the torque imposed on a wing due to flap and aileron operation, nor...
the rotational force imposed on a wing just from ailerons and flaps would further the argument
for a vertical web grain orientation.</p>

The reason I had to add my two cents is that Id guess 90% of the planes Ive built and seen
built fall into the catorgory described above. I dont know many people that take the time
to build that "perfect on paper" wing.</p>

The grain goes normal to the spars.</p>

Cheers,
James</p>

Per gsoav8r:</p>

"(#2) also a wing in flight under dynamic loads arent really comparable to a static truss. The FBD's given dont consider the twist or torque imposed on a wing during flight (lift),nor the torque imposed on a wing due to flap and aileron operation, nor...the rotational force imposed on a wing just from ailerons and flaps would further the argument for a vertical web grain orientation".</p>

First off, I agree with you, for the purposes of most R/C aircraft, vertical grain balsa shear webs work well enough. However, I question if they workbetter for the dynamic loads imposed on a wing during flight. I see the job of the wing spar assembly as exactly the same as a truss.</p>

Yeah I see what your saying. A big typo there. I should have wrote:</p>

(#2) a wing in flight under dynamic loads arent fully comparable to a static truss with a single load.</p>

I dont disagree at all that the spar asm. isnt a truss. Ive read about wings folding because webs were accidentally omitted during construction.</p>

I believe if someone where to look at the FBD's given prior and then think about  a FBD showing rotational forces normal to the spar that the argument for webs with grain normal to the spars will still hold for my less than perfect wings with "designed in" shear cracks.</p>

I think you get the best of both worlds. OK truss strength with the rotational forces normal to the fuse and then good strength with the rotational forces normal to the spar/truss. With rotational forces normal to the truss and the web grain normal to the spar, the webs are probably at full effeicency in compression and tension.</p>

Fun stuff.</p>