Carbon Tube spars

[GNieuwoudt]

What would be a suitable size (ID and wall thickness) for a carbon tube spar of a 3.3m glider like an Aegea. Is there a way of calculating this.

Any advice will be apreciated.

Cheers

Gert

[Phil Barnes]

I can't tell you any theoretical way to determine this but I can offer some advice based on experience. I have made quite a few Aegea Mantis wings with tube spars and have posted sopme details in the SALgliders YAHOO group;

http://groups.yahoo.com/group/SALglider/

Check messages 4170-4174. You should understand that the carbon tubes are not the only source of bending strength for the wing. Much, if not most of the bending strength comes from the carbon skin.

The Aegea Mantis wings use the airfoil and planform of the 130" span AEGEA wing designed by Mark Drela and posted in the files sectiom of the Allegro-Lite group.

http://groups.yahoo.com/group/Allegro-Lite/?yguid=163476728

The tubes that I use are found on the stock tubing page from the link below and are very similar or identical to the tubes sold by ACP and CST.

http://www.macqc.com/

Phil

[Sled Driver]

...I have made quite a few Aegea Mantis wings with tube spars and have posted sopme details in the SALgliders YAHOO group;

...You should understand that the carbon tubes are not the only source of bending strength for the wing. Much, if not most of the bending strength comes from the carbon skin.

Phil,

If you'll describe what you use for a layup, I'll model it in a spreadsheet and post results that present the distribution of load supported by the CF tube and the CF skin.

Jay Decker
Kennewick, WA

[Phil Barnes]

Phil,

If you'll describe what you use for a layup, I'll model it in a spreadsheet and post results that present the distribution of load supported by the CF tube and the CF skin.
I would be very impressed and gratefull if you could and would do that. I have often wondered how much the tubes actually contribute to the stiffness of the wing. I have no ability to do theoretical design and have derived the structure purely by empirical methods.

I will describe the structure and layup for the center panel only since the tip panels are defintely way overbuilt and not very interesting to analyze.

The center panel is 58" span with a 10" AG40d airfoil at the center and 9" AG41d at the ends.

Tube structure;
There is a full span 1/2" ID tube spar centerred 4" back from the wing LE.
There is a 9/16" ID tube spar centerred 2 5/8" back from the wing LE which spans 15" to each side of the wing centerline.

Skin;
There is a full layer of 3/4 oz bias glass on top and bottom.

There is a six inch wide layer of 4.7 oz carbon/ S glass running full span top and bottom which begins at the wing LE and extends back towards the TE.

There is a small 4.7 oz carbon/S glass doubler on top only which is 5" wide at the center and 2" wide at the tips and extends 11" each side of the wing center line.

Aft of the carbon is a layer of 1.7 oz bias Kevlar which extends all the way to the TE.

Additional details about the materials used;
The foam used for the core is Foamular 600.

The tube spars have wall thicknesses of .030" for the 1/2" tube and .033" for the 9/16" tubes. Additional material properties are listed at the Maclean Quality Composite site listed above.

The carbon/S glass is the stuff commonly called "8020 carbon" and has about 4 ounces of unidirectional carbon with about 0.7 ounces of S glass woven in the fill direction. The fabric is woven with 16 strands of 3K carbon tow per inch.

I anxiously await the results but will understand if this is actually a little more complicated than you might have anticipated.

Phil

[Sled Driver]

... I anxiously await the results but will understand if this is actually a little more complicated than you might have anticipated.

Phil,

I think that you've provided everything needed. I'll try to get it done tonight.

Jay

[Red Baron]

Jay,

I am very interested in how you perform these calculations. I have often wondered (worried?) about the strength of some of my models when im doing some aeros, and I would be grateful if you could show me some of the basic methods you use for this modelling. It would be nice to be able to do this, even if I'm just getting a rough estimate.

Regards,
Conor

[Sled Driver]

The attached figure depicts how much bending load (moment) is supported by each structural component in Phil's Aegea wing. Here's key that describes what each component labeled in legend at the bottom of the figure actually is:

Spar: There is no spar, so it supports zero moment across the half span.

Skin: The 3/4-ounce FG skin material.

