carbon tow question

[glasflugel]

I understand that the lower spar on a glider wing is in tension while the upper spar is in compression when in a loaded condition (flight).When carbon tow is added to wing spars, it is added to the outside of both upper and lower spars. I'm having a hard time understanding why carbon tow is added to the upper surface of the top spar to mitigate the compression load. Shouldn't the tow be added to the lower side of this spar to off-set the compression factor? Is the answer - to apply structural rigidity in both positive and negative g-loadings. What am I missing in this picture? I need some illucidation from someone who has a lot more gray matter than I,but, can explain the solution in a manner that my simple mind can wrap around. Thanks,Gary

[slopemeno]

Compression loads are higher and harder to deal with than you think- the tow added to the bottom of the wing can probably be 25% of what's on the top of the wing.

Think of it this way- if you take a carboard tube and try to flex it like a wing- what happens? Does the bottom in tension fail first, or does the top "oil-can" and fail in compression?

[Mark Miller]

On a sailplane we are concerned with positive G's. That is why the spar is usually designed with thicker top cap than bottom. If we were an aerobatic type plane doing positive and negative G's it would be different. The spar is designed for launch loads pure and simple.

Mark

[glasflugel]

Slopememo, I'm following your line of reasoning most of the way --when I put excessive force in an upwards direction on a cardboard tube the lower surface of the tube (under tension) remains relatively unchanged, but the upper portion of the tube can either "v" (under compression) or oil can-bulge. The natural tendency, then, is to always bulge. This would explain why the carbon tow is added to the top side of the upper spar-to resist the bulging tendency. I confused myself into thinking that the compression load on the upper spar would always cause the spar to "v" instead of ,correctly, bulge.My left-handed brain sometimes gets in the way of "right" thought. Thank you for a succinct answer to a vexing, albeit simple, question.

[glasflugel]

Thank you ,Mark, for responding to my inquiry. I'm one of those people that feels that the only stupid question is the one that isn't asked. I know this can be tedious at times to some but it's all part of the learning process

[Christian Baron]

In a bended spar (normal flight force) you have tension (bottom) and compression (top spar) loads that are equal!!!

The problem is the compression strength and behaviour of the material you use. The compression strength of the most materials used for wing spars are lower than the tensile properties. Carbon fibre spar caps are therefore thicker at the top. A carbon spar cap made of HT-fibres could be desiged only 10% thicker at the top, but a carbon spar cab out of UHM-fibres should be 60% thicker because of the bad compression behaviour.
Christian

[atjurhs]

wrapping the top and bottom with kevlar/carbon tow, or fiberglass cloth, or some other means to "tie" the top and bottom spars together will make them even stronger. The more that the top spar can be prevented from sliding inward (toward the root), and the bottom spar outward (toward the tip) the stronger the overall spar will be,

[glasflugel]

Thanks, Christian, for your input. I have followed your generous and knowledgeable contributions on Curtis Miller's scratch BS-1 build. Thanks, also to Atjurhs for his contribution. I'm getting smarter by the seconds! While I was familiar with the "how-to's" and the "what-fors," it was the "why's" I was fuzzy on. Gotta love the internet, Gary

[williamson]

Slopememo, I'm following your line of reasoning most of the way --when I put excessive force in an upwards direction on a cardboard tube the lower surface of the tube (under tension) remains relatively unchanged, but the upper portion of the tube can either "v" (under compression) or oil can-bulge. The natural tendency, then, is to always bulge. This would explain why the carbon tow is added to the top side of the upper spar-to resist the bulging tendency. I confused myself into thinking that the compression load on the upper spar would always cause the spar to "v" instead of ,correctly, bulge.My left-handed brain sometimes gets in the way of "right" thought. Thank you for a succinct answer to a vexing, albeit simple, question.

The top of a spar under compression can fail either by "Ving" downward or bulging upward. The "Ving" downward is constrained by having adequately strong shear webs. End-grain balsa (grain running vertically) has great compression strength and is the usual recommendation to avoid "Ving" downward. Bulging upward is constrained by wrapping the spar, usually with Kevlar tow.

The third type of spar failure is shear forces between the ribs. These shear forces are strongest near the center of the wing. To visualize a shear failure, look forward at the rectangle formed by the top carbon fiber, the bottom carbon fiber and the ribs on each side of a bay. With shear forces, this rectangle distorts into a parallelogram. One way to retard this shear distortion is to use stronger shear webs. However, this can add a lot of weight. The technique used in Mark Drela's Allegro and Bubble Dancer designs is to wrap the spar for several bays near the center of the wing with several layers of fiberglass cut on a 45-deg. bias. The fiber glass is epoxied to the spar before the spar is wrapped with Kevlar.

[lincoln]

Actually, the compressive strength is pretty close, if I remember correctly. HOWEVER, under compression you can have buckling, which you won't under tension. To resist buckling, you need something which is stiff. If you make the top cap thick, it will be stiffer and less likely to buckle. Think about leaning on a 3 foot piece of 1/8" music wire or a 3/4 inch square piece of 10 lb balsa. They both weigh about the same, and the music wire is much stronger, but the music wire will buckle easier than the balsa, because the balsa is stiffer.
In a bended spar (normal flight force) you have tension (bottom) and compression (top spar) loads that are equal!!!

