flat framed construction

This thread I've created as a topic for Cox International's "Quarterly Newsletter". I'm also saving as I write; to an MS Word doc. for Bernie to use if needed. The closing paragraphs are not really relevant to the actual "construction" methods.. but rather outline the configuration of the Cox Bee engines that I used on the models.

This method of construction is something I've experimented with on several 1/2A sized profile models of my own design. If you don't.. or have no desire to scratch-build, then this probably wouldn't interest you. The flight characteristics will be like any sheet-wing.. except there is a weight-savings over a solid-sheet design.. as well as some other benefits.

This method uses CA glue (thick or thin) exclusively.. and Top-Flite's Mono-Kote iron-on covering (or similar-type/weight product)

It's basically the same as a solid balsa sheet-wing (flat..   ) as supplied with many kits, except I build a balsa perimeter-framework consisting of a leading-edge, trailing edge, and wingtips, with perpendicular bracing in between. Depending on the design.. I may include a spar made of hardwood, and glue it to the rear of the leading-edge for rigidity. the open structure is then covered with Mono-Kote iron-on film.

In my experience; building a sheet-wing "framed-up" has had some advantages over a solid structure. They're lighter in weight.. and I haven't experienced any warping in the wings that I've built.. I believe it's because of the multiple grain patterns/directions and joints. The balsa pieces can all be made from 1" (or even less) width; 1/8" sheet. One piece of 1/8" x 1" x 36" will supply the stock the L/E and T/E for an 18" span model. The chord-wise bracing/gussets are made from scrap that many people would throw away. The braces only need to be as long as needed to arrive at the wing's chord; less the two inches of the L/E and T/E. You can use 1/2" width stock for the bracing if you want. If you buy two pieces of 1" x 36".. you can increase the span.. and rip one of the cut-offs in half to make 1/2" width braces.. and triangular gussets for even more support.

All the pieces need to be cut accurately. The braces should be cut with nice square edges.. because the pieces will all be butt-jointed together using the CA glue. It's easier to cut all the braces exactly the same length. I like to use a razor-saw set-up in in a miter-box with a stop-block clamped to set the brace-length. You can build a tapered T/E by adding it afterward if desired. Plan for it when laying out the frame's chord. It's easiest to keep the inner perimeter of the frame symmetrically rectangular. This allows for simple 90 degree joints.

Assemble the wing on as flat a surface as possible. Plate-glass would be ideal.. but I use a piece of 3/4" particle-board smooth-faced with melamine. The replacement closet-shelves you see at the home-improvement stores would work fine.. and a 12" x 36" one is all you need for a wing this size.

The building-surface must be clean and flat. Run a straight-edge over it (on-edge..) to make sure it doesn't drag on any scale or glue-drops. Cover the surface with wax-paper and tape it so it doesn't move. I like to trial-fit the perimeter pieces first.. and check for overall "squareness" using a framing square. Glue this outer perimeter "box-frame" together first. Apply CA to one edge and assemble the two pieces together by pressing them both down firmly on the building board and pressed firmly "together" for at least 10 seconds. Once the perimeter is built, install the remaining internal bracing in the same way. Braces should fit snugly.. but shouldn't need to be forced between the L/E and T/E. If they're too tight.. swipe one edge across a fine sanding block once.. and check the fit again. If there's a gap.. cut a new longer brace.

A 1/8" plywood bellcrank platform can be installed in a few different ways. It takes the place of a balsa brace. The better way to install it is to cut the plate 1/4" longer than a brace.. and cut notches 1/8" deep, into the L/E and T/E on their inside edges.. for the plate to sit "into" as opposed to a butt-joint. (see the tri-plane photos below) A butt-joint can be used.. but should have gussets installed at all 4 corners where the plate meets the frame. If your skeptical about either of these methods, you can opt to install a plywood-plate that extends through the wing-root.. with the wing-slot joint in the fuse for extra security.. but at the expense of added weight. If the iron-on covering is applied properly.. it's going to assist in locking-in a plate that's reinforced with triangular balsa gussets at the corners.

That's really all there is to it.. for building a wing..   Think about trying some framed (or cored) construction with a profile fuse too! If your using conventional ply nose-doublers.. the balsa between them can be cored using a hole-saw of the appropriate diameter, with a series of holes that can remove a significant amount of material. It will be just as strong with the doublers.. with less weight in both balsa "and" glue. The tail-section might have enough area to be cored as well, depending on the taper back from the wing's T/E. You can build your stabilizer as a flat-frame. With small models; the stab is generally flat-construction anyway.. why not save some weight?

Here are some examples of models I've built, using this type of construction.

My "Custom-Stunter"..   (wishful thinking..)



This model has a 24" wingspan 5.5" constant-chord for 132 sq. in. yet it weighs-in (less it's Bee engine) at just over 5 oz. and would be well under 5.. if not for the experimental/adjustable lead-out guide.. and tip-weight to compensate for it.



This model incorporates a "total" framed-up construction throughout, with Top-Flite Mono-Kote covering of the early 90's. The fuse consists of top/bottom 1/4" sq. Spruce rails nose to tail.. with 1/4" balsa between for the wing-slot and extending "solidly" all the way forward with 1/32" ply nose-doublers extending back 1.5" rearward of the wing's L/E. The fuse is only 1.625" in height, but unlike most of my designs; I didn't draw the fuse-detail on the plans unfortunately. The stab is flat-framed as well.

I apologize for the lousy quality of these drawings.. they were never completed or inked.

The Custom Stunter's.. built-up stabilizer.. and the second photo shows the plywood bellcrank-mount; a large plate extending into the fuse's wing-slot




The photos below of my "Custom Tri-Plane" drawings are a bit better.





The first photo shows a bellcrank-mount consisting of a 1/2" wide piece of 1/8" birch-ply.. set into notches cut into the main-frame. All three wings have 2" wide 1/8" sheet balsa braces at the root.. and not shown are outer wing-struts, consisting of 3/16" hardwood dowels. The fuse has chord-length pieces of 1/4" balsa supporting the top and bottom wings.

The tri-plane has recently been modified to remove the non-functional flaps from the top/bottom wings.. reducing it's weight (less Bee engine) to under 6oz.



These two models were built months apart in the early 90's..



Both models were a bit of a challenge for a reed-valve Cox Bee engine to pull around.. but they did fly pretty good. I should add that I flew them on 25 foot lines.. which was all the flight area that I had at the time. Both models utilized identical engine/tank configurations. I used standard "horseshoe" backplates.. and small Perfect wedge-tanks in the 8-10cc range of capacity. The engine-cylinders were of the standard "product-engine" type of the early 90's.. (non-sub-port induction slit exhaust-port no-frills cylinders..) running standard glow-heads on Cox Super-Fuel, Cox Racing-Fuel, or Sig "Champion "25" or "35". Propellers were generally two-blade 6" diameter. I preferred the 6" x 3" "Zinger" wood prop. I had several 3-blade 5" x 3" Cox "black" props.. which likely would have run better.. but I didn't have the knowledge at the time. In "general".. props that were sold as ".049" size props.. often assumed that a Cox rotary-valve Tee Dee or Medallion would be used.. rather than the reed-valve engines that I had.

Try adopting a framed-up construction to one of your next builds. A stabilizer would be a cool "ice-breaker".. as they're usually flat-sheet construction anyway.