The effectiveness of Individual Blade Control systems (IBC) to reduce N/rev vibrations (N = number of blades) and noise were already proofed by full-scale wind tunnel and flight tests. Due to the system architecture and the resulting blade-individual control capability other newer applications like Performance Improvement, In-Flight-Tracking & Balancing and Reconfiguration after partial loss of control system performance or blade damage can be addressed. The objective of the program NABIS was to provide evidence that IBC systems provide positive effects regarding these new applications by means of full-scale wind tunnel tests with an UH-60A rotor at the National Full Aerodynamics Complex (NFAC). Within the wind tunnel tests a special emphasis was laid on the development and applicability of closed loop control algorithms. For the specific and new application of In-Flight-Tracking and Balancing new electrical blade root control actuation systems adapted to the low bandwidth and low authority application should be developed and tested within a laboratory environment.
An existing servo-hydraulic IBC system, developed, manufactured and tested for an UH-60 rotor, was modified regarding the control electronics and the software to provide the functionalities and the closed loop control capability necessary to investigate the new applications of IBC systems. After functional and qualification testing the modified IBC system was integrated into the Large Test Apparatus (LRTA) test stand used to test helicopter and tilt rotors in the NFAC. After various check-out and envelope expansion tests the IBC system was used to investigate the effectiveness of blade individual control systems regarding the various test objectives like required power reduction, pitch link load reduction, in-flight rotor track & balance etc. According to the test objectives tests were performed at various representative flight conditions with different thrust levels and advance ratios up to 0.4.
- Open Loop IBC inputs were shown to provide improved rotor performance. Power Reductions up to 5% and L/D improvements up to approx. 8% were found using 2/rev IBC inputs at high forward speed.
- The IBC closed loop controller was able to essentially eliminate single parameter, single frequency hub loads.
- The IBC closed-loop controller was able to simultaneously reduce multi-frequency pitch link loads with multi-frequency IBC inputs. These reductions were shown to have only a small effect on non-controlled pitch link harmonics.
- Open-loop IBC inputs were shown to reduce the negative acoustic peak pressure that dominates low frequency harmonic content nearly 50% for one of the in-plane microphones. Although additional data analysis is required at other microphones, flight conditions, and IBC frequencies, these results suggest that IBC can be beneficial for point reduction of in-plane noise.
- Closed-loop testing showed that a 1/rev rotor imbalance (in-flight tracking & balancing objective) could be essentially eliminated using appropriate individual blade IBC offsets. The required blade pitch offsets to achieve these results were different at different flight conditions, thus demonstrating the benefit of the in-flight tuning capability.
- For the application of in-flight tracking & balancing purely electro-mechanical blade root actuation systems were developed and the test results confirm that the developed technology is suitable for this application.