The problem is the compression strength and behaviour of the material you use. The compression strength of the most materials used for wing spars are lower than the tensile properties. Carbon fibre spar caps are therefore thicker at the top. A carbon spar cap made of HT-fibres could be desiged only 10% thicker at the top, but a carbon spar cab out of UHM-fibres should be 60% thicker because of the bad compression behaviour.
Christian

[atjurhs]

I could be wrong but I think it is the prevention of buckling that kevlar thread wrapping helps most with. There are also forces that try to laterally shift the top and bottom spar caps along the wingspan line and the the tread wrapping should help inhibit that movement as well.

[schrederman]

It's not the loads, as they are equal. The concern is how the spar cap material handles the loads. Every spar failure where the caps are of equal size will be a compression failure of the top cap, unless the material is equally strong in compression and tension. Since most fibrous material has only about half the compression strength as tensil strength, to have equal strength in positive load, the top cap needs to be twice as thick as the bottom. I don't know what the ratio is with carbon, but being fibrous, I use that rule of thumb with my designs. The Houston Hawk uses .060 X .5 for the top spar cap, and .030 X .5 for the bottom. I've bent 1/2" joiners with these wings, but never had a spar failure. Wrapping does help with bursting failures, as it helps with keeping the bond between the caps and webs from being overstressed. It does NOT add to the tensile and compressive loads on the spar caps. Bottom line is that if the caps aren't strong enough to carry the respective loads, the spar will fail.

JW

[Mr B....]

Last year I posted some pic's on this very subject!

I mfg this spar for my contest woodie 3M with Clear Doug Fir 3/16 X 1/2 and 1/2" shear webs. I laminated three strand's of 12 K carbon tow to the bottom of the bottom spar and the bottom of the top spar! Notice the compression on the top spar above the zero of the 10 !



Here is a link with my findings.
http://www.rcgroups.com/forums/showthread.php?t=735814&highlight=Merrill

Just this last May 08 I did it again! I have two ready to test spar's if anyone wants to do there own testing! :)

Merrill

[mlachow]

The normal design tradeoff for sailplane wing spars is how much will the wing bend. So there is usually far more carbon than is needed for compression or tension.

[williamson]

I could be wrong but I think it is the prevention of buckling that kevlar thread wrapping helps most with. There are also forces that try to laterally shift the top and bottom spar caps along the wingspan line and the the tread wrapping should help inhibit that movement as well.

The lateral shift of the bottom and bottom spar caps along the wingspan line is the shear distortion that I mentioned in a previous post. Wrapping the spars does little of reduce this type of distortion. Wrapping with fiber glass cut on the bias does reduce shear distortion.

I tested a wing with carbon fiber on the top and bottom of the spar. The wing also had fiberglass wrapping of the inner bays. The wing was supported about 1 m each side of center. This spar did not fail when subjected to well over 100 lbs. on the tow hook. Without the fiberglass wrapping, the spar failed in the inner bays due to shear distortion. The failure occurred with about 70 lbs. on the tow hook.

[glasflugel]

.Remember,all, that my question had to do with the placement of the carbon fiber tow on respective spars. I think Merrill has answered my question best. Since carbon fiber tow is highly resistant to stretching, it is most logical , to me , to put it on the bottom of both the upper and lower spars. Jack, you are absolutely correct in regard to carbon cap strips and their respective dimensioning when designing a spar. Great example Lincoln. Williamson, you have reminded me that I neglected the shear-webs in my v-ing-bulging logic.Gadzooks, I am flabbergasted at how you "heavy hitters" have come to my aid. In homage to Hercule Poirot, you have stimulated my little gray cells.

[schrederman]

Quote> I am flabbergasted at how you "heavy hitters" have come to my aid.

The free interchange of ideas here is probably the best thing to happen to modelling, ever. I have learned sssoooooo much here from Mark Drela, Tony Estep, and so many others... Never been called a "Heavy Hitter" before... Ok maybe in church softball league... but not in modelling... Thanks for the kind words...

JW

[atjurhs]

Wrapping the spars does little of reduce this type of distortion.


Ahhhhhhh, that's what I said.

[BigTilly]

Sorry to not see this sooner. The "real" reason for putting the CF on top of the upper cap and on the bottom of the lower cap is pretty simple. In a beam in bending, the resistance to bending of the beam is dependent on a quantity called the moment of inertia (abbreviated "I") commonly (it is really the second moment of area). If you look up how to calculate this, you will find that you can maximize the value of I by putting the stiffest portion of the beam (whether the stiffness comes from a change in material stiffness or a change in width) the furthest from the shear center (the midplane in a symmetric beam). The caps of a beam "work" much more in bending than the rest of the beam. However, you still need a means of carrying the shear load down the beam, and that is what the web is for.

So if you built a beam (spar) with CF in the center and balsa at the caps, it would be very compliant in bending. If you reverse that, it would be very stiff in bending. Conveniently, the resistance to buckling is also dependent on I (look up Euler buckling for a quick tutorial on what maters in stability).

Make sure that you read this post carefully and recognize that I haven't used the word "strength" once. I'm not telling you how to make a beam stronger, merely stiffer.

[glasflugel]

Thanks,Big Tilly, now I see that I have been looking at this question from only a strength point of view and neglecting the moment of inertia shared by the upper and lower spars.Great stuff!! Gary