The present invention relates to the art of press moulding, and more particularly to a method of manufacturing hollow pieces.
The method according to the invention may be most advantageously used in the manufacture of the blades and spars of the helicopter rotors and steering rotors, the blades of the aircraft propellers and fan blades.
Methods of manufacturing hollow pieces by pressing a blank are widely known. The pieces obtained by these methods are of a constant cross-sectional shape over the entire length.
Furthermore, the methods of manufacturing metallic spars for the rotor blades of helicopters by pressing a blank with subsequent treatment thereof are also widely known.
By this method the blade spars made of an aluminium alloy are manufactured in the following manner. An aluminium alloy ingot is placed into a press chamber and is heated up to the plastic state. The press chamber is provided with a cavity corresponding to the shape of the outer surface of the spar with predetermined allowances for subsequent machining, as well as with a mandrel having a cross-sectional shape corresponding to the cross-sectional shape of the spar cavity. The mandrel and the cavity are disposed relative to each other so that the space defined therebetween corresponds to the spar wall's thickness with an allowance for machining. The blank portions, which are sectioned by the mandrel partitions, are pressed over the inner surface of the spar, whereafter they are welded under pressure and heat during the shaping of the outer surface in the press chamber cavity.
The resulting blank comprises a hollow press shaped piece of a constant cross-sectional shape. In order to obtain a spar from this piece, the latter is subjected to a number of production treatment steps including heat treatment, dressing, machining (milling, grinding, scraping, polishing), as well as twisting about the longitudinal axis, surface hardening, application of electrolytic coating. These steps are required to obtain sufficient mechanical properties and dynamic strength, a predetermined aerofoil shape, as well as corrosion resistance.
The above-mentioned production treatment steps require a large number of complicated, cumbersome and expensive equipment. A large volume of wastes results from this treatment.
In addition, by the above-described method a spar is obtained with a constant cross-sectional shape of the cavity over the entire length thereof, whereby rational arrangement of the blade based upon mass, aerodynamic and elastic characteristics determining the rotor efficiency and service life depending upon the dynamic strength conditions is limited. These characteristics include an optimum mass distribution over the length and width of the blade, a variable shape of the blade in a plan view, a variable arrangement to aerofoils as to the type and thickness thereof, great blade twist angles, elastic properties of the blade variable with the length, which are determined by its cross-sectional shape and wall thickness also variable over the length.