Technion-Israel Institute of Technology with civil Aviation authority of israel (CAAI)
Numerical Simulation of Flutter Tests
The study examines novel flutter prediction methodologies that support and enhance the safety of flutter flight tests. The project, done in collaboration with Dr. Matthew McCrink in the Aerospace Research Center of Ohio State University, includes the design and flutter testing of a dedicated UAS configuration. The testbed vehicle is designed to be a research and technology demonstrator, pushing the flutter envelope. Hence, some crashes in flight tests are expected. Taking advantage of 3D printing technology, the vehicle is designed for rapid manufacturing and assembly, in a low cost. The vehicle will be instrumented with inertial measurement units (IMUs) that measure three-axes rates and accelerations and with strain gauges. Data collected in flight will be used for the two proposed flutter prediction methods, which are briefly presented in the Introduction section of this report. The report summarizes the structural and aeroelastic analyses performed for aircraft design.
Methodology and Devices for Safe Flutter Test
The safe-flutter-test research project was intended to be one of 4 research programs to be carried out by the ASSURE consortium of US universities and the Faculty of Aerospace Engineering at Technion, supported by the US Federal Aviation Administration (FAA) and the Civil Aviation Authority of Israel (CAAI). The final proposal of the current research was submitted to MOT/CAAI in August 2017, after conducting initial discussions with a test group at Ohio State University (OSU), a member of the ASSURE program. The proposed plan included 4 parts:
- Method advancement: (a) further development of the PFM methodology, simulation tools and application models; (b) an additional 3D test to be performed at Technion; and (c) summary of the related wind-tunnel tests. (6 months)
- Flight test planning: Simulations to investigate the intended system performance with a generic UAV model and available shakers in collaboration with the ASSURE partner. (6 month)
- Flutter tests and data reduction to be performed mainly by the ASSURE partner. (6 months)
- Impact on flutter test procedures regulations: exploration of ways to improve the flight-test procedures in terms of safety, duration and cost. (6 months)
The first year of the proposed research (Items 1 and 2 above), has been performed successfully and exhibited very promising results. However, since OSU participation has not been formally approved and financed yet by FAA, the data obtained from them was limited to conceptual design data the UAV wing.
The second year of the program started with selecting a proper shaking device and its mounting location. An existing shaker/accelerometer device of about 70 gr seems to be suitable for the task when located at a front location in the wing-tip store. An additional device of about 200 gr, with a battery, driver circuit and reference accelerometer, may also be added at another location where it has small effect of critical flutter characteristics. When it became clear that OSU are not going to start their project soon, we changed our test-case model to be the Active Aeroelastic aircraft TestBed (A3TB) vehicle developed at Technion by Prof. Raveh as a student project. This UAV is currently at its preliminary flight test stage and it was designed such that it supposed to meet flutters in it flight envelope.
The PFM flutter analysis method, on which the planned flutter tests of our project are based, was expanded to accommodate a sensitivity study for flutter characteristics vs. added mass magnitude and location. Such sensitivity study with the A3TB vehicle showed that the original flutter velocity of about 24 m/s can be changed to about 34 m/s with 300gr mass added at the leading edge of the wing-tip sections. Both velocities are inside the flight envelope, so this case may become an excellent experimental testcase for the safe flutter test methodology.
The PFM method was further expanded to accommodate a simulation of experimental noise generated by air turbulence. It was shown that the combination of the aircraft response to intentional excitation by a shaker, at the point of added mass, with turbulence noise, may still provide adequate measurement signals for the extraction of flutter margins with which the test may proceed safely.
Aeroelastic Sensing and Control Using Optical Fibers
The goal of the research program was to develop, implement, and test a methodology for aeroelastic shape sensing and control that is based on strain measurements in ﬁber-optic sensors (FOS). Past studies of the author demonstrated how FOS strain-data can be used to reconstruct the static and dynamic deformed shape of an elastic wing and predict the ﬂutter onset speed. The current study further develops these capabilities and demonstrates wing shape control. Wing shape-control can be used to optimize a vehicle performance. While it is applicable to all winged aircraft, it is mostly valuable in ﬂexible conﬁgurations, in which wing ﬂexibility can be leveraged to achieve optimal performance, while avoiding adverse aeroelastic phenomena. Speciﬁcally in the current study, FOS-measured strain-data was used in an optimization scheme with a target of keeping a wing’s elastic deformations small, below a user-deﬁned threshold, while maintaining a constant lift value (as required for trimmed ﬂight) and minimizing the control-eﬀort.
