Wing Spars Dimensions/routing/material?

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Ethan Hopper
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Joined: Thu Nov 02, 2017 12:27 pm

Wing Spars Dimensions/routing/material?

Post by Ethan Hopper »

Hello I am building an Air Camper in Ontario at high school as a bit of a grade 12 wood shop project. I have experience with engineering and fabrication, I also have flown and have a grass strip behind my home so I'm not in the project blindfolded. I am very willing to learn though and always look for help and experience. I have the fuselage mostly built with help from some friends and the rest is mostly fiddly single man jobs so I'm going to get a bit of a start on the wings while I have the help.

My question is with regards to the wing spars, the originals are routed 1" spruce, however I'm building a 3 piece wing and will be using 3/4" spruce. A local wood supplier had an excellent selection of quartersawn clear Douglas Fir at a great price so I bought enough for the wings. Fir is on average %18 heavier than spruce but has a %25 higher strength. A solid 3/4" Fir spar is the same strength as a solid 1" spruce spar but %7 lighter. I know that a 3/4" solid spruce spar is perfectly acceptable so a solid 3/4" fir spar would be %25 stronger but %18 heavier, I couldn't find an example but would it be ok to route a 3/4" Fir spar with 1/2 thick center pockets like the 1" spruce? (1/8" router pockets on either side of the fir spar). If I routed away %18 of the material it would make it the same weight as a solid spruce spar, but still theoretically %7 stronger than spruce. Thoughts?
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taildrags
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Re: Wing Spars Dimensions/routing/material?

Post by taildrags »

Ethan;

It's not clear from your post as to whether you are one of the high school students or if you're the instructor. Can you elaborate? In any case, I recently sent the BPA Newsletter editor a draft of an article that I wrote on this subject, and if there is room in the next issue and it passes technical scrutiny, he may publish it. In the meantime, I'll take a look at the spar configuration that you're asking about and will offer my opinion. My article is actually the first installment of what I've planned to be a mini-series where I reverse-engineer the design of the Air Camper and the first article is an analysis of the original 1" thick, solid spruce spar to establish the basis for studying variations of that spar. The installment that follows will examine alternate configurations including 3/4" solid, routed, built-up "I" and "C" configurations, the extruded aluminum spar, and then a subsequent installment will look at deeper airfoil sections such as the USA35B and Riblett that lend themselves to altering the basic spar depth of 4-3/4". The spar depth is the dimensional characteristic that most influences the resistance of a non-tubular spar to bending and thus goes a long way towards determining the ultimate 'G' loading of the wing.

In my little series of articles I'm trying to avoid heavy technical jargon, a lot of math, and rigorous engineering analysis. It's intended to be winter fireside reading and not rocket science ;o)
Ethan Hopper
Posts: 4
Joined: Thu Nov 02, 2017 12:27 pm

Re: Wing Spars Dimensions/routing/material?

Post by Ethan Hopper »

I am a grade 12 student still in school, the airplane is my project and I only chose to do it as a school project because I get time everyday to work on it, a workspace, and machine and tools to use, the school doesn't have any ownership or responsibility for my plane. I guess to simplify the question down I just want to know if routing out a 3/4" spar in stronger douglas fir is acceptable, my original question just is a little more elaborate on the theory behind it. All the 3/4" spruce spars I've seen are solid with no router pockets, a 3/4" fir spar should be plenty strong if routed to bring the weight down to match a solid spruce spar.

I also don't get any newsletter or articles, I'm new to the community side of pietenpols so I'm not sure what I should or shouldn't sign up for. I love reading though and any information or technical data is always useful and interesting.
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taildrags
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Re: Wing Spars Dimensions/routing/material?

Post by taildrags »

Ethan; congratulations on taking on a project like building your own airplane so early in life! Excellent. About Doug fir, I've been working on a light single-place composite high-winger and am using Doug fir for all of the wood framing and wing spars since it is readily available here in Oregon. It has its little peculiarities and difficulties of working with it. The most annoying thing about working with it is trying to drill a small hole in it if I don't drill directly across the grain. Even using a drill press with a very sharp bit and slow steady feed, if I try to drill at any angle to the grain, the bit wants to deflect off-axis as it goes through one layer across a harder layer of grain. The fir I've been using sometimes has alternating layers of wood grain that are very different in hardness. Fir also seems to want to split and splinter much easier than spruce.

I'll work something up on the spar configuration you're asking about and post it here for discussion.
Ethan Hopper
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Joined: Thu Nov 02, 2017 12:27 pm

Re: Wing Spars Dimensions/routing/material?

Post by Ethan Hopper »

Thanks very much. I've always been fascinated with flight and engineering and my family has been into planes for a long time so the infrastructure and support is already there. I find the biggest part of doing things like this at a young age is doing your research and staying humble. Know the process and what your getting into the best you possibly can and always be prepared to take suggestions and learn from others who have more experience.