Tube Spar 1: The 1/2-inch ID CF tube that extends 42-inches from the center toward each tip (including 12-inches into the tip panel).

Tube Spar 2: The 9/16-inch ID CF tube that extends 15-inches from the center toward each tip.

Dart 1: The Uni-CF that extends most of the way from the LE toward the TE across the entire half span and varies in width. Note that Dart 1 has an doubled, as Phil describes above, the first 11-inches from the center toward the tip.

Dart 2: The bias Kevlar that extend from the Uni-CF to TE across the entire width.

The spreadsheet produces other result if you interested, e.g., wing deflection, allowable stress, bending stiffness distribution, etc., if you are interested.

Jay Decker
Kennewick, WA

[Phil Barnes]

Great job Jay. At first glance, to a non theorist, the results look realistic and support the idea that the tubes carry a small load compared to the skin.

Further enlightenment could be obtained if the spreadsheet could answer the following questions:

1) What is the maximum winch line load the wing can produce and where will the wing break first? Max winch line load would be most interesting since we could then compare this number to calculations for other wings such as Drela's Aegea or Supra wing.

2) What max winch line load could be produced if the short tube spar (tube spar 2) were removed?

3) What carbon skin layup would be needed to match the strength of the existing wing if there were no carbon tubes at all?

Please don't knock yourself out doing all these calculations unless you really feel like it.

Can you tell us where to get the type of software you used for this spreadsheet? I wouldn't mind learning how to produce spreadsheets like that myself. It could be a great tool to use in designing future wings.

Phil (maybe I should become a theorist afterall) Barnes

[Sled Driver]

1) What is the maximum winch line load the wing can produce and where will the wing break first? Max winch line load would be most interesting since we could then compare this number to calculations for other wings such as Drela's Aegea or Supra wing.

Phil,

The figure below depicts the amount of bending load (stress) on each component relative to where they where they should theoretically fail (break) for a 150 pound winch load load (this is the ratio of applied stress to allowable stress expressed in percent). A couple notes:

1) The maximum allow able stress is 90% for the Uni-CF, Dart 1, which carries most of load. To calculate where the wing would theoretically break, you divide winch load of 150 pounds by 0.9 (i.e., 90%) and you get 167 pounds, which is the theoretical winch load where the wing would break based on this analysis (this load would increase the allowable stress to 100% at that location).

2) Mark's Aegea wing/spar can take "a few times" as much winch load. As I recall, and I might not remember accurately, Mark's Aegea CF spar caps are loaded approximately 25% of the allowable stress for the same winch load. While the spar probably would not break at such a high load, there would probably other problems due to the flex of the wing.

3) I hesitate to make to comments that could be interpreted negatively regarding your Aegea wing, but I'll point out a couple things that I hope you will take constructively: a) Mark recommends that stressed skins designs not allow the top skin loading to exceed 80% of the allowable stress. b) Stress discontinuities, while realistically unavoidable, are not good, particularly when they are close to allowable stress, i.e., this is were the wing would mostly like break first.

Jay Decker
Kennewick, WA

[Sled Driver]

2) What max winch line load could be produced if the short tube spar (tube spar 2) were removed?

139 pounds

[Sled Driver]

3) What carbon skin layup would be needed to match the strength of the existing wing if there were no carbon tubes at all?

If you kept the same style of lay-up, e.g., 80/20 from the LE back roughly 60% of chord toward the TE, your 80/20 layers would need to look roughly something like this:

Layer 1: Root to tip.

Layer 2: Root to 31-inches toward the tip

Layer 3: Root to 18-inches toward tip.

Layer 4: Root to 8-inches toward tip.

These are rough estimates and somewhat conservative estimates that could probably be refined a more. Also, you could probably eliminate the 4th layer at the center and some of the other layers if you were to switch to Spyder or HiLoad 60 foam for the center section – this would increase the maximum allowable stress of the top skin.

Jay Decker
Kennewick, WA

[Sled Driver]

Can you tell us where to get the type of software you used for this spreadsheet? I wouldn't mind learning how to produce spreadsheets like that myself. It could be a great tool to use in designing future wings.

The “software” is a roll-your-own MS Excel spreadsheet. What made this spreadsheet possible for me is Mark Drela’s sharing of formulas and materials data combined with some other basic engineering definitions and formulas and a few numerical approximation methods. Thank you Dr. Drela!

