Mejzlik Propellers s.r.o. 28x9.4 2B MC Folding

The Mejzlik 28x9.4 2B MC Folding is a folding two-blade composite propeller from Mejzlik Propellers s.r.o. (Czech Republic) for heavy multicopters. The designation gives diameter and pitch, 28 in and 9.4 in, and the blade count as 2B. It is made by wet layup and can be folded for transport.

The 9.4 in pitch is one third of the diameter: flat, laid out for hovering under heavy load. The weight is 147 g with a tolerance of 10 per cent. The centre hole measures 10 mm; the hole pattern provides four M3 screws on a 20 mm circle and four M4 screws on a 23 mm circle. The maximum speed of 6,400 rpm is, according to the datasheet, the hover limit, bounded by blade-tip speed; in forward flight the limit drops. The maximum thrust of 30.0 kgf is simulated.

Both values hold only with a given motor, voltage and operating state and are comparable only within one manufacturer. Legally the aircraft counts: heavy multicopters operate at the edge of the open category, which ends at 25 kg take-off mass including payload. Above that the specific category applies, in which operation rests on a risk assessment. At 28 in, spinning blades demand distance from people and a de-energised drive before any work at the propeller.

On a folding propeller the blade seats are the wear point: they must be clean and free of play and open both blades evenly, otherwise imbalance results. The hinge, blade root and edges are checked for cracks and delamination, visible as frayed edges and dull-sounding areas. Propellers are replaced in pairs, and the fixing on the shaft is checked before every use.