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Will it hold or will it fold?!?!?!?!

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Will it hold or will it fold?!?!?!?!

Old 02-15-2004, 09:10 AM
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Kenny R
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Default Will it hold or will it fold?!?!?!?!

I have set a personal goal of building a 40sized 3d profile for less than 4 lbs using conventional materials. My latest project is 2 oz's too heavy.

My question is ...on the wing pictured there is a fulll lenght 1/16" aircraft ply spar, with as much of the wing as i have cut out do you think it would work if i removed the spar?
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Old 02-15-2004, 10:48 AM
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Default RE: Will it hold or will it fold?!?!?!?!

It would fold withit completely emoved.

Go to a 1/16 X 1/4 spar cap top and bottom with 1/16 balsa shear web in the form of an I beam. Lighter and you get the strength where it does te most good. The ply that is more than 1/8 inch from the skin on that wing... may as well be balsa... its not taking much load.
Old 02-15-2004, 12:45 PM
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Tall Paul
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Default RE: Will it hold or will it fold?!?!?!?!

If the ply spar is full depth, it's overkill, which means it's too heavy outboard of the 2nd rib bay. It could benefit from having lightening holes cut in the web between each rib leaving a certain amount of material at the top and bottom, and where each rib is.
If it's flat, with the width chordwise, it's almost worthless.
The simplest and lightest construction for a fun-fly is 1/4" sq balsa spars, doubled for the first two rib bays. With 1/16" sheet vertical shear webs, either between the spars or glued to the front or rear.
The leading edge need be little more than an 1/4"x1/8" spruce strip, with the 1/4" dimension along the chord.
The leading edge sheeting is wrapped around the leading edge from one spar to the other.
It's difficult to find a lighter and stiffer construction.
To lock the wing into total rigidity, a built-up trailing edge rather than a tapered spar, with the front of the two sheets again joined with a vertical grained shear web makes the wing unbendable..
The popular Stik-It series is built this way. Mine weighs just over 3 pounds!
Here's a partially constructed example of this on a profile version of the SIG Somethin' Extra..(The material unique to the SE adds a lot unnecessary weight)
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Old 02-15-2004, 02:30 PM
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Default RE: Will it hold or will it fold?!?!?!?!

I doubt it's going to fold, but it's in the wrong place. The main spars should be at the thickest point of the wing for maximum strength. Plus it's a heavy spar that doesn't pull it's own weight. Next time use 1/4" x 3/8" balsa with some webs and you'll save a lot of weight and it will be stronger.
Old 02-15-2004, 06:02 PM
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Kenny R
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Default RE: Will it hold or will it fold?!?!?!?!

Tall paul the spar is vertical and goes from sheeting to sheeting...so its overkill. ok...

The reason the spar is that far forward in the wing is this is a prototype and i thought i might need the extra "pocket area" for radio gear to get the cg right...i hate dead weight. Will be moved to the thickest part in the next wing.

Can I change the question..If i built a 18" spar with 1/4x1/4 balsa and used shearwebs made the neccasry cutouts in the foam for the spar to sit in the thickest part and goe from sheeting to sheeting, do you think it would hold?

Nice work TallPaul, and thanks to everyone for the comments.
Old 02-15-2004, 06:08 PM
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Default RE: Will it hold or will it fold?!?!?!?!

Kenny - I built an extremely fast .40 size model with a 10% thick wing having 1/8" x 1/4" spruce spars with 1/16" webs. It worked fine. If you add webs, you can downsize your spars a bit. Personally, I like wide, thin spars - width = twice the thickness. I always use webs and I've never had a wing fold.

For that model I would use 3/16" x 3/8" or 1/4" x 3/8" hard balsa with straight grain. The weight is almost negligible and you won't fold the wing. In fact, you don't even need a joiner. Just some glass around the center and it will withstand anything reasonable you do to it include square corners.

- Paul
Old 02-15-2004, 06:14 PM
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Default RE: Will it hold or will it fold?!?!?!?!

You want the lightest that will do the job?

1/8X1/4 spruce, and at 50% out from the fuselage, taper to 1/8 square at the tips. Use that for the upper and lower spars. Then use 1/32 ply shear webbing. in the 2 bays each side of the fuselage. The shear web on the outer of the 2 bays would get lightened, but I'd have to draw the shape of the ct-out. (its a curvy V on its side. No corner at the "base")
Old 02-15-2004, 06:38 PM
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Default RE: Will it hold or will it fold?!?!?!?!

You got good advice.
The best spar is a built-up I-Beam. THe wing sheeting is part of the flanges of the I-Beam, but adding another layer to stabilize the edge is a good plan. THe wood vertical component is not highly stressed..it just keeps the flanges (the part aligned with the surfaces of the wing) from buckling. That's why balsa shear webs work well. An I-Beam truss, using triangulated sticks instead of shear webs, would be really light. Just for peace of mind, you could go to a solid shear web on the inner 2 bays. You're trading complexity for ease of construction, though.
Old 02-15-2004, 08:25 PM
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Tall Paul
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Default RE: Will it hold or will it fold?!?!?!?!

A thick wing (15% or so) if built with the center section in one piece, spars one piece, not joined at the middle, with the sheeting continuous from one rib across the center to the other side can get away with very little "conventional" bracing.. ply braces etc.
Being a belt-and-suspenders man myself though, I like to put a full-width shear web, the width the same as the spar at least in the first bays out from the center section. And sometimes glass cloth outside. And 1/16" sheet shear webs. I've done a couple of wings with diagonal bracing between the ribs as Jim suggests. These apperr to work OK, but the planes weren't hard-duty types.
The strongest wing I ever made was from 3/32" sheet for everything except the leading edge. The ribs slotted into the spar, and with the usual sheeting the thing was unbreakable due to any flight mishap.
As the flight loads themselves drop to zero at the tips, there would be no structrural reason for shear webs all the way, if the plane were a full-scale, but since we fliers tend to land on the tips sometimes, it helps to have the sturdy construction go all the way to the tip. It doesn't add much weight(inertia) but does cut down on unintentional damage.
Old 02-16-2004, 10:50 AM
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Default RE: Will it hold or will it fold?!?!?!?!

On a plane that size, there's no good reason for hardwoods in the wing anywhere but at the mounting points. As long as you make sure the skin is bonded to the spar/shear web for the whole length, you could replace the ply with vertical grain balsa and it would be fine. This way, the skin itself acts as the top & bottom flange on the "I-beam". Then if you are still worried and want to save weight, get some .007x1/4 CF strip and CA to the top & bottom of the wing over the spar. One other thing would be to put a very light layer of fiberglass (3/4 oz) over the center wing saddle where the wing contacts the fuse. This keeps the wear & tear of the fuse on the wing skin from weakening the wing.

Now, if you are really concerned about overall flying weight, you need to look at the components that make up the heaviest parts of the airplane. The radio, engine, and landing gear. Look at it this way, if you beat your head trying to save 10% of the weight of a 8 oz wing, you save .8 ounce. Saving 10% weight over your existing design would be doing well too.

Changing wheels can get you an ounce if the current ones are on the heavy side. On a bigger plane, changing wheels once saved me a pound. You can always look into CF landing gear too. On the radio, the battery pack is key. You can get a Nimh battery pack of the came capacity and save more than an ounce there. Or go with a smaller pack and charge more often for even better weight. Servos can also save weight. Hitec HS-81/85 is a mini servo with standard servo torque at less weight. The motor is sort of a fixed quantity, although a mousse can muffler can add performance while weighing less that the stock muffler.

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