The spreadsheet is not “done”, and not that it ever will be. The weight estimation portion has not been finished and neither are any user data input forms. So. it is not at all “user friendly”.

Jay Decker
Kennewick, WA

[Phil Barnes]

Jay, I am very impressed with your calculations. All of your results continue to look realistic compared to observations of actual wings.
if you were to switch to Spyder or HiLoad 60 foam for the center section – this would increase the maximum allowable stress of the top skin.
I think you may have missed this on my initial posting about the details of my AEGEA wing construction. My AEGEA wing uses Foamular 600 for the entire wing. This is equivalent to Hi Load 60 and almost as good as Spyder Foam.

I wonder if it would be easy to just change one number and re-run the program to see how much that affects the outcome.

Phil

[Phil Barnes]

If you have been following this thread then you have probably figured out that carbon tubes are not a very efficient spar system in terms of strength to weight considerations. So you might wonder why anyone would want to use such a system.

Carbon tube wings are most often used on production wings which must be made with minimal man hours of work. Tube spars are very fast and easy to install and require no time at all to make. They are fairly inexpensive to buy especially when bought in quantity.

The carbon tube also provides a convenient place to insert the wing joiner. Round wing joiners can be made or bought cheaply also.

Carbon tube wings also have a carbon skin which carries most of the load. In the event that the carbon skin should buckle then the carbon tube is there as a backup spar system. I call this "carbon tube insurance". You may occasionally see a carbon tube wing crease but you've never seen one completely break. This provides the opportunity to fix the wing before the entire model is lost.

A model with a "real" spar tends to have a rather thin skin. Carbon tube wings have thicker skins and so tend to be more resistent to "hanger rash" or "tree rash".

If you are out to build the most efficient wing possible in terms of strength to weight then you probably can't do better than a Drela design. His new Supra F3J model weighs about 48 ounces. If, however ,you don't need or want a 3.? meter model that is that light but you would rather have a model that is faster and easier to build then you might want to try a carbon tube wing.

My Aegea model weighs about 64 ounces which is as light as most people want for a model that size under almost any conditions. A model with a real spar can launch harder and weigh less but if all you are trying to do is AMA thermal duration with winches then the carbon tube wing may be all you need.

Phil

[Phil Barnes]

If anyone is interested in the tip layup for my AEGEA wing, here it is. I had sent this to Jay in an E mail earlier. This may also help if you are trying to understand Jay's results;

The tip panels also have carbon forward and bias Kevlar aft.

The bias kevlar is 3 1/4" wide at the root end and 2" wide at the tip. This Kevlar is actually two layers of 1.7 oz. That is done for torsional stiffness on the ailerons.

Forward of the Kevlar is 3.5 oz carbon/S glass. This is the same as the 4.7 oz "8020" carbon except that it has only 12 3K carbon tows per inch instead of 16.

There is also a 1/2" ID carbon tube 12" long in each tip.

There is also a bias layer of 3/4 glass on the entire tip.

The tips also use Foamular 600 for the core and the layup is the same top and bottom.

[markdrela]

I think you may have missed this on my initial posting about the details of my AEGEA wing construction. My AEGEA wing uses Foamular 600 for the entire wing. This is equivalent to Hi Load 60 and almost as good as Spyder Foam.


I'm curious why you use Foamular 600 rather than Hi-Load 60. Cost? Availability?

Here are the relevant numbers that you measured with the 3-point bending tests:

foam rho E33 E33/rho
type (lb/ft^3) (psi) (in/10^6)
---------- ------- ------ -------- ------------
Hi Load 60a 2.30 6212 4.67
Hi Load 60b 2.30 6412 4.82

Foamular 600a 3.04 5108 2.90
Foamular 600b 3.04 5251 2.98


Hi-Load has 1.2x the stiffness but only 0.75x the weight of Foamular 600.
Seems like Hi-Load is the obvious choice.

Here's the data for all common foams:
http://groups.yahoo.com/group/Allegro-Lite/files/Skin_Stress/
file Foam_modulus2.txt

[markdrela]

If, however ,you don't need or want a 3.? meter model that is that light but you would rather have a model that is faster and easier to build then you might want to try a carbon tube wing.

