Hi all:
I've been working on and off for the past few years on a smaller test-bed twin design that I call "Double Take." The idea was to find out how a smaller (read that easier to build, less expensive, less intimidating…) electric twin would perform, and just how light a power package could be achieved while still having plenty of power.
The Double Take was built using what I call the “Lost Sheeting” method of wing construction. The Combat guys have for years used raw foam wings covered with various thin, iron-on films very successfully. Some years ago I decided to build a few Slow Combat ships and opted to use .02/oz. carbon mat in place of the film. I applied it using water thinned Titebond II glue, and it worked to perfection. Phil Cartier stopped by my house to pick me up to go with him, Gil Reedy and Bub Reese (both now passed…) to a Slow Combat contest in New Joisey. When he came into my shop and saw the two “Slip Knots” (really just aesthetically adjusted Gotcha 500s) he said that I couldn’t fly them at the contest. “They are too beautiful to fly in Combat” he said.
In the Lost-Sheeting method of building, a foam core wing is cut and sanded to as smooth a surface as possible. Then 1/8 x 3/8-inch slots are cut from root to tip in each panel, top and bottom. These slots will eventually accept full span 1/8 x 3/8-inch basswood strips. A full-depth spar slot is then cut to accept a 1/8-inch Lite-Ply spar. In the case of the Double Take this spar extends out past where the profile nacelles will reside on the wing panels. A couple of shear web slots are also cut outboard of the full depth spar to grip the basswood surface spars and prevent “racking.” In inside maneuvers the forces tend to compress the top spars and “stretch” or tension the bottom spars. The shear webs help prevent these forces from collapsing the wing. In normal foam wing construction the balsa sheeting and the full depth Lite-Ply spar does this job.
In the case of the Double Take I chose to go with foam flaps, and so cut the panels with the flaps integral. They were separated and framed after the carbon mat was applied. And, I double covered just the flaps for added rigidity. Trust me, they are more rigid done in this manner than balsa covered flaps!
The control system was installed and the panels joined prior to gluing in the basswood surface spars and applying the carbon mat with the thinned Titebond II glue. Balsa tips were added, and the wing was ready for installation in the fuselage.
The remainder of the plane was constructed in the normal manner with a fuselage crutch and molded balsa shells top and bottom. The stab and elevators were made from sheeted foam.
I did try something different when I made the large canopy/battery hatch. I didn’t want to “waste” a big block of light balsa on a simple test-bed model, so I made the large canopy using the Dow Buoyancy Billet foam that I have been using for years to make my fuselage mold bucks for the top and bottom shells. I glued a piece of the Buoyancy Billet foam to a firm balsa base, tack-glued the assembly in place on the fuselage, and carved and sanded it to the desired shape. The I applied two coats of Z-Poxy finishing resin (sanded between coats). Next the canopy section was covered with a piece of 1/2-ounce glass cloth, also applied with Z-Poxy. After trimming the excess glass from the edges and sanding the surface smooth, I removed almost all the foam (left about a 1/16-inch wall), and the result was a very light and smooth piece. I'll most certainly use this technique to make the large canopy/hatch sections on future contest models.
The wood parts of the airframe were given two coats of Z-Poxy done in the following manner: A coat of resin was applied to small sections at a time, heating and wiping all excess material off the airframe. A light sanding was done between coats. This yielded a very light satin finish that will protect the model’s wood parts from moisture. I decided to also coat the wing with Z-Poxy in the same manner to guard against moisture (read that, rain…) from softening the Titebond II Glue that was used to apply the carbon mat. And again, every bit of excess glue was wiped off the wing, leaving just an extremely thin film of epoxy to form a barrier. That coat added almost an imperceptible weight gain.
Vital stats:
The Double Take mounts two E-Flite Park 450 motors fitted initially with two Zoar 9 x 5 wooden props (one tractor and one pusher). The all up weight is 45 ounces, including the battery. I’m using two Castle Creations Talon 25 Amp ESCs and a Fiorotti timer with twin function. The plan is to try it on .014 braided lines that are 64-feet from center of the model to center of the handle and adjust from there as required.
The initial spin-up and first flights are scheduled for tomorrow. I’ll give a full report on how it went tomorrow night (unless I’m depressed for some unexplained reason…). I’ll also add to this thread a lot of the construction technique photos for the processes mentioned above as I get some time.
My plan is to develop this concept as a model with which modelers can try an electric twin at minimal cost in time and materials.
I won’t be getting too much sleep tonight…
Later - Bob Hunt
Topic: Eve of a spin-up (Read 2567 times)
