Sheeting: Balsa or Glass or Ply or Obechi?

[green66]

Hi all,

I'm doing my first foam-cored own-design, and wonder if anyone can summarize the pros and cons of the various sheeting methods (balsa vs. 1/64" ply vs. glass vs. obechi vs. ?), in terms fabrication, weight, and structural issues.

As an aside, what weights of glass cloth are typically used on various components, e.g. wing, fuse forward section, fuse boom section, tail feathers, etc?

Any advice much appreciated!

[Ollie]

Wings have to withstand three kinds of forces: torsional, bending and drag. Drag forces are seldom a problem. The type of foam core and and skin must complement each other for best results.

First let's consider balsa. Balsa is highly variable in strength and density with densities fro 4 to 24 pounds per cubic foot. It is available in a wide range of practical thicknesses. Its strength is roughly proportional to its density. It gives very good strength to weight ratios for low aspect ratio wings with thick airfoils and when used with beaded foam cores. To improve the strength to weight ratio use dense balsa on top and over the thickest part of the airfoil and light balsa elsewhere. It is difficult to get a strong, thin trailing edge with balsa.

Obeche is only readily available in one or two thicknesses. It is about as dense as the densest balsa. It is easily split and requires careful handling. It is suitable for use with beaded foam. For aspect ratios above eight or so and airfoil thicknesses below about 12 % a supplimental spar system is needed to carry the bending loads associated with vigorous launches.

Ply skins are like the other wood skins but with poorer bending strength to weight ratios. their only advantages are dent resistance and torsional rigidity.

Wood skins can be successfully pressed without resorting to vacuum bagging.

Fiberglass-epoxy skins are best with higher density closed cell foams and thin airfoils. When the cloth is laid on with the fibers at 45 degrees to the spanwise direction it gives much better torsional rigidity than wood skins. Glass cloth is availabel in weights of .75, 1.5, 2 and 3 ounces per square yard. The skin weight can be varied along the span by using one layer over all, two layers out to 2/3 the semi-span and three layers over the inner 1/3. The weight of cloth can be taylored to the size and purpose of the model. Vacuum bagging is the most practical way of applying glass-epoxy skins. Some supplemental spar system is almost always necessary with glass skins.

Kevlar-epoxy skins are like glass-epoxy skins except they have much better strength to weight ratios. Kevlar is much, much more expensive than glass and it is much, much harder to work with. It has little strength in compresion and almost always requires a supplemental spar system for bending loads.

Glass-epoxy-unicarbon skins are the simplest to construct and give very good strength to weight ratios without the need for supplemental spars to carry the bending loads. They are my personal favorite except where the application is most demanding.

Rules of thumb will get you only so far. Beyond that engineering methods must be employed to determine the minimum weight of materials required for the strength required in a specific application.

The first priority is to make the wing strong enough to cary flight and launch loads. The second and third priorities are to make it easy enough that you are willing to build it and light enough that it flies well.

BTW, the wing structures with the best strength to weight ratios that I know of, are Dr. Drela's, using conventional balsa ribs and sheeting with tapered unidirectional carbon spar caps. These designs have been meticuously engineered and are most worthy of study.

[green66]

Thanks a bunch Ollie, xlnt very informative response!

Question re finishing glass - Is the cloth texture sanded out or is some sort of filler applied before sanding? Although I'll probably do a balsa-sheeted wing, I'll do a glass fuse via Lost Foam technique, and would like to avoid show-thru of the cloth weave. How is this done?

Rules of thumb will get you only so far. Beyond that engineering methods must be employed to determine the minimum weight of materials required for the strength required in a specific application Yes, I'd like to run some numbers but, without advanced methods, FEA, and some empirical factors thrown in, seems that even a qualitative assessment of stress, strain, and weight issues is futile when considering such a complex structure ==> A highly non-axisymmetric thin-walled shell loaded in combined bending and torsion, whose primary failure mode is compressive buckling on one side, where the shell is supported by a soft interior - Yuk! :eek: I'm tempted to build some sheeted cores, then destructively test them just to get an idea of their load capability.

[Ollie]

A vacum bagged, stressed skin over foam wing doesn't necessarily need finishing in the traditional sense. Here is how it is done:
1. Cut the wing cores from stiff, strong, closed-cell foam like Spyder foam.
2. Cut two 14 mil thick mylar molding sheets to the planform of the wing panel but with 1/4 inch extra chord.
3. Hinge the mylar molding sheets together at the trailaing edge with tape.
4. Lay the open book of mylar on the bench with the tape side down and wax and buff the mylar surface. Use a silicone free wax like Johnson's paste floor wax.
5. Spray paint the waxed surface. To avoid fish eye on the waxed surface apply several light dusting coats and allow to dry before applying a wet coat.
6. Apply four bias cut strips of 1.4 ounce (0.002 inch thick) fiberglass cloth around the leading edge of the core with laminating epoxy. Make the strips of varying width form one to two inches wide.
7. Lay down two plies of 1.4 ounce fiberglass cloth in epoxy on the painted surface of the open mylar molding sheets. Orient the fibers at 45 degrees to the spanwise direction.
8. Lay down two long triangular gores of 2.9 ounce uniweb carbon (0.005 in. thick) in epoxy to the mylar molding sheets. The gores are oriented with the fibers spanwise and placed so that the wide end is at the root and the pointed end at the tip. The chordwise placement of the gores is such that they will fall at the thickest part of the airfoil when the mylar, with its layup, is folded around the foam core.
9. Blot off the excess epoxy with paper towels.
10. Lay the foam core over one side of the mylar with its layup. Position the core chordwise so that the hinge line falls about 3/8 inch behind the trailing edge of the core and the leading edge of the mylar falls about 1/8 inch back from the leading edge of the core.
11. Fold the mylar over the core and place the loose sandwich of mylar, layup, core, layup, mylar into a vacuum bag with paper towel air bleeders.
12. Seal the bag and place the shucks from the foam cores over and under the bag. Align the shucks to the work in the bag and weight the shucks sown to a flat surface to hold everything flat and in alignment.
13. Apply the vacuum for two days while the epoxy cures.
14. Open the bag. Strip off the paper towel air bleeder and peal off the mylar molding sheets leaving a perfectly smooth, waxed pain finish on the wing panel except at the leading edge.
15. Trim the 0.008 thick trailing edge to within 1/8 inch of the foam core.
16. Carefully sand off the flash at the leading edge where the paint and mylar did not cover.
17. Mask and paint the leading edge or cover it with a strip of sticky monocoat trim.

