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Scratch built Slingsby King Kite 1/4

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Joined: 30 Sep 2017, 12:43
Location: Hillegom

Scratch built Slingsby King Kite 1/4

Post by Bovin »

Hello everyone,

To start with, English is a foreign language for me,sorry for the mistakes!
Its almost winter again and it started to itch, what should I build? Whilst building the Slingsby Gull I had the possibility to read the book “ Slingsby Sailplanes ” by Martin Simons, writing about the development of all their gliders. There was a wonderful story about the Slingsby King Kite T- 9. This was a innovative design from 1936/37 with the aim of participating in a major international competition at the Wasserkuppe in July 1937. It was said that gliding would become Olympic. This glider was equipped with flaps and built to be able to fly fast. Because there was little time 3 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 and with such an enlarged vertical stabilizer he finally flew and did reasonably well. Afterwards it turned out that there was a mistake in the building of the wings. Which mistake has never become completely clear, but what I understand is that the wing twisting was not well built (in all 3!) And that the center of gravity was quite far back. It is a wonderful story , it can also be found on this website : articles / rise and fall ., wonderful to read!
Because this glider was covered with plywood (I like that) was innovative and had good flying possibilities, I decided to build it in 1: 4 scale. The wingspan will then be 3875mm and I hope to keep the weight below 4.4 KG
Now the making, a different story. I grew up with pencil and drawing board and now I wanted to draw this glider digitally. I bought the drawing programs Devwing and Devfus4, just affordable . It took me quite some time to learn to work with it, but gradually I managed to do useful things sometimes with frustration, but in the end gave me a lot of satisfaction.
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I also had a problem what to use as wingjoiner. After breaking the carbon 8mm round wingjoiner (it was designed for a steel rod, which is has it now) of my Nemere 1/6, 330cm wingspan and with some warnings on the Retroplane forum, I had my reservations about carbon wing joiners . A steel wing pin is quite heavy and I really wanted to keep the weight down. Due to the gull wing there is quite a bit of dihedral in the middle part of the wing , which did not make it any easier.
On the Dutch modelbouw forum and on the Retroplane forum I stumbled in an excel sheet with wing spar and wing joiner calculations. That was nice, I'm educated in Kilogrammes , later Newtons and finally Mpa's , and I was afraid to make a mistake by a factor of 10 or something. With this program I could calculate the wing spar and also a wingjoiner. ... ngeron.htm
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I realized that if I made a wing joiner of UD glass/epoxy myself I could make it with dihedral built in . This means , moreover, that the wing joiner could only be held in place by the (reinforced) skin of the fuselage, a tube is not possible because of the built-in V . I bought 20 X 20mm square aluminum tube which will fit nicely between the 20 X 4 mm upper and lower girder of the main spar . Now I had to make a 17 X 17mm square glass rod so it would fit into this square tube . I thought let's try to make that. From spruce battens and a board (covered with packing tape to get a non sticking surface) glued together with hotglue, I made a simple mold.
I took four pieces of UD glass tape, 7.5cm wide (600gr / m). Impregnated that with epoxy resin and folded lengthwise twice double (4 thick) , placed on the bottom of the mold, and after that the two upright sides in a similar way. In between those upright impregnated glass 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. Thereafter, the last layer of glass on it, and closed it with a batten with the tape to get a flat surface. Now I got 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 stand 27G according to the above calculation program
The weight of this joiner is 168 g, 60 cm long. I was glad with this experiment, now I could start the building of glider itself.
There is very little info of this glider, all info is very welcome!
To be continued.