Investigation of the dynamic properties of engine fan titanium rotor blades in a high manoeuvrability aircraft in FOD aspect

A current problem concerning the use of military and civilian aircraft is the damage caused to turbine compressor blades by ‘foreign objects’. Here the term ‘foreign objects’ means small stones, pieces of metal, cement pitch, asphalt, etc., left on runways and taxiways. Foreign objects also include ice and iced lumps of snow as well as birds sucked into aircraft engine air ducts. All such objects pose a serious threat to proper engine operation. They are very harmful in two respects. One is the direct danger during flight when a bird or some other object is sucked into the engine. The other danger is in what might occur in later flights if the engine, especially the compressor and turbine blades, are not inspected for durability reassessment. This paper presents an analysis of how the size and distribution of defects on blade edges affect, the frequencies and fatigue strength of titanium blades in the first four stages of a high manoeuvrability jet engine fan (low pressure compressor). In particular, damaged high manoeuvrability aircraft fan stage rotor blades and measured natural vibration frequencies and logarithmic decrement of damping of high manoeuvrability aircraft fan titanium blades are presented in the paper.