The Slingsby King Kite
Part III: The Fuselage
This is the third part of a six part series. Readers may want to review previous parts of this series before proceeding with this article.
I drew the fuselage with DevFus (see Resources, below). After a lot of messing around on the screen with terms I’d never heard of, I got familiar with it! Unfortunately, sometimes drawings were suddenly (perhaps magically?) put in a different folder and I could no longer open them in the drawing program (stop laughing!) That’s why I drew the hull about three times. Practice makes perfect they say. Gradually I found out what was possible. It’s much easier to make a hull straight in a drawing than to sand frames down or glue extra material on!
I thought it would be handy to use a temporary central spar to align the frames. My friend Adri Brand kindly offered to mill the frames from 3mm plywood and again he did an excellent job. As with the Gull, I made a building jig from a board onto which I screwed 10 x 10mm battens at frame spacing. I deliberately made a ‘jump’ in this as I wanted to cover part of the fuselage on the construction jig. Because I wanted to use thin stringers this was necessary to keep it straight. It takes some effort to save weight!
Aligning those stringers took time and I wanted to get on with it, but mistakes in the stringer arrangement are hard to correct later. So patience was essential. The temporary central girder rested on the construction jig, so I didn’t have to screw down the frames, clamps were sufficient.
To reduce the ballast, I wanted to keep the tail surfaces and tail boom as light as possible and place all servos and receiver batteries as far forward as possible. For the fuselage stringers I looked at the real plane; those stringers are really minimal, so I decided to use 3x3mm and 4x2mm spruce stringers. Together with the 0.6mm plywood skin this should be strong enough. After having sorted this out I could start to build.
I first glued in the two stringers, left and right, turned the building board over and then the lower stringer, which flared out to the wide keel, where the skid would come later on. It was great that the frames were aligned so there was very little sanding needed! The design had taken some effort, but this went very well! When that was done, all kinds of details had to be made.
First the wing joiner reinforcement. On the two main frames I glued some massive spruce, forming a square hole so the curved wing pin could be slid through. Reinforced with M2 threaded rods to transfer the forces to the fuselage.
For the tailskid I laminated the lower girder and made an extra frame, I had to deduce from the photos how that had been constructed. It was fun to find out, but it took some time.
No details of the fuselage-horizontal stabiliser connection could be found on any of the photos of the full-size aircraft. I suspected that the way of assembling had been similar to that of the Gull I had built before. That was fixed with four horizontal pins that were inserted from both sides. In the model I solved this with two continuous 1mm steel wire pins. That determined the shape of the fuselage at the leading edge of the horizontal stabiliser. First I made a filler piece according to the drawing. I had to remake it three times before I felt it to be right. Knowing this I laminated from balsa and 0.4mm plywood curved plate, sanded at the outside to align the other frames. In front of the fin, there appeared to be also some kind of spacer, visible on the photo.
The placement of the servos came next. I wanted them well in the front for the centre of gravity, also low in the fuselage to make the cockpit interior to scale. I could place the three servos, namely the rudder servo which works with two pull-pull cables, the tow release servo and the elevator servo, under the ‘knees’ of the pilot.
After this research, it was time for the front of the fuselage. First I cut off the top part of the fourth frame (canopy) and glued the angled rear cockpit frame in. Then I covered the inside of the frames with 0.6mm plywood. I now had still good access and the front of the fuselage was dimensionally stable.
My intention was to cover the top of the fuselage up to the cockpit with plywood, with the fuselage still in the construction rig, to keep it in shape. After that it could be removed from the rig and finished.
Before I continued with the cockpit, I had some doubts about the size of the stringers in the fuselage. They were about the same size as in the real one, but my landings are often a bit rougher than in real life. On the other hand, a light tail boom gives much less ballast.
I decided to put a single glass roving with epoxy on the inside of the left and right stringer, the stiffness of glass is closer to that of wood than that of carbon. I hoped that would work out well. I drilled 2mm holes in the frames right next to the stringers and with a double folded 0.2mm steel wire I pulled the dry glass roving through the holes and impregnated it with epoxy. After curing, the stringers felt much stiffer and not much weight was added.
Back to the cockpit. I first fitted the pilot (borrowed from my Gull) and after the necessary fitting he sat neatly in his seat. Now I could made a backrest, seat and floor with plywood/balsa sandwich. All fastened with screws in plywood supports, so I could reach the servos later.
Then the canopy. In those days ‘bubbles’ could not be made, so the original canopy was made of curved panels of Perspex resulting in lots of character! First I laminated the three frames and glued the whole together using the fuselage as a mould, of course with plastic foil in between. To keep it dismountable I made two pins at the front and in the back, between layers of 0.6mm plywood, a couple of strong magnets, both in the canopy and in the fuselage. From 0.6mm plywood I laminated the longitudinal beams. That was a fiddly job, but now the canopy frame was ready to be glassed.
In the next instalment coming up next month, I will kick off with details of gluing the ply sheeting. I passed on cyano for this, because I wanted to try another long-known technique: the heat of a covering film iron and dried white glue. But for that, you will have to join me next time. In the interim, if you have any questions, please leave them in the Responses section below and I’ll do my best to answer them.
Until next month!
©2022 Vincent de Bode
Resources
- devFus — From the website: “our innovative application to draw fuselages and other similar parts…”
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.
