Before I followed the plan and just glued the shear web in the way it is shown there I researched this forum a littleand found a lot of very helpful posts.
I summarized the posts as : most manufacturers tell you to glue the web at the back of the spar. I assume that this is just the easiest way to do it. I also learned that an I-beam kind of construction would be the most effective. Thinking about that I decided ti go with that. There has to be a reason for I-beams being used that much in general construction
It was a real pain to get that right. I ended up measuring between 2 ribs, trying, sanding until I finally had the right fit. I just cut a piece of 1/16 balsa with the grain running vertically ans used the same material with the grain running horizontally for the top and bottom piece. Slow CA to have time to fit it and accelerator once it fits work wonders!
The result is a very solid structure and I am very happy.
I do believe that it has to be a tight fight because of load distribution and I made sure everything fits at least 100%.
I am very happy with the result. I believe it was orth the time.
The advantage of building yourself is that you do not end up with the standard product and have room for improvements.

Cheers

Steff

Steff,

I use both methods of construction depending on the configuration of the wing. If the wing is sheeted between the LE and Main Spar I glue the shear webbing to the back of the spar, if the wing is open between the Main Spar and the LE as on my "Bingo" I use the I-Beam.

I have been building this way for more than fifty years and have never had a wing failure. Using the D box construction (sheeted between Spar and LE) is much easier and faster to build.

Steff,

Actually, it is probably best to glue the shear webbing to the back of the two spar members, as your plans suggested. The job of the shear webbing is to carry the shear load ( the force that makes the two spar members move relative to each other when the wing is loaded ). The glue joint that you get when you lap the shear webbing to the back of the spar members is much better than the joint you get when you butt it between them. Steel I-beams are different in several important ways, which is why their construction is different. Your wing will probably be fine, but sometimes the easy way is also the best way.

banktoturn

I agree with banktotem, the amount of material that overlaps the spars makes for a much stronger bond .

Highlander

In built-up r/c sailplane wings, which have to take winch loads of hundreds of pounds on launch, the standard practice is to use vertical grain balsa shear webs the width of the spar on the inner part of the wing (where bending stress is highest) and to thin the shear webs from full-width to 1/16" from the inner part to the wingtips to save weight. On the inner part, the spar/shear web is also wrapped in Kevlar thread to keep the spar/shear web from "bursting". Epoxy thickened with microballoons is used to assemble the shear web/spars.

You judge a strong launch by observing how much the wings flex on launch and also how much wrinkle you get in the covering.

Hmmm, I would say that getting a good bond between web and spar is more important that getting the web at the spar centerline. In other words, a poor glue joint will drastically reduce wing strength while an off-center web will probably have very little effect. Remember that you have a lot of ribs keeping things lined up fairly well.

I have built glider wings as BillHarris describes. 1/8x3/8 spruce spar, 3/8 vertical grain balsa web, carbon filament wrap. This was eight years ago. I have no idea what "modern" design details are.

steff,
The SPARS are meant to be the top and bottom of the I-beam. No need to make a seperate I-beam.

For most applications attaching the shear webs to the back of each spar is more than good enough, assuming a quality glue job.

I feel statements that back-of-the-spar shear webs provide a stronger glue joint are misleading. With an I-beam shear web the glue is only necessary to keep the shear web from moving out of place, there isn't nearly as much stress on the glue joint as the back-of-the-spar approach.

SheldonYoung,

Shear webbing needs to carry a shear load ( you can think of this as a sliding load ) to prevent the top and bottom spar members from moving relative to each other in the spanwise direction. If you use an 'I-beam' configuration, you still need the web to be securely joined to the top and bottom members. Those joints require glue to carry the shear load. Actually, the lap joints you would have with the webbing glued to the back would carry less stress, since the joints would have more surface area over which to distribute the same amount of force. Generally, to have a stronger glue joint, you want a good fit, and enough surface area. It is easier to get both by lapping shear webbing to the back ( or front ) of the spar members than by edge-gluing the webbing in between.

banktoturn

banktoturn,

If that was true the grain of shear webs would run horizontally instead of vertically. Instead, I suggest shear webs prevent spanwise stress before it even becomes an issue by maintaining the distance between the spars during bending loads (compression).

If you think of the rectangular space between the spars and ribs where the shear webs fit, when the wing bends under load this rectangular area wants to become a skewed parallelogram. When this happens the spars shift lengthwise relative to one another, hence the term "shear".

However shear webbing also carries a compression load. When the rectangular area skews the top and bottom spars want to move closer together. In this case vertical grain webbing between the spars can sustain the compression load better than webs glued to the backs of the spars. If the spars can't compress into the space between one another then they can't shift lengthwise in a shearing direction either.

The strongest shear webbing (of a wooden type) would be a laminate of two plys crossed at 90 deg and then cut on the bias. If this is were placed between the spars then the compression load would dominate and the glue strength is less critical. However although the glue joint strength is less critical, the fit is more critical than webs lapped to the sides of the spars.

SheldonYoung,

Your point about the orientation of the grain is incorrect. As the name of shear webs suggests, they are intended to bear a shear load. Specifically, they keep the upper and lower spar members from moving relative to each other in the spanwise direction. For a shear load, wood is strongest across its grain, which is why shear webs are oriented with their grain vertical.

