Don Wells,

The thread started about adding carbon to Sky Bench kits and the need for carbon added to the spars based on how the plane was going to be launched determined if the kit actually needed the addition of carbon. The kit ribs are designed with 1/8 deep spar notches, the launching method determines if the builder should waste time and money adding carbon. Half the guys building my kits these days are installing e power, no carbon needed. So the kits will remain with 1/8 deep spar notches in the ribs. If the builder decides to add carbon, the next question he faces is what thickness of carbon.

Based on how Steve built his OLY lll using .014 and the way he describe his robust launching from an electric winch, my application of .060 thick carbon is over kill. My experience using the .060 carbon and zoom launching from very strong winches tells me that epoxying the .060 carbon to the vertical grain 3/8 wide block shear webs and capping them with plywood spacers to fill the gap is the quickest way to add carbon to my kits and obviously more than adequately strong.

I'm not addressing the optimum method of scratch building a spar, I am trying to be helpful to the builders of my kits. Using the .060 thick carbon and 1/16 thick plywood spacer system as I describe in my post is the quickest and strongest way to add carbon to my Sky Bench kits.

Ray LSF 803
Sky Bench ... Woodys Forever
http://www.skybench.com

Thanks Ray, understand about the top and bottom of the spar. My question was about CF on the sides of the spar as you asked about the value of CF on all four side, unless I misunderstood. There is not a doubt in my mind that, out of the box, stock, D-Tube constrution of your kits with spruce caps is stronger than heck when assembled like Sean says, "no gap, well fitted parts".

I have to agree with Owl though about the difference in adhesion of CF to wood vs. wood to wood. If I were to go with CF strips directly on top of the shears, I would wrap them. Actually, If I were to build another O3, I would not CF cap the spars.

Some Notes:
1) The carbon should be on the outside of the spar caps. A spar's stiffness and strength is proportional to the square of its depth; you can lose 25% or so of the spar's load capacity by placing the carbon against the shear webs as opposed to placing it on top of the standard spruce caps.

2) It should be tapered in thickness. Loads reduce with the square of the span; there is little need for full thickness carbon all the way out. And, there may be some stress-concentration advantage for the reduced stiffness of the outer portion of the wing panels.

3) The wooden spar caps do continue to share the loads, so, spruce is better than balsa or other stuff.

4) The top carbon should be twice the thickness of the bottom since, like other spar materials, it has twice the strength in tension as it does in compression.

All this has been covered or referred to in RC Groups. Do a search for Dr. Mark Drela's comments (mdrela, I think). Even better, search for Oliver Wilson's comments (Ollie, I know).

Ollie makes the most lucid and useful arguments about carbon spar reinforcements I have read. He has much more to say that is useful to sailplane builders and you could do worse than read all of them. Unfortunately Ollie suffered a stroke a few years ago that left him with aphasia, a terrible blow for such a clear thinking and talented engineer, modeler and writer.

Finally, Ray Hayes is the authority regarding the capabilities of his designs. He is informed, experienced and correct. His planes do not need carbon spar reinforcement for anything we can reasonably expect them to do. Save your money and spend the extra time making sure the spar caps are bonded to the shear webs with no voids in the glue.

BTW: If you do use carbon spar caps, use tapered ones (0.060 to 0.014" top and 0.030 to 0.007" bottom). Glue them to the 1/8" standard spar caps. Then, sand the assemblies so that they are 1/8" thick for their entire length. You'll then have spar cap assemblies that are of uniform thickness for easy installation with all the strength of carbon spar caps -- which you don't need nohow ;o)

Joe

Originally Posted by Joe Minton

Some Notes:
1) The carbon should be on the outside of the spar caps.

Don's explanation also backs this up.


2) It should be tapered in thickness.

Maybe so, but it is a Royal PITA in my opinion and I'll suffer the extra grams. I know, a true aficionado, craftsman, and competitor would do so, I'll pass.


3) The wooden spar caps do continue to share the loads, so, spruce is better than balsa or other stuff.

Besides I have the spruce.


4) The top carbon should be twice the thickness of the bottom since, l

Check

All this has been covered or referred to in RC Groups. Do a search for Dr. Mark Drela's comments (mdrela, I think). Even better, search for Oliver Wilson's comments (Ollie, I know).

Agreed and I have read their posts. But not every tid bit of wisdom is gathered in once place or thread. So sometimes locality of reference makes find information easier. I wish I had the bandwidth to start a woodie FAQ. FINDING information in the fire hose that is RCGROUPS can be daunting.


Finally, Ray Hayes is the authority regarding the capabilities of his designs. He is informed, experienced and correct. His planes do not need carbon spar reinforcement for anything we can reasonably expect them to do.

Agreed. But then you've not seen me launch. Given the low cost of this "insurance" I think the extra 3 or 4oz, if that, are worth it. Especially if you look at the cost in time of rebuilding. I have enough rebuilding practice thanks to my landings!