I think there's a number of mods one can do to lighten a Phil-type wing, without incurring too much extra work:

1) Hi-Load 60 rather than Foamular 600
2) Less carbon and more Kevlar. I'd move the joint forward to maybe 50% at the center, and maybe 20% at the extreme tip.
3) Weblets on the upper surface to stiffen skin against buckling. Space them maybe every 1". The narrower carbon will then suffice.
3) Doubled Kevlar only over the front half of each flap or aileron.
4) 1.0 oz Kevlar replaces 1.7 oz Kevlar on the outermost parts (see Supra wing).

I'd also reduce the sweep somewhat -- something closer to the Supra. This will reduce the washout that appears on a swept flexing wing.

[Sled Driver]

I think you may have missed this on my initial posting about the details of my AEGEA wing construction. My AEGEA wing uses Foamular 600 for the entire wing. This is equivalent to Hi Load 60 and almost as good as Spyder Foam.

Used a value of 5,000 psi for Fomular 600 in the calculations. I use a modulus value of 5,000 psi for Foamular 600, a modulus value 6,000 psi for HiLoad 60 and a modulus value of 6,500 for Spyder foam when calculating compressive allowable stress for a stressed skin. These values are from the Mark's table posted at the Yahoo Groups Allegro-Lite website. I also intuitively find Foamular 600 "to feel a little softer and more flexible" when handling it.

[Sled Driver]

2) Less carbon and more Kevlar. I'd move the joint forward to maybe 50% at the center, and maybe 20% at the extreme tip.

Phil,

Working on this premise is how I originally got into creating this spreadsheet. The idea was that if I only applied carbon in the form of "Darts" centered over the highpoint of the wing, I could save weight and without having to build and "grout" into the wing what many of us have come to affectionately call the "Drela spar". If you look at a numerical integration of the area moment of inertia for a typical airfoil, the result that jumps out is that carbon applied to the thickest 40% of the wing chord takes most, over two-thirds, of the bending load.

I suspect that application of this principle resulted in the design of Frank's Millennium wing spars.

Jay

[Phil Barnes]

I think there's a number of mods one can do to lighten a Phil-type wing, without incurring too much extra work:

1) Hi-Load 60 rather than Foamular 600
I was actually still under the impression that Hi Load 60 and Foamular 600 were equivalent. I was wrong about that. The current batch of Foamular 600 that I have weighs 2.72 lbs/cubic foot. I still prefer the pink foam because it produces fewer whiskers when hot wiring it and may warp less when cut.

The remainder of Mark's points are also valid but mostly for non production wings. They are some things that can be done to inprove the strength to weight ratio of the wing without incurring too much extra work and are steps that are somewhat short of a full blown Drela wing.
2) Less carbon and more Kevlar. I'd move the joint forward to maybe 50% at the center, and maybe 20% at the extreme tip.
For production, I'll keep a constant width carbon piece on the center section. It is faster and easier to cut and layup non taperered carbon pieces. For the tip, there are also considerations of what happens with wing tip landings. I think you want more carbon at the tip than you strictly need for air loads.
3) Weblets on the upper surface to stiffen skin against buckling. Space them maybe every 1". The narrower carbon will then suffice.
The weblets are something I need to try even for production. I'll post something more about weblets later.
3) Doubled Kevlar only over the front half of each flap or aileron.
For production, the full doubled kevlar aft of the carbon has a number of advantages; It is far easier to cut and layup than separate small doublers. It is difficult to accurately place doublers, especially semi chord doublers. Cosmetics are more important on production wings. Doubled kevlar that buts right up to the carbon skin produces no bump or visible line in the finish surface. Doublers are very "noisy" and leave obvious lines. All I do is stick two layers of Kevlar together with 3M77 and then cut the pieces out and lay them up as if they were a single piece of fabric.
4) 1.0 oz Kevlar replaces 1.7 oz Kevlar on the outermost parts (see Supra wing).
Clearly not a production option but something to consider for personal airpalnes.

