RE: Sheeting Foam wings
 

Bob, yeah...deal with it...I wasn't completely awake yet!

RE: Sheeting Foam wings
 

But it is 500 sq/in both sides of the wing. Maybe it is easier to understand if you visualise the wing suspended in mid air. The bag will collapse all round it, not just on one surface.

Gary,

Have we been here before? With a porous wing in the bag, all of the atmosphere is not removed with the vacuum I apply. If it was the foam would be crushed. If the bag is completely emptied of atmosphere then the total surface area of the bag would be subject to full atmospheric pressure. which is around 14.7#/sq.in. If there is nothing in the bag to "Store" atmosphere as it were the bag will probably collapse completely with a low vacuum setting.

Do you ever wonder why we do not implode? It is because we have atmosphere inside us pushing outwards, with the same pressure as the atmospheric pressure outside of us pushing inwards trying to squash us.

Ed S

RE: Sheeting Foam wings
 

It seems that Scorpion Racing and I are on the same wave length with vacuum bagging. We even use the same epoxy resin. We can prattle on about pressures and in which direction as much as we like.

All I can say is that my bagged wings come out of the bag light, straight, strong and stiff with no glue oozing out of anywhere.

Ed S

RE: Sheeting Foam wings
 

You are right Ed, but the table also pushes up with 2000# also.

I think we did this discussion two years ago.
I guess we are all big gas bags.

RE: Sheeting Foam wings
 

High Plains,

At last we are in agreement.

Ed S

RE: Sheeting Foam wings
 

I no longer sheet wings without vacuum. You can use less resin to get a good bond because the pressure (no matter how you calculate it) is dead ***** even over the entire surface.

RE: Sheeting Foam wings
 

It is actually amazing how little glue actually is used when bonding skins. I mix about 2-OZ of West systems for an entire wing. I squeegee it all off the skins until it is barely visable as wet. I will brush a little back on the leading edge and trailing edge CF and fiberglass(i like a really stiff trailing edge). It is amazing how much ends up back in the cup! To get a good bond I like to lightly sand the cores and skins smooth(vacumn/tack rag the dust). Some people spray the skins with hairspray to prevent the epoxy from penetrating too much and using even less glue.


I have seen many foam core wings post crash bonded from every method descibed in this thread... they all look the same in the can... and delamination is never a problem. Build your wing using whatever method fits your time and budget, go fly, and don't worry about it too much!

RE: Sheeting Foam wings
 

I was thinking there are some with a little too much atmosphere trapped inside. Don't get any on ya!:D

RE: Sheeting Foam wings
 

Guys I really appreciate this discussion even if it's old hat for some of you. It has had my mind working on how I can do this myself with what is available and I understand all the practices you have shared.
I came up with a question on the ride home tonight about what you guys put under the skin...some use fiberglass tape and others carbon tow. My question is are all of these laid in a straight tip to tip line like a spar? Does anyone or is there anything to gain from running an X across the wing for torsion stiffness? I look forward to your answers.:eek:

RE: Sheeting Foam wings
 

Vic, can't say that I built a lot, but the two or three old Formula one wings I built were carbon tow running spanwise, about the high point of the airfoil, between the sheeting and the cores.

And they were built without a vacuum pump, and it was my dumb thumbs that did them in, not wings folding or delaminating!

RE: Sheeting Foam wings
 

I have used .007'' x 1/2'' wide carbon strips for spars. I have heard others in the forums question the need for this, and I suspect that they may have a point, but have not had the guts to leave them out. Making an X may increase torsionsal stiffness, but the sheeting makes them plenty stiff enough.

If you use the CF spars, make a shallow groove for them to sit in, or you will get a 1/2'' wide bump running the length of the wing. What I do is make a sanding block that is about 2'' wide and has a 1/2'' wide strip of 80grit sandpaper glued to it. Use double sided tape to attach a straight edge to the core so that when you put the sanding block against it, the 1/2'' strip is positioned where you want the spar to go.

Now sand the entire span until the sandpaper cuts to the maximum depth.