The whole process has about 2 hours of direct labor not counting drying and curing time.

I tested a 2-meter wing with this construction to distruction. The chord was 9.5 inches. The airfoil was the SD7037 (9.2% thick). The top and bottom carbon gores were 3 inches wide at the root tapering to zero at the tips with two plies on top and one on the bottom. The secon top gore was only 18 inches long to the point. I supported the wing between two tables 36 inches apart and applied lead pigs to the center of the wing. The wing failed at a load between 83 and 90 pounds. That's a bending moment at the center of about 800 inch pounds. The model that the wing came form had a flying weight of two pounds four ounces. Therefore, the wing was capable of 40 positive G maneuvers. The wing weighed 16 ounces and had 612 square inches of area.

Re lost foam fuselage pods: I use from 12 to 24 plies of 1.4 or 2 ounce glass cloth for a bullet proof pod. The last layer or two are sacrificed to wet sanding to achieve a smooth paintable surface without fillers. Three or four layers of 1.7 ounce kevlar will do a lighter and tougher pod but fillers are required because of kevlar's poor sanding qualities.

[green66]

Ollie,

Great stuff - the kind of info that isn't otherwise readily obtainable. Goes w/o saying that your experience is a genuine asset on these boards (and at rcuniverse.com), which I'm sure many, especially those newer to the hobby, appreciate. Thanks again!

[Duke58]

Thanks Ollie , you have given me answers to a few questions about Vacuum Bagging also.Perhaps I could plead for a bit more.

I have several old Scale glider kits that I want to build and I'd like to improve on the wings so as to make them more durable and flyable. I thought of building new with something over foam or maybe using the build up wings but completely covering them with sheet and then a light glass over to get a smooth tough finish . I have done some vacuum bagging but not on a build up wing , Any thoughts?

Sorry Green66, I'm not really trying to Hijack your thread but this was just the info I was looking for and well , ya gotta do what ya gotta do ;)

Earl

[Ollie]

Earl,

It is not practical to try to vacuum bag a built up wing because the force of vacuum required to do any good will flatten the structure. Even beaded foam can't take more than about 1/3 to 1/4 of an atmosphere without permanently deforming. Most closed cell foams are strong enough to not permanently deform under full atmospheric pressure. If you want to glass balsa sheet do it before you assemble the sheet to the ribs. In most cases you are better off covering balsa sheet with a light weight film like Ultracoat Lite or Oracover than with light glass cloth and epoxy. Also, light glass cloth in polyurathane varnish makes a tough surface that is lighter than the same cloth with epoxy.

I could offer a more specific opinion if you gave me the span, chord(s) and airfoil thicknesses you have in mind. Also, the intended use of the sailplanes such as thermal, slope, aero tow, etc. would help me form a better opinion.

[Duke58]

Ollie,
The pressure would have to be light ofcoarse , but the area between the ribs could be deformed, I've baged wings with light foam cores and balsa covering with good results (apx 6-7# on the gauge).

The reason I want to cover with glass is to make them resistant to Ridge Rash, the other planes I've built with open structure covered with film require constant repairs (Dried weeds are tough here ). The glass cloth and urathane sounds like the ticket or just the film over the balsa sheet. I may try to bag a build up wing though just to see how much it does deform.

I have an old Graupner Mini Nimbus kit with a 3.3m wing apx 8" chord and a Sterling Schweizer 1-34 kit (90" span)that has a Wortman airfoil . The 1-34 needs a more modern airfoil for flying slope so it is a candidate for a new foam wing , I'm just kicking ideas around now I may even have ribs cut and build up a wing with sheet covering for it . I like sport flying and I'm not interested in trying to go fast or winning contests , most of my flying is inland slope/Thermal.

Thanks
Earl

[Ollie]

Here are some things to consider. On a high aspect ratio wing like the Mini Nimbus it is critically important to keep the wing tips light to preserve maneuverability. The addition of a few grams of mass near the wing tips will cause a noticable degradation in the roll rate. This will adversely affect your ability to maneuver smartly in the landing approach. It will also reduce the plane's ability to signal lift. Inherently fragile planes like the Mini Nimbus are just not well suited for some slopes with rough landing areas.

If you do deside to glass the builtup wing, limit the glass to the extreme leading edge, use the lightest glass you can find and polyurathane it without a vacuum. Even the weight of paint at the wing tips is detrimental.

You are firmly stuck between conflicting objectives on this one. Use the Mini Nimbus for thermal flying and the 1-34 for the slope.

[Duke58]

"You are firmly stuck between conflicting objectives on this one"

How right you are, but when I here that hisss of a scale ship cranking through on a good day , wings flexing , clipping the grass and snaping down wind turns it GETS GOOD TO ME. Been doing it long enough to learn some keen rebuilding techniques too! Spoilers and/or flaps are very nice if you fly slope with 100" plus models , the Nimbus may end up with both.

Thanks
Earl