[Phil Barnes]

I'll first describe what I think I know about weblets. Much of what follows I learned from Mark Drela and the weblets idea is also Mark's;

The failure mode of a stressed skin wing will almost always be a buckling of the top skin when it is subjected to compression loads. The skin can either buckle inwards by compressing the underlying foam core or buckle outwards by either debonding from the core or stretching the foam. Debonding can be prevented by carefull use of epoxy and avoiding dry spots in the layup. Use of stiffer foam will greatly improve the wing's strength by better resisting either compression or stretching of the foam core and thereby preventing the top skin from buckling.

The above is why we use higher strength foam for foam cores in highly stressed areas of the wing. There are such things as foams that are even stiffer than Hi Load 60 or spyder foam (such as Hi Load 100 or Foamular 1000) but current thought is that these are too difficult to hot wire and/or too heavy. Another way of preventing the foam core from compressing or stretching is the use of weblets. Weblets are a way of stiffening the foam with very little addition of weight.

Weblets are just thin strips of material (carbon?) about .007" X 1/8". You just make slits in the foam core with a razor blade or knife and insert the weblets into the slits.

Now some questions for Mark;

Should I think of the weblets as "stiffening the foam"? In that case I would think they would need to be bonded well to the foam and should be spaced more closely than one inch. Should I think of the weblets as being bonded to the skin and forming little T shaped pieces of skin that would resist buckling much better than a flat plate skin? In this case I would worry about there being such a small bond surface between the weblet and the skin. Also intuition would tell me that the weblets should be spaced about 1/4" or 1/2" apart to be effective. Can you help me in getting a better understanding of how the weblets work?

What material should the weblets be made out of? I imagine making cured sheets of fabric and then cutting into strips. The problem is that I don't understand how the weblets are functioning structurally and so don't what material (carbon or fiberglass) to use or the proper fiber orientation.

I think that the weblet idea may be the next big improvement in production wings. I just need to get a clear picture of how they work before attempting a test wing. If they do work they may allow wings with just a minimal tube that would act as a joiner receptacle and as "carbon tube insurance". They might also allow a skin with no doublers which would save time and look better.

Thanks for your time and great ideas, Mark

Phil

[bigjohn]

I would like to hear Mark's ideas as well. But my own idea, with the recent availability of micro-sizes of carbon rod & tube would be to zap grooves into the foam core with the hot wire and lay in a rod or tube that would then be bonded to the skin during bagging. The grooves could be precisely sized using a cutting template. This is assuming that the 'weblet' is essentially a stringer for anti-buckling purposes. Not necessarily the best strength to weight, but maybe a compromise between little slices and extra layers.

[markdrela]

the weblets idea is also Mark's


Actually, I stole the weblet idea from Chris Kaiser's Kahu DLG that Joel Foner posted at the CRRC site.

The weblets primarily stiffen the skin, although the foam is certainly still very significant. The foam has several functions. One function is to stabilize the skin+weblet structure against the usual inward or outward buckling. Another function is to hold the weblets perpendicular to the skin. WIthout the foam, the weblets themselves would buckle and flop over when loaded in compression.

Theoretically, the effect of the weblets is huge. I calculated that they quadruple the buckling load of the cap spars on the SuperGee-2. Pretty good deal for only 2g added to the glider. One weblet is plenty adequate for the 0.014" carbon caps on the SG2.

The 1" spacing for the Phil-type wing is a gut-feel guess. You want as few weblets as possible for simplicity, but they must be spaced close enough so that the skin between them can't buckle at too low a load. The weblets have some chordwise reach via the skin's small but significant chordwise stiffness. I think the only way to rigorously find out the ideal spacing is to make a few wing samples and bust them with a 3-point load setup.

The weblets should be bonded well to the skin. Their bond to the foam is much less important, and may not be necessary. When putting the weblets into the slice in the foam, I don't push it down flush with the foam, but leave it protruding slightly. A good way to do this is to insert the weblet into the slot, place a strip of mylar along the weblet, and push it down flush with the mylar. This leaves is 0.014" above the foam surface. The weblets get pressed down flush by the mylar+skin under vacuum, which then ensures a solid contact between the weblet and skin. Going over the protruding weblet edges lightly with the epoxy roller before placing down the mylar would further help ensure a good bond.



