Helicopter power transmission: changing the paradigm

Abstract

In the conventional helicopter, the transmission and powerplant systems are the major production and operator direct operating cost drivers. Additionally, they are related to helicopter safety and reliability concerns and impose performance boundaries. As a consequence, they need to be addressed.

Adapting the transmission and powerplant systems with the introduction of more electric technologies, which are reported to be more reliable and cost friendly, involves a gross weight penalty, which cannot be accepted from a performance viewpoint. The implementation of liquid hydrogen, with the objective to introduce a sustainable energy carrier and free cold source for the high temperature superconductive devices driving the tail rotor, appears unattractive, from either a weight or a exploitation standpoint. Biodiesel could be an alternative to Avgas driven configurations, but at the moment, it has questionable chemical characteristics and is therefore discarded. Conceptual alternatives to the conventional helicopter explored in an attempt to verify their ability to overcome the stated performance, safety and cost aspects, are subjected to the same problems, the Turbine Driven Rotor (TDR) helicopter configuration excepted. The TDR helicopter drives a coaxial rotor configuration by means of a rotor embedded Ljungström turbine, omitting the need for a mechanical transmission system.

Three TDR helicopter thermodynamic cycles are proposed. The piston engine powered TDR cycle shows to be of interest for the low weight class helicopters. The turbofan powered TDR cycle is preferred in the mid and high weight categories, benefitting from its configurational simplicity. The more complex turboshaft powered TDR cycle requires a heat exchanger, is heavier and thus not recommended.

With respect to the Ljungström turbine, the loss models of Söderberg and Ainley and Mathieson used to establish its geometry and performance characteristics are generally acceptable and appear coherent, while a deviation angle correction is developed to cope with the radial outflow configuration of the turbine. Similarly, loss models for the internal leakage and disk friction are proposed. However, these models could not be substantiated by means of experiments. A design methodology to implement the Ljungström turbine in the helicopter rotor head is presented and allows adjusting the thermodynamic cycle characteristics such as to maximise the performance gain with respect to the conventional helicopter. For nominal operating conditions, ISA SLS, a VLR-class TDR helicopter shows to bear a performance gain of 10% over a conventional helicopter when equipped with an Avgas engine and 14% when a Diesel engine is used. Hereby, the cycle pressure ratio remained low, i.e. approximately 1.25, allowing a turbine polytropic efficiency of 87%. An identical study with a NH-90-class TDR helicopter proved to offer a performance potential of 50% at a cycle pressure ratio around 1.6 and a turbine polytropic efficiency of 90%. In all cases, the gas temperature at the inlet of Ljungström turbine remained below the rotor bearing temperature limit of 400 K.

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