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-   -   How do you sheet without gaps? (http://www.rcuniverse.com/forum/questions-answers-154/3529595-how-do-you-sheet-without-gaps.html)

99GrandTouring 11-07-2005 01:29 PM

How do you sheet without gaps?
 
Just curious as to what methods most use to sheet over wing ribs or sheet between spars to get and exact fit?

I'm pretty decent sheeting wing ribs as the edges can be sanded down flush, but sheeting between to wing spars for strength sometimes there are gaps, so what techniques do you reccomend?
Thanks!

MinnFlyer 11-07-2005 01:50 PM

RE: How do you sheet without gaps?
 
I'm not exactly sure what you mean by "Sheeting between two spars for strength". If you mean shear webs, gaps don't really matter. If you mean sheeting an entire wing (As opposed to just the Le and TE), I usually join several sheets BEFORE applying them to the wing.

99GrandTouring 11-07-2005 03:02 PM

RE: How do you sheet without gaps?
 
yes shear webs... placed between an upper and lower spar to make it like an I beam...
What do you mean gaps do not matter? I though this is what creates the strength in the wing?
Thanks!

jagnweiner 11-07-2005 03:09 PM

RE: How do you sheet without gaps?
 
I'm sure Mike will probably answer with a drawing, but in my experience you don't generally install shear webs "in between" the spars in the manner of an I-beam. You're right, that could be tricky to do without gaps. Instead, you either glue them to one or both sides of the spars, to make them like a "C" or an "O". Because you are overlapping the spars, there is no problem with gaps and they are plenty strong. Probably stronger than the "I-beam" because you have a greater glued surface in contact with the spar.

I'm no expert, so I might be wrong.

-Scott

99GrandTouring 11-07-2005 03:32 PM

RE: How do you sheet without gaps?
 
hmm... I double checked the plans to make for sure I didnt' screw it up and they are shown in the center.... like and I beam...
Its a glider so I don't know if that makes a difference?
I could see how boxing it or doing just sheeting on the front making a C would work, that would be easier as you could just sand them flush...
Thanks!

jagnweiner 11-07-2005 03:44 PM

RE: How do you sheet without gaps?
 
I'm not sure if it makes a difference either. Like I said, I'm no expert. If your shear webs make a box or a "C", you don't usually have to sand them flush. You usually make them slightly smaller than the dimension from the top of the top spar to the bottom of the bottom spar, so they don't run past the edge. If I were doing the "I-beam" method you describe, I would probably try to sand them as close as possible to fit and then use a good fillet of glue to give them strength.

-Scott

dr_wogz 11-07-2005 03:51 PM

RE: How do you sheet without gaps?
 
Sometimes, the center shearweb is pre-cut, and teh top spar is glued on top. The notches in the ribs are sanded to meet the shear web (or visa versa) so taht all are the same, and the top spar sits nicely on top of it all. That's how my 4* is done, and a few others as well (GP do it on a few of their kits)

You can also just cut slightly oversized, then sand each to fit, so each is a 'custom fit'

Glueing slabs on the front (or back) of the spars also work, just ensure you get the grain in the right direction (vertical)

shd3920 11-07-2005 03:58 PM

RE: How do you sheet without gaps?
 
The strength of shear-webs comes from the vertical direction of the sheeting not the horizontal, so it does not need to fill between the ribs (there can be gaps) as long as they fit properly from the top spar to the bottom spar (that is where the strength needs to be).

99GrandTouring 11-07-2005 04:33 PM

RE: How do you sheet without gaps?
 
awesome guys! Thanks!

BWooster 11-08-2005 08:48 PM

RE: How do you sheet without gaps?
 
Shear webs between the spars is stronger than outside of the spars. The web's function is to maintain the separation of the spars, nothing more, so tolerances should be close on the top and bottom edges. But before people built I-beam spars, they always built box spars, and not a lot of aircraft fell out of the sky. So it's really a matter of taste, I think.

I cut a plywood template to exact size, then replicate the webs from that, for constant chord wings. For tapered wings, every space is custom fit.

multiflyer 11-08-2005 11:53 PM

RE: How do you sheet without gaps?
 
Regarding shear webbing. Some of the advice above is good and some isn't so clear. The spar is the structural member in the wing that resists bending. When you bend a beam, the surface on the inside of the bend experiences compression and the surface on the outside of the bend experiences tension. The material in the middle experiences "shearing" forces. If you hold 2 sticks parallel and bend both towards one of them, like would happen in a wing, you will see that the sticks slide or shear like a pair of scissor blades against each other. A "shear" web prevents the spar "caps" from sliding relative to one another and therefore prevent the assembly from bending. So the shear webbing maintains cap spar spacing but mainly creates bending stiffness.

Regarding box verses I beam construction. Balsa is strong for its weight but the grain strength fiber to fiber is not so much. If balsa is used for box webbing, only the surface fibers glue to the spar caps. The rest of the fibers are only attached by the grains natural strength. Increasing the gluing surface or web thickness only increases strength to a certain point. After that the web will fail because the wood grain itself shears apart. IF balsa is used for I beam webbing, the gluing surface is less, but all the grain fibers are individually glued to the spar caps. An I beam spar uses less glue too. So an I beam can be very light for the same strength. The box type spar is more suited for thin plywood webbing. Good aircraft plywood has much higher internal shear strength than balsa.

One additional consideration is the I beam does not resist twisting. The rest of the wing structure will have to supply the "torsion" strength if I beam spar is used. The box spar has torsional rigidity. Using a heavier box spar will allow the rest of the wing structure to be lighter.

The "C" type cross section is by far the poorest design geometry for use as a spar. The shear webs of an I beam must be individually cut to fit without gaps between the spar caps. It is easier to make box webbing, especially for a tapered or curved spar like in a Corsair wing. Oversized webs can be glued against the caps and trimmed after drying. The C type spar is the easiest to build.

Of course any type of construction will work if components are sized properly.

Multiflyer


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