On Friday, July 26, 2019 at 10:02:23 PM UTC-7, Sky Surfer wrote:

Interesting discussion. Another question for Bob K. and other experts: If you saw crosswise (cross section) through a typical sailplane's main spar tongue/stub what would it look like? Mostly foam surrounded by many layers of fiberglass? About how much glass versus foam? Just trying to envision a vital structural component that most of us take for granted and never see the inside of unless one is a designer or fabricator.

Spanwise fibers for tension and compression in the spar caps at the top and bottom to react accumulated tensile and compression forces due to bending. Bias fibers in between to react shear. Localized reinforcements to react discrete loads at main pins and root rib. Some manufacturers use some wood, but not me.

The shear web between the upper and lower spar caps is usually a composite sandwich similar to the wing skin; for most of the span it's 6mm to 10mm PVC foam with one to three plies of bias cut (+/-45) fiberglass or carbon on each side.

The unidirectional fibers in the caps have in the past mostly been fiberglass or carbon fiber tapes or raw tows. These days carbon fiber spars are usually made of prepreg carbon and autoclaved. In my shop we use strips of a pultruded unidirectional carbon fiber product called Graphlite.

Composites usually have lower compression strengths than tensile strengths, so sometimes the spar caps are asymmetrical in depth, with a deeper section for the upper spar cap than for the lower.

At the spar stub, the spar is wrapped with several plies of fiberglass or carbon fiber to react shear due to the moment applied by the force couple between the attachments between the two wing spars.

The wing spar tapers in depth (top to bottom), to fit inside the tapering wing. The spar caps are also usually tapered in one or both dimensions to tailor their strength and stiffness to bending loads that diminish rapidly as you go from root to tip. In my gliders the spar caps are about 0.37" deep from root to tip, and taper in width (front to back) from 1.76" at the root rib to about nothing at the end of the 18m tip extension. Out there the loads are so low that the wing skin itself has enough strength to handle bending loads.

If you want to see this stuff in action, come by our workshop in Arnold CA. We are just now dressing out a pair of spars to go into the next wingset.

Thanks, Bob K.