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Author Topic: Box Spar vs I-Spar  (Read 791 times)
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flydean1
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« on: May 24, 2018, 10:09:56 AM »

For you engineer types out there.  Many Gas designs and others specify an "I" beam spar with vertical sheer webs.  I find it difficult to get a precise fit, and instead generally opt for a box spar with sheer webs on either side of the flange with grain on the 45 angle.  The sheer webs are half the thickness of the specified vertical sheer web.

All this for torsional strength.  Am I hurting myself here?
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PB_guy
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« Reply #1 on: May 24, 2018, 12:11:25 PM »

From: https://www.reddit.com/r/AskEngineers/comments/4h2cs1/ibeams_vs_square_beams/
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Because I-beams have a relatively mush lower moment of inertia/second moment of area about their minor axis compared to their major axis they are susceptible to lateral torsional buckling if not adequately restrained laterally over longer spans. This is also compounded by the fact that they are not as good in torsion, in fact that's probably the main advantage of closed hollow sections.
So what they say is that I-beam construction is great for loads in one plane (vertical), but a box spar is better if Torsional loads come into play. But there are other hollow sections that would work better - elliptical sections ... But we aren't usually dealing with only using one support member in wing construction. Loading tests on sample structures would demonstrate deflection/torsion in reality.
ian
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flydean1
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« Reply #2 on: May 24, 2018, 08:45:03 PM »

Thanks PB,

I checked the link and it pretty well confirmed my intuitive notion.  In gas models, and possibly other high loaded wings (F1A on launch) torsion loads as well as simple bending loads are present.

I have seen I-spars used as the back end of a D box where the leading edge is sheeted top and bottom forward of the main spar.  The whole assembly resists torsion through the sheeting, and bending at the I spar.

Another variant I've seen is a single sheer web connecting the flanges forming a C-channel, or more properly, "]" and again paired with leading edge sheeting to form the D box. 

My longest-lived and most consistent flying T-Bird had box spars, and though I had to be very careful to launch it vertically to keep the airspeed in check, seemed to resist flight loads well, and survived many, many DT landings.  Of course, it had no LE sheeting, per NFFS Nos Gas rules.

I was hoping Josh Finn and Dan Berry would chime in, as well as Glidermaster, Pit, and others of this distinguished panel of experts.

Please one and all feel free to comment
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fred
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« Reply #3 on: May 24, 2018, 10:24:06 PM »

Box beam IS easier to cobble together than an Actual I beam.
 Every bit as strong possibly stronger.  Dead easy to glue together in a Model wing.  I haven't bothered to do an I beam in over 40 years... as I beams  are a genuine pain to build adequately.
Never had a failure with the box variant
Why the 45D grain orientations though.... unless they are opposite on either side ??
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flydean1
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« Reply #4 on: May 24, 2018, 11:00:49 PM »

There was a Sympo article back in 2003 I think.  It proved that the forces in the web acted on a 45 degree angle thru the web.  This was both in bending and torsion.  The conclusion of the paper was that vertical grain or horizontal grain made no difference.  The author made reference to making a balsa "plywood" with the grain on opposite 45 degree orientation, or 90 degrees to each other.

This is dealt with in the box spar with the sheer webs oriented opposite each other as you suggest.  Still much easer than an I beam.
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FF Bruce
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« Reply #5 on: May 24, 2018, 11:31:04 PM »

Dean, I have at least one I beam spar in every model I build. Most have two and one a class C model had three. I have found that I could use smaller lighter spars and get stronger, lighter wings and no bow in the stab. As for torsion I gain nothing that is impacted more with thickness and type of covering. Like you I fly mostly Nostalgia type planes so everything must under the rules of the event so adding Diagonals is out. I also never use any carbon (just don't like the stuff). I have on my Vintage FAI models used a boxed beam three or four rib bays out to help strengthening the wing in a hi load area. They can be a little time consuming but I find it well worth the effort. I have 4 models in the box with hundreds of flights for over 10 years and not one failure. All use vertical grain tip to tip with spruce spars on the wing (mains only) . One thing you can do is make a couple short test spars and brake them that is what I did.      Bruce         
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flydean1
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« Reply #6 on: May 26, 2018, 10:57:47 AM »

Presently building a DeLoach 202e.  Don furnishes the spar webs with vertically oriented grain.  The webs are exactly the correct height, reaching from the bottom spar to the top spar notch.  I decided, since the webs are already correctly sized, to go with I-beam on the stab.  First of all, I've developed a very slight tremor with age which is why I stopped building scale in general and peanuts in particular.  I found fitting the sheer web nearly as fiddly and any peanut I have built.  Still, it went together well.  I will do the same on the wing for the same reason.  Next on the board is a Maverick on which I will go the box spar route to compare building complexity.

