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Author Topic: How spars affect the airfoil  (Read 799 times)
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flydean1
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« Reply #25 on: December 13, 2020, 10:55:52 PM »

Uh...look at your broken wing for an excellent example of a buckling failure.
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a23smith
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« Reply #26 on: December 13, 2020, 10:59:01 PM »

Lincoln,

Thanks for the additional information on the airfoils you mentioned. I will probably follow up on that later. I was already in a black hole before I got back to model building this summer, so now is no different and the future probably won’t be either. My interest is the enjoyment of learning about the most important aspects of building a good flying free flight rubber powered model airplane and learning the most important aspects of getting it trimmed properly, even if I never fly it in a space any bigger than my local park. I didn’t realize how vast this hobby is, but I now understand that there is more than enough to keep a person busy an entire lifetime. Thanks too for warning me about Autocorrupt, but if I read something you wrote that I don’t understand I will probably attribute it to my comparatively meager knowledge.

Alan
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lincoln
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« Reply #27 on: December 13, 2020, 11:46:53 PM »

Have you ever stood on an empty soda can, which held you up until you touched the side and it collapsed? That's buckling failure. In other words, you have something that's strong enough to take a compressive load if it stays straight, but it doesn't stay straight. Like pushing a rope. Compressive failure is what you might see if you squashed a slice of cork with a vise. (For fun with compressive failures, I recommend the Hydraulic Press Channel, though one may see other types too.) Tensile failure is what you see when you pull on a rope until it snaps.

For buckling failures, stiffness is more important than strength. Consider two long, thin rods, of identical dimensions. One is cheap steel that's been annealed until it's quite soft. Maybe the yield strength is 35,000 psi. The other tube is of heat treated 7075 aluminum and yields at 75,000 psi. If you load them in compression, the steel tube will actually support a higher load before it buckles, because steel's elastic modulus is almost three times that of aluminum. On the other hand, if you made the two rods of the same weight, the increased diameter of the aluminum would more than make up for the lower elastic modulus. This us why balsa is so good. The low density allows bigger cross sections. A 1/4 inch square balsa stick might weight about the same per foot as 1/32 music wire. Which would you rather use as a pushrod?

I suppose elastic modulus may not be an obvious term. Say you had a 100 inch long, 1 inch square steel bar. A particular kind of steel might have an elastic modulus of around 28,000,000 psi. Say you pulled on the bar with a 28,000 lb force, or 1/1,000 of the elastic modulus. Each inch of that bar would stretch by .001 inch, for a total of .100 inches. It works going the other way, too, as long as it doesn't buckle. An aluminum bar, with an elastic modulus of around 10,000,000 psi, would stretch almost 3 times as much. Then again, it only weighs a third as much.

Autocorrupt is in fine form tonight. I missed a space and "autocorrupt is" became autocratic. Sometimes it comes up with proper names that aren't spelled anything like what I typed.
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lincoln
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« Reply #28 on: December 14, 2020, 12:04:22 AM »

I can't remember if you mentioned McComb's Making Scale Model Airplanes Fly. If you can get yourself a copy, and a magnifying glass, you will find it informative and useful. I think it used to be advertised in NFFS Digest. It's only a bunch of mimeographed paper stapled together, and it's ugly, but it's great. A beautiful and useful model airplane book is Ron Williams' Building and Flying Indoor Model Airplanes. But read the other one first so you aren't disappointed with the design of it. Also, the Williams book covers different material which won't help you much with an old time rubber model.

