Spruce for wing struts?

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rtfm

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Hi,
All planes use either steel or aluminum for wing struts. So my question is, what is wrong with spruce? Just a relatively small cross section is sufficient for spars, so why not use a significantly larger cross section of this material for wing struts?

Just askin'

Duncan
 

wsimpso1

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Struts are limited by critical load in compression. Look up Euler criteria and Johnson criteria - Shigley covers it well. You usually see compression in struts in negative g. Pcr is mostly made big enough by using enough E*I. E is material, I is shape and size. E of sitka spruce is about 1.15 Mpsi, aluminum is 10 Mpsi, steel is 30 Mpsi. So a wooden strut would need about 26 times as much I. Besides the metal struts having a lot less aero drag, thin wall streamline tubes might even be lighter than a solid wooden strut. Then there is keeping a long slender wooden strut straight as it takes up and gives off moisture...

Billski
 

rtfm

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Another question about wing struts.
Well, not struts exactly. I was thinking of the single masts as used by Croses on his Fleas. Here's a picture to remind you:
https://imagearchive.com/proxy.php?image=http%3A%2F%2Fassociationconstructeursamateurs.e-monsite.com%2Fmedias%2Fimages%2Fen-croix-sur-roues.jpg&hash=dbac6796f50192d3f34d1ad1e141b206

These are constructed from wood. Possibly with wood encapsulating a steel insert?

In light of Billski's remarks above, I'm still worrying this idea. I'd love to have single cantilevered masts on my Fleabike, but any input on how exactly to build/mount these would be welcome.

Duncan
 

wsimpso1

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As usual, I answered your question. I even hinted at how it could work, but you need to know how I works. To increase I of a shape by 26 times, you need something like.2.26 times the diameter while holding proportions.

That looks like a substantial wood strut with foam fairings. The chord is quite large. Given that a min drag teardrop has a chord about 3 times its thickness, this does indicate a pretty thick strut. So, that wood and foam strut indicates that somebody else thinks so too.

Billski
 

rtfm

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As usual, I answered your question. I even hinted at how it could work... [snip]
Hi Billski. Do I detect a hint of annoyance in your reply? Hey, mate, I'm not in your league with this stuff, and I'm struggling to get my head around this. Apologies if I am trying your patience. Certainly not intended.

Thank you for the additional info.

Duncan
 

mcrae0104

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I think if I were dead-set on a wood strut, I’d look at fabricating a plywood tube, tapered at the outer thirds of the length. (A solid tube with sufficient radius of gyration would be carrying a lot of dead weight in the middle when in compression.) I suspect the fabrication, probably involving multiple steam-bent spiral wraps scarfed along their length, would not be worth the trouble.
 
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challenger_II

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Duncan, wood struts have been used since the Wright Flyer. However, bear in mind that wood takes the loading better in compression, than in tension. By the time you fabricate a wood strut that will accept the tension loads you will incur with a high-wing aircraft, the weight will be such that a metal strut would be more beneficial.
 

wsimpso1

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Hi Billski. Do I detect a hint of annoyance in your reply? Hey, mate, I'm not in your league with this stuff, and I'm struggling to get my head around this. Apologies if I am trying your patience. Certainly not intended.

Thank you for the additional info.

Duncan
Sorry to let it show. My apologies to you Duncan.

Many times I answer a question in engineering shorthand and even provide a pointer to the engineering text that covers the topic. This may seem to be jargon and books used to insulate the cognoscenti from the unwashed masses. I assure that is not the intent:
  • Engineering shorthand because it is way more convenient and generic to do that as well as much more precise in applying the topic by the reader. Folks who bother to become educated on a topic WILL become exposed to the engineering shorthand for the topic and then learn its benefits;
  • The engineering texts are great in that all terms are defined somewhere in the book or in the prerequisite courses, examples given and worked, etc. The book I cited is used/translated/published worldwide, even illegally copied/translated/published in some countries and panned off as originating there (I know folks who learned Mechanical Engineering Design in both Communist China and the USSR who know the book this way). Shigley covers this topic (and many others) excellently and should be on the shelf of anyone doing any kind of machine design, including airplanes.
Then we routinely get subsequent responses where people are behaving as if I said "It will not work". Sometimes "It will not work" is the right answer, sometimes "It will be challenging", and sometimes "here is how to learn the topic". This time I said a wooden strut will need to be bigger and perhaps even heavier than the thinwall metal tubes commonly used and cited the text for those who want to learn.

