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Old Nov 28, 2003, 10:32 PM
David Cairns
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Orange, CA
Joined Apr 2002
1,095 Posts
B-52 Project

I am finally getting back to my B-52 project. I now have cut a 2.5 foot test section from blue foam. Next I fiberglass it run it through rigorous torture testing (kick it around the garage) to see if it will be strong enough when it is 9.5 ft long. In this short a length, the blue foam seems pretty strong. I am sure that would be different when built out to 9.5 ft. I have been making the box with four pieces of blue foam, glued together in a box shape with 3M spray glue. It did not seem to stick together that well, but I didnít let it dry overnight like I should have. To impatient, I guess. Once the fiberglass is on, the glue between the foam wonít be that important, and it will have plenty of epoxy oozing into any gaps. The plan is to put two or three layers of heavy fiberglass and one layer of light on the bottom. Let that dry, sand off the rough edges, and apply one or two layers of heavy and one layer of light fiberglass to the top and sides. This is going to be a real goopy mess!! For the real thing, I will make 4, two foot sections like this, glue them end to end and add the nose and tail sections to make a complete fuse.

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Old Nov 29, 2003, 06:59 AM
No fuse too fat
slopeiron's Avatar
USA, CA, Redondo Beach
Joined Apr 2002
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Wow. That's going to be a big plane Dave!! It sounds to me like it should be plenty strong the way you plan to build it. With that much glass on it, it should be as solid as a rock. Can't wait to see more on this project!

Russ.
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Old Nov 29, 2003, 09:37 AM
Moderatee buncha places
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Southern California
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i would use something other than the 3m spray. i just melted a set of new blue wing cores with the stuff , i called TFLG and he said the acetone in the 3m is not compatible with the blue foam.

Tawg
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Old Nov 29, 2003, 10:07 AM
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b-52

what is the span going to be on that monster

Bill
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Old Nov 29, 2003, 01:55 PM
David Cairns
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Orange, CA
Joined Apr 2002
1,095 Posts
Quote:
Originally posted by seafury_fb11
Wow. That's going to be a big plane Dave!! It sounds to me like it should be plenty strong the way you plan to build it. With that much glass on it, it should be as solid as a rock. Can't wait to see more on this project!

Russ.
Do you think I am overdoing the glass? I have not built anything this large, so I am just guessing on how much glass to use.This does not need to be built like a Fermin Flyer, but strong enough to handle the landing areas that we have in SoCal. I was going to go thicker on the underside, particularly on the nose area. It will probably be something of a hangar queen, since it will be such a big production to take it flying.


My gut feeling is that the blue foam is pretty strong, and that even a small amount of fiberglass will add the stiffness needed. But how that works out over 9.5 ft of fuse, is another matter .

The span will be 11 feet. Wings will detach at the inboard engine nacells, and the tail section will need to detach so it will fit in my truck. I am also thinking of making the elevator a full flying elevator. Less worry about setting incidence and having enough throw to make it handle.
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Old Nov 29, 2003, 05:02 PM
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San Diego
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b-52

You are going to have to keep us posted on your progress
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Old Nov 29, 2003, 09:32 PM
That Freeking Laird Guy
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United States, CA, Riverside
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I don't think you are underdoing the glass. With a fuse that long it will be easy to snap it if you get a good side load. My caravelle fuse broke in front and behind the wing when it wrecked at Utah. Of course that was after a midiar busted the wing off. The plane was not very high though and probably only fell about 20 feet. I thought that fuse was bulletproof with 3 layers of 6 ounce cloth on it. If you had something running the length of the fuse in the middle like a cardboard tube that would stiffen it a bunch. That sucka's gonna cost you a fortune in glass and resin!

TFLG
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Old Nov 29, 2003, 11:18 PM
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Lakewood, Colorado
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Something some folks around here have been using to strengthen fuses and wings and everything on built up foamies, is banding material. The stuff that they wrap around lumber on pallets and such. (it's not metal) It's very strong in tension, doesn't stretch, can be found for free in pretty long lengths (and can be bought in huge rolls if it turns out to be useful). If bonded to something dense like basswood, it's also pretty strong in compression so you can build shear web spars out of the stuff. Oh, and did I mention that the stuff is damn near indestructible?

My concern with a 9 foot long fuse would not be how stiff it was in terms of flexing, but how well it'd take an impact to the side after this 50 lb beast catches a tip and comes in sideways. A sudden large bending load. The banding material struck me as a good way to add strength to a long fat fuse. Only trick would be finding a way to bond it to the blue foam if that's what you're going to stick with. Adhesives like Goop will kill blue foam, and as mentioned above, 3M Super 77 has been reformulated and now has Acetone which also eats foam.

ian
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Old Nov 30, 2003, 03:16 AM
Whadda YOU lookin' at?
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United States, CA, Folsom
Joined May 2002
2,555 Posts
"rivet" your blue foam!

If you want to make a REALLY strong glue joint with blue foam just use and awl and poke a zillion holes into the opposing parts to be glued... it is like putting littel rivet joints in them. I learned this from Olg, the guy who makes the Taboo DLG.

Here I am using his technique on a wing repair for a Hobie Hawk.


CJS
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Old Dec 01, 2003, 11:24 AM
David Cairns
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Orange, CA
Joined Apr 2002
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3 layers of 6 oz, huh? I guess I better get some heavier cloth an experiment some. I did a test section with a layer of 4 oz and 3/4 oz (had some laying around) but that was easily punctured with a whack from my exacto knife handle. So I better do at least 3 x 6 oz. Does it matter much how you divvy up the cloth application? That is to say, would two layers of 9 oz be the same as 3 layers of 6 oz? I would prefer to keep the layer count down so I don't spend all the resin's working time putting the layers together. I got the 24 hr hardener, but working time is still around 40 min. and on a fuselage this size, it will be a bit of a rush to do it all.

I have been considering some other stiffing techniques. Carbon fiber strips under the fiberglass or carbon fiber tubes in the foam. It looks like the fuse may be stiff enough, once an adequate thickness of fiberglass is applied. I am most worried about the area between the cut out for the wing and the cut out for the bomb bay. I am thinking that a diagonal line between them is going to be the weakest area of the fuselage and a good place for reinforcement.

I have experimented with both the new and old 3M 77 on the blue foam, and the new stuff does eat the foam a bit, but not enough to be a problem for what I am doing. It only seems to eat in about 1/32 of an inch, when sprayed heavily, and since I am joining blocks for hot wire cutting, this in not a problem. The property of the spray glue that allows the passage of the hot wire cutter though the glue joint is more important.

I use that technique of "poking holes in the foam" when joining wing halves with epoxy, too. More surface area = stronger joints. I am not sure how well it would work with the spray glue thing, but the strength of the spray glue joints is not what I am concerned about. Once it is covered with fiberglass, the primary strength will come from the fiberglass, the foam will just help with holding the shape when squeezed, etc.
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Old Dec 03, 2003, 09:49 AM
David Cairns
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Orange, CA
Joined Apr 2002
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I did some test patches of 3 5 and 7 layers of 6 oz and a outer layer of 3/4 oz.
The 3 layer looks like it would be the minimum. It passed the "whack" test but pressing hard with my finger, I was able to leave a slight dent. The 5 layer test resisted finger denting much better, and the 7 layer was impervious to finger denting. So I am thinking 3 layers of 9 oz will do the trick. Because of the size of the thing, I want to keep the layers to a minimum, so going to 9 oz will give me the equivilent of 4 or 5 layers of 6 oz.

I was re-reading an article on Simon Cocker's similarly sized B-52 from several years ago, and he used blue foam with SolarTex over the foam. So structurally, I think I will be ok. I just don't have all that soft green grass to land on like they do in the UK, so the fiberglass will be needed to keep rocks from making holes in the plane.
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Old Dec 03, 2003, 10:53 AM
No fuse too fat
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USA, CA, Redondo Beach
Joined Apr 2002
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Just keep in mind that the force of impact goes up by the square of the weight of the plane (err, somethin' like that ). So if you triple the weight of the plane, then the force of impact is 9 time greater. Then the question is, did you increase the strength by a factor greater than 9? Even if you did increase it by a factor of 9, then you've still only broken even as far as survivability goes. You would actually have to increase the strength by a factor greater than 9 to gain better survivability. That's the catch 22 of trying to build the plane stronger - you really can't increase the strength enough to keep up with the increase in force of impact, so heavier planes always have less survivability.

Russ.
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Old Dec 03, 2003, 11:27 AM
David Cairns
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Orange, CA
Joined Apr 2002
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Yeah, any crash with 20-30 lbs of plane is going to be ugly. That is why I am concentrating on things like rock hits on landing. But one thing I learned a long time ago, "build it to fly, not to crash" It makes for a better flying plane, until you do crash. Fortunately, I never crash...
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Old Dec 03, 2003, 11:36 AM
Previously plasmasb...
Santa Barbara, CA
Joined Jun 2003
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Quote:
Originally posted by dcairns
Fortunately, I never crash...

...Very very famous last words.
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Old Dec 03, 2003, 01:28 PM
That Freeking Laird Guy
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United States, CA, Riverside
Joined Feb 2002
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Several layer of thinner glass is better since you can alternate the direction of the weave. This will give you a much stronger finished product. put the first layer on straight then put the next at 30 degrees to that one and the last another 30 degrees off. Puncture resistance will be the same but it will be less likely to snap the fuse if you alternate the direction of the weave. If you really want it bulletproof put a layer of kevlar in the middle. Robert at CSD has some great prices on glass, carbon and kevlar cloth by the way. He has a source that gets close outs and overruns. He sells it for pretty darn cheap! You may want to check out what he has or give him a call. He's a guru when it comes to composites!

rcglider.com


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