The study was numerical and experimental. In year-one, a dedicated ﬂexible wing was designed, built, and analyzed. The wing was ﬁtted with four trailing-edge control surface that were used as trim eﬀectors, controlling the wing shape and resulting lift. A ﬁnite-element model was generated and validated through a series of ground-vibration tests that were based on measurements of both strains and displacements. The mode shapes and frequencies from the ﬁnite-element model were used in the aeroelastic analyses, in the commercial ZAERO software, based on linear panel aerodynamic model. Aeroelastic analysis was performed to compute the wing’s deformation and aerodynamic lift under certain, nominal ﬂow conditions. Aeroelastic analysis was also performed to predict the wing’s ﬂutter onset speed, which, together with stress analysis, was used to set the envelope of wind-tunnel testing conditions. A trim-optimization algorithm was devised to control the wing shape based on strain-data inputs, such that under the nominal testing conditions the wing deformations are kept small while the aerodynamic lift is retained, using minimal control eﬀort. The computational phase of the study was designed to mimic the steps of the experiment, to demonstrate, computationally, its feasibility.
In year-two, the wing model was constructed, wired with sensors, and tested in the subsonic wind-tunnel in the Faculty of Aerospace Engineering, Technion. Two optical ﬁbers, each holding 10 Fiber Bragg Grating (FBG) strain sensors were attached to the wing’s main and rear spars, over the wing span. The collected strain-data were translated to modal deformations and used to compute the sensitivities of the modal deformations to control-surface deﬂections. The latter, together with lift sensitivities, were used in the trim optimization scheme to compute the optimal control surface deﬂections that will reduce the wing deformation to 70% of its nominal value, while keeping the lift ﬁxed. This was demonstrated in the optimization runs, in the wind-tunnel. The validation of the deformed shape was done with a motion recovery system, comprised of three cameras that follow infrared reﬂectors glued onto the wing surfaces.
The study demonstrate, both computationally and experimentally, how FOS can be used as wing deformation sensors, and how such an information on the deformed wing-shape can be utilized to optimize the wing performance (in the case of this study, the wing shape). Future studies can attempt online closed-loop shape control, building on the open-loop optimization of the current study. The study opens a path to using FOS (which are currently already implemented on several UAVs) to optimize winged UAV performance, and to the design of lighter (and thus more eﬃcient) UAV platforms.
Aeroacoustic Signature of Quadrotors
Urban air mobility faces increased concerns with respect to public acceptance of small scale un-manned aerial vehicles (UAVs) due to noise pollution. These vehicles, commonly referred in the literature as drones, are designed to provide thrust and torque with rotors, propellers, and fans, which are required for forward ﬂight and/or take-oﬀ and/or landing vertically. UAVs have had a signiﬁcant impact on civil and military aviation; however, noise due to rotating blades is limiting their further spread.
The high level of noise, generated during multirotor ﬂight, is a major concern for the aeronauti-cal industry. In the case of civil applications, noise has a more comprehensive range of implications due to the sustainability of air traﬃc growth. Within the scope of the Civil Aviation Authority of Israel (CAAI) research program our main objective is to collect, process and archive noise signature from a single and multiple number of propellers. The collected database can be used as basis for future deﬁnition of the required regulations for such vehicles.
The research project focuses on advancing our knowledge in understanding the acoustic sig-nature of single and multiple propellers. The research targets acoustic measurements in static conditions in an anechoic chamber. The research plan is split into four phases: (1) perform a literature review and acquire the necessary equipment; (2) Collect and process acoustic signature of a single propellers; (3) Collect and process acoustic signature of multiple number of propellers;(4) Analysis of the acoustic data and preparation of acoustic database.
The ﬁrst year of the proposed research (items (1) and (2)), has been performed successfully and exhibited very promising results. During the second year, eﬀorts were focused on preparing the experimental setup for a four propeller conﬁguration (items (3) and (4)). Noise suppression method, based on phase synchronization, is proposed and implemented. Experiments were performed in newly established anechoic chamber at the Technion – Israel Institute of Technology. It has been demonstrated that phase synchronization can lead to a signiﬁcant noise attenuation.
European Union Aviation Safety Agency (EASA)
Urban Air Mobility (UAM)
Urban Air Mobility is expected to become a reality in Europe within 3-5 years. New technologies such as electric propulsion and enhanced battery capacity, applied to vertical take-off and landing systems, make this possible. The first commercial operations are expected to be the delivery of goods by drones and the transport of passengers, initially with a pilot on board. Later remote piloting or even autonomous services could follow. Several pilot projects are under way and some European manufacturers have already applied for certification, including for piloted vehicles for passenger transport. EASA is working with them on the airworthiness of the vehicles. The EU, and EASA in particular, have an important role to play in enabling this breakthrough and so helping European industry be a first mover at global level.
Study on the Societal Acceptance of UAM Operations
Citizens’ acceptance and future UAM users’ confidence will be essential to the successful deployment of Urban air Mobility in Europe. EASA conducted a comprehensive study on the societal acceptance of UAM operations across the European Union to guide this work. The study was carried out together with the consulting firm McKinsey & Company and the Arup Sound Lab between November 2020 and April 2021. Based on thorough research, literature review, local market analysis, surveys and interviews, the study examined the attitudes, expectations and concerns of EU citizens with respect to UAM and revealed interesting insights that will help EASA prepare the future regulatory framework.
