Click HERE to go back to the top Vacuum Foamshell page
Click HERE to go back to the main Projects page
Click HERE to go back to John Tetz's Projects
Click HERE to go back to the Foamshell Links Page
Turtledeck is a term used in open cockpit airplanes to describe the section of fuselage behind the pilot’s head. This section describes how to hand-form compound curves without a mold.
The vacuum molded foam in the turtledeck area was not usable, mainly because the main shell-to-tail split line cut through the sides of the turtledeck. Also, the mold wasn’t high enough to leave enough usable material, and what was there was fairly distorted.
Before cutting new material, I find I need to sketch some ideas to scale, complete with dimensions, to see what the requirement and possibilities are. With the side, top, and end views on paper, I can now cut the first piece of material.
Here I am heating the area that needs to match the curvature of the trailing edge of the canopy. Apply heat in a constant motion covering about 12” x 6-8” for at least 30-45 seconds, until the foam is fairly floppy. A handheld heat gun cannot sufficiently heat a larger area if deep heat is needed. To get deep heat, alternate heating both the inside and the outside surface, but heat the inside surface last. This allows the outside to cool somewhat and become less sensitive to surface imprints from hands or table surfaces.
This shows hand-forming both the canopy curve and the tail curve (foil shape). The curve to match the canopy is formed by placing the foam up against my body, while the tail curve is formed with the left hand lifting the edge and the right hand pressing down.
Here is the canopy curve fit, which generally will require a couple of tries. Don’t try to get an exact shape at this point, because forming other shapes will affect this curve.
More bending further along the tail curve. And again, the continuation of the canopy shape is being formed up against my body, while the tail curve is formed with the hands. These curves don’t show up very well in the photos because of the white surface, but they are indeed there.
This is a better view.
This shows bending a concave curve near the top of the turtledeck. Here the tabletop is being used as a surface to push against. The concave section will only be about 8” in length.
It’s time to mark and do the first trim of the bottom edge in relation to the tail shape. The turtledeck will plug into the top edge of the tail using the tongue on the turtledeck and groove on the tail, as described in the Tail section.
Trimming the fit to the canopy. Again, this will not be the final cut, so leave enough material for further trimming. I could write a whole section on fitting and trimming. Basically, what I have found is that after testing the vehicle on the road with the bare minimum of a shell, some adjustments are invariably needed. So the previous well-fitted and trimmed joints are not tight anymore. I had to retrim the tail-to-shell fit several times. And because the nose turned out to be slightly high and the tail end low, I had to jack up the tail. More trimming. Therefore, the rule is to leave enough material to retrim.
Both right and left sides are temporarily taped on the tail. This clearly shows the concave section that is to be close to the head in an attempt to get the air to follow along the turtledeck surfaces. I say an attempt, because the air coming off the trailing edge of the windshield is extremely turbulent and tends to circle forward behind the windshield, much like water swirling in behind a rock, possibly forming a virtual foil shape bubble. With all this turbulence, it is unlikely that the air has a serious chance of being cleaned up. However, without the turtledeck the drag would be higher.
I first sat in the bike and used a mirror with an extension to locate the helmet position.
Here I have added black strips to continue the black & blue stripe from the nose to the tail. The strips are made from 0.2” HD80. I cut a step in the white material of about 0.25” x 0.2” so the black lies flush with the white, providing a larger glue surface. But before cutting the overall shape of the top edge of the white sides, I spent a lot of time marking lines and sitting back to view them. This took several tries until the shape was pleasing. Generally waiting another day with a fresh look helps. Both aerodynamics and aesthetics are considered, aesthetics because this vehicle is a road streamliner running various errands and will be seen by many people.
Here is the blue section. For a decent fit to the black, I cut angles on both the black and blue so that the final visible edge is a single line. This requires quite a bit of precision. I would suggest practicing on scrap material first to get the matching angles correct. Use a very sharp razor blade. I keep a sharp razor in reserve just for precision cuts.
The final results with the black & blue trim down to the tail section. In order to get decent control when hand-forming, keep the parts to be formed small, as large pieces are difficult to control without use of a mold.
This shows the inside. The black along the bottom edges is the tongue that slips into the groove of the tail. It is made from 0.2” HD80, which helps stiffen the edge. Note the four latches (see Canopy for a description of the latches) and the rear hinge, which allow the turtledeck to be swung open for easy access to the cargo area. The hinge at the very back is fabricated from aluminum with a removable pin so the turtledeck can be removed.
Next section: Finished streamliner