My thoughts on the design and development of a modern F3J model

After visiting a F3J contest in 2005, I decided to design a Glider for my own purpose since the currently available models did not meet my expectations of a F3J sailplane.

To Start my construction and design Ideas and to learn more about the demands of a F3J ship, I constructed a very light set of wings from my “old” T-Warp mold set and combined these with the fuse of the F3B Model “Galaktika”. During this experimentation, I confirmed my ideas about the basic workings of a new F3J Model.

The dimensions of the model were found relatively quickly. 345cm wingspan, and a very thin fuse with cross tail. The wings were designed as 2 piece. The thoughts over the individual design Ideas are as follows:

345cm Wingspan

I was designing a model for all conditions, so I chose the middle size span of 345cm. I believe various flying and weather conditions, e.g. floating or wind, should be dealt with by adding ballast and different stiffness and strength model lay-ups. In my mind, switching models during contests is not optimal since the combination of model/pilot would constantly change. In addition I was looking for a lively and agile model.

Two Piece Wing

There are multiple reasons for two-piece wings; the most obvious benefit is the ability to center most of the mass in the CG range. The weight and construction difficulties of a bolt on wing are also nonexistent. By the chosen span of 345 cm, the transport also does not become an issue. Each wing half is 170cm, just slightly longer than the 160cm long fuse.

Fuse

The basic fuse was derived from our F3B model, the Altus. The minimal frontal area was maintained, but the length was matched to the new wing. The nose of the fuse has a stronger slope than the F3b version to assist with precision landings. The traditional nosecone was changed to a canopy system and the overall strength of the fuse was enhanced to meet the demands of F3J landing tasks.

Rudder

For the rudder profile and shape, I utilized the rudder from the “Skymax 4”. This rudder was a perfect and seamless fit. Phillip Kolb provided the rudder, and I thank him for this contribution.

Dihedral

The wing was given a triple dihedral break to maintain stable thermal turns. The central dihedral joint is provided by the 2-degree wing joiner, and each wing half has an additional dihedral break at the flap/aileron joint.

Elevator

The elevator of the High End maintains the geometry of the wings, but is reduced to approximately 10% of the wing area. For the airfoil, I chose the Selig 8025.

Wing (geometry and airfoil)

The wing geometry is centered on a stretched elliptical wing to give the model forgiving elevator tendencies and stable thermal turning capabilities. With a root depth of 250mm and with the combination of the stretched elliptical wing, the resulting wing area was 71.6dm2. I chose the smaller aspect ratio wing to ensure that the RE-numbers at slower airspeeds would still be sufficient. At the same time, this wing plan form results is a sufficiently strong yet light wing. The wing airfoil design was taken over by Andreas Herrig. We chose a thinned MH32 airfoil as a starting point, which I asked Andreas to optimize for the demands of a F3J model. The resulting AH151 tapered to AH151a showed very promising results on the polars. This airfoil is optimized on a control surface depth of 25%, which I incorporated into the wing design. At this point I have to thank Andreas for his work. He started with the MH32 and modified it to exceed my expectations. In over a year of flying the model, I must say that Andreas’ airfoil does everything better than the MH32. The flap and aileron effectiveness as well as the flat turning capabilities of the High End has set a new standard. The model is extremely neutral with no tip stalling tendencies, even in weak weather.

I am sure Andreas would like to write more about the airfoil himself.

Construction

The wings and elevator were CNC cut and the fuse plug was made from already available materials. From this the molds were created. The wings were then laid out in full carbon construction. It is my experience that a carefully built carbon wing is not heavier than a glass wing, but is many times stronger. This relates directly to a stiffer wing, which enables higher launches and longer flight times. This experience was proven in praxis. My lightest High End weighs 1940g and has a wing loading of 27g dm2, stronger lay up versions weigh 30g dm2 and are with the additional carbon “windproof”.

Dieter Perlick