The Quarter Scale Slingsby Gull
Building a faithful replica of a classic design can have its ups and downs.
After 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 (see Resources below). I like gliders with varnished plywood sheeting. Talks with friends started about the different flying characteristics of the vintage gliders. Because I usually fly on a flat field that is an important matter. The Fokker FG-2 is beautiful to watch it fly, but it comes down very quickly. What do you expect, with all those rigging wires? On a slope with a lot of wind it’s okay, but on a flat field it gives not much flying joy. A friend mentioned the beautiful Fafnir from the early 30’s. I started reading more about it. The gliders from that period had very thick wings, they were made more for minimal sink rate than for speed. The Fafnir and the Petrel have a profile of almost 20% thickness with a very blunt nose. To get better flight characteristics in a model, you would have to choose a thinner profile. Unfortunately the aircraft will look very different then.
Thanks to Martin Simons’ very nice book Slingsby Sailplanes I did find a few pre-war, clear-varnished plywood gliders with more modern profiles. I chose the Slingsby Gull T-12, partly because of its gull wing. It also has spoilers, for me, a must to land such a glider. Another nice thing about this plane is that the first ten were made with struts and a the last Gull produced flew without. This was the Gull III, the T-15 or the Cantilever Gull. This was also the first British glider to cross the English Channel.
The documentation is rather limited: a description, some pictures and a drawing in A4 format in the Martin Simons’ book and info on the site of Scale Soaring UK (see Resources) .
This one I wanted to build, but how? Unfortunately digital drawing is not yet in my toolkit — I should really do something about that! I did it the old fashioned way; I enlarged the A4 drawing to the one-to-four dimensions, but unfortunately the lines became very thick. From this drawing I started building, with the construction details of the fuselage in mind but not on paper yet. The order of building is unfortunately a bit chaotic, you can’t finish the fuselage without wings and wings without fuselage. However, in this report I put them in a sort of logical sequence.
Wing
Rob Ten Hove had already started to digitally design the ribs of the wing. The real Gull has a profile of NACA 4416 up to the aileron and then tapering to RAF 34 at the tip. It is nice if you can use a thickness similar to the real thing. Rob has reduced the thickness from 15% at the root to 12% thickness at the bend, which is camouflaged by the bend itself.
In addition, the two inner ribs behind the main spar are ‘stretched’, just like the real Gull. This is actually the start of the fairing to the fuselage. Rob’s proposal was to build the wing spar from a lower and upper spar, stiffened with 0.6mm plywood web plates.
Wing Joiner
Because of the large dihedral in the centre section the positioning of the wing joiner proved to be tricky. The upper and lower spars are each 10mm thick, so there’s not much length left for a solid straight wing joiner. I decided to place one joiner in front of and one behind the spar. The brass tubes of the wing joiner (10mm round steel/carbon) are glued to the web plates of the main spar with glass tape and epoxy.
I can use carbon rods combined with struts for light winds and steel rods only with more wind, flying it as the Gull III. They are about 400g heavier and that’s not bad with strong winds. After these important and decisive first steps I started building.
Spars
The upper and lower spars are built up from 10x2mm strips, laminated from five layers at the root to one layer at the tip. I had to have lengths of almost two metres. I made these by scarfing two 1m battens and gluing them with epoxy thickened with wood dust.
From these battens I laminated the spars with PVA (that is, white wood glue) incorporating the ‘gull’ curve. Fortunately I remembered just in time that the upper and lower spars are not interchangeable. I made a drawing on which the wing could be built. Because of the bend in the wing I had to make a building board with a bend. First I made bearers of two battens (42x18mm), which I glued on each other to obtain that bend. This bend runs over 75mm, so I rounded off the bearers. On these two battens I screwed two pieces of plastic coated chipboard of 400mm wide and 20mm thick, not over the bend of course. On top of the chipboard I glued — with dots of contact glue — 4mm poplar plywood, with the thread across. One piece nicely curved over the open part of the bend. After these steps I could start building.
Ribs
My friend Adri Brand was willing to CNC mill the ribs. He suggested to do that from 2mm poplar plywood and the five root ribs from 3mm plywood. I first thought of building the ribs from spruce battens but Adri also suggested to make large lighting holes in the plywood ribs and glue thin battens over them. When Adri had milled the ribs, it turned out to be easy to glue the battens on; it makes them stronger and it gives a lot of building satisfaction. After all these preparations the wing was glued together quite quickly.
Trailing Edge
Because the trailing edge is curved as well, I build it as a sandwich; 0.6mm plywood bottom, 3mm balsa; sanding it down and covering it with 0.6mm plywood on top.
