How do we create the unobtanium spar?

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NAES
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How do we create the unobtanium spar?

Post by NAES »

Well it looks like just about everything else in our planes is either fixable or replaceable except for the spar. When that breaks, major surgery or the trash can is inevitable.

I've been doing the 1/8" fg rod thing for a long time and am now ready to move on to the next latest and greatest. I did make one spar as a tester that truly was unreal. The problem was it weighed in at 1oz. per foot so it left a lot to be desired. Took a whole lot of punishment though.

Tufflight has a pretty good idea with their spar but man it takes a lot of work to get it done, very messy and cumbersome.

I'd love to hear some ideas, what choo got?
sgilkey
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Post by sgilkey »

Having built/flown several Predator Xs, their spar construction is really good, Ihave yet to break a spar in several punishing direct hits that have otherwise torn up the plane pretty well. I'd say their spare is nearly indestructible. The weak point of their construction, however, is how it is joined to the cores. For those not familiar with the Tufflight spar, it is a lamination of wood (i think it's bass) and plastic (I believe polycarbonate), capped with 1/8 f/g rods and wrapped with heavy thread along the whole length. The thread is the problem, I believe. It keeps the rods from bowing out in an impact, which is great. The solid spar keeps them from bowing inward. But the heavy thread makes a large gap between the spar assembly and the foam core, and if you don't use a lot of PU glue, you will not get a strong bond between the spar assy and the cores. This is where we messed up. So either use a lot of glue (and deal with the resulting ooze) or ditch the full thread wrap and just do it in local areas, say every 8 inches or so.

I think the 1/8 f/g rod spar setup in an EPS wing is very good, the biggest problems we have had are the spars breaking loose of the foam grooves (not a glue joint failure, but the foam itself fails), and the spars bowing inward and crushing the foam between the rods. I think a possible solution to this would be to make a 1/8 balsa or bass web that goes between the rods, cut the web to the root and tip thickness minus 1/4" (for the top and bottom spars), cut the cores in two pieces (front and rear), and either pre-build the spar asm, or do the whole wing asm in one step along with the cores. after the glue is cured, go back and sew or zip-tie the spars every 6-8 inches to keep them from bowing OUT.

BTW back to the TufFlight spar, the first one we built was messy and I CAd myself in so many places I had glue globs to eat off my skin for days afterward (yum). Subsequent ones went better once we got the hang of it.
THend
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Post by THend »

Sean, I don't know. I have actually changed my game a bit so I avoid more midairs, read as stay out of the Furball!
Lou Melancon
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Post by Lou Melancon »

Sean,
For several years we used hard materials in our leading edges. We have abandoned that in Georgia because we have found a better way. The hard leading edges transmitted the shock into the foam. Going to harder foam only made the problem worse.

Last year at Paris, we got some EPP/Blue foam cores from Roy Appleton. Since getting the cores from Roy we have switched to soft leading edges. Survivability is way up and wing write offs are way down. I don't think we will use hard foam leading edges again. The epp foam is covered with one to three strips of bi-di tape on the LE to prevent it from splitting. Wing to wing hits are almost always survivable.

We still use the 1/8" glass rods top and bottom as spars. We locate them about half an inch rearward in the blue foam. We have found that securing the rods every four inches or so with either carpet thread or tie wraps prevents the rods from popping out of the foam and gives a lot of additional strength. Also if the rods are secured you can put tape over the poly-urethane glue and keep on working.

Nothing really revolutionary, but the evolution of construction practices and changing to epp has worked really well for us.
mad
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Post by mad »

Sean, I use a method that doesn't rely on the glue to foam joint. which is your weakest link. Just drill a 1/8 inch hole from spar grove to spar grove about every inch down the legnth of the wing.
Then when you put your pu glue and spar in the glue expandes thru the holes and creates an I-beam. you no longer are relying on the glue to foam. I also wrap a piece of 1/4 inch strapping tape compleatly aroung the spar starting at the root on top and going out to the tip , under the bottom and back to where i started with a couple inch lap over.
I have taken motor strikes on the spar where it broke the fiberglass spar in half yet did not break the bottom or the top loose. Repiar was simply cutting out about 6 in of broken spar and glueing in a new piece Hope this helps
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Dane McGee
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Post by Dane McGee »

Great thread!!

I have been searching for the unobtanium spar for years and have yet to try anything that I am fully satisfied with.
I believe that if we can hammer this question out we will have gone a long way toward offsetting "constant building burnout".

Questions:
What are some of strongest lightest materials know to man?
Composite or metal/alloy or other?
How expensive is it?
How readily available is it?
Alex Treneff
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Post by Alex Treneff »

Spider webs have among the best tinsel strength/weight ratio of any substance on earth. Spider webs are much stronger than braided steel per weight. Find a way to harvest the web and make it into a rod-like spar and you'll not only have one super combat plane, you'll be a billionaire!

I'm assuming the groove you're putting the spar into is a very tight fit? The only actual spar failure we've has was a T-bone from above, and the F/G spar broke. Now we use high strength hardwood slightly forced into the grove.
NAES
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Post by NAES »

I keep thinking that an I beam of sorts is the smartest way to go.

Lou, I too have done away with solid LE for the most part. I keep having a spar dislodge issue and a few of my wing "taps" have had the spars pop out and take a ziptie or 2 with it. The last 2 wings I've severely damaged are only panel replacements as opposed to full wing replacement.

