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Open-loop hover experiment of a Mach-scaled SNUF rotor for active vibration control
Get PDFDesign and open-loop hover experiment of a 4-blade Mach-scaled Seoul National University Flap (SNUF) rotor equipped with an active trailing-edge flap is attempted. Its test is performed on the Seoul National University Rotor Test Stand (SNURTS). First, the rotor hub is improved to withstand the centrifugal loads. In it, the original soft in-plane hingeless rotor hub used for blade optimal design is replaced by a rigid hingeless rotor hub. The multi-body dynamic analysis DYMORE and cross-sectional design tool VABS are used to design the new rotor hub adaptor for the flap blade by utilizing time-marching free-wake analysis. Preliminary hover test using two-bladed OLS teetering hub showed excellent repeatability of the SNURTS, but a torque level of 400N-m should be considered. Rotor test stand modal test response and servo kinematics are fully adjusted into the analysis model. Next, a single active flap blade test is performed to assess the flap driving component. The hysteresis of the flap driving mechanism is identified in a static bench test. Bench dynamic tests of the SNUF blade active flap showed that present flap actuation system has 4/rev bandwidth. In the future, flap deflections will be recorded for the 1-5/rev at 1300 RPM, where the tip Mach number is 0.6. The frequency response of the flap mechanism will be identified under hover centrifugal loads while the blades are still in-track
Increasing the damping properties of carbon laminates by flax fiber hybridization for rotorcraft applications
No full textVibrations play a crucial role in the design of helicopter structures and are in many cases the cause for structural changes in the late development phase. Often, dynamic interactions and excitations can only be determined in flight tests while structural dynamic simulations can only make inaccurate predictions. This is even more true for unconventional designs such as eVTOLS, for which practical experience is limited. One possible solution to minimize risks in the design is to increase the global damping in the helicopter structure, for which natural fibers offer a promising approach. This paper lays the groundwork for further investigations using flax fibers in hybridization with carbon fibers for selective damping enhancement of helicopter structures. For this purpose, the damping of flax, carbon and flax/carbon hybrid laminates is investigated at laminate level. High-performing flax and carbon prepregs were used to manufacture test specimen in a vacuum/autoclave process. Experimental modal analyses, using a contactless measurement method and impulse technique, were performed on hybrid and non-hybrid laminate test specimen with different stacking sequences and flax contents to determine their dynamic behavior. The damping is calculated using the Half-Power Bandwidth Method and set in relation to their mechanical properties, quantifying the damping potential for different load cases. The superior damping potential of flax was confirmed, especially in fiber direction. The distance to the neutral axis was shown to be the main influence between the oppositely behaving stiffness and damping in the laminates. However, the use of flax fibers in the hybrid laminates made it possible to achieve an advantageous ratio in favor of the damping. Flax fiber orientation in the direction of loading (UD0) is particularly suitable for this purpose
Sustainable aviation fuels for helicopters: Challenges, opportunities, way forward
No full textReduction of CO2 emissions is key in limiting the effect of climate change and Sustainable Aviation Fuel (SAF) is one of the major enablers to achieve the challenging targets of the aeronautical industry. Since the use of blended SAF with conventional JET A-1 is already approved at a ratio of up to 50%, it is necessary to look ahead to understand how to make daily 100% SAF flight possible. Both drop-in and non drop-in pathways can be envisaged. The latter offers additional opportunities, like reduction of non CO2 emissions, but also presents additional challenges. A first flight on a H225 helicopter using 100% HEFA fuel on one engine was performed in November 2021 followed by a second flight in May 2022 using 100% SAF on both engines. Preliminary findings indicate that even if the overall behavior was found acceptable, and that no immediate detrimental effect of the 100% HEFA fuel was observed, some differences in the engine gas turbine temperature dynamics during starting phases, or in the helicopter gauging system were identified. Some were expected due to the fuel characteristic differences, others were not and will need further investigations. In addition, mid and long term impacts of a daily usage of 100% SAF need to be further assessed. Airbus Helicopters already identified several topics among which are engine operation, gauging systems, material compatibility, fuel flammability and volatility, or flame characteristics. Airbus Helicopters’ roadmap to achieve 100% SAF by the end of the decade does not privilege any solution and considers both drop-in and non drop-in pathways. For the first one, activities are centered on the work performed by the existing ASTM task force while for the second one, synergies within Airbus Group will be used to further investigate on the main topics identified previously
Generic eVTOL aircraft preliminary sizing method for AAM/UAM missions
Get PDFAdvanced Air Mobility (AAM) or Urban Air Mobility (UAM) presents many challenges to the aircraft design. The concept not only requires operations over urban airspace but also without the use of traditional airports with long runways. Additionally, the new aircraft designs must do so on electric power as much as possible with low chemical and noise emissions. Hence the term electric vertical takeoff and landing or eVTOL is popularized. This paper presents a novel method for preliminary sizing of eVTOL aircraft of arbitrary architecture. The methodology allows the conceptual analysis and initial trade-off studies independent of the aircraft configuration. Aircraft is considered as the sum of building blocks like rotors, propellers, wings, and several other subsystems contributing to mass, energy, power and drag estimates. Results from this study permit a quick evaluation of configurations and missions with an inevitable degree of approximation. Finally, the main features of the method are discussed in the paper with a relevant validation exercise for various eVTOL aircraft considering mass and other performance metrics