Joe

Thanks for the excellent info! Jeff

To whomever asked. When I was referring to the shear webbing I am referring to the Houston Hawk style. The (normally) vertical grain wood that the lower spar cap is glued to the bottom of and upper spar cap is glued to (and rests on) the top of. I am not sure if there is a terminology difference to the wood used to make 'box' style spar cross sections.

I've been following this discussion from the sidelines, but the last comment prompted me to chime in. Regarding the "box" style spar cross section, I was strongly cautioned against this at one time. After some reflection, the reason why seems rather obvious....

In a traditional "I-beam" cross section, the vertical shear webs provide support for the upper spar cap against compression failure. It is the vertical grain that gives it the necessary strength to do this. That's why you don't see horizontal-grain shear webs. They would not offer the needed strength.

Looking at a "box" cross section, with the webs glued to the outside of the spar caps, the only thing preventing the spar cap from buckling is a thin layer of glue.... which probably won't hold up too well under a high load. One exception to this that I have seen work successfully is making a box spar at the root, carefully wrapped with kevlar tow, and used as a receptacle for a wing joiner, where, if the fit of the joiner is sufficiently snug, allows the joiner to effectively become the shear web.

Perhaps the easiest way to learn about how to do 'proper' wing spars is to go through the various threads about the Bubble Dancer by Mark Drela. I cannot think of a better source regarding contemporary woody sailplane design. Although, in fact -- there is little wood in the thing beyond ribs, shear webs and the bits at the back. LOL

Dr. Drela states that the Dancer can be pointed straight down, that it will reach 65 MPH by doing so, then -- one can give full up elevator with no worries.

One Bubble Dancer (BD) builder visited our club with his beautiful example. He proceeded to break our new 300# winch line twice. That winch and line had destroyed three other well-built planes.

LVsoaring: You're right, Box spars are weak. They depend on glue and no one has ever built a bridge out of that stuff.
It surprised me to learn that, according to Dr. Drela, horizontally-grained shear webs aren't all that bad but are more dependent on the wood's density and grain consistency than our traditional vertical-grained webs. He stated that vertical shear webs can also fail in shear along the grain. He further stated that the strongest way to do it would be to make each shear web section in two (vertical) layers such that the grain of each layer would form a 45-degree angle between the upper and lower spar caps and a 90-degree angle between themselves. It is notable that he didn't go to all that trouble when he designed the BD and it is the strongest piece of wood in the air.

gdjsky01: I hear ya' man! Three or four ounces and an extra four or five hours isn't much compared to breaking your plane that cost hundreds of hours and dollars -- plus doing it in front of your friends who probably bought moldies that don't break. About the only way to destroy those things is lay them across a pair of power lines. I've seen that and it ain't purty -- good light show though.

;o)

Mt way od doing shears is to use aircraft ply that is about half the thickness of the called for balsa. I cut them so the grain runs at 45 degrees to the spars. I've yet to have a wing built this way let go on me during a launch. YEs, the wings do come out a bit heavier, but the added strength is well worth it. If you use a good wood glue you can keep the added weight to a minimum.

Joe....Don't tell me thqat was you that caused that blackout in newbury Park. ;O) I do remember someone around here telling of a Nova that hit some high power lines and became a super nova.

NOTE TO SELF: Try not to answer posts to the threads while in the work truck on a laptop! My typing is bad enough as it is.

Originally Posted by Joe Minton

..... He further stated that the strongest way to do it would be to make each shear web section in two (vertical) layers such that the grain of each layer would form a 45-degree angle between the upper and lower spar caps and a 90-degree angle between themselves. .....;o)

I remember reading that, and as a matter of fact, the CF shear webs that CST has are on the 45 bias. http://www.cstsales.com/carbon_shear_web.html

Wasn't me. I dont own a moldy, cant afford one. Likely though it was one of the TOSS (Thousand Oaks Soaring Society) members who made the papers.

TOSS flies out of a school yard which has a set of power lines along its eastern edge. They (I am no longer a member) have had several encounters with that set of power lines.

TOSS can only use the school yard on Sunday. For six hours, 8am till 2pm, the yard is theirs; no one else can use it. They've had this privilege since, I think, 1968. The rule is that they cannot fly anything with a propeller. The neighbors across the street seem to have liked the idea since there are no screaming children for a few hours each week.

However, those next to the field on the East side have had the occasional issue. It seems that the power lines feed electricity to a local church. Said church uses lights so everyone can see (they are short on windows), also, their minister preaches through a wireless speaker system -- no electricity, dark room and no one can hear the preacher. You can see where this is leading.

