Several partners are designing a single-propeller aircraft, which is powered by a pair of turbine engines, transmitting power through a common gearbox. One of the challenges in the design and analysis is to accommodate design iterations and multiple load cases that effect the propulsion mounting system. An ability to quickly analyze design changes and multiple load cases will mitigate the risk of failure and facilitate design optimization. A methodology has been established to systematically analyze the entire propulsion mounting system with various mounting configurations such as failure cases or geometric changes and flight maneuvers.
In this installation, the propulsion system mount points are at the gearbox mount pads. The gearbox, in turn, supports two gas turbine engines by means of two torque tubes, attaching the engines' front frames to the gearbox input flanges, and a welded space frame that supports the engines mount brackets at the gearbox mount pads. The propeller is fully supported by the gearbox through its internal bearings and internal structure. Therefore, all external loads exerted on the propulsion system such as inertial, thrust, torque, gyroscopic loads, etc. are reacted at the gearbox mount pads.
Complexity in the gearbox stress analysis arises from several sources: prop loading, engine mounting, and aircraft mount truss reactions. The complexity in predicting stresses in the gearbox housing from propeller loading is due to the need to properly distribute bearing and thrust loads through the multiple helical-cut gears, roller bearings, and thrust bearings supported within the gearbox. The engine mount loading adds to the gearbox loading at the input flanges and mount pads through a structurally redundant system. The aircraft mounting structure that supports the gearbox is a complex, statically indeterminant system of brackets, strut tubes, horsecollars, and elastomeric isolators.
Consequently, an analysis system was created to allow single-point load application at the propeller along with inertial loads to automatically account for the complexities listed above and accurately distribute load from input points to the mount pads. This paper will discuss techniques used to simulate the entire engine mounting system such as brackets, linkages, struts, torque shaft, gearbox, gear meshing, spline gear, and horse collars for stress analysis. In addition, the paper shows the comparison between analytical and actual testing data.