GeraldO,

There certainly can be a tendency for the upper and lower spar members to move closer together when the wing bears a bending load, but it is not true that the members can't move relative to each other in a spanwise direction without being compressed together. In any case, wood is very strong in compression in the direction of its grain. Shear webs with vertically oriented grain, and glued to the back ( or front ) of the spar members will do an excellent job of bearing the shear loads and the compression ( & tension ) loads that result from a bending load on the wing. The superiority & simplicity of the lap joint make it very easy to realize those benefits. Moving the shear webbing between the upper and lower spar members would not meaningfully improve its ability to bear the compressive loads, but would degrade its ability to bear the shear load, which is more important.

I don't know about the superiority of your proposed 90 deg. laminated shear webs, but on a strength-to-weight basis I am a bit dubious. Wood shear webs with vertical grain are already making excellent use of the wood's shear and compression strength, and the small gap between the upper and lower spar members virtually eliminates buckling as a concern.

The easy way still seems best to me.

banktoturn

The stress in the web has one component due to the shear load, and another due to the bending load. These are perpendicular, so the resultant stress is at an angle.

If one purchases carbon fiber "shear webbing" the fibers are oriented at 45 degrees.

Which does lends credence to the two ply approach.

My understanding of web loading is exactly like banktoturn describes. In terms of aligning the wood fibers to the loading direction, I think vertical is optimized. With vertical webs, the fibers are parallel with compressive loads and at a right angle to shear loads. This is the best alignment.

My $.02

Banktoturn is very correct. You must have good contact between the webing and the spars or the webing is nearly useless. Jim is also correct, the maximum stress is at an angle; neither perpendicular nor horizontal. For maximum strenth the grain should run at 45 degree angle but; it shifts 90 degrees when the load shifts from positive to negative. For that reason, horizontally grained sheer webbing as as good as vertical grained webbing. For max strength, you would have to have a ply webbing with the grain of each ply at 45 degrees to horizontal, one ply to the left, the other to the right. Take a look at any mechanical engineering book and you will see what really happens.

The stress on the shear webbing is certainly not limited to pure shear stress in the spanwise direction, but it is also not necessarily at a 45 deg. angle. The exact nature of the stress is affected by the geometry of the structure, the relative properties of the different members, and probably some other things. My gut feel is that this situation is affected by the fact that our material is highly non-isotropic as well, but I'm getting in over my head. If I get the kids to bed early enough tonight, I may have to dig out my Mechanics of Solids book and see what I can dredge up.

banktoturn

Seems to be quite a controverse topic!
I am in no way an engineer. But one thing seems to be clear to me: the load will ONLY be distributed if the webs are tightly connected.
I agre it would be even better to just put a web between the spars. With a 1/16 vertical grain bals piece it seems to be easier to do it the way I described. Ultimately, because of the tight fit, the loadwill be distributed over all the webs.
You are correct that if you have a vertical and a horizontal force the result will be what I know as a vector and by no means in either of these directions. Since you can use wood only with the grain vertically or horizontolly there is little you can do other than maybe using the above mentioned carbon webs.

The structure seems to be very sturdy and solid with little added weight. I'll let you know if it did not work ok, promise

Cheers

Steff

Steff let me mention that for thermal sailplanes (they have relatively LONG spars) the suggestion (and practice) is to use carbon fiber material of a given thickness on the BOTTOM of the spar system and use material of the same type but twice as thick on the TOP of the spar system to prevent spar damage when using 'zoom launches' which very much stresses a sailplane wing structure.

So besides this interesting debate about the best type of shear web construction (I have had interest in this topic for years) you might look into the reinforcement I mention here since you may find it of interest as I have.

On many occasions, when driving down the highway, I have convinced myself that I really need to make a number of different spar systems using the materials with which I had worked (Aliphatic resin, spruce and balsa) and attempt to break them under controlled conditions (including low temperature and high temperature) just to figure out what works best with these materials. I have yet to actually do it. 8^)

I have built with vertical balsa between the spruce spars, balsa in front of the spruce spars, balsa behind the spruce spars, balsa in front of and behind the spruce spars (both on one spar system) and balsa in between, in front of and behind the spruce spars (all on one spar system). And I placed ply where required by the wing connection systems). I have not broken a spar yet so I assume each was adequate for the stresses that the wings encountered. But I am still curious about what others find out about building different spar systems. With the new materials used in sailplane structures there has been vast improvements in the strength of sailplane spar systems.

In particular I have wondered if adhering the shear webbing to the sides of the spar actually did or did not provide a superior shear web due to the increased area of contact. That strength is derived from the quality and penetration of the adhesive into the balsa and the balsa material transfering the stress out away from the glue joint. If the shear webbing is placed in the center of the top and bottom spar the adhesive is not transfering the stress in the exact same way and I believe this method provides a stronger spar. But I will watch this thread to find out if anyone finds results of actual tests performed in the past.

