Full-Core Composite Sandwich Wings

July 1981

RAF pioneered the structural method of using the hot-wire styro block to form full-depth foam core wings in 1974. We have built over 60 flight-hardware flying surfaces using this method in the development of the S.P. VariViggen, VariEze, Quickie, Defiant, Long-EZ, AD-1 and other aircraft. The method has since been used on other types, including an STCed vertical fin for the older Mooneys.

It is estimated that approximately 500 full-core aircraft are now flying, logging over 100,000 flying hours. The major advantages of full-core are the ease of moldless construction, the accurate contour maintenance under airloads, and elimination of moisture traps. Critics have claimed that full-core is heavier than the hollow wing with standard skins. Our analysis has shown the weights to be very close.

However, we have built and tested wings designed to the same criterias (hollow vs full core) and have found the hollow wings to be heavier. In addition, the hollow structural configuration is more susceptable to workmanship errors that can result in structural failure. This is due to the presence of peel loads and blind rib closeouts. In addition, the hollow structure flexes, has more points of concentrated stress and is more prone to catastrophic failure should a joint open up (leading or trailing edge).

A builder who had built a VariEze, Quickie, Long-EZ and Adventure is now building a homebuilt with molded wing skins. He reports that despite the large molded parts, the man hours in the wing are at least 50% more than for both full-core Long-EZ wings. This is due to the many ribs, jigs, control system parts etc.

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