Airframe Modification

Last weekend I started on the airframe modification to the Stryker for the speed competition. The first step was to smooth out some lines on the front end of the plane. There are some cosmetic air intakes along each side of the canopy. The fronts of the intakes are flat and have a significant impact to the orthographic projection of surface area for drag. I took my handy wire cutter and streamlined the profile freehand. Some final forming was done using a medium grit sand paper. The imperfections were then filled with a lightweight spackle and sanded to a smooth finish. The overall area reduction is not huge, but at speeds in excess of 100 miles per hour, it will have an impact.



The standard Stryker has 4 cavities in the foam at each training edge to accommodate the elevon tabs. Since I am making custom elevons out of balsa (with control hinges), I filled the cavities with spackle as well. Once filled, the training edge was sanded smooth.



During the modification I added a 6mm carbon tube to the underside of the wing. The spar was set into the foam using my hot wire cutter and epoxied in place. The position of the spar is just aft of the servo pockets, extending from wing tip to wing tip. I have see some discussion in regards to reinforcing this airframe with additional pieces of carbon, but it is my belief that with a fiberglass surface there is more than enough strength in this airframe.



With the spar epoxied in place, I sanded the entire surface of the airframe to remove any molding imperfections in the foam. I then wiped all surfaces with denatured alcohol to prepare the surface for glassing.



Last night I began the glassing process on the bottom of the airframe. Using a .75 oz fiberglass cloth, I cut out a single piece that would cover the entire bottom surface. I then mixed some water-based polyurethane with a small amount of water. This was done to prolong the dry time of the PU, so I could work the surface smooth. With the glass laid in place, I worked the PU over the surface and into the weave. I started the application at the fuselage, and worked my way out to the wing tips. With the surface whetted, I used a business card to squeegee the excess PU for an overall smooth surface. With the PU drying, I took an exacto knife and trimmed the leading edge of the wing. I then went back and applied more PU along the edge to verify that the glass was secure.



Pictures to be added soon…