Well: on three occasions, someone has landed a carbon fiber moldy across the power lines and blown out a couple of square blocks of the power grid. No football and no preacher. The folks were pretty tolerant the first two times, after all is was Sunday, until -- a fellow was standing in his back yard when a carbon moldy landed across the power lines in front of him. There was, I am told, a great flash of light and a cracking burst of sound. After which, flaming wreckage fell at his feet; in his back yard; on Sunday. That might have been okay except that he, for a moment, thought that Armageddon had truly arrived and that he had not been ready for it.

When the truth dawned, this citizen was angry, very angry. He and some members of the church filed multiple complaints with the authorities, both on Earth and elsewhere. TOSS nearly lost their field.

They no longer fly over the power lines to the east and station a geezer with one of those can-powered horns to warn off anyone who might be about to pass over the lines. It was a close call -- I did not know that it made the news.

Joe

Guys, here is the situation-we are talking about the Skybench Bird series and specifically the Big Bird XL 110" span. The way the wing goes together you lay down the leading and trailing edge sheets. You use ribs to locate the sheeting and spar in their notches, then glue the spar to the leading sheet first. There is no easy way to build the spar and rib assembly separate from the bottom sheeting that I can see in order to wrap the spar with Kevlar. You could use a needle and thread through the sheeting, but seems like it would make mess. So what you are face with it deciding to place the CF on the outside where there will more strength or the inside against the vertical webs, then fill with wood. I think the logic is the carbon will be retained better being captured against the webs. Ray has been successful with this method. Question is will it come loose if not wrapped on the outside? Will placing toward the inside be stronger than no CF at all. Those are the two questions to decide on this particular plane. They are already strong with no carbon. I would be interested in Mr Stackhouse's view because he really understand the engineering behind all this. Then maybe we move on to something else. These are great kits and they fly awesome so I don't want to give anyone the wrong idea.

All one has to do to wrap that spar is to build the wing on the spar that has been spaced up 1/16". This assembly is lifted off the plan, wrapped, and glued down to the leading edge sheeting once wrapping is complete. I generally sand the sheet in the wrapped area to prevent the extra thickness from creating a bulge, and to keep the wraps from holding the ribs off the sheeting. If you are paranoid about straightness, like me, then build it against a straight edge. I build this way up against a 1/2 X 3 X 60" piece of 2024T3 aluminum.

If you are going to use tapered cf, glued to spruce, figure out where each layer ends. They are typically .007 thick. Fiddle with your jack plane a little until you are cutting .007 deep... NOT that hard to do... and make passes beginning where each layer begins. Your spar cap will be the same thickness as the original spruce, and all will glue up perfectly. With that method, you can put the cf on the outside where it best does it's work.

This is good stuff, I am very much enjoying learning these new techniques. Guess we have to drill deep. Couldn't be this good anywhere else.

Originally Posted by CloudSniffer

This is good stuff, I am very much enjoying learning these new techniques. Guess we have to drill deep. Couldn't be this good anywhere else.

True dat.

I love this stuff.

Woodies are just too much fun. (Watch it guys). I mean, every aspect of RC soaring is worth a lifetime of learning but woodies have endless "tweak" possibilities. I just knocked off a few grams on a HL project that already has been proven to be nose heavy without lead. I'll lay in bed tonight figuring out how to shave the weight I have "in the bank".

Originally Posted by seanpcola

True dat.

I love this stuff.

Woodies are just too much fun. (Watch it guys). I mean, every aspect of RC soaring is worth a lifetime of learning but woodies have endless "tweak" possibilities. I just knocked off a few grams on a HL project that already has been proven to be nose heavy without lead. I'll lay in bed tonight figuring out how to shave the weight I have "in the bank".

If it has a noseblock on it, Sean, just get out the razor saw and starting trimming the front of the block away. (Don't cut until it's balanced, though, because you want to leave yourself room to sand an aesthetic shape onto what remains.)

Nightowl

This is all good stuff to read, I haven't read a thing that wasn't without merit in being proven or sensibly plausable. My 0.014" caps were all outside on my 142" Oly3, and that glider took at least a couple of hundred launches often into a stiff sea breeze often using a high spec Hi-start using braid line- not a mellow experience like stretchy mono line. But on my somewhat souped up 2m Lil Bird the caps I applied to the outside on the bottom spar and inside (shearweb side) on the top spruce spar. In addition the inner panels were fully sheeted on that one, the glider would take balls to the wall F3J winch launches and full stretch 3m histart launches into 12 knots of wind with frenzied aplomb, but there was a bit more to the wings build than only it's 0.014" CF caps. The strength of any sailplane is dependant on a whole chain of build factors that depend on its builder rather than the kit they recieve and in retrospect, they are all subtle accumulative things that add up to something much greater when it comes to the moment of high stress, and she either holds up or doesn't and that's the $64000 question in autopsy.

A good list of broken RC aircraft behind me when I think about it, attributed to stress.