What material should the weblets be made out of? I imagine making cured sheets of fabric and then cutting into strips.


Unidirectional CF is what you want, maybe with 0.75 glass to hold it together if splitting is a problem. I think Chris Kaiser laid up 2.9 oz Uniweb and sliced that up. I used CST's precured 0.007" or 0.005" CF. You want the edges very straight to ensure good contact with the skin.

[markdrela]

with the recent availability of micro-sizes of carbon rod & tube would be to zap grooves into the foam core with the hot wire and lay in a rod or tube that would then be bonded to the skin during bagging.

The flat weblet is vastly more efficient than a round rod. For example, a 0.1" x 0.005" weblet has the same weight as a 0.025" diameter rod, but the weblet is 20x stiffer in vertical bending. However, if you want to also add overall wing bending stiffness because the skin is marginal, then the rods make more sense.

Making the slices for the weblets is very easy. I attach a soft balsa depth stop to a single edge blade, and just slice into the core along a straightedge. A single-edge blade makes a groove just about the right thickness for 0.005" carbon.

[chlee]

I've learned a tremendous amount from this thread. If you don't mind another question, Prof. Drela:

You said that the weblet-skin bond is crucial. But what do you do about the discontinuous surface at panel breaks?

If I use a single CF strip for a half-span, the weblet will probably have a radius at each panel break where it can't adhere to the skin. Is this o.k.? (Or, are you using separate weblets for each panel?)

Thanks again,
Chung

[markdrela]

You said that the weblet-skin bond is crucial. But what do you do about the discontinuous surface at panel breaks?


I think that using the Mylar or the sparcap to press down the weblet will give a minimal gap at the slope discontinuity at the panel break. On the SG2, I coated the weblets with epoxy, pushed them into the slot, and glued on the sparcaps, all in one operation. There was enough epoxy on the weblets to fill any such small gap. On the SG1, I put a bit of extra epoxy on the weblet at the panel break to ensure that the gap would be filled. May be overkill if the caps are a bit on the wet side.

[chlee]

Aha. I won't worry too much about this gap, then.
Thanks,
Chung

[Ricardo RW]

Is there any place to see a photo or a drawing showing a weblet? I'd like to see about what are you talking about.

Regards.

Ricardo.

[Sled Driver]

Is there any place to see a photo or a drawing showing a weblet? I'd like to see about what are you talking about.

The first two photos below are the weblet installed in "wingeron" slope wing under construction. The second the third photo show the slot being cut into the foam wing core and last photo show top and bottom weblets in a DLG wing core.

There are more photos at:

http://www.rchomepage.com/~jdecker/gallery/rotor

http://www.rchomepage.com/~jdecker/gallery/wing01

Jay Decker
Kennewick, WA

[Ricardo RW]

Jay,

Thanks,... I was very confused, thought weblets were winglets... :o :o
Now I'm off my ignorance. :)

Thanx again.

Regards.

[Ben Roberto]

Working on this premise is how I originally got into creating this spreadsheet. The idea was that if I only applied carbon in the form of "Darts" centered over the highpoint of the wing...
Jay

Jay,

Can your spreadsheet calculate the effect of weblets?

-- Ben

[GaryO]

Sled driver,

Phil Barnes has posted the layup of his latest wing, which now incorporates a .060cf spar, bonded to the top layer and the cf skin. Also some other changes in the layup.

Would you consider running the new layup thru your spreadsheet and share the results? I'm just about starting a build and would like to see what the numbers are.

Gary

[Sled Driver]

Sled driver,

Phil Barnes has posted the layup of his latest wing, which now incorporates a .060cf spar, bonded to the top layer and the cf skin. Also some other changes in the layup.

Would you consider running the new layup thru your spreadsheet and share the results? I'm just about starting a build and would like to see what the numbers are.

Gary

Hi Gary,

I'd consider doing that... is the information posted anywhere?

Jay

[GaryO]

Jay,

Should have included this link: http://www.rcgroups.com/forums/showthread.php?t=412772

In the sailplane forum.

Gary

[Phil Barnes]

Gary

Check your email. I sent you a private message through RCgroups......unless I screwed up. I think I sent it to your private email rather than the internal RCgroups email. Its too long to rewrite.