Sheeting
The D-box had to be sheeted with 0.6mm plywood. On the pictures of the restoration you can see that the plywood panels stretches over three ribs. So there will be many joints in the wing, with additional joints in the ‘gullwing bend’. Here again the question: butt or scarfed joints? Butt plates in the wing are difficult, because the joints have to be in line with the wing rib, which means the ribs have to be 0.6 mm smaller.
I then remembered that in the construction of wooden sailing boats (real ones that is) a lot of glass epoxy reinforcement is used. Glass tape with epoxy is much thinner than wood and this is how I tried to get around this problem. I laminated strips of glass tape with epoxy on a flat surface, let them harden and then cut them in half, and used this as a butt plate. Such a strip is very flexible and I could easily glue it to the rib with thick CA.
The sheeting with 0.6mm plywood went as follows. I made a template from thick paper, and using that template, I cut out a slightly larger piece of 0.6mm ply. I bent it while running boiling water over it and dried it with a hairdryer.
After that I sanded it to a nice fit. I put tape along the outside of the panel to avoid cyanoacrylate (CA) glue getting on the ply. I first glued the leading edge (LE) with thick CA and put the panel in place. After a minute or so, the glue was partly hardened, I bent it open on the underside, applied glue to the ribs and main spar, when pulled and pressed the panel into place, the top in the same way. Then I cut off the excess plywood behind the main spar. Somehow I got used to doing that with CA, there are many other ways such as with white glue (PVA).
Ailerons
I built the wings including the ailerons to cut them off later. I made hinges from 1mm epoxy plate with a glued-in nail as hinge pin. I had drilled the holes for the aileron hinges had glued the hinges on the ribs with the pins through the holes, before assembling , so I didn’t have to worry about the alignment. After the wing was sheeted, I cut all the ribs through, sanded the aileron leading edge round with a long sanding batten and the ribs of the wing with a power file hollow. I covered the front of the aileron with pre-bent 0.6mm plywood, light and torsion-resistant.
For fidelity to the original design, I wanted the aileron to be operated with pull-pull wires. These had to go around 90 degrees with pulleys. After thinking long and hard, I decided to put a large servo in a kind of box in the root of the wing, so it could be removed. Later I made an inspection hatch in the root of the wing, so I could adjust the tension on the wires when the wing was finished.
I made the horns at the aileron and their controlling servo arms with the same throw so the cables will always be taut. Afterwards, there appeared to be quite a bit of friction because of the flexing of the wires over the pulleys, so the servos didn’t come very well to their neutral position. I then made the servo lever throw half as large. To get the correct geometry I made an angle in the servo lever, resulting in better centering.
Struts
I wanted to make the struts of steel tube with a wooden streamline body (just like the real thing). I had some 4mm thin-walled stainless steel tubing (from the Fokker FG-2) lying around, which would do nicely. I measured the length, cut it off and also two short pieces for the mounting M3 bolts. With silver solder I soldered it together. The tube felt a bit flimsy and was afraid to bend it while handling the plane. I glued 1x4mm carbon strip on top and bottom of the tube and balsa at the front and rear. and also a spruce leading edge (LE). I planed and sanded this all into a teardrop shape and sheeted that with 0.4mm plywood.
Empennage
Because I didn’t succeed in making ribs for the horizontal stabilizer the old fashioned way, I went for a shortcut: I pinned strips of 2mm balsa, slightly higher than the profile is thick, upright on my building board, with glued paper rib templates on the inner and outer strip.
With a long sanding stick and several other sanding tools, I sanded the notches for the 2x2mm spars, and LE. I glued in the spars and sanded the whole assembly in profile. Then I sheeted the nose with pre-bent 0.6mm plywood. On the centre bottom of the horizontal stabiliser I glued four vertical epoxy strips, with 1mm holes that just glide outside similar four strips on the fuselage. With two long 1mm wire pins I can attach the horizontal stabiliser to the fuselage. To mount it, the stabiliser slides under the front of the fin and there is just enough space to hook the control rod to the control horn of the elevator.
The rudder is similar to the stabiliser, except for the trailing edge. Instead of a sandwich of 0.6mm plywood and 3mm balsa as core, as for the wing and stabiliser, I laminated the trailing edge from three strips of 2x3mm and let it continue all around the rudder. Of course I sanded it down later. The hinges are basically identical. I did make it easy to disassemble, for ease of transport.
