This invention relates generally to turbomachine airfoils and, more particularly, to an inlet guide vane structure for a turbofan engine.
The invention herein described was made in the course of or under a contract, or a subcontract thereunder, with the United States Department of the Air Force.
In turbofan engines comprising alternate stages of stationary and rotating airfoils, it is desirable to have the stages relatively closely spaced in the axial direction in order to minimize the length of the engine. This is particularly true where there is a plurality of stages and, therefore, multiple axial spacing.
The effort to minimize the axial spacing, however, is somewhat limited by the requirement for blade deflections during conditions in which foreign objects such as birds and ice are ingested into the main airflow stream. Since these ingestion loads generally exert both a circumferential and an axially forward component of force to the blades, an accommodating axial clearance must be provided which results in increased engine length and weight, or alternatively, an interference will occur between the rotating and stationary airfoils. If the latter circumstance is allowed to occur, the impact between these interfering elements tends to cause significant damage to both the rotor and the stator blading and is likely to cause secondary damage resulting from the passing of severed parts through the system.
A particular turbofan structure which is susceptible to this phenomenon is a multiple-stage fan having a plurality of inlet guide vanes located upstream thereof for controlling the direction of and possibly the volume of air which enters the system. Upon the ingestion of a significant foreign object, the stage one rotor blades tend to deflect forwardly causing them to strike the downstream portion of the stationary inlet guide vane. The impact may result in significant damage to the rotor blade, in which case the engine would have to be shut down because of the imbalance problem or, alternatively, it may cause significant damage to the inlet guide vane which would tend to decrease the effectiveness and efficiency of that airfoil. In either case, it is likely that large sections of either the rotating blade or the inlet guide vane would pass through the system and cause significant secondary damage to the downstream system.
The problems brought about by ingestion loading become more pronounced in aircraft which are designed to operate at high velocities and low altitudes where the number of foreign objects encountered tend to be numerous. If sufficient axial clearance is provided to allow for the blade deflections, the resulting increase in engine length and weight may very well be unacceptable.
It is therefore an object of this invention to provide a turbofan engine with reduced sensitivity to foreign object damage.
It is another object of this invention to provide a turbofan engine with minimum engine length and weight characteristics.
It is yet another object of this invention to provide for the axial deflection or deformation of rotating blades within a turbofan engine without causing substantial damage to the engine.
It is still another object of this invention to provide for axial deformation of blading and resultant impact thereof with the stationary airfoil without causing substantial secondary damage to the system.
These objects and other features and advantages become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings.