Grain orientation on wooden box spars

[bruesch38]

Hi All,
I am building a wing for a Serenity (formerly Warner) "Spacewalker," which uses a boxed (partial) I-beam spar on the outer wings. My question involves the selection of lumber for the built-up (three layers) spar caps at the top and bottom of the outer spars. The spar is a boxed I-beam (with three laminations of spruce for top caps and three for bottom caps) for the first two feet, and then transitions to no "I" web (but still boxed) for the remainder, with the center cap being 7 ft long and the outer caps being 10 ft long. Like so:
____________________________
___________________
______
XXXXX
___________________
____________________________

There are stabilizing blocks in the outer sections beyond the "I" web. My question is about the necessity for vertical grain in each of the spar caps. Is it alright if one of these (3/4" thick top to bottom x 3-1/4" wide) is not "vertical" (less than 45 degrees)? To me it is hard to comprehend the benefit of "vertical grain" in each laminated spar cap, when the plywood (outer) shear plates help bear the bending loads and shrinkage is somewhat better controlled by the lamination process. I've looked in the FAA Advisory Circular and in Bingelis' book for the answer but no joy. Does anyone know of an authoritive document on selecting and orienting lumber for built-up (laminated) spars of different types?

[WLIU]

Stop. Do not pass go. Do not collect $200.

Grain orientation is VERY important. The spar caps carry the wing's bending loads. The plywood shear webs hold the spar caps apart.

The grain of the layers of the spar caps must be as close to the axis of the spar as possible. Every bit of angle away from the axis of the spar is reduced strength. Aviation grade spar blanks can have the grain slope a small amount and the strength values used by designers accounts for this.

If you really really want to understand the topic, get the USDA Forest Product Laboratory Wood Handbook, or read it online.

And don't go to Home Depot for wood for your wing.

Best of luck,

Wes
N78PS

[Bill Berson]

Grain direction matters in the usual Champ type spar. But a lamination is a different matter.
I think if good wood is used, and the grain directions are alternated, a three ply lamination should be very good.
Others may have a different view.

ANC 18 is probably the book you need. Might be online.

[bruesch38]

OK I'm on board with what BOTH of you are saying, but still need some clarification:
Wes,
What is the "axis of the spar?" up and down or front-to-back?
Bill,
I know that lamination is a different matter, but it's hard to find good info on this. I did review ANC 18 and there is a lot of good info there, but am still having trouble seeing the exact answer I am looking for (is vertical grain of the laminations critical, or tolerant as you suggest?). On the other hand I do always alternate grain directions (seems the right thing to do from both a shrinkage and strength perspective).

By the way, I use "aircraft grade" spruce (not home depot, as indicated by the price!) but even when buying from a well known supplier (e.g. Wicks, McCormick) the grain still runs toward 45 degrees from vertical sometimes, especially as the width of the piece is increased (i.e. might be considerably less than 45 degrees near one edge but close to (or even more than) 45 degrees at the other edge). This is the issue I'm addressing at present.
Thanks for your comments!
John

[WLIU]

So it sounds like the question that is asked is based in not understanding how wood grows, and how trees are cut into boards. You are likely seeing CAM 18 talk about growth rings and grain slope but have yet learned how the manner in which boards are cut from a log gets you the characteristics that you want.

Not sure that all of that can be explained in a forum post. Prof Hoadley at the University of Massachusettes published Understanding Wood that explains more than you want to know.

The short version is that you should look at the end of the board for the growth rings. Aircraft wood is cut from a log "quarter sawn", which means that the end of the board should show the growth rings near vertically across the thickness of the board assuming that your board is wider than it is thick. We like to harvest older trees that have a wide girth and that gives us boards where the growth rings look almost flat (long radius from a wide tree trunk). Now looking at the edge of the board, we want the grain to run straight the length of the board. Since we don't see exactly straight in nature, we accept some slope to the grain and design around it. The face of the board is another matter. If the tree trunk that the board was cut from did not grow perfectly straight up, you can see grain patterns that look like elongated ovals. This is due to the tree growing one way for a while and then back the other way but the sawyer cutting a straight board across that growth. I'll stop now...

So read the references. Compare your boards to the specs, and cut out the parts that you need according to the plans and build instructions.

If you are building a wood airplane, I suggest that first you build a small, all wood not plywood, cabinet for your spouse or for storing tools. Stop by a Woodcraft or Rockler store. Wood working is its own hobby. And if you want to learn enough to build a wood airplane you might discover that building some cabinetry for your spouse and your workshop, will make the project go more smoothly.

Best of luck,

Wes
N78PS

[George Sychrovsky]

This question comes out periodically , the answer is it does not matter which way the grain goes

[martymayes]

By the way, I use "aircraft grade" spruce (not home depot, as indicated by the price!) but even when buying from a well known supplier (e.g. Wicks, McCormick) the grain still runs toward 45 degrees from vertical sometimes, especially as the width of the piece is increased (i.e. might be considerably less than 45 degrees near one edge but close to (or even more than) 45 degrees at the other edge). This is the issue I'm addressing at present.
Thanks for your comments!