Fuselage
The starting point are the fuselage frames. I wanted to laminate them, because I have some experience with that process and it is good engineering. They are laminated from 5x1mm spruce strips, the main frame from 10x1mm strips. Lamination goes like this: I put cling film over the drawing of a frame and then stick in pins on the estimated inside of the frame, adjusting them later, so the outside of the frame fits the drawing. I bent a package of five strips around them, the larger frames I can bend dry, with the smaller frames I have to use boiling water to avoid breaking. How is that done? With a kettle and a flat oven dish, I pour boiling water over the strips in the flat dish and bend them into the right shape, sometimes I have to repeat this until they are right. It’s a good idea to try it with some strips to get an idea what you can do with wood. Be careful not to burn your fingers — if you are afraid of that it might be a good idea to wear household gloves. With the help of more pins and clamps I make sure that the bundle of strips comes into shape. Then I apply a generous amount of thin CA and after a few minutes I can separate the frame from the cling film.
Before I removed the frames from the drawing, I drew the centre lines on them. Also a horizontal line where the 10x10mm battens were to be placed to hold the frames in place. I glued 10x10mm strips and which in turn were screwed on a building board to complete the setup.
I had to adjust some frames because the lack of accuracy through the enlarging of the plans. Checked them with a long thin batten (called a spline in shipbuilding) to find high and low spots. I sanded the high spots down with a long sanding stick and had to fill up some low spots with some extra strips glued on the frames. Now I had a nice, sleek skeleton ready to sheet.
I wanted to make the joints of the plywood on the fuselage also as a butt joint. A scarfed joint, if done properly, is technically the best solution. The advantage of a butt joint is that you get an optically clean joint and it is easier to make. My idea was to make a long butt strip for the horizontal joints instead of stringers and glue butt strips over the frames for the vertical joints.
I glued the horizontal butt strips of 0.6mm plywood over the frames. To make that more rigid, I glued pieces of 4mm balsa between the frames, sanded the balsa smooth with the inside of the frames and glued a spruce batten of 8X2mm on it, making these sandwich stringers very stiff.
Next was the bottom centre stringer or keel, 2mm thick spruce, wide at the front where the skid should come. After chamfering the stringer I glued plywood strips to it as well. The fuselage was now so stiff that I could take it off the building board and turn it around, enabling me to make an epoxy fillet between the keel and the plywood strips.
Before sheeting I needed the wing-fuselage connection. I made a sort of centre wing piece, consisting of the metal wing joiner tubes glued between ply plates, and two 3mm thick ply root ribs. This centre piece fits over the mainframe. Having this sorted out I sheeted the top of the fuselage with 0.6mm ply glued with thick CA, leaving some slots open for the wing centrepiece. I glued in pieces of 3mm balsa sheet to support the ply sheeting later. On this base I glued pieces of pre-bent 0.6mm plywood, to make a nice fairing.
With this complete, I went on with the front part of the fuselage. I sheeted the inside with plywood and the servo tray’s of poplar ply were glued in.
Next, the nose cone. A tricky part, with all those narrow curved panels. I read that one restorer of a full size Petrel spent six months on it. I started with a hollow cone made of ten layers 10mm balsa. The glue joints give, as with plywood, an idea of what you’re doing! After roughly sanding it into the right shape, I tried to sand it so the panels will fit, these can only be bent in one direction. For the rear half of the nose, the panels go from rounded to flat, forming a decagon. At the front, the flat panels curve to the nose point. I chose veneer for the sheeting, you can sand it without showing the layers as in ply.
Interior Details
In my opinion, no scale project is complete without a detailed, authentic, scale interior — which includes the pilot of course. Thus:
Pilot
To make the interior of the cockpit I needed the pilot. I made it from 10mm and 20mm thick balsa. Knee and elbow joints are Robart hinges, the hip and shoulder joint I made from a piece of shock cord under some tension. My sister Hans made the clothes for the pilot. I made the hollow head out of Super Sculpey, and my wife painted the head (see Resources section for the great tutorial from Josh Foreman).
Seat and Canopy
Next the backrest , the seat and the floorboard. I made the safety belts, the ‘Sutton harness’, from woven elastic tape, sewed leather strips from an old wallet on. Then I pressed in copper rings, I found the copper bushes that you get with servos very suitable. From balsa I made a mould for the canopy and had it vacuum formed on it.
Instrument Panel
I couldn’t find a picture of the instrument panel at first, but thanks to Scale Soaring UK, I got the right image. Actually it is not a panel but a kind of box. Nice place for a receiver battery I thought. Also I saw in the few pictures of the interior that there is a kind of narrow table in front of the instrument panel. I made a kind of shelf with the on/off switch under it. The box itself I made of 0.6 plywood, except for the back where the instruments should come, that became three layers. With a circular cutter, a kind of compass with a knife, I cut round holes in the plywood for the instruments. The visible side the hole corresponding with the instrument. In the second layer I cut holes that were a few mm larger (that’s where the photo plus glass fits) and behind that a closed layer. I glued these three layers only at the sides and the top, I didn’t glue the bottom. I cut out the ‘instruments’ out of a photo of the instrument panel.