My fuselage is built out of sitka spruce which is a beautiful material, However it has just recently become even more expensive due to wildfires and other sad factors along the west coast. A local wood store had some excellent quartersawn douglas fir I got for a great price. The grain is very square to the board and very little to no warp and twist so should be good for the spars and rib material. My woodshop teacher while not a plane builder still can help lots with the woodworking side of it so manipulating the fir and getting a good low split finish shouldn't be overly difficult. Let me know about the spars when you can as I can square them but wait on routing them till I'm sure if it should or shouldn't be done. Thanks
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taildrags
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Re: Wing Spars Dimensions/routing/material?

Post by taildrags »

Ethan; here's a summary of my pencil calcs on the spar configuration you asked about. You'll have to read my upcoming article to get the calculations and assumptions that I made to arrive at the design basis, but if I assume an Air Camper with a maximum gross weight of 1,080 lbs and an unrouted 1" thick solid spruce spar, I have arrived at an ultimate loading of 5.88 positive Gs. If the positive G loading for certificated aircraft in the Normal category is 3.8, then the stock 1" solid spar has a safety factor of about 1.55 (55% margin of safety).

The routed 1" spar made using Douglas fir will have about 97.5% of the strength of the solid spruce spar, so an ultimate loading of 5.73 G and a safety factor still about 1.5.

Now to directly answer your question, a routed 3/4" Douglas fir spar that retains a 1/2" thick web and 3/4" thick spar caps will have an ultimate load strength of 4.57 G for a safety factor of about 1.2. Note that "margin of safety" is the safety factor minus one and is expressed as a percent, so the margin of safety of this spar is 20%. Still within design loading, but with a slimmer margin of safety. Also bear in mind that my calculations have been made in the comfort of my home, at my computer, where nothing much more serious than a paper cut or a broken pencil lead can happen if my numbers aren't exact or the data I used for strength of materials and the calculations for bending stress were made in a casual fashion. Well, I guess spilling an entire cup of hot coffee on my workstation and lap would be pretty significant too, but none of those things compares to the serious consequences that marginal wing spars on an airplane in flight might create. If you're only planning to build the airplane one time and you have solid 3/4" Douglas fir spars available, consider saving yourself the time and effort involved in routing them. The solid spars might provide a safety factor of about 1.36- a margin of safety of 36%- nearly double the margin of the routed 3/4" fir spars that you propose.

I did not spill my coffee while writing this, and I hope you don't break anything on your airplane, either. Best of luck with your project!
Ethan Hopper
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Joined: Thu Nov 02, 2017 12:27 pm

Re: Wing Spars Dimensions/routing/material?

Post by Ethan Hopper »

Thanks very much, I understand what your saying and that leaving it solid is the better option. I was only looking at it from the aspect of weight as fir is heavier. I just ran some of my own simple calculations though and a 3/4" fir spar will only be a little over 7 lbs heavier than a 3/4" spruce spar, definetley less than I was expecting. With that small margin I'm definetley not interested in sacrificing any safety and strength for 7 lbs! It's probably easier to shave 7 lbs off the pilot and have a little less for lunch than worry about it in your flying machine when your in the air. Amazing sometimes how the small weight saving nearly halves the safety margin; but I guess it's like that in everything, the small benefits we see at hand have large cons not as apparent without doing the math.

I'll have to make another thread and post some build progress soon. Thanks
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taildrags
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Re: Wing Spars Dimensions/routing/material?

Post by taildrags »

OK, I'll admit it... I'm a numbers addict. I just had to run through the numbers regarding weight savings gained by routing a 3/4" spar. Routing 1/8" by 3-1/4" off the face of each cheek of the spar, times 28 ft total tip to tip length of the spar, yields 0.158 cubic feet of wood if they were routed from tip to tip, but they're not. If they were, and if Douglas fir has a density of 33 lb/cu ft, then the weight of wood removed would be 10.4 lb. Ethan has apparently accounted for the areas that are not routed, arriving at roughly 7 lbs of weight removed per spar. However, there are two spars in each wing so the total weight saved would be 14 lbs. My strength analysis leads me to want to leave the main spar alone and not route it, preferring the tradeoff of an additional margin of safety to carrying a few extra pounds on the airplane for the rest of its life.

So now, how about a compromise? How about leaving the main spar solid, but routing out the aft spar? Again, my upcoming article will suggest that the rear spar doesn't carry anywhere near the load that the main spar does, so I would feel much better about routing that one down by 7 lbs than I would the main spar. Ethan, if you still want to go through the effort of routing pockets into spar faces to save a little weight, this is an option you may want to pursue. It may also give your airplane the distinction of perhaps being the only Air Camper in the fleet with a solid main spar and a routed aft spar ;o)
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taildrags
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Re: Wing Spars Dimensions/routing/material?

Post by taildrags »

Oops... those numbers can bite you sometimes. If the savings for routing two spars is 7 lbs, then just routing one would yield 3.5 lbs.
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