RE: Sheeting Foam wings
 

If you put a carbon strip on your wing, put it on the bottom side of the wing.
Since racing stresses on the wing are mostly positive, the bottom is where you will
gain the strength. Imagine pushing a string and you can see that adding it
to the top is not much help. If you are sheeting the wing as 1 piece, full span,
most folks cut a v shaped piece of fiberglass cloth (or 2, with the first one bigger
than the 2nd) and put this between the core and skin. On the bigger one, make it
about 14" wide at the l.e. and 6" at the t.e.. Probably about 8 and 5" for the 2nd one.
Cut this cloth on the bias (so that the fibers are running 45 degrees to the span) to
get the most strength. If you have room for extra weight and want a really stiff
wing, add a layer of 3/4 ounce cloth, full span and chord, between the skin and core.
brad

RE: Sheeting Foam wings
 

Hey Vic, I use a strip of .007 x 1/2" wide 48" long carbon fiber on top & bottom at the max thickness of the wing as a spar. I also use 2 strips of 2oz fiberglass cloth 2" wide x 52" long and 1 strip of 2oz fiberglass cloth 1" wide x 52" long on the trailing edge to create a very stiff and sharp TE, and to stiffen the ailerons up some (The glass allows me to sand it sharper than balsa or plywood, and it is very stiff with the MGS resin.) . I also add a few pieces of 2 oz fiberglass cloth in the center section (under the balsa skins) as added insurance against flex and wing mounting, the same as most people do on top of the skins. I do all of this under the skins so that the outer balsa wood shell is smooth, as I like to leave the balsa natural looking and clear glass it with .5 oz cloth and just do some trim and lettering for decorations.

I think torsional stiffness is fine due to the core and balsa contact being constant. The only way it would flex would be if you cut out pieces of foam or the glue was missing in areas. I also had more flexable wings when I used poly glue or that ZAP laminating resin. West system was stiffer, but melted to foam a little bit more than I liked.

But as they say, ... your results may vary!

RE: Sheeting Foam wings
 

Excellent info! Thanks guys.

I thought you guys might get a good laugh out of this repair I did on my Predator a while back. A few pc of carbon, gorilla glue, balsa, and a fiberglass patch. It looked like the unibrain for a while but worked just fine.[sm=bananahead.gif]

RE: Sheeting Foam wings
 

Actualy this is not the case. For a wing to fold the top skin/spar has to compress. It is far more difficult to compress material than it is to stretch it and cause failure.

Ed S.

RE: Sheeting Foam wings
 

Correction...it's much "easier" to compress material than it is to stretch it.....

I can't say I ever remember seeing a wing fail from the bottom letting go, it's always been the top failing in compression.

RE: Sheeting Foam wings
 

This is usualy the case, but a strip of.007'' CF isn't really adequate to prevent buckling, which is the true culprit in this kind of failure. In my previous post I implied CF top and bottom, but I was thinking in terms of aerobatic aircraft where the loads are +/-. For a pylon wing, the bottom will see the tension load. The top sheeting is what is taking up the compressive load.

EDIT

as far a compressive vs. tensile strength, it depends on the material. Wood is generaly a little stronger in compression than in tension, Concrete is a lot stronger in compression than tension, which is why it is often ''pre-stressed'' with steel cables (which have very high tensile strength) to keep it in compression at all times.

As far as buckling goes, lay a .007'' CF ctrip on a flat table them try to push the ends together. It is hard to do , until it flexes up from the table just a little bit. At this point the load is no longer purely compressive, but bending stress is introduced, and buckling commences. Try this with a tube, and you will need a hydraulic press to get it to buckle.

RE: Sheeting Foam wings
 

Yes, which is why many people use 3/32" sheeting for the top skin of pylon racers. The thicker sheeting will do far more in compression than a strip of .007" carbon.

RE: Sheeting Foam wings
 

Gary,
You need some engineering knowledge. If a wing is about to fold but is caught before it does, inspection will show a crack along the bottom at the fracture point and no indication on the top.


Gary

Make up your mind. I agree, the thicker skin on top of the wing for the reasons stated. I also agree that .007 carbon is basically useless. However if somebody insists on using .007 carbon strip it also is more use on the top of the wing for the same reason as the thicker balsa.

If you are just trolling then have your fun.

Ed S

RE: Sheeting Foam wings
 

I probably wasn't clear enough in my comment, but I meant that it's easier to fold a wing in compression (top skin buckling) than in tension (bottom skin stretching and fracturing).

