The Slingsby King Kite
Part II: More Empennage Design and Construction Details
This is the second part of a six part series. Readers may want to preview Part I, before proceeding with this article.
The Horizontal Stabiliser
I drew the stabiliser and rudder with devWing which I introduced in Part I. After some practice I managed to get some use out of it and convert the resulting files to DXF.
My friend Adri Brand was helpful by milling the ribs and now the stabiliser could be built. I wanted to keep that light as well; I chose 0.4mm plywood for the covering and web panels, combined with 2x2mm spruce spars, that should be strong enough.
I glued the hinges to the ribs before placing them on the building board. Then I glued the spars in place and also the web plates. To get stability when removed from the building board, I filled the space between the two main spars of the elevator with balsa. In the nose, I temporarily glued diagonals and covered the top of the D-box with ply to form a kind of temporarily D-box. The lower ply can only be put on when the stabiliser is off the building board. The trailing edge is made of two layers of 0.4mm plywood with balsa in between. After removing the stabiliser from the building board, I cut the ribs to separate the elevator.
The nose of the elevator was covered with a strip of pre-bent plywood, bent with boiling water. On these pre-bent ply strips, I marked the ribs on the inside to make the nose with the various gusset plates in one piece. In real life, the gusset plates were separate, but I wanted to keep it as light as possible. I glued these pre-bent strips with cyano over the main spar of the elevator, thus forming a torsionally rigid round nose.
At the location of the hinges, I milled openings in the ply nose of the elevator and now it had to fit to the fixed part. With some fiddling and measuring I succeeded. But later, when the the fixed part of the stabiliser was sheeted with ply, it turned out to be quite awkward to unlock the elevator hinges with a screwdriver to keep the spring-loaded hinge pins aside.
After some thinking I made a sort of angled screwdriver from 1mm iron strip, which I could fasten with a piece of tape. With a few of these iron strips, all the hinge pins were ‘unlocked’ and I could slide the rudder into place. To unlock I turned the iron strips a quarter of a turn back (at maximum deflection of the elevator) and the hinge pins sprang back. That worked nicely and I was very happy with this, because it could also be applied to the ailerons and make them removable for covering and varnishing.
Attachment to The Fuselage
On the centre ribs of the stabiliser I glued four vertical epoxy strips with a 1mm hole that could slide over similar strips on the fuselage. Two long 1mm steel pins slide through them to secure it, almost the same as in the original. The mounting and dismounting of the elevator control was also a bit of a problem, you can’t reach it. So I made a hinge point in the push/pull rod of the elevator, so that the metal clevis of the elevator could be easily disconnected and connected. When mounted, the hinge in the push/pull is straightened and becomes stiff. After this, I sanded all the triangular gusset plates neatly into shape and the stabiliser was ready to be covered. It weighs 118g, and feels very stiff.
The Vertical Fin
After setting up the ribs, I made the top edge, laminated from 3x1mm spruce, which was bent after being poured over with boiling water and then glued with thin CA.
The curved trailing edge was made from three parts of 0.4mm plywood glued together with some scarfed joints and then cut into shape. Between the ribs I glued pieces of 3mm balsa on top and sanded it in shape. On top of that I put a similar strip of 0.4mm plywood, forming a sandwich. Just as the horizontal stabiliser I cut the ribs at the hinges in two and now the rudder could be removed by sliding upwards. Handy for storage and transport. With a horizontal sliding steel wire at the bottom of the rudder it is locked. The fixed part of the fin is covered with 0.4 mm plywood, which, together with the main rudder spar, makes a light and stiff tube.
Then I put capstrips on the ribs and sanded the gusset plates into shape. The rudder was now basically ready, and weighed 50g.
After finishing the tail planes it was time to digitally draw the fuselage. But we’ll start that in Part III coming up in next month’s issue of the New RC Soaring Digest. For now, thank you for reading and if you have any questions, please leave them in the Responses section below.
Until next time!
©2022 Vincent de Bode
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.
