Wing Spar Design

For less than $1 and less than an ounce of weight gain you can double the bending strength of your built up wing.

Under positive G bending loads the top spar cap always fails in compression before the bottom spar cap fails in tension if the spar caps are of equal crossection and material. This is because fiberous materials like spruce and other woods are about twice as strong in tension as in compression.

The cambered airfoils used in gliders can usually produce more than twice as much lift upright as inverted so, there is no penalty in biasing the structure for positive G loads. There are two ways to do this in a built up wing structure that uses top and bottom spar caps with shear webs. You can strengthen the top spar cap by using a stronger, denser wood than the bottom spar cap or you can double the crossection of the top spar cap using the same type of wood.

Because the bending load half way from the center to the tip on a wing is less than 1/4 the bending load at the center, only the inner panel's top spar cap needs to be strengthened. For example, if you replace the top sparcap of spruce with Ramin of the same crossection you get an 80% increase in the wing strength for a 60% increase in the top spar cap weight. If you double the top spar cap of spruce you will double the bending strength of the wing for less than an ounce weight gain.

Models that don't lend themselves to this approach are models that already have a larger top sparcap than bottom and models that have a single stick rather than a built up spar.

Wing Analysis and Design

Ok, let's go with 76 pounds. Therefore, each wing half has to produce 38 pounds of lift. The center of lift of the wing is less than half way between the root an the tip. To be conservative, we'll assume half way. The bending moment at the root is 29 inches times 38 pounds equals 1,102 pound-inches. The spar will be a bit less than one inch thick so we'll assume the spar depth at the root is .75 inches. The top spar cap must carry a compression load of 1,102/.75= 1,469 pounds. Sitka spruce has a compression strength of 5,610 pounds per square inch. The required top spar cap crossection is 1469/5610=.26 square inches. The nearest standard size stock is 1/4 x 1 inches. This is a bit large so lets consider carbon fiber.

The compression strength of prepreg unidirectional carbon is about 200,000 pounds per square inch. The require top spar cap crossection is 1469/200,000=0.0073 square inches. Spar caps of tapered carbon 1/4 x 0.028 x 48 inches tapering to 0.007 inches thick will do the job. Four of these prefabricated spar caps will cost you about $10. The shear webs should be 1/4 inch balsa. use hard balsa at the root and gradually transition to soft balsa at the tip. The last 11 inches of the tip needs no spar. The spar caps should be epoxied to the vertical grain shear webs and the whole assembly wrapped in about 4 layers of 1.4 ounce fiberglass in epoxy. Taper the vertical height of the spar so that it fits in the wing. Rout a 1/4 inch slot in the bottom of the wing. Install the spar with micro balloons in epoxy and cover it with balsa sanded flush to the obeche.

Wing Repair

Remove the broken plywood joiner completely. Clean off the fore and aft edges of the top and bottom sparcaps so that new ply joiners will make a good glue joint to the sparcaps. Remove a slot in the balsa sheeting if you have to in order to clean off the spar caps. Make two new joiners of 3/32 birch ply. The joiners must have the correct dihedral angle and be high enough to glue to the front and back of the top and bottom sparcaps. Cut out a half ellipse in each end of each joiner so that the top and bottom edges come to a point. This will prevent a stress rise at the ends of the joiners. Make one of the joiners an inch or two shorter than the other, again to prevent a stress riser. Trial fit the joiners to ensure a good enough fit to ensure good glue joints. Remove any material necessary to get as perfect a fit as you can. Glue the wing halves together at the propper dihedral angle. When the glue dries, cut slots in the bottom sheeting so that the new dihedral braces can be inserted. Trial fit again. Glue the new dihedral braces in place and wedge and pin them to hold them in tight contace with the sparcaps. Repair the sheeting. Glass the center section with light glass in low viscosity laminating or finishing epoxy. Cut the glass on the bias to prevent fraying. Use several plies of varying width, starting with the narrowest. Blot off the excess epoxy with toilet tissue until no more epoxy can be removed. Sand and recover. Thursday, 19-Oct-2000 07:09:25 EDT