The Slingsby King Kite
Part I: Recreating Fred Slingsby’s Star-crossed Design in Quarter Scale
This is the first part of a six part series. See the Resources section below for links to Vincent’s other magnificent projects previously featured in the pages of the New RCSD.
Winter was near and I got that building itch again. What to build this time?
First, I like to refer to the Slingsby Gull that I built before and wrote about here in the New RCSD. It turned out to be a bit heavier than expected and maybe partly due to the choice of airfoil, the flying characteristics were a bit disappointing. For slopes this is not a problem, but I fly mostly on a flat field and then this is a pity. A lighter plane with a better airfoil would be nicer.
Whilst building the Slingsby Gull, a friend lent me the book Slingsby Sailplanes by Martin Simons, writing about the development of all of Fred Slingsby’s gliders. There was a wonderful story about the Slingsby King Kite T-9.
This was an innovative design from 1936/37 meant to participate in a major international competition at the Wasserkuppe in July 1937. It was expected at the time that gliding would become an Olympic sport. So this glider was equipped with flaps and built to fly fast. Because there was little time three (!) prototypes were built simultaneously. During the test flights, the aircraft unfortunately turned out to be extremely sensitive to spins. As an emergency measure, all kinds of enlarged vertical stabilizers were built. With such an enlarged vertical stabilizer it finally flew and did reasonably well. Afterwards it turned out that there was an error in the building of the wings. What went wrong never became completely clear, but what I understand is that the wing twisting was not well built (in all three!) and that the centre of gravity was quite far back. It is a wonderful story: Slingsby — The Rise and Fall, can be found on the Scale Scoaring UK website which is linked in Resources below.
Because this glider was sheeted with plywood (which I like), was innovative and had good flying possibilities, I decided to build it in quarter scale. The wingspan would then be 3875mm and I hoped to keep the weight below 4.4kg, that is the original weight, 280kg divided by 64 (4 x 4 x 4) = 4.4kg.
The Wing Connector
I grew up with pencil and drawing board and now I wanted to draw this glider digitally. I bought the drawing programs devWing and DevFus (see Resources)and it took me quite some time to learn to work with them. Gradually I managed to do useful things, often with some frustration, but in the end with a lot of satisfaction. Moreover I was unsure about a couple of things; the wing connector, the hinges of the control surfaces, the glueing technique and how to keep the weight as low as possible.
First the wing connector. After breaking the round carbon 8mm wing joiner (originally a steel pin was planned) of my Nemere 1/6, 330cm span along with some warnings on the Retroplane forum, I had my reservations about round carbon wing joiners.
An aside: what I may have overlooked was that putting a wing joiner in rigid tubes in the fuselage and wing introduces stress points. A steel wing pin is unfortunately quite heavy. And then there’s the V-shaped arrangement in the middle part of the gull wing, which makes it impossible to insert a long straight joiner, especially with a somewhat thinner profile.
Fortunately, on the Dutch modelling forum (and on the Retroplane forum) I came across an Excel spreadsheet with spar and wing joiner calculations: Calcul d’un Longeron (Dédié Structure Bois) which I have linked in Resources, below. That was convenient, because being brought up with kilogram, later newton and finally millipascal units, I was afraid to make a mistake of a factor ten or so. With this programme I could safely calculate the wing spar and also a wing joiner.
I realized that if I made a wing joiner of unidirectional (UD) glass/epoxy myself, I could make it with dihedral built in. This means, moreover, that the wing joiner could be held in place solely by the reinforced skin of the fuselage, a tube being impossible because of the built-in V. I bought 20x20mm square aluminum tube which should fit nicely between the 20x4mm upper and lower girder of the main spar. Now I had to make a 17x17mm square glass rod so it would fit into this square tube. I decided to try and make that first. From spruce battens and a boardcovered with packing tape to get a non-stick surface and glued together with hot glue, I made a simple mold.
I took four pieces of UD 600g/m² glass tape, 7.5cm wide, impregnated that with epoxy, folded it lengthwise twice double (four layers thick, total) and placed it on the bottom of the mold. And after that the two upright sides in a similar way. In between those upright impregnated tapes, I first put 2mm balsa strips of 11mm, each against one side and then I put 1mm balsa strips between the other balsa strips to get the impregnated glass in place without touching it and surely make a mess of it. Thereafter, the last layer of glass on it, and closed it with a batten wrapped in tape to get a flat surface. After the resin had set I had a square glass/epoxy ‘tube’ with a balsa core. The wall thickness turned out to be more than 4mm, making the joiner much stronger than the wing spar. It should be able to resist 27G according to the Calcul d’un Longeron spreadsheet. The weight of this joiner is 168g, 60cm long. I was glad with this experiment and now could go on planning the glider.
Hinges
The next problem: all horizontal steering surfaces are ‘locked in’. Because of the planform they cannot slide sideways on hinge pins and I wanted them to be removable. I solved that with hinge pins which were sprung. I made strips of 1mm epoxy plate and drilled 1mm holes in one end. To compensate for the thickness of the epoxy sheet, I glued a piece of spruce to the fixed part of a rib and then attached the epoxy strips. From 1mm steel wire I bent a long hinge pin, so it could spring. If lifted with a screwdriver, the hinge is released. If I take the screwdriver out again, the hinge pin will click into the hole.
Coming up in Part II of The Slingsby King Kite in next month’s issue, I move on to further construction details of the empennage. For now, thank you for reading and if you have any questions, please leave them in the Responses section below and I will do my best to answer them.
©2022 Vincent de Bode
Resources
- Slingsby — The Rise and Fall — Excerpt: “F.N. SLINGSBY commenced the manufacture of training type gliders in 1931 at Scarborough. In 1932 on the personal advice of Gunther Groenhoff…”
- 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…”
- 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…”
- devFuse — From the website: “our innovative application to draw fuselages and other similar parts…”
- Retroplane — The extensive discussion regarding the Slingsby King Kite on the Retroplane forum.
Vincent de Bode’s Other Projects Featured in the New RCSD
- The Fokker FG–2 — Excerpt: “The challenge for Retroplane 2017, which took place in Vauville (France), came at the exact moment I finished my 1/6th scale Nemere, built in ply. The challenge was to build a glider of which the prototype was built before 1925…”
- The 1/6th Scale Nemere — Excerpt: “After writing the article about the Fokker FG–2, I would like to tell a bit more about the scale gliders that I built earlier and, at some point in the future, about the ones I built after the FG–2. I thought it might be a good thing…”
- The Quarter Scale Slingsby Gull — Excerpt: “fter I had finished the new fuselage of my small 1/8th-scale Nemere, I was looking for a new project , a vintage glider of course. First I wanted to build the Fokker FG–1, but it looked very similar to the FG–2 which I had already built…”
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.
