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Old 11-08-2005, 11:53 PM
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Join Date: Oct 2005
Location: simi valley, CA
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Default 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.

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