

Home > Articles & Tips Index > Model Design > Zoom Proof RES Wings [Courtesy of Oliver Wilson, ocwilson "at" sunline.net, December 1999] There is no need to go to exotic materials to achieve a zoom proof wing!! With rational design it can be done with wood and without excessive weight. The bending moment of a wing is maximum at the center and increases from zero at the tip at approximately the square of the distance from the tip. This assumed load distribution is conservative. One spar cap geometry that matches the bending load distribution are spar caps that are liniarly tapered in both width and thickness. Compare a typical spar design which has a uniform srength along its span to a spar whose caps are tapered in width and thickness. For the same weight in material, the tapered spar cap case will be three times stronger at the root. Still, one half of the weight of the bottom spar cap is wasted because it is never stressed beyond half its strength during the zoom. So, the bottom spar cap can be halved in crossection compared to the top sparcap. There is a tropical hardwood called Ramin that is 80 percent stronger than Sitka Spruce and only 60 percent heavier. Its strength in compression is about 10,000 PSI compared to about 5,600 PSI for Sitka Spruce. By using rational design and the best wood available it is possible to build a wood wing that is at least five or six times stronger than a typical wood spar at little gain in weight. Best of all the center of mass of the sparcaps will be 1/3 the distance from the root to the tip instead of half the distance with a typical spar design. All it costs is the price of the Ramin and the construction effort to taper the spar caps, shear webs and rib notches. The strength requirement of the shear web is a linear function of its position along the spar so, it can taper as the width of the sparcap. Happy New Year, Ollie 