John, 45 degrees is still considered vertical grain. With the wide side of the board on the bench and measuring from horizontal or flat side, anything 45 degrees or greater is vertical grain. 44 degrees or less is flat grain. On a solid spar, only 2/3rds of the spar height has to be vertical grain. So for example, for a 5 inch high spar only ~3 3/8" of vertical grain is required to be acceptable as a solid spar in a certfied aircraft. Based on what you have said, I would imagine your wood is acceptable for solid spar material and should be acceptable for a box spar/laminated spar cap as well unless otherwise specified by the designer.

I would use the vertical gain wood you have as long as it otherwise meets aircraft standards with regard to annular rings, grain runout, knots, compression wood, decay, etc.

[bruesch38]

Hi Wes, George and Marty,
Thank you all for your helpful replies. I think we're getting somewhere!
Wes, no problem with my skills (furniture maker since 1968, aircraft woodworker since 1996). My problem is knowledge of the design guidelines for LAMINATED spars (vs. solid) and (to a lesser extent), the terminology used when describing grain characteristics (and especially how these relate to a laminated spar instead of a solid one).
George, thanks for your direct answer - I'll combine it with Marty's and shoot for vertical grain wherever possible.
Thanks guys!

[WLIU]

Good to hear that you have a handle on the topic. Communicating via the internet often is incomplete. I don't think that one aspect of building laminated wood structure has been mentioned. Cut the blank into laminations on your band saw then swap the laminations end for end when gluing up parts. That greatly increases strength by taking the defects or sub-optimal areas of wood, segmenting them, and moving those segments into locations surrounded by better wood. The Steene folks make spars that way. If you dig on the internet you can find photos and descriptions of tests of solid wood and laminated beams in testing machines, demonstrating the improved strength of the laminated structure.

Best of luck,

Wes
N78PS

[colinc]

Hi,

a major consideration in choosing the orientation (in any wood construction) is the way wood changes shape with changes in moisture content. Most of the dimensional change is around the circumference of the annual rings - they lengthen and shorten and effectively the ring radius changes. If the rings are tangential to the long face of a board then it will cup in one direction or the other as the moisture content changes and the ring radius changes. That also changes the board width by a significant amount. If the rings are tangential to the short edge then the long edge stays flat and the thickness varies but only by a small amount (the same % of course).

In a box spar, if the top and bottom boom's depth were to be changing in relation to the spar web depth then they are stressing the glue bond between their edges and the ply. If the rings are perpendicular to the glue joint then there is insignificant movement in the glue line. The aim therefore is to have the rings at 90 degrees to the ply face, but of course we have to be realistic and use the best wood we have which will usually be a bit of a compromise.

Grain orientation does affect bending stiffness, try bending a square section and you will see that it bends easier in one direction.

I would recommend that you look up ANC-18 (Design of Wood Aircraft Structures) and ANC-19 (Wood Aircraft Inspection and Fabrication) both may be downloaded from here: http://www.lonesomebuzzards.com/cgi-bin/forum/Blah.pl?m-1352247687/ They contain very clear explanations about the requirements for (and advantages of) laminating spars.

regards,

Colin

[Clark Savage Jr]

Grain orientation in box spars is, and has been, a hot-button topic since Orville and Wilbur. Many years ago, the plans vendor for Piel aircraft designs in North America, Gene Littner (an aircraft engineer and designer specializing in wood airframes) maintained that spar caps need to be built from flat grain rather than vertical grain laminations. Here is an excerpt from his Emeraude newsletter . . .

http://i256.photobucket.com/albums/hh171/Clark_Savage_Jr/Aviation/Sparwood_zps1046b4a7.jpg (http://s256.photobucket.com/user/Clark_Savage_Jr/media/Aviation/Sparwood_zps1046b4a7.jpg.html)

This statement sparked a controversy which lasted for months, until Gene finally refused to discuss it further. I built mine with VG orientation because that's the way my rough spruce was sawn.

[Mike Switzer]

Ok, how about if I throw another log on the fire. It has been a while since I did anything with the THEORY of wooden structures, because in the real world we use span & load tables for 99% of all wood structural design, and if that doesn't cover it I get a structural engineer to do the job.

I got to thinking, all #1 Southern yellow pine (pretty much the standard for structural work) is cut with more of an edge grain/vertical grain than anything else. I am attaching a pic of a piece left over from construction of a large header. Granted this is not cut to the same standards as aircraft wood, but you can see the grain is mostly vertical.

I am also attaching the first section of the Wood Structural Design Data manual. Read & discuss. :)

[Bill Berson]

Vertical grain is liable to split easier than flat grain. So I agree with Clark's drawing # 2, the VG box spar could split lengthwise from a torsion force.
For these reasons, a laminated cap of three layers to prevent splitting, should be excellent.