The next response was a "here is an example of what I was talking about". I admit to not knowing the motive this time, but I have had quite a few where someone set out to prove it does so work when I never said it would not work. The photos might be "yeah Billski is right, look at how big that wooden strut is" or it might be "here is one and it does not look huge". I said a wooden strut needs to be bigger than it would be in metal, and look at that, the example IS bigger than a typical metal strut.

This is an internet forum. Someone other than Billski and rtfm will read this thread and who knows what they will take from it? And I try to write for all of the readers who might later search on wood struts and find this and conclude wrongly. Without a further comment on the topic, I assure us all that some readers will scan this thread and come by a conclusion that "Billski said it won't work and here are other airplanes that use them". I do get irritated when someone thinks me an idiot.

If the photo provided was to support the point I made, the OP could point out just how big that wooden strut is compared to metal ones. If the photo was to refute my comment we could get into typical sizes of metal tubes used in the same airplanes to see if the wooden one is bigger and by how much, as well as to see if the airplane was successfully flown or not. Sounds like the photo provided might have even been a subject change.

So, the irritation shown was not at you Duncan, but reflects how often I make a post and see it argued with by people who need to work on both engineering and reading comprehension... My apologies to you Duncan, not my intent to take issue with you.

Billski
 

wsimpso1

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Another question about wing struts.
Well, not struts exactly. I was thinking of the single masts as used by Croses on his Fleas. Here's a picture to remind you:
https://imagearchive.com/proxy.php?image=http%3A%2F%2Fassociationconstructeursamateurs.e-monsite.com%2Fmedias%2Fimages%2Fen-croix-sur-roues.jpg&hash=dbac6796f50192d3f34d1ad1e141b206

These are constructed from wood. Possibly with wood encapsulating a steel insert?

In light of Billski's remarks above, I'm still worrying this idea. I'd love to have single cantilevered masts on my Fleabike, but any input on how exactly to build/mount these would be welcome.

Duncan
Sounds like a subject change. If yes, a new thread is most appropriate. I will move it to a new thread if you like.

Putting steel inside wood is the heavy way to get enough EI of strut. EI is bending stiffness.

Now the term "single cantilevered masts" will need some clarification before we can go further on this... masts? A cantilevered wing is straight forward and easier to analyze than externally braced wings , but typically requires a beefier spar toward the root. Perhaps what is being talked about is the distinction between single struts without bracing, double struts without bracing, and double struts with jury struts?

Billski
 

mcrae0104

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If the photo provided was to support the point I made...
I think you're referring to Duncan's linked photo, but in case it wasn't clear, the Albatross image was intended to demonstrate the larger size, and that sometimes a material constraint can lead to beautiful solutions, not to refute the valid engineering point.
 

Hot Wings

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Hey, mate, I'm not in your league with this stuff, and I'm struggling to get my head around this
Not directed at rtfm in particular..............

I'm not in his league either, but I generally can follow his logic pretty well. To me this whole engineering thing is fascinating and worth the time to learn. The learning is much like the curve of an endothermic chemical reaction curve. There is a pretty steep up slope of energy input at the start of the process but the rewards can be great. That is just the way life is.

If one wants to design EABs efficiently, and without having to use well versed individuals as a crutch, it may be of benefit to spend some time learning the basics of structures and materials. It doesn't take math skills much more than basic algebra unless you need/want to know the derivation of the formulae.*

More knowledge also helps to know when to ask questions. I've also found that by crafting a well tought out question the answer becomes apparent. The question no longer need be asked.
Two good YouTube sources:
Jeff Hanson
Michel van Biezen

Plan on spending a couple of months at an hour a day ............................. initially. I still pick up a few bits from Michel van Biezen.



*I struggle with the math because I've found it impossible to memorize trig identities - which makes integration a little harder/slower for me.
 

TFF

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Sopwith used wood. They are beefy chunks of wood. Probably expensive chunks of wood.

Here is the issue I see; wood fuselage and wood struts or wood fuselage and metal struts; both usually require metal work. Not that you couldn’t glue wood struts, but you can’t scab on with wood, they will have to be deep in the fuselage structure. If the struts are not built into the fuselage, you will need metal no matter what.
 

Chilton

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DH 53 like most of the biplanes also has pretty big wood struts. I dont have suitable pictures here, but comparing the Tiger Moth struts with those on a Stampe SV4 which are steel tubes, is an interesting contrast.

Certainly with the current cost of aircraft timber the steel tube struts are very much cheaper!
 
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