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Laminated, box or I beam wood spars for Cub?

Drew S

Registered User
Several designs use laminated, box, I beam or other manufactured wood spars, with reinforced wood webs in high stress areas like the strut or base areas.Has anyone encountered, or used one of these methods for building an experimental Cub? Thanks!
 
Original spars were laminated; later ones were one piece. I am unaware of a Cub wood spar failing in flight. The Scout had maybe four in-flight failures after hitting things, and now the entire fleet goes through a rather expensive annual spar inspection. That tells me the FAA is unaware of Cub spar failures.

But I agree - go aluminum. Stronger, and way better at recover time. Readily available. Don't re-invent stuff just to make it heavier.
 
Original spars were laminated; later ones were one piece. I am unaware of a Cub wood spar failing in flight. The Scout had maybe four in-flight failures after hitting things, and now the entire fleet goes through a rather expensive annual spar inspection. That tells me the FAA is unaware of Cub spar failures.

But I agree - go aluminum. Stronger, and way better at recover time. Readily available. Don't re-invent stuff just to make it heavier.

Are wood spars heavier than Al?

I really doubt a box, I beam or D section wood spar would be heavier than a solid chunk of aluminum
 
That 'solid chunk of aluminum' isn't much of a 'chunk'. It's extruded I-beam, basically. And it doesn't weigh as much as wood.
 
I've also never heard of an in-flight failure of a wood spar on a cub, but I have heard of finding cracked spars during a rebuild. Peace of mind is worth something, if you ask me. Not to mention, aluminum spars make a plane much more sell-able.
 
If you go for box or I beam wood, you really ought to get an engineer involved to verify the strength. Solid or laminated solid is a proven product.
 
With alum. you don't have to be as concerned about birds, mice, or other wood chewing critters doing damage easily. And if exp. you can off space ribs-compression members etc. and still use the wood wing parts on an alum. spar. Easier maintenance visually and better resale value down the road. I have done them both ways and prefer the alum. but everybody to their own. Have fun; I find building nearly as much fun as flying. Well maybe not quite, but I like to build.
 
I have bare wood and aluminum spars in my shop right now. I'll weigh them and report back Sunday or Monday.
 
The boxed wood spar is really stout. First used by Anthony Fokker in 1917 in the D8, so strong it was strutless. Stampe also used them in there aerobatic biplanes.

Glenn
 
The boxed wood spar is really stout. First used by Anthony Fokker in 1917 in the D8, so strong it was strutless. Stampe also used them in there aerobatic biplanes.

Screenshot_20200926-204856.png

This was the Carbon Cub of it's day, 900lbs with 160hp of literally a fire breathing spinning dragon up front

Glenn
 

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The boxed wood spar is really stout. First used by Anthony Fokker in 1917 in the D8, so strong it was strutless. Stampe also used them in there aerobatic biplanes.

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Glenn
Yeah, that's what I was thinking about.

I have plans for a D shaped box spar that incorporates the leading edge, made of plywood.

Looks light and stout and would fit on a cub or cub like aircraft.

The carbon fiber aircraft I work on incorporate that design and I always wondered.
 
It would be easy to double the strength of a Cub - or for that matter, a 737. Remember, these things are only built strong enough to meet specifications with a safety factor. After that, all you are doing is adding weight and decreasing performance.
 
A D shaped box spar is a) at least as light as the stock cub arrangement, and potentially lighter, and would b) substantially strengthen the wing at the point where one would put slots and slats. D shaped box spars can be made from aluminum, plywood or carbon fiber, and are in other Cub-like aircraft.I thought I was posting in the "Experimental Cub" section. My mistake. This is obviously the "Cubs cannot be improved upon" section.Thanks for the input.
 
Drew S, Go ahead and try the D shaped box spar. But unless you are one and know how, please get an engineer involved to be certain the proper places have adequate strength to handle the loads involved. I'd hate to learn some little part failed at an inopportune time.

When you do build the spar be certain to take the proper washout into consideration as you will not be able to twist the wing for rigging purposes. A D shaped spar is extremely resistant to twist.

And be certain to keep us informed of your progress as this is an interesting project in an entirely different direction than others have attempted.

By the way, a Cub does have a box spar as the leading edge skin when properly installed becomes part of the spar increasing the twist resistance.
 
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It all comes down to this. Do you want to build to fly? Do you want to build to design? On Cubs everything has been done, there are no ways to reinvent the wheel. The lightest best Cub wings that I know of are Carbon Cubs. Cub Crafters took a design and used modern manufacturing, CNC and carbon fiber to reduce the parts count and lighten up the structure. Good luck in your endeavor.
 
A D shaped box spar is a) at least as light as the stock cub arrangement, and potentially lighter, and would b) substantially strengthen the wing at the point where one would put slots and slats. D shaped box spars can be made from aluminum, plywood or carbon fiber, and are in other Cub-like aircraft.I thought I was posting in the "Experimental Cub" section. My mistake. This is obviously the "Cubs cannot be improved upon" section.Thanks for the input.