One clear advantage of the I-beam is that it is not dependent on the glue joint.  The glue just holds the web in location.  On the box beam, the glue is subject to the same stresses and is depended on to keep the sheer webs functioning.

Don't plan on "test breaks" as I get so little building time.

Figured Maxout will have chimed in now.  He is a for real engineer as are several others here in Hippocketland.

Thanks for all your comments, both now and in the future.
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Rich Adams
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« Reply #7 on: May 27, 2018, 09:58:31 AM »

I know a little about engineering and will chime in here. A box spar is definitely better in both torsion and bending. For pure torsion, nothing replaces a tube section, but it is not as good as the other two for bending. This is why you wouldn't see an I-beam used as a drive shaft on your car. Plus it would look ridiculous. The other advantage to the box beam is that you have two glue joint per side to take the shearing forces of the beam under bending loads. An I-beam gives you one per side.

Years ago, the discussion of best web orientation came up. I refer to a document published by the FAA titled AC43.13, "ACCEPTABLE METHODS, TECHNIQUES, AND PRACTICES - AIRCRAFT
INSPECTION AND REPAIR". Chapter One deals with wood aircraft structures and specifies that all web related repairs or replacements shall be performed using plywood with the face grain oriented in the direction of the spar. Now, aircraft plywood has many plies, the stuff we use in modeling is a good example. 1/8" thick plywood has five plies with a slightly thicker center ply. Plywood was our first attempt to make what we call a semi-homogenous material and is now replaced by synthetic composites in the aircraft world. However, based on the preferred orientation of the face grain and the amount of material in aircraft plywood oriented in that direction, it would be a safe bet to say that the preferred orientation is lengthwise.

My argument is that in a vertical orientation, shear in a beam is trying to pull the grain, or the cell fibers, apart in tension whereas in the lengthwise direction the cells are trying to be pulled apart in shear. Balsa is nothing more than a series of lengthwise tubes (see picture of cross section) and it would stand to reason that they are stronger lengthwise than transversely, otherwise the tree would not resist the large bending forces in a strong wind.
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mike
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« Reply #8 on: May 27, 2018, 02:32:18 PM »

Pure shear is tension on one 45 deg. diagonal and compression on the other 45 deg. diagonal, so, unless the web is thin enough to suffer buckling under load, web grain along the spar or across the spar is the same for pure shear.  I put the grain across the spar ('vertical') to resist the tendency of the spar flanges to close together when bent.  (Google 'Brazier effect')
A two-ply web with grain at +/- 45 deg. would have merit. A carbon cloth web should always be laid at +/- 45 deg.

As for box vs I it depends on the rest of the wing structure.  A D-box LE butted to top and bottom spars would likely be fine with a full-depth shear web on the back of the spars.
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flydean1
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« Reply #9 on: May 27, 2018, 02:47:47 PM »

Thanks Rich and Mike.

You have confirmed what the Sympo article stated in the direction of forces in the sheer web, and the superiority of box beams in torsion.  I oriented the grain of my sheer webs at 45 degrees on my T-Bird.  Each web was oriented opposite of each other.  However, the Sympo paper agreed with Mike in that the grain could be run either horizontally or vertically.

My next project, a Maverick will have a box beamed main spar.
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Rich Adams
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« Reply #10 on: May 27, 2018, 03:54:42 PM »

I agree with Mike's assessment. 45 degrees is the way to go especially in a web that may experience buckling or a tension field. Interestingly enough, balsa has better properties in shear across the grain versus with the grain after doing a bit of research. I would not have expected that.
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FF Bruce
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« Reply #11 on: May 27, 2018, 07:02:42 PM »

I use spruce spars because they have better properties as far as compression and stretching than balsa so the vertical grain is only to keep the top or bottom spar from crushing the web. With it going horizontal  the balsa simply crushes and you have failure. I've seen some use just a couple blocks between ribs with success. I think there is no argument that a box spar is stronger it is. But a well made I beam works well for what we do, much better than no web. I weighed a spar I had planed on using on a 1\2 A of balsa I then weighed the same spar made of spruce, almost the same but the spruce was so much stronger that I have stayed with spruce ever since. I guess I just got lazy always had box spars in my A-2's back in the day.
 
