+/- 45-degree unidirectional carbon fiber orrientation: Is that to the main spar or airflow? 1

[Broncazonk]

A hypothetical wing has a -/+ 5-degree trapezoidal hi-aspect cord and the main spar has a similar 5-degree taper past the landing gear attachment point. The rear spar is full length and tapered forward 5-degrees. Unidirectional carbon fiber is selected, as is a +45/-45/+45/-45 orientation. But is that 45-degree orientation based on the main spar or the airflow?

Thanks!

Bronc

 

[Andries]

Hi Broncazonk,

The +45/-45 directions are mainly to carry shear loads in the wing skins and spar webs. Looking at the spar web the directions must make +45 and -45 degrees to the "centre line" of the web.
For the wings the +45 and -45 directions must be relative to the direction of the spar(s), not the airflow.

Regards,

Andries

 

[RPstress]

A beam will normally bend about its neutral axis and twist about its torsional axis and the two should be quite close, especially for a wing. Any ply angles should be roughly relative to the NA. The wing will usually have a nominal drawing axis which approximates the NA (although the wing drawing axis for a cover is sometimes along one of the spars where it intersects the skin). One could make the argument that torsion plies should be 45° to the torsional axis and bending plies should be parallel to the NA.

I'm not too sure why an all-45° antisymmetric pattern is suggested (though see below). A spar web might be mostly 45 away from the caps but all-45 is not very sensible for a wing cover, even for a small wing. A wing's main loading is bending and that means the skins should resist spanwise tension and compression, which needs spanwise plies. A common layup on airliner wings is something like 40% plies at 0°, 40% at ±45° and 20% at 90°. In the past (late 19-teens, the 1920s and early '30s) it was the practice to put most of the bending resistance in the spar caps but this was mainly because the skins were fabric and could take no endload. Once skins could take endload (mainly 'cos they were metal but see the Mosquito's endload-carrying sandwich upper skin) they were used for that.

On the other hand if the spar caps are chosen as the main bending capability for some manufacturing reason then ±45° for the skins makes sense to carry the torsion (the spar webs, being mainly for vertical shear, have a lot of 45° plies and are good for carrying the torsion with the skins as a box; very efficient for torsion).

If the skins are like old-style fabric with no shear capability, then the torsion is carried by differential spar bending which is not very efficient and proved in dangerous in the first monoplane fighters of WWI. Having too little shear capability in the skins tends towards this situation. (With non-endload-and-non-shearload-carrying skins, ribs were often diagonal rather than chordwise (the only passably efficient way to carry torsion for an open section); see the Hurricane's fabric-covered tailplane (see attached (though not a very good view) and also see

http://weldingmag.com/blodgett/no_twisters_swingers_allowed_0501/).)

±45° is ok for skins as long as there're some pretty meaty spar caps for the bending.

I've used the terms skins and covers interchangeably; the cover of a big wing usually also includes stringers. 'Skin' in this case includes any such capability for carrying endload (such as the Mosquito's sandwich upper skin/cover).

 

[Broncazonk]

Aviation Week had an interesting article a couple months back about the cancellation of the mostly composite Bombardier Learjet 85:

http://aviationweek.com/business-aviation/bombardier-learjet-85

From the article:

“For the Learjet program, Belfast fabricated the upper and lower wing panels with integral stringers, along with both wing spars, mainly from dry carbon-fiber non-crimp fabric sheets that are precision cut by automated mills. Then, resin is infused during an autoclave cure process. The wing ribs, though, are made from aluminum because that metal has better shear strength properties for the weight than composite materials. Spars, skins and ribs then are assembled into complete wing structures.”

It's the "integral stringers" part of the sentence that I'm interested in. Instead of utilizing a full fabric in the skin layup, the design utilized a series of thick unidirectional carbon-fiber tapes to form traditional stingers? (Or stringers were in addition to full fabric laid at the same orientation?)

Really interested in this "integral stringers" concept, does anyone have info or even speculation?

Thanks!

Bronc

 

[SWComposites]

Integral means the stringers are cured/bonded with the skin. They are multidirectional lay ups as are the skin laminates.

 

[Broncazonk]

SWComposites wrote, "Integral means the stringers are cured/bonded with the skin. They are multidirectional lay ups as are the skin laminates."

Yeah. I think we all know what the word, "integral" means. You have direct knowledge that the stingers are laid up multidirectionally? Because a multidirectional stinger defeats the purpose of a stringer does it not?

