Before a repaired rotor blade is delivered to the customer, it must be balanced in order to adjust the rotor blade in terms of weight distribution according to the factory values. This is important for conformity with the other rotor blades. The better the balance and aerodynamic shape of the rotor blade is adjusted after repair, the fewer flights are needed at the customer to balance the rotor blade set. However, traditional static balancing only considers blade mass centre of gravity measurements and adjustments.
Innovation Track 3: Developing a testing facility to reduce maintenance costs
The purpose of the project is to develop an innovative helicopter rotor blade static testing facility capable of measuring the properties of rotor blades. The facility will measure all the blade properties that are relevant for the blade dynamic behaviour. The aim is to use this information for a blade adjustment advice, prior to the dynamic Rotor Track Balancing (RTB) of the blades. The ultimate goal is to reduce the number of required flights for RTB and thus to reduce helicopter maintenance costs for Defence.
Computer simulations done by NLR in 2019 have resulted in a ranking of blade parameters that are most significant for efficient Rotor Track & Balance. The ranking is adopted by the Royal Netherlands Air Force (RNLAF) for assessment in a test phase and implementation in a certification trajectory. On the basis of NLR’s findings a precision measuring facility was developed and commissioned by SPECTO.
The measuring facility consists of balancing equipment with purpose-made precision scales on either end. A 50 kg scale is positioned at the root side of the sub frame and two 35 kg scales are positioned at the tip side of the leading edge and the trailing edge of the sub frame. This allows for three-point measurements to be carried out to determine the centre of gravity of the propeller blade. The scales can only be used correctly after calibration. The calibrated weight needs to be verified before each use and the scales need to be calibrated once a year.
The analyses and state-of-the-art measuring facility have proven to support a more efficient Rotor Track & Balance procedure for RNLAF Apache helicopters. Proper balancing of a rotor blade decreases the required number of Rotor Track & Balance (RT&B) flights.
Runtime: May 2016 – May 2019
The Boeing AH-64D Apache helicopter has been chosen as demonstration platform for the analyses.
Base specifications for a static rotor blade measurement facility are available, enabling the start of the facility’s design and development.
The simulation model for the calculation of the structural blade and hub loads in an isolated rotor has become available, enabling the start of the rotor balancing analyses.
Simulation model for the calculation of loads and accelerations in a complete helicopter has become available, enabling analyses of the rotor track and balancing activities.
A full set of the aerodynamic properties of the apache rotor blade, including the trim tab influence, has been provided which provides the input for blade aerodynamic influence on the RTB process.
The properties of recently introduced ‘trailing edge wedges’ on rotor blades have been included in the flow analyses.
Virtual RTB and optimisation is developed and available in the rotor simulation tool. Validation with data from operational RTB sessions can now be done.
A new design based on latest finding of the simulation and sensitivity studies has led to a more efficient, lean and more functional design of the measuring facility. The functional design is frozen and is set to be manufactured.
The test facility will be available and will be able to measure the relevant properties of the rotor blades in the RNLAF inventory, to support more efficient Rotor Track & Balance procedures.