Were you looking for input from other builders/designers/mechanics or were you looking for simple agreement with your ideas? While new materials continue to be developed, there are very few truly new ideas regarding the design of these aircraft. If you dig into the past history, I'll guarantee the design you are considering has been tried on a Cub or a similar aircraft. As was pointed out above, the wing only needs to be as strong as necessary plus a safety margin. Bellanca made a wood box spar that broke off a telephone pole without breaking. Strong? Oh yeah! Necessary for a 200 mph aircraft but way stronger than needed for a Cub, which translates to being heavier than desirable.

You asked for input and now have some. If you still believe that you have a better idea, then go ahead and work it. But do it wisely and with plenty of testing. And be honest with yourself. If you can build this spar as strong or stronger and with less weight, then show it off. If the design is heavier or prohibitively expensive then admit it and learn from it.

But reread Skywagon's post #15. Did you realize that the current spar and leading edge formed a D spar?

Web
 
The stock cub spar and leading edge do form a "kinda, sorta" D spar. Yes, I know that. I've built three sets of aluminum wings and one set of wood wings for Piper aircraft as an A&P.The rest of you, with some exceptions, are reading way too much into my question. I simply want to know what has been done before; and if possible if there is something interesting to imitate, if I decide to build. I have been out of the civil aviation/small plane world for about 12 years, and have noticed a lot of Cub-like airplanes that weren't here when I left. I think some of you are pre-loaded to just blow up at posters when triggered. In my few days on this forum, I've seen more testiness and outright hostility than in any other place. Life is too short to hang out in a place full of negative people. Good day.
 
...I think some of you are pre-loaded to just blow up at posters when triggered. In my few days on this forum, I've seen more testiness and outright hostility than in any other place. Life is too short to hang out in a place full of negative people. Good day.
It is sad that you have formed this opinion. While there may be a few negative comments, most should be taken with a grain of salt. This group is one of the best with more accumulated knowledge freely given than anyone could imagine. I have minimally connected with some other sites where the collective civility is minimal at best. You've only been here a few days, sit back and observe. Give us more time, you will find this to be the best group of people in the industry.

Where are you located? Find other members in your vicinity, get to know them. You will lose that bad taste in your mouth. We are politically neutral, sj has seen to that. Whatever our politics are, they are left at home. Perhaps that is where you have been elsewhere? This is a pro-aviation group with members across the entire spectrum.
 
Drew, I’ve been where you are with this site. My first post was looking for an STC to put an 18 tail on my J-3. The first half dozen posts were telling me I shouldn’t do that and all the reasons why I shouldn’t. Now, I grew up around Cubs in Alaska, and knew what I wanted to do. I didn’t ask for anything other than where to find an STC.. I got my STC and my 18 tail, and decided this site wasn’t for me. I started lurking, and not participating. Well, 17 years later I’ve decided the occasional aggravation is well worth the good info I get here. This site is just like life; there’s good info, bad info, and the occasional guy that just likes to stir things up. Just scroll past what you don’t like and press on. I’m sure you have something to contribute that may help someone on this site!
 
I didn't see a whole lot of offensive posts above. When you post on a forum you are looking for opinions, almost by definition. You got some.

When I post a question, I generally need opinions, and I get them. These guys have saved me thousands of dollars on simple mainenance hints. I have been over my head several times, and responders in this thread have previously helped me "dig out."

On another forum, folks offer things for sale, then take offense at comments. One guy said "Cub for sale." Then he said he wouldn't tell us about it because we would comment. Again, ask a question on a forum, expect some answers. Choose the ones you like. Just like Mam says.

I don't remember his PA-18 tail, but I bet I participated. I flew one such mod and hated it. To each his own.
 
Must be a Democrat!

The stock cub spar and leading edge do form a "kinda, sorta" D spar. Yes, I know that. I've built three sets of aluminum wings and one set of wood wings for Piper aircraft as an A&P.The rest of you, with some exceptions, are reading way too much into my question. I simply want to know what has been done before; and if possible if there is something interesting to imitate, if I decide to build. I have been out of the civil aviation/small plane world for about 12 years, and have noticed a lot of Cub-like airplanes that weren't here when I left. I think some of you are pre-loaded to just blow up at posters when triggered. In my few days on this forum, I've seen more testiness and outright hostility than in any other place. Life is too short to hang out in a place full of negative people. Good day.
 
On the original topic, I just wanted to add that the rear spar is where the work needs to happen.
 
Wood spar is "as removed" so it includes plywood plates. Aluminum spar is new from D&E.
front wood spar: 1.05# per foot
rear wood spar: .69# per foot
front aluminum spar: 1.01# per foot
rear aluminum spar: .66# per foot
 
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