Current design methodologies as applied to rotorcraft transmissions yield reliable, safe, and weight effective systems. In general, weight and performance (i.e., torque transmission per pound of transmission weight) are the primary active variables while safety is addressed by monitoring the design so that conventional criteria, such as calculated stresses, are kept within established allowable levels. This procedure is almost universally applied and has been quite successful both at Boeing Defense & Space Group, Rotorcraft and at other major rotary wing firms.
It is possible, however, to raise these already good safety characteristics to ultra safe levels through the use of certain design approaches. UltraSafe gear system design will require a basic change in the overall philosophy of design as it is practiced today. This change must be revolutionary rather than evolutionary.
In order to develop an UltraSafe transmission a basic paradigm shift will be required. In addition to addressing the design process from the typical "prevent a failure from occurring" point of view, it will also be necessary to view the system from the point of view of "what happens when a failure occurs".
The current State-of-Technology is such that the concept of safety is addressed during the design process in an indirect rather than direct manner. Major design goals are achieved by designing components so that they meet the applicable allowable stress levels in order to insure that the possibility of a failure occurring is reduced to a finite but acceptable level.
Since it is not possible to design, manufacture, build or use any machine with absolute 100% certainty that a failure, at some point, for some cause, in some manner, will not occur, the design process must address the problem of failure in a more direct manner. The current State-of-Technology does not adequately address this facet of the design process.
The objective of this program is the development of a set of design guidelines and specific design techniques which will make a dramatic improvement in the failsafe operation of helicopter and tiltrotor aircraft transmissions while not significantly affecting either performance or weight characteristics. This paper presents the initial results of an ongoing program aimed at achieving this objective.