The ECT rotor disk is machined as one piece, with blades and disks integrated, from a solid stainless-steel forging (material 17 – 4 PH). The method used for manufacture is 4-Axis CNC milling. This multi-axis milled bladed disk is termed a “blisk” and is the latest trend in aircraft engine compressor wheels. Conventional competitor turbines have separate disks and blades, with blades inserted into retention slots in the disk.
The blisk method of construction results in an exceedingly robust rotor, capable of withstanding the stresses at over-speed of 150%. This compares with the blade-release over-speed of <50% for conventional inserted blade designs. Further, the material chosen for the turbine blisk has an elongation of 13% before failure. This means that the disk would grow about 30 mm radially before rupture. Because the gap between blisk and turbine shroud is only 1 mm, this elongation would cause a severe rub, followed by seizure and stopping of the turbine well before it approaches the burst speed. (This approach is used in aircraft engine air-turbine starters, as a safety measure to prevent rotor burst in case of a control failure). This form of intrinsic safety is unique to ETGroup ECT.
The accuracy of blade profiles in a blisk is generally in the 20-30 micron level. This compares very favourably with inserted blades, which have tolerances in the 200 – 300 micron range. Further, unlike inserted blade designs, which often have “missing” blades due to the locking features required, the blisk is always fully bladed. Thus, blisks offer the potential for fully exploiting the advanced aerodynamic design methods used in the design of the ECT.