Minimizing Dynamic Response of Counter-Rotating Engines Through Optimized Node Placement

It has been previously proposed that a low-speed rotor balancing procedure can be suitable for supercritical shafting (GT2008-50077). That paper documented the necessity of taking into account nodal locations in the bending mode shapes of a supercritical rotor when designing an optimum balance process for such a rotor. This is due to the fact that balance correction forces (or for that matter, any forces) have the least impact when applied near the nodes of a particular mode. This result led to consideration that node location optimization could help with another issue, i.e. the excitation of backward excited whirl modes in a counter-rotating system. When designing a two rotor gas turbine, there are distinct advantages to having the two rotors turn in opposite directions. Among these are the ability to shorten and lighten the engine by reducing the length of the engine since a row of static turning vanes can be eliminated. The engine can be further lightened by inclusion of an inter-shaft bearing which eliminates static bearing support structure. Additional reduction in gyroscopic maneuver loads and deflections can also be achieved, thus resulting in multiple benefits to a counter-rotating system with an inter-shaft bearing. Unfortunately, the excitation of backward whirl modes of one rotor, which would normally not be a major concern in a co-rotating engine, can be a significant issue when excited in such a counter-rotating engine through the inter-shaft bearing, which serves as a conduit for forces from the other rotor. However, the logic of the earlier statement regarding the effectiveness of forces applied at, or near, a nodal point led to the hypothesis that optimizing the nodal locations relative to the interface points between the rotors could minimize the responsiveness of the system. This led to the hypothesis that by optimizing the node placement relative to the inter-shaft bearing, it should be possible to minimize the excitation of the backward modes. This paper examines that proposition and demonstrates that considering this aspect during the design of such an engine could lead to significant benefit in terms of minimized dynamic responses.