The main carrying part of Mi-8 helicopter main rotor blade is the spar made of duraluminium. Strength of the spar is increased by cold hardening method (steel shot peening) on inner and outer surface. At the spar root area a steel fitting (journal) is attached with bolts and bushings to provide connection with main rotor hub. Blade segments (21 off) are bonded along the spar, forming together with the spar an aerodynamic surface of blade. Blade segments are made by bonding a duraluminium skin, two side ribs and aluminium honeycomb filling.
Blade calendar life is conditioned by ageing of adhesives and sealants, or, respectively, by increased risk of segments separation from the spar, as well as separation of segment skin from the honeycomb filling.
Since blade service life of 2000 hrs most frequently is not a limiting factor (as calendar life of 5, 6 or 7 years mostly is), depending on exploitation conditions, revitalization of blades is done by replacement of blade segments, limiting the calendar life due to ageing of adhesives and sealants, preserving the most stressed elements of structure such as spar and fitting. By complete replacement of the existing segments with segments made of composite material, a problem of blades limiting calendar life has been solved.
Blade segments are made of prepreg and aramide honeycomb filling bonded with film adhesive in specially prepared dies. All materials and agents directly or indirectly used for preparation and revitalization of blades must have certificates of quality and operating life.
After placing prepreg and filling in the die, a polymerization process is done in NC-polymerizer according to pre-defined parameters. Fabricated segments comply with the original Russian segments by geometry and weight thus preventing impermissible changes in aerodynamic and mechanical characteristics of revitalized blades.
A detailed inspection of each blade segment after fabrication also covers a hardness test of honeycomb filling-to-skin joint in specially designed devices.
Blade forming is done in a jig by placing segments onto the spar and by polymerization in NC-polymerizer according to the set parameters. Prior to assembling, a detailed defectoscopy of each spar and fitting is performed.
Apart from the standard inspection procedures, prior to and upon revitalization, each blade is subjected to torsion and tensile test, position of elastic axis and frequency response test on a specially designed test bench.