P.S. Mike mentioned end grain. End grain is only use for balsa core, as far as I know.

[Mike Switzer]

P.S. Mike mentioned end grain. End grain is only use for balsa core, as far as I know.

OOps. Meant edge grain. Editing..

[Mike Switzer]

I suspect that this may be a case where the correct answer depends entirely on how the spar is loaded - do you need the greater strength in the direction of thrust, or the direction of lift?

[WLIU]

If you make your spar caps or spars out of single planks, then you worry about movement with changes in humidity and things like splitting. Now I will point out that the widths of materials used in aircraft actually are narrow enough so those issues do not exist for practical purposes. Cabinetmakers worry about wood movement because we make wide panels or table tops. Movement and stress is a factor of width. A 1" wide spar cap is unlikely to move enough with humidity changes to cause glue joint problems.

Now you solve a whole bunch of problems when you build up your part using 1/4" thick laminations. I have a wood spar hanging on my hangar wall that is massive, built up from 1/4 laminations. I believe that it is 40 years old and the original glue shows no signs of distress anywhere.

I would not worry about these issues in a wood spar of the dimensions that we use in Pitts, Skybolts, etc. A 1" x 1" box spar cap is not large enough in cross section for the vertical or flat orientation of the grain to make a practical difference in strength. Theoretically there is a difference, but not really in the real world. The NTSB reports are not full of Piels that fell out of the sky.

Now a box spar is designed to try to load the spar caps in pure tension or compression. So the stress and strain is intended to be along the length of the spar caps. So vertical or flat, the internal stresses of the spar caps should be lower than what is required to cause catastrophic shearing along the grain.

On the topic of Southern Yellow Pine, I installed an doubled LVL header when I turned a doorway between my kitchen and dining room into an 8' wide opening at the request of my wonderful wife. Interestingly, LVLs are composed of a large number of vertical laminations. Very strong. Very hard to drive a nail through, even with the air pressure turned all of the way up on my framing nailer. Go to your lumber yard and get a brochure that includes the span tables for the things. Compare those span tables to the building code span tables for dimensional lumber. You'll be a laminated beam convert.

Best of luck,

Wes
N78PS

[Mike Switzer]

Wes - over a certain length it gets to the point where LVLs are more cost effective (or just plain easier to use, especially if you don't have a lot of room). I have the tables around somewhere but when that sort of work comes up I'm usually working with an architect that does the structural work.

Personally, I prefer steel I beams, but then i have a lot of time doing industrial work. I am planning on making my spars out of welded steel tube (the inner sections, anyway) so I hadn't given this discussion a lot of thought.

[Hiperbiper]

AC 43.13b has a great section on wood aircraft components, wood substituions and grain direction, grain pitch and general woodworking procedures. No need to re-invent the wheel! Most of the info has been around since the Spam-Can fleet was made of wood (Spam Crate?).

Once you get past all the FAA-speak (numbering, etc.) that one book has an amazing amount of useful info even if your bird is an Experimental.

Chris

[Bill Berson]

Mike,
My spars are welded steel tube also, light, strong, cheap and simple. Don't use wood much.
But that ANC-19 ( link above) was interesting. The part about making wood props was useful. I saved the entire 336 page book on my iPad.

[WLIU]

Well, nothing in aviation is flawless and that includes welded steel wing spars. Go look at how Beechcraft fared with the -18 wing spars. Read the AD that resulted in most of the Beech 18 fleet getting spar straps. Periodic X-ray inspections anyone?

Pick your poison and study to fully understand your chosen subject. In the acro world, wood beefed up with the judicious use of carbon fiber looks really good. I am not aware of anyone, no matter how hard they have abused the airplane, breaking one of Jon Staudacher's wings. And I believe that the Edge's have the similar construction. I know pilots who think pulling 10G's is an easy practice flight. If you are still working on the drawing board, it might be worth looking at.

On the topic of residential construction, the use of wood vs steel appears to be regional. When I lived in upstate NY, it seemed like everyone had a steel carrying beam under their house. In New England all of the houses have a big wood beam. I can only guess that availability of of steel beams was less (distance to mills?) and around here wood is plentiful. We laugh at city folks who think that trees are endangered. More recently, residential construction is moving to engineered beams for greater stiffness over longer spans. But the fire departments hate I joists because the come apart really quickly in a fire making it very dangerous to enter a burning house. Dimensional lumber structure keeps its integrity a lot longer in a fire. Commercial construction building codes drive the use of steel in office buildings and factories these days, but the residential code still does not weigh in favor of steel studs and the like.

Best of luck,

Wes
N78PS

[Bill Berson]

Wes,
Thanks for heads up on the Beech 18 spar problems. I read the AD and other stuff here:http://www.twinbeech.com/beech18sparconcerns.htm

I am not very concerned, since the Beech had landing gear stress (that I won't have). And I don't expect to be around another 60 years anyway.
Sorry about thread drift, but I think the original question was answered. ( I learn from thread drift)