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lincoln
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« Reply #12 on: May 31, 2018, 06:26:34 PM »

If the web is sized by buckling considerations, as it may be in full scale airplanes, then an I-beam will be MUCH better than a box beam with two webs, each half the thickness. I'm not sure how often that's the case with models. Considering that the full scale VJ-23 hang glider uses 1/32" aircraft ply for a shear web, I suspect that most shear webs on models are NOT sized according to buckling. +/- 45 degree grain orientation will be significantly stiffer in shear, but again, I'm not sure if that's really relevant.

I've had a model or two where I glued balsa shear webs on the side of spruce or basswood spar caps. In that case, the shear webs would sometimes separate from the caps, leaving just a little wood behind. I suspect that if end grain were butted against the spar caps, as in an i-beam with vertical grain webs, this would be much less of a problem. Alternatively, some other shear web materials are stronger across the grain than balsa is. 1/64 ply would probably work better and be about as heavy as 1/16 medium/heavy balsa.  1/32 heavy balsa would probably be better used this way than 1/16 light balsa.

The ideal shear web would probably be adjusted for density so that it was just strong enough when made exactly as wide as the spar caps. The Sky Pup, a homebuilt ultralight design with a good reputation for safety, has Styrofoam (that blue stuff used as insulation) for shear webs, full width of course, for most of the wing.
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« Reply #13 on: May 31, 2018, 08:47:03 PM »

Quote
If the web is sized by buckling considerations, as it may be in full scale airplanes, then an I-beam will be MUCH better than a box beam with two webs, each half the thickness. I'm not sure how often that's the case with models. Considering that the full scale VJ-23 hang glider uses 1/32" aircraft ply for a shear web, I suspect that most shear webs on models are NOT sized according to buckling. +/- 45 degree grain orientation will be significantly stiffer in shear, but again, I'm not sure if that's really relevant.

From what I've seen on balsa built up wings - the shear web material is usually over strong in terms of buckling - particularly for balsa due to it's lower density and thus extra thickness for the same weight. The early manpowered aircraft often used thin plywood for the shear web which had to be strengthened by vertical spruce members for buckling.

I think perhaps the real issue between using I beams versus Box Beams in models is one of craftsmanship and strength of glue joints. Balsa I beam webs should be wide enough to ensure a decent end grain glue joint and on the other hand the mating faces on the flanges of box spars need to be accurate to provide a strong glue joint with the webs.

I feel the best spar arrangement is to have a LE D Box with an I beam spar or rear web on the spar flanges. However the extra weight may be a problem for some of the FF classes but it has been used for years in CL and RC - including gliders.

John
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lincoln
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« Reply #14 on: May 31, 2018, 11:38:44 PM »

Gluing shear webs to the sides of the spar caps, with the grain direction vertical, puts the parts of the shear webs adjacent to the glue joint in rolling shear. Wood is much weaker in rolling shear than in other directions. This probably explains why I was getting shear web failures. So I guess, if you really must put your shear webs on the sides of the spar caps, use horizontal grain, not vertical. Maybe pages 21 and 22 of the following will clarify:

http://www.dot.state.mn.us/bridge/pdf/insp/USFS-TimberBridgeManual/em7700_8_chapter03.pdf
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flydean1
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« Reply #15 on: June 01, 2018, 09:54:14 AM »

Thanks to all for the excellent input on this subject.  Keep them coming.  Waiting for Maxout and DanBerry to chime it.

I have some 3/32 Rohacel (tm) foam 2.5 lbs density that would make a dandy shear web.  Suggestions?
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danberry
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« Reply #16 on: June 01, 2018, 10:32:23 AM »

Don't over-think this. It's toy airplanes.
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OZPAF
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« Reply #17 on: June 02, 2018, 07:13:37 PM »

Interesting info Lincoln. However I'm inclined to think that in general the spars and webs are still oversized and that glue penetration into the cells would make a considerable difference to the cells collapsing.

John
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lincoln
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« Reply #18 on: July 17, 2018, 06:26:35 PM »

Don't over-think this. It's toy airplanes.

Yes. It's toy airplanes. It's supposed to be fun, and part of the fun is over-thinking it.

If wood spars on outdoor ff are oversized, particularly if they don't taper, then some weight can be saved by making them smaller. In RC soaring, wood spars aren't usually oversized. I've seen some of them break. The two exceptions I can think of are a 2 meter wing I built that had 1/2 X 1/4 maple spar caps on the center section, and another 2 meter wing where I made the top cap 5/8 X 1/8 for. Perhaps the latter wasn't really oversized. I broke it on the winch, on purpose.

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