Determination of Service Life in a Poorly Designed Composite Blade

Design of a composite blade requires some iteration from initial design to test and final production. Special attention should be given for composite parts which require long service life. If a poor designed composite component shows damage in the service, it becomes more challenging to investigate the cause and determine the repair procedure. Determination of the maintenance intervals becomes important too. Certification requirements for a manned rotorcraft mandate Fatigue tolerance and Fail-Safe evaluation for a flight structure. However, there is no specific requirement for unmanned air vehicles. NATO STANAG 4671 Airworthiness requirements for UAV's Custodial Support Team have recently extended the scope of study to rotary aircrafts and light UAV's. Airworthiness requirements for rotorcrafts larger than 150 kg shall be covered and base for this standard is selected as CS 27. This paper is an example of poor designed blade which can be used for a ducted fan type UAV in the absence of certification requirements.

In the scope of this study, a damaged composite blade which has deformation in the perimeter of root has been investigated. Manufacturer analyses have been reviewed and flaws are identified between manufactured part and the analyzed part. Basic FEA model has been prepared; test reports are reviewed and used for initial analysis verification. NDT methods are used to examine failed portion and destructive inspections have been done when necessary. With the depth knowledge of the situation, design has been improved with structural requirements given in airworthiness standards. Repair / reinforcement procedures are developed for the other blades which are in service. In the end of the study, very widely discussed issue of using manned aircraft standards in the UAV world is answered with numerical advantages. Cost is always an important parameter for UAV's, but inappropriate flight structures shall cost more in the long term.