Aother thing is that since I scratch build I cut out shear web material using a table saw so that each piece is a 'standard' length and each corner of each piece is 90 degrees. Then I place a rib on the bottom sheeting then a precut shear web, then another rib then another precut shear web, etc. The precut shear webs brace the ribs 'vertically' and I have a near perfect fit (tight) of the shear webs to the ribs because I install them one right after the other. This has saved quite a bit of time. In those areas where the wing tapers I trim the excess (above and or below the spars) after the 'rib/shear web' installation dries.

Interesting thread.

The question of maximum principle stress angle is a good one. I opened up my mechanics of material book and the example problem for wide flange beams shows to assume the shear stress parallel with the beam and normal stress perpendicular. I didn't take the deep dive on the fundamentals, so this is just a regurgitation of printed material. The example they gave to visualize why was a leaf spring. Not too much help really.

The nature of the material (anisotropic) is also key, as banktoturn points out. Balsa has very low shear and high tensile strength parallel to the grain. The opposite is true perpendicular to the grain. Ideally, you want the shear and normal stresses aligned accordingly. As far as my text goes, that would be vertical in a wide flange beam.

OK, so I did a little more reading. The assumption of my previous post was a beam in pure bending. For a distributed load like an airplane wing, the principle load directions have to consider the beam section properties and can be resolved using Mohr's circle.

I did learn that earning a living in the real world makes you stupid over time. I knew this stuff inside and out when I was in school. Not any more.

I appreciate all the valuable thoughts gentlemen.

I will try to do what I always do if I can not answer a question. I will go back to the basics, which in this case is physics. I have a friend in Germany who may be able to help me. I hope. My times in school when I knew a little about that are to far back. :cry:

At this point it appears that the only way to really solve the problem is testing. Which is one of the most difficult things to do. You would have to apply the same conditions to the same materials to get comparable answers. I doubt taht anybody is equipped for that.

I summarize all I learned like this:
There are different ways to build shear webs. As long as the fit is tight they all seem to work. That being said it may be left to your own preference which way you go and how much effort you want to put in it.

I enjoy building and if I find a way to build more stable without adding weight I WILL go that way, no matter what the effort is. I see a point in what some of you said about getting the same result with less effort.

I did not only look in reeinforcing the spar with carbon as you suggested Keith, I actually did that before I even installed the shear webs. If that spar ever breaks there will be nothing else left to repair anyway.

Cheers

Steff

Steff,

Testing is a very sound idea, if you really see the need. It wouldn't be all that hard. Just build several spars using different methods, and load test them. You don't need to build complete wings. This way, you can compare the alternatives according to whichever standards you want to apply ( strength, weight, ease of construction, cost, etc. ). You don't need any special equipment, and you don't need to worry about variability in the materials, as you are going to have the same variability when you build your planes. Whether it is worth it to you to do this is a matter of how badly you want to optimize your design.

I would ask one question. Are you having problems with spars failing? If not, what is driving your willingness to improve them "no matter what the effort is"? There are always other places to put some more time or effort ( practicing flying, reducing drag, ... ) , so I like to leave stuff alone when it is in less need of improvement than other issues.

If you do the testing, I'd love to see your results posted.

Good luck,

banktoturn

Actualy banktoturn I have no problems with spars breaking what so ever.I am just curious to find out something I can not figure out myself.
I did learn something from all these discussions : if there is a force that can make my wing break it will most likely be during the launch. Since my plane is electric I really do not need to worry about that. It is just my German urge for perfection that makes me build things 150% when 90% would be more than enough.

As to testing: it is not hard to build parts and break them. But how do you ensure that you use the same force in each situation? How do you measure that? How do you know that what you measure is a rule and not just a coincidence where forces come into play that you are not aware of?

The only acceptable test would be one that can be repeated by anyone else and would lead to the same results every time you do it with the same equipment. Impossible in a private environment.

I am not a big fan of learning by trial and error because it is inefficient. But that is the only option we have I guess.

Like I said before, someone who is really proficient in physics can answer that question. It is as far as I remember simply a matter of vectors. What is the result of a force in that direction and how is it distributed in which situataion. Beginner stuf for a physics major.

Steff

>>I did learn that earning a living in the real world makes you stupid over time. I knew this stuff inside and out when I was in school.<<

I respectfully disagree ilikeplanes!

Earning a living just does not leave you enough time to keep up with everything you once learned. Does that make you stupid? Hmmmm.... if specializing is a form of stupidity yes. You are right. But then again it may be better to know at least a litle about one subject then nothing about many

I was a very good musician 30 years ago. Today I am making a living as a Finacial Analyst. Just a different way to introduce the same theme in different notes and tunes at the same time, that's all


Steff

Steff

Here is an example of some simplified stress calculations for a model glider wing (courtesy of Dr. Mark Drela):

http://www.charlesriverrc.org/articl...par_sizing.txt

The numbers in the example are for a winched launched 2 m glider.

The shear load (at the wing root) is estimated to be 80 lb. The tension and compression in the spars (as a result of the bending load at the root) is estimated to be a whooping 1733 lb. Yikes!

The spars in the example are carbon wrapped balsa. There is another article somewhere on that site concerning their construction and testing.

There is a note at the end of the article about wooden I-beams as well.