Saab Draken (oz15627)

About this Plan

Saab Draken. Radio control sport scale model. Tractor prop layout. Wingspan 31.5 in, for .20 to .25 engine.

Quote: "Saab Draken. Lars Pilegaard, from the Viborg RC Klub in Denmark, gives RM his latest all-sheet version of the supersonic fighter, for .20 size high speec aerobatics.

The original Draken model was made in the early 1984 for club PR work. It was necessary to use a model construction method which would be easy to build, and could be equipped with old and different radio parts - because nobody wanted to take new radio gear from their latest mode!, even though it would be good PR and bring money to his club etc.

The models data are following: span 80 cm, weight 1200gm, engine size 3.5-4 cc and radio 3-4 servos for throttle, elevator, ailerons and needle-valve control (not necessary, but an advantage) - and of course, either electronic or mechanical mixer.

Flying the prototype model brought me a lot of experience, and the model l am now describing is, in fact, the fourth generation and more will come time after time. But take note from the outset - the Draken is not for beginners. Experience in aerobatic and/or Club 20 race flying is needed.

THE AERODYNAMICS BEHIND THE MODEL: This model is either what we call a flying flat-plate or a normal plane, as it is lifted in the air by a vortex, just like the real Draken. The vortices start as shown in fig.1 at the front of the delta wing and then it rolls out, growing bigger and bigger, along the leading edge of the wing, until it breaks off by the wingbend leading edge. by the "airleading-filler. Just after the fillet, a new and smaller vortex builds up, so that the wing has its maximum lift on the centre part, giving high manoeuvrability; as opposed to deltas with straight leading edges, where the vortex produces maximum lift at the wingtips. Of course, I have not seen the model Draken in a wind tunnel, but confirmation that the theory holds out in practice is given by missing paint on the top surface of the model's wings, which is evident on the real Drakens, too.

GENERAL TIPS BEFORE BUILDING: Only use wood of absolute first quality, or you may experience damage caused by both the engine vibration and the vortex. Keeping your eye on the weight is necessary too, as your model otherwise will lose its speed too quickly in the vertical climb. No speed - no control, because all deltas are unstable in low speed. For the construction you will need some pieces of beechwood and 2 mm plywood for the engine mount, some 1mm ply to reinforce the drop tank and the following balsa in 10x100 cm (4 x 36 in) sheets: 1 sheet of 4mm, 6 sheets of 5mm, 1 sheet of 6mm, 1 sheet of 10mm, 2 pieces of 5x5mm spruce.

If you make 2-3 models at the same time, you can save both materials and time. The prototype was ready to paint . after 2 days (in the same amount of time, two of us made three models!). Gluing work is mostly by cyano and a little epoxy and contact glue.

ENGINE MOUNT: The engine mount is made of: 4 pieces of beechwood 10x15 mm, 2 pieces of 2 mm plywood and one piece of 3 mm plywood (see fig.2). The plan shows a spacing for a Webra 3.5 speed, but of course you can make it to suit other engines too.

THE DELTA WING: The whole wing is glued by cyano, and the leading edge is reinforced by 5x5 mm spruce, which is sharpened to a knife edge by sanding. You can also build without spruce fillets, but the wing will get damaged very easily, especially when landing in long grass with seedstraw (once I saw such a straw cut off an entire wingtip!). The 'air-leading-fillets' and rocket pylons, which also give the necessary strength to the wingplate, are made from 4 mm balsa. Be careful with alignment. A little bit off-line here and your model will never fly straight. Make the elevons from very hard 5 mm balsa. The design is a little special in respect to the wing chord. Don't make them bigger, if you do, the larger elevons will act as airbrakes, sending you directly into a real 'superstall', when trying to roll fast.

FUSELAGE Glue the engine mount/firewall to the wing with epoxy. Then make the fuselage sides from 5 mm balsa, and glue to wing with cyano, and to the firewall with epoxy. When the epoxy has set, Fit F2 (cut with the grain horizontal) and F3. Cut the fuselage cover of 4 mm balsa, noticing that the front end sheeting is cross-grain to make it easy to bend the balsa. Now you can place a triangle fillet of 10 mm balsa along the fuselage/wing, and sand it to a concave section with some sand-paper on a suitable diameter piece of dowel.

The fin is 6 mm balsa reinforced at the leading edge with 5x5 mm spruce fillet as for the wing, and then epoxied exactly over the fuselage centreline. The last job, which will give the model the right look etc., is to make a canopy from a 'stick' of 3 x 6 mm balsa pieces cyano'ed together and epoxied to the fuselage..."

Direct submission to Outerzone.

Draken from Radio Modeller, February 1998.

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