EASA Regulatory Activities
The Agency has started creating the UAM regulatory framework, building notably on the results of 2021 UAM study on societal acceptance. Some building blocks have already been achieved:
- On airworthiness, EASA has been the first in the world to publish in July 2019 a Special Condition to authorise small VTOL aircraft operations, in 2020 for Light Unmanned Aircraft Systems operating in medium risk situations, and in 2021 Guidelines on the design verification of UAS operating in the specific category
- On operations and pilot licencing, in early 2019 it has launched preparatory activities that will lead to rules for the pilots/remote pilots of these vehicles, their operators and for the infrastructure, e.g. vertiport operators
- On airspace integration, EASA has prepared a worlds-first U-Space/UTM regulatory package (Commission Implementing regulations 2021/664, 2021/665 & 2021/666, adopted by the European Commission on 22 of April 2021; this package will become applicable early 2023 and will enable the safe integration of UAS operations in urban environment
- On the R&D side, EASA is also engaged in a large number of projects (AMU-LED; SAFIR-Med; CORUS-XUAM; AirMour and EASA may get involved in other additional ones such as GOF 2.0; TINDAIR; Uspace4UAM); it has also signed the Manifesto (of several cities) of the UAM initiatives by European cities (EU Smart Cities Marketplace)
National Research Council Canada
Assessment of Drone Impact on Commercial Aircrafts
A mid-air collision with an unmanned aerial vehicle could potentially be dangerous to an aircraft. The National Research Council of Canada (NRC) is collaborating with Transport Canada and other government departments to evaluate the risks associated to drones impacting aircrafts at cruising under 10,000 ft (3 km) altitude and approach phases. Since the 1960s, the NRC has been performing similar research in the context of bird impact tests on aircraft structures, windshields and engines.
Safe integration of Remotely Piloted Aircraft system (RPAS)
As the popularity of drones is rapidly growing, the National Research Council of Canada (NRC) is assisting Transport Canada to develop evidence-based regulations for safe integration and operation of Remotely Piloted Aircraft systems (RPAS) in Canada. The NRC is supporting this regulatory development by performing R&D planning and coordination and generating scientific data utilizing its unique facilities and expert researchers.
Icing Investigation for Small Unmanned Aerial Vehicle Rotors and Propellers
Icing build up on small unmanned aerial vehicle (UAV) rotors and propellers can be a serious threat. The National Research Council of Canada is investigating the icing of small propellers at high RPM to provide Transport Canada with scientific data to support the development of a safe and commercially viable regulatory framework for RPAS operations in Canada.
For information on all the other completed public reports, review the 2020-2021 Transport Canada RPAS R&D Yearly Progress Report.
Embry-Riddle Aeronautical University External Research
Visual Detection of Small Unmanna Aircraft: Modeling the Limits of Human Pilots
Dr. Gregory Stephen Woo
The purpose of this study was to determine the key physical variables for visual detection of small, Unmanned Aircraft Systems (UAS), and to learn how these variables influence the ability of human pilots, in manned-aircraft operating between 60-knots to 160-knots in the airport terminal area, to see these small, unmanned aircraft in time to avoid a collision. The study also produced a set of probability curves for various operating scenarios, depicting the likelihood of visually detecting a small, unmanned aircraft in time to avoid colliding with it. The study used the known limits of human visual acuity, based on the mechanics of the human eye and previous research on human visual detection of distant objects, to define the human performance constraints for the visual search task.
The results of the analysis suggest the probability of detection, in all cases modeled during the study, is far less than 50 percent. The probability of detection was well under 10 percent for small UAS aircraft similar to the products used by many recreational and hobby operators.
The results of this study indicate the concept of see-and-avoid is not a reliable technique for collision prevention by manned-aircraft pilots when it comes to operating near small, unmanned aircraft. Since small, unmanned aircraft continue to appear in airspace where they do not belong, regulators and the industry need to accelerate the development and deployment of alternative methods for collision prevention between sUAS aircraft operations and manned-aircraft.
The analysis effort for this study included the development of a new simulation model, building on existing models related to human visual detection of distant objects. This study extended existing research and used currently accepted standards to create a new model specifically tailored to small, unmanned aircraft detection. Since several input variables are not controllable, this study used a Monte Carlo simulation to provide a means for addressing the effects of uncertainty in the uncontrollable inputs that the previous models did not handle. The uncontrollable inputs include the airspeed and direction of flight for the unmanned aircraft, as well as the changing contrast between the unmanned aircraft target and its background as both the target aircraft and the observer encounter different background and lighting conditions.
The reusable model created for this study will enable future research related to the visual detection of small, unmanned aircraft. It provides a new tool for studying the difficult task of visually detecting airborne, small, unmanned aircraft targets in time to maneuver clear of a possible collision with them. The study also tested alternative input values to the simulation model to explore how changes to small, unmanned aircraft features might improve the visual detectability of the unmanned aircraft by human pilots in manned aircraft. While these changes resulted in higher probabilities of detection, the overall detection probability remained very low thereby confirming the urgent need to build reliable collision avoidance capability into small UAS aircraft.