I'm always on the lookout for the next great thing.

BTW, as far as strongest materials that we have available.

FG rods-Flexy and strong
Lexan-flexy and strong but tough to glue w/o a mess
hardwood- takes a beating but will snap
CF-not so flexy and brittle
strapping tape-kida the duct tape of combat. BTW, has anyone done any tensile strength tests on the various tapes? I know bidi is the weakest but the cross fibers are great for hinges.

Keep the ideas flowing, NAES
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Dane McGee
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Post by Dane McGee »

Dunno how to harvest the a spider web, but there is a small diameter braided fishing line called "spider wire" and it absolutely will not break. I think it is some kind of kevlar material or something but it takes wire cutters to cut it. The wrapped type spar that is mentioned above would be a good application for this suff. It is light and strong and would not let a fg rod pop out if wrapped correctly

I had some 3/16 hickory spars made but never used them because they were twice the weight of basswood.....very very strong though.

Has anyone tried a basswood (or other wood)spar laminated on both sides with g-10 or something? Of course you would have to play with different thickness of the materials to get it to come out light enough, but it would be strong. You could even throw a couple of fg rods on top of the bass wood, laminate each side with thin g-10 and wrap the entire thing in spider wire and have a helluva spar maybe.
NAES
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Post by NAES »

I may do my own layups and try a G10 style spar going vertical.
danpow1973
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Post by danpow1973 »

Scott,
Anyway you can post some pics of the tuflite spar It sounds like it is close to unobtainium.
redfred1
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Post by redfred1 »

This might be an interesting answer...
http://www.ridoutplastics.com/500widflanbe.html

It is a 2"x2"x1/8" I beam of FRP.

It's only $3.26 a foot!

Maybe with a little lightening and shaving, you could have a very strong I beam spar!

I don't know... Just a thought!

FredD
aseaholm
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Post by aseaholm »

The perfect spar and wing design, now that's a thread baby! Forget about rules, let's get back figuring out how to make a better can opener, good deal!

Here's one I've thought about, the only problem is the cost and time.

<b>18 lb density 1/4" thick Urethane foam spar core
Cap stripped with .003"-.007" Unidirectional carbon
Boxed front and back with ~.003-.007" woven carbon or e-glass</b>

Talk about Unobtanium. The racers use end grain balso capped with uni-directional carbon for spars in the 2 part shell type composite wings. We have a lot of bending forward also though, that's why I think the high density foam would hold up better with boxed carbon design. This shouldn't cost much more then about $25 a spar plus an hour or two of labor per spar. I bet it would be tough though...

Tim Feathers came up with idea to add high tensile strength (300 lb.) strapping material behind the EPP LE. This seems to be working well for SSC, especially becuase the strength to weight ratio is pretty good. I think the soft LE technology is our best bet for future survivability, I believe Roy A. and Lee get the credit there.

In my opinion, flexibility is the key to survivability. But too flexible and they fly terrible because everything is flexing and moving under flight loads. That's what makes combat so much fun to try and design for, there's no other form of RC that has such a wide range of loads to consider.

Here's the strapping material. Someone may come up with a cool spar with this also...

http://www.uline.com/Browse_Listing_275 ... +Strapping

Here's a couple pics from the on-line instructions to show how we put it to use.

Image


Image

Most are probably aware of this but Matweb is a great source for material properties. It has basswood, balsa, even glass rod specs.

http://www.matweb.com

When you factor in cost, build time, and toughness, the high density foam glass rod reinforced wings are pretty dang tough to beat. Add the EPP LE to the equation and they get even better...
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Dane McGee
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Post by Dane McGee »

AJ thanks for the heads up on the strapping material........I've kicked this stuff outta the way for years and never thought of using it as a spar reinforcement, great idea Feathers[:D]

For those of us who use a full vertical wood spar, this material could easily be laminated to both sides of the spar with 3M-77 and should be really really strong.
Father O 5
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Post by Father O 5 »

I have just started building combat ships and i have now built 2 wings using the following method.

Trace the outline of your wing on coro, including the aileron. Make sure that the the leading edge of the aileron is lined up with a flute so that you can hinge it later. Cut the piece out of the coro. Do this twice.

Trace the outline of your wing onto a sheet of 1/2 inch blue styro but do not include the aileron. cut it out. Do this four times.

Sand 1/2 an airfoil into each piece of the blue styro. A "cheese grater" made for shaping bondo on cars works great for this. It takes me about 15 minutes to get one shaped. One top and one bottom for each side. Now glue to the coro. I am using Liquid Nails and that works great.

Viola. Two wing halves. Stip the flute at the hinge line to get the ailerons working. Bevel in some dihedral. Glue them together. Fiberglass some tape at the atachment point and off you go.

The first wing I did was for a 2610 ship and it is covered with strapping tape on the leading edge, packing tape all over and then momokote. The one I am finishing now is for a 2548 (both P-47s) and has yet to be covered. I will try and get some photos and weights posted tomorrow.

On the 2160 wing I was originally going with foam with fibreglass spars (arrows bought at the local sporting goods store) but two spars was the same weight as the coro center.

Neither of the wings have any flex in them. I am next going to try this on a 60" SSC wing.

Let me know what you think of this building method.
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