Thanks!!

Bronc

 

[Compositepro]

Broncazonk, you just dissed one of the most respected contributors to this forum. You had better be more professional, in this forum for engineering professionals, if you expect any assistance. Your questions come from ignorance and it also shows in how you interpreted SWComposites reply. He answered the question you asked you simply did not understand the answer.

 

[Broncazonk]

Compositepro: How could you read that post as a diss? I merely pointed out that everyone knows what "integral" means, and I asked two pertinent questions.

Oh, I get it. You meant that SWComposites was patronizing me--and I pointed that out, and I wasn't supposed to do that? Is that what you meant?

Bronc

 

[SWComposites]

Yes, I have direct knowledge of the Lear85 stringers, along with the configurations and layups on a bunch of other aircraft composite structures. And no, I'm not going to post the details here since they are proprietary.

Cheers,

SW

 

[RPstress]

Re stringer/stiffener ply directions and manufacturing, the stringers on the Global Express tailplane were simple blades, were integral with the skin and were roughly quasi-isotropic. They were UD prepreg laid up as long U-shapes and then placed on the (uncured) skins (with 'noodles' between them) so each flange of the U formed half the thickness of the blade. The base of the U formed part of the skin between the stiffeners. A fairly innovative glass-reinforced rubber tool was used to consolidate the inner surface with the stringers. That was 20 years ago in Northern Ireland just after Bombardier bought Shorts. I would think that they have developed both the design and manufacturing considerably since then!

I have seen designs (I think it was at Airbus) that used stringers with up to 80% of the plies at 0° (i.e., spanwise) but this is quite rare. A lot depends on the ratio of stringer area to skin area (A_str/A_s), which is a fairly high-level design decision made early on, affecting overall efficiency and manufacturing methods. The form of the stringer affects these too, of course; Niu (Airframe Structural Design about page 600) has some guidance about the efficiency of different stringer forms; the simple blades often used with composites get a '0.656' (lowest) but a 'Y' section gets '1.23' (highest). It's hard to understand but I think the stringer weight may be proportional to the 1/number (the number's called "ε_max"). The tooling for I and J stringers with a free flange is fiercely complicated of course.

Because of the cool-down from cure the stringers affect the final curvature of the cover, and it's often decided to keep the stringer layup the same as the skin to have the same CTE. A metallic skin is usually curved (especially if it's machined of course) so the design methods used to compensate for this usually apply to a composite cover.

Niu references the NASA Handbook of Structural Stability, which is online. (Seven parts with a supplement to Part III; I'm afraid I'm not sure which bit might be most helpful about stringer efficiency.)

 

[Broncazonk]

What is the 90° orientation actually doing in a wing? Not for tension or compression in span wise bending loads. Not for torsion. The other orientations seem pretty straight forward, but I can't visualize the purpose of the 90°.

Thank you!

Bronc

 

[Compositepro]

It keeps the spar from splitting down its entire length. There are almost no real world parts that are made from 100% uni. The difference in 0 and 90 degree thermal expansion alone would make it impossible to bond the spar to the skin.

 

[SWComposites]

90 deg fibers are needed to avoid splitting under impacts and loading, improve crippling strength, improve bearing and bypass strength at fastener locations, and to make the stiffener poisson's ratio closer to that of the skin which helps skin-stiffener disbonding and skin panel damage residual strength, etc.

 

[RPstress]

On a big wing with stringers the basic skin panels beam lift loads to the stringers which causes chordwise bending. Also the ribs build up chordwise forces as lift loads are moved to the spars as shear in the rib webs. These chordwise forces need chordwise fibers.

Also, a lot of discrete positions need a lot of 90° fibers, such as flap track connections, aileron actuators, spoiler connections, gear beam connections. Lots of rear spar stuff needs chordwise strength. Also engine pylon connections towards the center and front, as well as slat track forces.

If you have sandwich skins with no stringers the basic skin panels mainly beam lift loads to ribs by spanwise bending, so there's less need for 90° fibers, unless you go the whole hog and have sandwich skins with no ribs, in which case most of the air pressure is beamed to the spars which causes chordwise bending in the skins. (Ribless sandwich skins are a serious suggestion for major airliner parts. The Germans are looking into sandwich skins with fewer ribs on Airbus fins. That'll need more chordwise fibers in the fin's skins. However, The British have chickened out of ribless wings. Interestingly the Mosquito was designed with ribs in spite of sandwich skins.)