Druine Turbulent D31 Libelle (oz8216)

About this Plan

Druine Turbulent D31 Libelle. Radio control scale model, for .61 power.

Quote: "R/C Modeler Magazine is proud to present one of the finest scale aircraft ever published. An exact reduction from the plans of the full size Turbulent. This R/C aircraft is totally scale in every detail.

Introduction: During a big meeting of home-built full size aircraft on the Airport of Wangen/Lachen at the shore of Lake Zurich, I was anxious to see the simultaneous flight demonstrations of a Tipsy Nipper and a Turbulent: I only had heard of the Turbulent but never seen the full size aircraft, so I looked forward to the show. The first fly-by made me completely change my mind and put this plane into the first priority of models I wanted to build. Later, I talked to the pilot and obtained the address of the owner of the aircraft. After sending him a letter it did not take long to get the full set of building plans and parts lists as well as the authorization to completely check and take pictures of the original plane at its home airport.

I don't know too much about the history of the Turbulent. It was developed in the early 50's by a Frenchman by the name of Druine. It soon became very famous as a home-built aircraft. In Germany the slightly modified Stark Turbulent was built in a lesser quantity. The original of our model was built by its owner entirely by himself in a time span of about 7 years. It is powered by an original 30 HP 1953 VW engine. A lot of cross country trips, especially to the northern countries like Denmark and Sweden, prove that more than fun-flights in the neighborhood of the airport are possible.

The Model: I decided to build a model that was truly scale in every detail directly from the full size plans. Not only the dimensions, but I also wanted the materials to be true to the original. To avoid excessive transportation problems I decided to build it to a scale that would result in a wing span of less than 2m (79in). With a scale of 1/3.33 the wing span resulted in 195cm (77in). The plans did not specify an airfoil, so I photographically reduced the full size profile. The ribs are built up from spruce strips in a jig, taking about 1 hour per rib, and using 25m of 2 x 4mm spruce strips. The main spar is fabricated, as on the original, from spruce, but on this important sub-assembly I committed the worst error with regard to the weight. When I finally weighed the model it was 180 g (6.35 oz) too heavy and this meant for me; another 20 ribs and another wing! I was able to save 324 g (11.43 oz) with this new wing, but with a balsa main spar!

The wing sheeting is .6mm ply. The only difference from the original is the leading edge, because it was no longer possible on the model to pull the ply sheeting around the tight radius. Therefore, a balsa leading edge had to be used. The attachment of the ailerons is absolutely true-to-scale. The pulleys are specialty made from alumnum, the control cables are 19 x 0.1 mm CL lines. Of course the control stick moves with the controls and the tension of the control cables can be varied by scale tension blocks.

The wings are covered by Graupner artificial silk. An absolutely scale effect on wings, vertical and horizontal tail is achieved by the simulation of the rib lacing cords and the pinked-edge finishing tapes. With regard to the latter, I made an immense number of tests until 1 solved the problem with a saw-blade. I bought one in a department store with the correct tooth height. Since the teeth are not symmetrical, the first operation was to file them. into the right shape and a uniform tooth height of 2mm. After that I started a research program on the adequate paper quality. However, I could not obtain the proper result, since either the teeth were not properly formed or the remaining fibers were too long. I was just about to give up, when I saw my 7 year old boy, Renato, producing perfect pinked-edge tapes from some of my blueprints. At first I was angry, but then I realized that this was the paper quality I needed.

The technique is simple, all you need is a flat surface, pulling slowly and along the grain structure of the paper. The covered wing is doped twice, then the finishing tapes are applied with glue. Be careful, because bubbles could occur under the tapes due to the fact that the paper expands when it gets wet. The lacing cords are simulated by applying little 'worms' of glue with a hypodermic syringe. This operation has to be practiced, because the needle has to move all the time. After these operations, the wing is covered with dope in the conventional fashion.

The fuselage of the original is built primarily from 2.5mm ply. For the model this would mean .75 mm, which is not a standard thickness in Europe, so I chose .8 (.031 in) ply. There are spruce reinforcements on the fuselage and, where they are visible, I made them absolutely to scale dimensions. I used an additional nylon cover over the fuselage with finishing stripes at all fabric connections.

An HB-61 engine from Graupner was used with a 400ccm (13.5 oz.) tank. It would have been possible to lead the exhausts through the exhaust pipe of the dummy engine, however I decided not to do so because of the problems with smearing over the canopy..."

Hello Mary and Steve, Turbulent D31 Libelle. Full ScaleModel Size 1:3,33. Span: 1950mm. Engine: 10cc Two Stroke. Wight ca : 5kg. Konstrukteur Franz Meier. Greetings,

Note: this plan is in German.

Direct submission to Outerzone.

Supplementary file notes

Article pages, thanks to Hlsat, JHatton.

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