History of sandwich structures in aircraft flooring

First attempts to use sandwich structures in aircraft were in the Viscount and Vikings. Plywood skins, 3.97 mm thick were separated by aluminium top hat stringers. Fig. 19.6. 

However, due to the unsupported spaces between successive crests, the panels were easily damaged by sharp impact. Furthermore, the natural water content of the plywood caused corrosion of the aluminium stringers. 

To overcome these problems a similar construction was used but with 0.91 mm aluminium skins instead of plywood - Boeing 707, Douglas DC 6 and DC 9 (the DC 9 still uses this type of construction). However, the skins were still vulnerable to damage in areas between the stringers, especially from castor wheels of catering trolleys. 

Balsa wood is a naturally occurring material and therefore displays variability of properties. For example, density ranges from 7 to 12 lbs ft 3. In order to minimize the effect of density variation, panels were 

at this time, a wide range of core and skin materials were being used to produce a variety of sandwich flooring panels for aircraft. Individual companies concentrated on their own specialized products with little coordination of knowledge and effort within the aircraft industry as a whole. 

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