Banana Speed FX

Developed in 1985, the «Banana Speed FX» was the first competitive F3B model, developed at the Akamodell Stuttgart team. It was also the first all composite sailplane model of the group. The aerodynamic design was performed mainly by Werner Würz, using Multhopp's method for the optimization of the induced drag. The airfoil FX 60-100 has relatively high amount of camber, which gives it excellent endurance performance. For speed tasks, the camber can be reduced by the full span flap/aileron combination. Later contest experience showed, that better allround performance could be achieved with a less cambered airfoil, and led to the development of the Wizard. Nevertheless, the Banana was an important mile stone in the development of building techniques in the Akamodell. For the moulds for the wings and the horizontal tailplane, a building technique was adopted, which had been demonstrated at the International Segelflug Forum (ISF) in Switzerland. This technique is based on thin, preformed aluminum sheets, which are epoxied to foam moulds, which yields very quickly moulds for trapezoidal wings. It proved to be very difficult to create the desired airfoil shape, though, especially in the leading edge region.

Front view of the model, also showing the panel model.

After the first models had been built, I used a 3D panel method to analyze the flow around the model, with special interest in 3D effects and in the flow around the fuselage. Similar to 2D panel methods, as used for airfoil analysis, the 3D methods make it possible to find the flow field around arbitrary bodies. The effects of boundary layer and thus, friction have not been taken into account, though. The difficult part is the definition of the geometry, and the computations had been very time consuming - they were done on my PC with an 80286/80287 processor, which had been a top performer in these days...

The image on the right shows, how the front part of the fuselage and the wing are broken up into individual, flat surface panels to calculate the flow field around the model. The solution yields the velocity and pressure distributions on these panels, which can be used to calculate streamlines on the surface. Also, it is possible to calculate streamlines in the flow field, at a distance from the body.




The images below show some results for an angle of attack of 6º, which corresponds to a thermaling flight condition.

The velocity distribution along the center line of the fuselage shows the influence of the wing and also, that the flow around the front part is creating lift at this angle of attack. The flow over the tail boom of the fuselage is nearly symmetrical.

The streaklines on the surface of the forward fuselage show how the upwash in front of the wing forces the flow across the fuselage. Also, the flow distortion around the nose, which can be seen as a small suction peek in the velocity distribution, is clearly visible.

see also: 3-view drawings of modern F3B models.

Last update of this page: August, 28th, 1997.

Last modification of this page: 15.10.07

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