For the ‘glass’ I took 0.2mm of transparent plastic, left over from some package. After painting the whole thing black I could slide the cut out instruments with the ‘glass’ in it and because they are locked in the holes in the middle layer it didn’t need to be glued. Another nice detail is the pitot tube, a very clumsy thing with transport. For ease in transport I made it dismountable. I soldered the solid pitot tube in a brass square tube, which fits exactly in a larger brass tube, which I in turn glued in the fuselage. With a magnet it is held in place.
Finishing
There are many options to stain the plywood. I use bister, an old brown water-soluble pigment, available at art stores. I start with a very diluted solution, darkening is no problem, just do it again. Because it is so watery I apply it with a brush and spread it with a sponge.
After this I apply two layers of dope with filler and a layer of PU yacht varnish which is different from clear varnish. The covering was done with Diacov, a woven material.
First Flights
Quite suddenly, it seems, the Gull is ready to fly. Fortunately, after the COVID-19 limitations were eased, we were allowed to use the field again and the weather was nice too. Rob wanted to tow, Raymond equipped the Gull full of camera’s and I checked everything again — are the ailerons really going the right way? Always exciting the first time, but after a few metres in the air I noticed its responding well, some aileron trim and after uncoupling it needed some elevator trim, it flew fine! A beautiful sight, just like the real thing. The transparency of the covering is really nice. Because of its size and weight it flies like the real thing, with enough speed in a strong wind. Also when landing it looks more and more like a real plane, you can just see that there is more mass involved. Fortunately the relatively small spoilers do quite a bit. The second flight I had some thermals. The Gull circled in the lift nicely, leaving me with a satisfied feeling!
I hope to fly many hours with it, but it still needs a vario. Secretly I had hoped to keep it a bit lighter, that’s always a challenge.
I would like to thank Rob for drawing and towing, Adri for milling, Raymond Esveldt for the video, all the other club members as well as Retroplane and Scale Soaring UK for their support!
Epilogue
After flying a whole season with the Gull, not on slopes but on flat fields, I found the flying performance a bit disappointing. I guess it’s a bit to heavy and I am not happy with the choice of the wing profile. At Retroplane 2021 there was not much wind, I couldn’t fly the Gull, so it has not flown on a slope — more’s the pity.
That triggered me to build another glider, the Slingsby King Kite which will be the subject of a future article for RC Soaring Digest. I managed to build the King Kite lighter and with a better wing profile. It just made its first flights and it seems to perform much better, which is nice if you don’t have slopes close by — I live in the Netherlands after all!
©2022 Vincent de Bode
Resources
- Dimensions: weight 6200g; span 382,5cm; wing area 92.9dm²; wing loading 66.7g/dm². The ‘scale’ weight should be 4438g, so it’s a bit heavy.
- The Folker FG–2 — My first article which appeared in the New RC Soaring Digest.
- Scale Soaring UK — An invaluable reference resource. From their website: “We are a group of people with interests in Radio Control Large Scale gliders and Sailplanes and Tugs. If you have interest in the following, then why not join in with our discussions on the forum. Maybe you can even help others with your views and ideas.”
- Slingsby Sailplanes by Martin Simons — “This work describes every Slingsby sailplane and glider from the British Falcon of 1931 to the last motorless aircraft produced — the Vega. Each type is illustrated with a full-page three-view drawing, with photographs and text outlining the background to the design. The drawings are based on the original workshop plans.”
- Vliegtuigtriplex 0.6mm Buigen (Bending Aircraft Plywood 0.6mm) — This video shows how “I bent [the sheeting] under running boiling water over it and dried it with a hairdryer.”
- Bending 0.6mm Ply across the Grain — “How to bend 0.6 aircraft ply at right angles with the grain, in this case for making the fuselage/wing fairing of a scale vintage glider.”
- Sculpey 101 Class 1: Tutorial on How to Sculpt a Head with Polymer Clay — “We start with building the basic structure for a humanoid head. I tried to come up with a way for beginners to be able to get the proportions correct.”
- Slingsby Gull at Halle — “Scale model Slingsby Gull III at a retro aerotow meeting in Halle, Germany.”
- Slingsby Gull Scale 1/4 — As it originally appeared on Retroplane.
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.
