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Tandem light aircraft proposal Shehak.
In an effort to create a cheap, foolproof light aircraft, D. Schehak, Hamburg, carried out model tests with a tandem. Extensive aerodynamic refinement was dispensed with from the outset in favor of the simple structure and a low price. The Tamdem design was given preference because it offers advantages in terms of overall dimensions and control.
After flight tests with a model with a wingspan of 1.5 m, the following points for the construction emerged: 1. The surfaces should not be placed in the same plane, preferably the rear one is higher than the front. The difference in altitude allows the horizontal distance to be reduced. 2. The position of the means of pressure is determined in the same way as in duck fliers, it lies more or less in front of the middle between the means of buoyancy of the two surfaces, depending on the deflection. 3. For longitudinal stability, a deflection of the surfaces by 1° is sufficient. Angle of more than 3° causes wave flight (over-attenuation). 4. The lateral keel through the hull is sufficient, a
special keel fin behind the second surface is not required. 5. The tandem construction method is not very sensitive to changes in loads. Even with strong tail heaviness, there is only a long wave flight without sliding sideways. 6. The damping of vibrations around the transverse axis (longitudinal stability) is
very large. The model reaches normal flight attitude after a short flight distance, regardless of the position in which it was launched. Slipping over the wing could not be detected even when taking off with an inclined position.
Compared to these advantages of the tandem design, there are two disadvantages: 1. Higher power requirement than with a single wing. 2. Higher stress on the fuselage due to bending and torsional forces.
The construction of a single-seater with 20 hp built on the model tests was omitted due to Shehak's death.