I am more of a business guy than an engineering guy, but I haven't seen many with a crack along the bottom (unless they came apart where the two halves were joined poorly). I have, however, seen several with signs of the top skin buckling prior to any other signs of potential failure. I've even seen many that partially folded but didn't come 100% apart. In each time, it seemed more like the bottom skin was mostly intact, and was what helped keep the wing from coming completely apart, while the top skin was buckled.

It all depends on the particular failure mode, your "engineering knowledge" should tell you that ;)

And going back to the original question on this thread......I still say that no, you don't need to use a vacuum bag to make a good wing. Does it help? Yes. Is it needed? No. For somebody new to racing, the $ for a bagging system would be better served by being used for practice fuel and glow plugs.

RE: Sheeting Foam wings
 

Well Gary I am a lot more engineering guy than business guy. 40 years of it, and you are absolutely correct. As the wing is being bent, top surface under compression load, at some point the shape of the wing begins to deform because the foam core is not able to maintain the shape. As this occurs the integrity of the bottom surface load path generally remains good but the top surface load path starts to loose the integrity of the load path and that surface starts seeing bending in the deformed area and fails in bending. That's the surface itself. What you called buckling. One of the things we can do to increase the load we can carry before the onset of severe deformation is to add carbon fiber to the upper surface. As usual I typically over design. Two 1 inch wide .014 bidirectional strips on top and one on the bottom.

Denis

RE: Sheeting Foam wings
 

With the world economy in it's present shape, I am shocked that anyone would claim business knowledge.

As to engineering knowledge, the more you know, the more you don't know. It's also known as Paralysis by Analysis.

The localized bending of the upper surface is usually caused by poor bonding to the core, though the foam compression can also allow the problem. There is a special type of foam available (spider foam) that has greater compression strength in one direction (it is an anisotropic material).

However a slightly thicker skin offers several advantages. Comparing 3/32 to 1/16" balsa:

With equal density, the thicker skin has 50% more compressive strength before failure - obvious to the casual obsever.

But more importantly, the stiffness of the balsa skin is proportional to the square of the thickness. So increasing the thickness by 50% increases it's resistance to bending by more than double.

Compressive strength in wood is generally directly proportional to density. So the top wing skins should have the dense heavy wood from the leading edge back the first third to half of the wing chord. With most sheets of balsa, the density varies from one side to the other. Try to place that higher density where it does the most good when planning out the wing skin.

With the 3/32 top skins, it is not necessary to use any carbon under the top skin. I did use the 1/2" by .007 on the bottom 1/16 skins, but only 36" in length since the loads drop off to nothing as you move away from the center.

Staggered glass (2 oz) works well as posted above. Having seen plenty of wing failures over several decades of racing, I always examined peoples failures when ever possible. A single layer of 6 oz glass on the center was quite common, with the failure at the transistion. Staggered layers of 2 oz almost never fail and is a lot easier to finish. If you know how to use wax paper, almost no sanding or filling required.

As to bonding the skins, the lightest method is to use 3M 465 acrylic tape. A quickie wing picks up 0.8 oz with this tape. Since I was having to add up to two oz of lead to reach 56 oz weight, saving weight is not all that critical with the newer rotund models of today's rules. As a side note, that was with 1/8" top skins and 3/32" bottom skins. 6 to 7# wood.

RE: Sheeting Foam wings
 

" the more you know the more you don't know"

It's a process of evolution. I have said often that when I graduated from St Louis University 50 years ago with a degree in Aeronautical engineering I knew almost everything about almost nothing.:eek: 40 years latter when I retired I knew almost nothing about almost everything.:D

Denis

RE: Sheeting Foam wings
 

Bob, there are a lot of people with business knowledge...unfortunately they're not the ones making the decisions regarding the economy.

RE: Sheeting Foam wings
 

I used 3M Acrylic tapes on a non-RC application a few years ago. The stuff failed miserably in shear. I did a peel test on it and was not impressed. We wound up using liquid C.A. Using a thin film of epoxy with Vacuum is plenty light, and I have never had a delamination.

There are a lot of people with buisness knowledge. That doesn't make them honest.