The Slingsby King Kite
Part IV: Fuselage Sheeting and Main Wing Structure
This is the fourth part of a six part series. Readers may want to review previous parts of this series before proceeding with this article.
A New (To Me) Method for Attaching Fuselage Skin
I didn’t want to do this with cyano, but wanted to try another long-known technique: with the heat of a covering film iron and dried white glue. This is how it is described: “apply white glue to both pieces of plywood to be glued, let dry, then press them together and heat it with a hot foil iron.” Sounded hopeful, but new to me.
I started sheeting the top of the fuselage, while clamped on the building jig again, which was easy with the central support batten. I made a template of strong paper and transferred it to the plywood. Then I cut and sanded it. Now I had to sand the bevels (for the welds), a time-consuming job, especially if the panels are not straight. Some more about this technique can be found on videos on Scale Soaring UK (see Resources, below).
After sanding and fitting, glue could be applied which had to dry until it was no longer white. After drying the process was repeated to get more glue thickness. The gap of the scarfed joints, which are also curved, was critical and I didn’t know if I could work accurately enough.
A tricky thing was the horizontal (longitudinal) seam, which was difficult to get straight; it was hard to get each panel exactly in place, the technique with the dried PVA made that easier than with cyano. When in place I heated it with a foil iron in the middle position (150C). After cooling, it was firmly in place. Because the front of the fuselage was already sheeted with ply on the inside, the fuselage was now rigid enough to be removed from the building jig.
Before sheeting the bottom of the fuselage I made the elevator control, a 1mm steel wire push/pull rod, in a double plastic guide. To be able to assemble and disassemble the stabiliser, I first made a hinge/swivel in the rod at the tail side. I bent a very small eye in this rod, in which a pin of a quick link fitted tightly. Then I made a short rod from two quick links and a piece of M2 threaded rod. I clicked one quick link over that eye and thus obtained a hinged steering rod. This piece could be lifted and then attached with the quick link to the rudder horn of the elevator. As soon as the stabiliser was lowered in place, the hinge was ‘stretched’ and thus became a stiff steering rod. The rudder was operated by two steel braided pull/pull cables. These were attached to the rudder hinges of the rudder by means of self-locking steel wire hooks so that the rudder could be easily dismantled for transport.
Now I sheeted the whole bottom of the fuselage. Then the nose, which had to be covered with narrow strips of ply, as with the original. A firm base was needed for this. I filled the spaces between the frames with 4mm balsa and sanded it smooth. I built the nose cone from 20mm balsa planks, roughly cut in shape according to the side view. Before gluing the balsa block, I had already hollowed out the inside before gluing the planks together, and glued a plastic jar in it. That jar can hold 130g of lead so after finishing the plane I could put 130g of lead in the jar, measure how much ballast I needed for the most forward centre of gravity and fill the nose with lead shot and epoxy resin. This would give the possibility to remove the ballast without having to chop it out.
After gluing the nose cone on, I sanded it to shape as best I could and then sanded in the faces where the sheeting had to be glued on later. It was hard to see, with a flat pencil I could mark it. Below the largest width the strips were vertical, above they were horizontal, tapering to the point. With light at a flat angle I could see where the faces had to be. I got the idea to mark the faces with pinholes in the balsa cone, so I could sand a bit without removing the marking. At the connection to the cockpit canopy, they followed the rounded frame shape. Further forward, the strips became flat so that they could be bent towards the nose tip. Double bending was out of the question. The strips were one-by-one glued on with thick cyano. I used veneer of 0.5mm instead of ply, that can be sanded without showing it.
Starting The Wings
The airfoil choice was difficult. The full-size King Kite had a NACA 23021 which looked quite symmetrical. The profile and thickness determine the look of the aircraft and so I ended up with an HQ 2.5 airfoil with a thickness of 12% decreasing to 10% at the tip (link in Resources). With the flaps (and with the ailerons too, not prototypical) I could make more camber. I then drew the wing with devWing (see Resources, below).
My friend Adri Brand was kind enough to mill the ribs and the web plates. The root ribs in 2mm plywood and the rest in 2.5mm medium-hard balsa. With the program Calcul d’un Longeron (link in Resources) I calculated the main spar: 24x4mm spruce on top and bottom at the root to 2x10mm at the tip. It should be able to take 10Gs, not including sheeting.
I built the spars from 2x10mm and 4x4mm spruce, with scarfed joints glued with thickened epoxy resin. The spars themselves were laminated with white glue. As the model has a gull wing, I made a building board at an angle and laminated the spar at that angle too. The wing was quickly assembled. I had to improvise a bit with the servo attachment because the original had a small rib distance and the servos didn’t fit between the ribs.
The wing is completely sheeted with 0.6mm plywood, like the real one. I had already made a glass/epoxy wing joiner with dihedral that fitted in 20x20mm aluminium square tube. The space between the aluminium square tube and the main spar was filled with an epoxy/micro balloons slurry. Quickly it started to look like a wing, so the other wing went without delay and both wings could be fitted on to the fuselage.
And at that moment I discovered a huge mistake! In my enthusiasm I had made the dihedral in the wing equal to the dihedral of the wing connector. It had to be half of that — oops!
It was very annoying, to say the least, but I thought: “better now, than later in the construction”. So I cut out the curved section in the top spar and with my powerfile I bevelled both sides 1:10. I changed the angle in the build board (extensively checked now!) and glued two 1x10mm battens under the upper spar to get a stable situation. I made suitable pieces of 2x10 mm spruce and glued it between the spar with 24 hour epoxy with wood dust as filler. At least now the King Kite looked good and I proudly took it to the garden for a few pictures as shown above the title of this article.
With the basic structure of the wings now ready, it was time for the ailerons and flaps. But for that, tune in same time, same place next month!
©2022 Vincent de Bode
Resources
- Scale Soaring UK — From the website: “We are a group of people with interests in Radio Control Large Scale gliders and Sailplanes and Tugs…” This link will take you directly to the plywood panelling videos mentioned above.
- devWing — From the website: “innovative application to draw ribbed wings…you can create in a very simple way a ribbed wing drawing…and cut [it] using a step-by-step approach. No CAD skills are required…”
- Calcul d’un Longeron (Dédié Structure Bois) — Translated (by Google Translate) as ‘calculation of a spar (dedicated wooden structure)’: “This spreadsheet was mainly dedicated to the composite construction of the spar. The formulas and the structure have been modified to adapt more particularly to wood construction…”
- HQ 2.5/12 AIRFOIL — As found on the Airfoil Tools website.
All images by the author unless otherwise noted. Read the next article in this issue, return to the previous article in this issue or go to the table of contents. A PDF version of this article, or the entire issue, is available upon request.