Speaking of the size of the hobby, I probably have enough kits in my basement to keep me busy for the rest of my lifetime, and enough balsa to keep me busy for years after that. At the rate I'm going now, it will take me the rest of the millennium to get through it all.
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a23smith
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« Reply #29 on: December 14, 2020, 01:23:34 AM »

Lincoln,

I’m impressed. Thanks for the information about failures. It was entertaining to read it and now I think I have a basic understanding of the difference between buckling and compression failures, although I’m not sure I could distinguish between them in practice. I imagine failures aren’t purely one or the other, like most other things. And I think I understand that elastic modulus is a measure of how well a particular material sticks together, rather than its stiffness. Thanks for telling me about the books by McCombs and by Williams. I was aware of McComb’s, but haven’t been ready to search for it. There is a lot available from free articles online and I’ve been letting my particular issues guide my searches for the most part. You are lucky to have so much balsa. The best source I have locally doesn’t stock 1/2” sheet and when I commented about how expensive balsa has gotten lately the woman operating the cash register there told me all of the balsa is now going to the companies making the huge blades for the wind powered electricity generators. I ordered 2 sheets of 1/2X3x36 from Specialized Balsa Wood and it cost me almost $45. I misread your statement about Autocorrupt being in fine form tonight. I thought I saw “Autocorrupt became autocratic” and I started to worry about you. You have probably read about Elon Musk having some concerns regarding AI and currently I am watching a Netflix movie called “Tau”.

Alan
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PB_guy
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« Reply #30 on: December 14, 2020, 10:55:47 AM »

Lincoln, if you are using Word you change the autoCorrect features, or turn them off entirely: https://www.computerhope.com/issues/ch000564.htm and I am sure you can do similar for other WP's.
ian
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TimWescott
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« Reply #31 on: December 14, 2020, 11:46:55 AM »

... And I think I understand that elastic modulus is a measure of how well a particular material sticks together, rather than its stiffness. ...

That's backwards.  Elastic modulus is how stiff it is before it takes a set.  Strength (i.e., tensile strength, compressive strength) is how much stress it can take before it bends permanently.

Somewhat counter-intuitively, the elastic modulus of nearly all steel alloys is pretty much the same.  What's different is how soon it'll stay bent if you bend it.  A spring made of soft wire has the same spring rate (force vs. distance pushed) as one made of spring steel -- it's just that the soft wire spring will permanently deform much more easily than one made with spring steel.
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a23smith
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« Reply #32 on: December 14, 2020, 11:51:05 AM »

Tim,

Thanks for the clarification.

Alan
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bentodd
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« Reply #33 on: April 05, 2021, 08:40:45 PM »

This is a very interesting train to read.  I will add a few more concepts to consider. 

For a basic beam, in this case the wing, both the stiffness, and also the stress on the structure are related to the height of the beam cubed.  If your wing was 2 mm thick, it would have a stiffness factor of 2^3=8.  If it was 4 mm thick, 4^3=48.  And as I said the stress on the wing also goes up by this factor, for the same loads, the bending would be much less and the stress would be much less.  So older wings with only a balsa structure tend to use thicker wings, 8 to 10% thick, modern wings, with carbon fiber can are around 6% thick.  And for this same reason, it is best to put a pair of spars, top and bottom at the thickest point of the airfoil.

When the wing is normally loaded in flight, the top spar will be loaded in compression and the bottom spar in tension.  Balsa is stronger in tension so it might be better to make the top spar a little bigger than the bottom spar.  But thinking about it, most plans I see tend to have the top and bottom spars the same size. 

Just guessing, from personal experience, you wing failed when one tip hit the ground first.  This would put the the top spar in compression again and the bottom in tension.  Since both top and bottom spars failed, it is hard to say which one failed first.  When one failed, all the load would transfer to the other and it would break too.

http://www.mh-aerotools.de/airfoils/  suggests a interesting theory.  The sag of the covering between ribs reduces the span-wise flow of air that normally occurs.  So a little sag might be a good thing.
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lincoln
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« Reply #34 on: April 05, 2021, 09:53:06 PM »

Actually, for a rectangular spar of constant width, the stiffness will go up by the cube of the depth, and strength by the square. If you vary things in other ways, it gets more complicated. It also gets more complicated if the spar is long and skinny and poorly supported enough to twist or buckle. The criteria for buckling are tricky.
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