Fabrication and static testing of a high-speed morphing rotor blade
Get PDFHigh-speed rotorcraft experience regions of reverse flow over the inboard sections of blades. In these regions, because of the reverse flow, the rotor blade experiences high drag, large dynamic pitching moments, and significant vibrations. If the shape of the rotor can be changed, or “morphed” in the inboard section, a number of these adverse effects can be minimized. The shape modification which provides optimum benefit between high-speed forward flight and hover was outlined in an earlier paper presented at The Vertical Flight Society's 76th Annual Forum & Technology Display [1]. This paper further builds upon the work presented at the Vertical Flight Society’s 77th Annual Forum & Technology Display [2] and details the fabrication and demonstration of the full-scale rotor blade section capable of meeting the shape change requirements with adequate stiffness and stability. The fabrication of the sub-components and the final assembly of the morphing rotor blade section, which is full scale in the chordwise direction but limited in the spanwise direction, is described. The demonstrator is equipped with a rotary actuator to morph between the hover and high-speed cruise flight conditions. The morphing composite blade demonstrator meets the shape change goals while preserving stiffness and structural integrity
Comparison of optimization based inverse simulation methods for helicopter maneuvers
Get PDFIn a helicopter certification process, aviation safety agencies want to be sure that the helicopter can safely fly all maneuvers defined in its usage spectrum. Therefore, loads engineers carry out all these maneuvers for each appropriate combination of weight and center of gravity. Moreover, this maneuvering and load analysis process should be performed in the most efficient way. For this reason, this article works on two different algorithms, the gradient-based Symmetric Rank-One (SR1) and commercial optimization tool Siemens HEEDS, to perform desired helicopter maneuvers. In this study, helicopter pushover maneuver is carried out and the results for each algorithm are compared as an example. However, different maneuver results are also added to show applicability of the solution algorithms. Furthermore, rotorcraft simulation and modelling software FLIGHTLAB is used to simulate the maneuver. Rotorcraft is modeled as rigid and uniform inflow is used for the calculation of rotor aerodynamic loads
Rotor blade modeling in a helicopter multi body simulation based on the floating frame of reference formulation
Get PDFThe Floating Frame of Reference formulation was chosen to include the Beam Advanced Model in DLR’s Versatile Aeromechanics Simulation Tool. During the development and concurrent testing of the model in the field of helicopter rotor dynamics, some particular shortcomings have become apparent. These mainly – but not exclusively – concern inertial loads affecting the flexible motion of beams. This paper treats the related physical phenomena, and proposes enhancements to the model which remedy the deficiencies of the baseline method. Particular attention is given to the introduction of rotational shape functions to account e.g. for the propeller moment and the consideration of an accelerated Floating Frame of Reference to address the blade attachment’s radial offset from the rotor center in the centrifugal field. Furthermore, the application of external loads (e.g. airloads) away from the beam’s nodes or off the beam axis is addressed as a prerequisite for independent structural and aerodynamic discretization. Finally, the modal reduction under centrifugal loading is considered. The individual model upgrades are verified based on analytical reference results of appropriate rotor dynamics test cases. The enhancements are necessary for simulating flexible helicopter rotor blades within a Multi Body System – a feature required for sophisticated simulation scenarios in which the limitations of conventional rotor models (e.g. constant rotational hub speed) are exceeded
Understanding the effects of rotor dynamics on helicopter incremental non-linear controllers
Get PDFWith an increasing trend towards automatic flight control system applied to rotorcraft, the goal of the present paper is to understand the effects of rotor dynamics on the design of robust incremental non-linear controllers such as INDI (Incremental nonlinear Dynamic Inversion) and IBS (Incremental Backstepping Control). Nonlinear dynamic controllers are a desirable solution to helicopter flight control as it can solve its highly nonlinear dynamic behavior. However, conventional nonlinear controllers heavily rely on the availability of accurate model knowledge and this can be problematic for rotorcraft. Therefore, incremental control theory can solve the modelling errors sensitivity by relying on the information obtained from the sensors instead. The paper will demonstrate that for helicopters the incremental nonlinear controllers depend on the delays introduced in the controller by rotor dynamics. The paper will show how the residualization and synchronization methods need to be applied to an IBS controller in order to remove the effects of the flapping (disc-tilt) dynamics from the controller. This indicates that the incremental nonlinear controllers can have relatively small stability robustness margin when subjected to rotorcraft time delays and unmodelled dynamics that influence the feedback path and should be therefore carefully applied