The Role of Shot Peening in the Design of an Innovative Twin Engine Pack System (TEPS) for Helicopters

Author:  A. Gilioli, D. Braconi, S. Bagherifard, H.Y. Miao, M. Bandini, F. Pozzi, R. Papetti, M. Guagliano
Source:  ICSP-13
Doc ID:  2017066
Year of Publication:  2017
Abstract:  
Introduction: TEPS is a twin-internal combustion engine developed at Robby Motor Engineering in order to motorize a twin-engine helicopter in the category of Very Light Rotorcraft CS-VLR. More in detail,despite coming as a “single unit” (single engine body), the TEPS motor is the rational integration of two engines capable of operating in-sync in a modular manner, but when it is required, also individually and independently. With this architecture, helicopters equipped with the TEPS are allowed to fly over built-up areas in Europe and other countries where strict regulations do not allow light helicopters with a single engine to fly over such areas. With respect of twin-turbine helicopters, TEPS allows to strongly reducing the operative costs and, at the same time, by including the two engines in a single body, reduces the final total weight of the vehicle, allowing an increase of the pay-load. This makes the TEPS a very attractive solution even for new possible applications. Among these latter, pilot training and territorial and environmental monitoring, can be considered the most immediate ones. Another advantage of TEPS is the ability to use automotive gasoline instead of Avio gas, making easier the refilling. Inside the Pack unit, the two engines are placed in parallel, while the cylinder arrangement is horizontal and opposed. Each engine consists of a group of four cylinders and is equipped with its own lubrication system, power supply and electronic control. Bearing in mind the final application of TEPS, its performances are critical, especially concerning the weight reduction and the reliability. In order to enhance the performances, it is necessary to pay a great attention at the design of the system and of its components, by adopting all the possible techniques to improve the mechanical properties of the machine elements. In this paper, the attention is focused on the crankshaft, made with the low-alloy steel 39NiCrMo3, the most critical part with respect of fatigue design due to its complex geometry and the multiaxial stress state. The design of the crankshaft has been carried out by finite element simulations; for the improvement of the fatigue behavior, the effect of shot peening, nitriding and their combination has been investigated. Particular attention has been devoted to the design of a specimen able to replicate, as close as possible, the stress state in the most stressed geometrical detail of the crankshaft, in order to get results that can be directly used for the fatigue strength assessment of the crankshaft under the expected loads.