𝐃𝐚𝐫𝐤𝐀𝐞𝐫𝐨, 𝐈𝐧𝐜 𝐫𝐞𝐩𝐥𝐚𝐜𝐞𝐝 𝐛𝐮𝐭𝐭 𝐣𝐨𝐢𝐧𝐭𝐬 𝐢𝐧 𝐭𝐡𝐞 𝐜𝐞𝐧𝐭𝐞𝐫 𝐰𝐢𝐧𝐠 𝐛𝐨𝐱 𝐚𝐬𝐬𝐞𝐦𝐛𝐥𝐲 𝐭𝐨 𝐢𝐧𝐜𝐫𝐞𝐚𝐬𝐞 𝐭𝐡𝐞 𝐟𝐥𝐢𝐠𝐡𝐭 𝐭𝐞𝐬𝐭 𝐞𝐧𝐯𝐞𝐥𝐨𝐩𝐞 𝐢𝐧 𝐡𝐢𝐠𝐡-𝐬𝐩𝐞𝐞𝐝 𝐠𝐮𝐬𝐭 𝐥𝐨𝐚𝐝 𝐜𝐨𝐧𝐝𝐢𝐭𝐢𝐨𝐧𝐬 𝐟𝐨𝐫 𝐢𝐭𝐬 𝐃𝐚𝐫𝐤𝐀𝐞𝐫𝐨 𝟏 𝐩𝐫𝐨𝐭𝐨𝐭𝐲𝐩𝐞 𝐜𝐨𝐦𝐩𝐨𝐬𝐢𝐭𝐞 𝐚𝐢𝐫𝐜𝐫𝐚𝐟𝐭.

The original stressed-skin wing relied on bonded honeycomb-cored panels that formed simple butt joints. Formalizing the flight envelope showed that higher positive and negative gust loads would create out-of-plane tensile stresses at the curved lower wing skin near the fuselage. These stresses exceeded the demonstrated strength of the existing butt joints.

Pi joints were introduced to convert the loading into double lap-shear. Element-level tests recorded more than 300 percent higher out-of-plane first-failure strength and more than 250 percent higher ultimate strength compared with the original butt joints. Full-assembly proof load testing confirmed the redesign, while wing-tip deflection per g decreased by more than 25 percent.


source : CompositesWorld /DarkAero Inc

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