Development and validation of hybrid propulsion system components and sub-systems for electrical aircraft

Final Report Summary - HYPSTAIR (Development and validation of hybrid propulsion system components and sub-systems for electrical aircraft)

Executive Summary:
The HYPSTAIR project is a concrete example from the aviation industry about how to implement the more efficient use of energy. The development of environmentally friendly technologies and products in transport is not only encouraged, but necessary for achieving sustainable transport. As outlined in the European strategic paper (White Paper “Roadmap to a Single European Transport Area”, 2011) the vision is to build competitive and sustainable transport system which could be achieved through reducing emissions among others also with developing and deploying new and sustainable fuels and propulsion systems. A significant reduction of fuel consumption and associated reduction of carbon dioxide emissions can be achieved by use of hybrid-electric and all-electric propulsion. With all-electric propulsion severely limited in range with existing battery technologies, the hybrid-electric aircraft currently represent the best efficiency versus range compromise in the light aviation segment. Especially serial-hybrid aircraft represent a promising architecture for future aircraft since their propeller is driven purely by an electric motor powered either by batteries or by an on-board generator used to extend the range when necessary.
To draw a path towards more environmental friendly transport, the HYPSTAIR project main scope was to design and validate serial hybrid drive components for the use in light aircraft and moreover to develop a human-machine interface (HMI) to simplify operation of a complex hybrid system. The project objectives were focused towards five goals:
• Definition of hybrid propulsion aircraft concept and requirements;
• Development of hybrid system components;
• Assessment of applicable certification regulations;
• Development and design of the HMI for hybrid aircraft;
• Sub-system level integration and testing of components;

Although emissions reduction is an immediate benefit of the hybrid propulsion system, the technology is rather new and its implementation into commercial products is limited by the lack of certification requirement. Nevertheless, the HYPSTAIR project has made the first and important step towards commercialization of hybrid aircrafts by exploring applicable certification framework for hybrid drive systems and applying them selectively in the component design and development. This approach is paving the way towards a competitive supply chain for hybrid drive components and will reduce the time to market of hybrid drive systems for light aircraft. Furthermore, by increasing fuel efficiency of hybrid and electrical components and consequently reducing travel costs, HYPSTAIR is encouraging more environmentally friendly mobility.

Project Context and Objectives:
1.Key challenge and driver of the project

The main driver behind the HYPSTAIR project was to develop a new, greener propulsion system to pioneer one of the aviation industry’solutions to reduce the environmental impact of air travel. The propulsion system developed in the project is suitable for powering four-seat general aviation aircraft, a market segment which is currently relying on piston-powered engines, where the leading engine manufacturers are providing units whose basic technology, although constantly updated and reliable, is now over 50 years old and, in most cases, still needs lead fuels to operate. Due to diminishing reserves of fossil sources of energy together with the environmental impacts of their use and geopolitical issues, the use of more environmentally friendly technologies and products in transport is not only encouraged, but necessary for achieving sustainable transport. As outlined in the European strategic paper (White Paper “Roadmap to a Single European Transport Area”, 2011) the vision is to build competitive and sustainable transport system which could be achieved through reducing emissions among others also with developing and deploying new and sustainable fuels and propulsion system. Among the future propulsion systems, hybrid-electric powertrains represent a concrete option to reduce fuel consumption and associated carbon dioxide emissions. However, the development of these technologies for the propulsion of airborne vehicles did not follow the automotive trend, mostly due to very strict requirements of light weight and small volume of the energy storage devices which poses serious challenges to designers. Fortunately, it is in the light aviation segment where the application of all-electric aircraft technology, including propulsion, can be best applied and can give significant benefits. Although, the fully electrical and hybrid airplanes represent a serious challenge, especially when light weights and small volumes are strict constraints. Pipistrel faced this challenges with production of the first electric two seater aircraft, the Taurus G2, and, more importantly, the first electric four seater aircraft, the Taurus G4. The latter aircraft also won the Green Flight Challenge sponsored by Google 2011 and with the prize provided by NASA. In 2015, Pipistrel flew its second electric model, the Alpha Electro, a two seater aimed at reducing costs of ab-initio training initiating a new market. Also Siemens, Diamond aircraft and EADS developed the first serial hybrid aircraft DA-36 E-Star in 2011 and in 2016 flew the all-electric aerobatic aircraft Extra 330LE powered by the recently developed 260kW e-motor. Although the idea of the hybrid propulsion system is present for decades and led to several successful research projects, none of them were able to reach the production-ready stage. With the emergence of all-electric powered aircraft, notably the single-seat Lange Antares 20E (2006) followed by the two-seat Pipistrel Taurus Electro (2007) and Yuneec E430 (2007), shortcomings of battery-powered electric aircraft became apparent. While these aircraft were revolutionary, unprecedently quiet, extremely efficient and modern in all their aspects and technologies, the battery technology was and still is the limiting factor in proliferation of electric aircraft, especially when considering the comparison to common internal combustion engine systems. Likewise, the current technology of drives and converters is too heavy for use in airborne vehicles. Therefore, completely new electric components (motor, generator, inverter, battery etc.) with high power densities need to be designed due to stringent weight requirements.
The HYPSTAIR project partners addressed the research gap and set general concept of a serial hybrid aircraft. While a long term goal of project partners is an electric aircraft devoid of fossil fuel technology, it is the limitations of current electric energy storage technology that make an electric-only propulsion system unsuitable for long range flying. As battery mass is never spent while flying, it gives the aircraft a constant weight penalty regardless of its battery state. Using a serial hybrid drive, an aircraft can be built that has sufficient battery capacity for shorter electric-only hops, while for longer ranges and also for added safety, an on board generator driven by a combustion engine provides a weight efficient, even if somewhat less energy efficient, power generation solution.
Currently there are very few electric and basically no hybrid planes, so at the moment there is no certified hybrid propulsion aircraft in the market for general aviation. Lack of certification plan and norms for this kind of propulsion represents a big challenge for its introduction to the market. This makes the hybrid and electric propulsion for aircraft very difficult to develop. Firstly, the certifying authority (EASA) has to approve dedicated rules and requirements to award “type certificates” for hybrid-electric propulsion systems. Secondly, the strongly weight optimized technology for electric motors and inverters employed for airborne application is completely new and cannot be found in other applications. During the project, the partners have assessed existing regulations, interacting with EASA and contributing to industrial regulation committees (ASTM) in an effort to drive the standardization work undergoing in Europe and overseas towards a dedicated section in the new CS/FAR 23 revision. These efforts will help create a competitive supply chain for hybrid drive components and reduce the time to market of such innovations. Furthermore, increased fuel efficiency of hybrid and electrical components is one of key technologies to reduce travel charges.

2. Aim

The aim of HYPSTAIR project was to develop of electrical components and necessary sub-systems comprising a serial hybrid aircraft propulsion system. Hybrid aircraft currently represents the best efficiency versus range compromise in the light aviation segment. It can be considered as an electrically powered aircraft with an on board generator used for extending the range when necessary.
The HYPSTAIR project scope was to design serial hybrid drive components specifically developed for airplane propulsion an validate them on ground in a test setup resembling a real aircraft installation. The components which were developed are the electric motor, inverter, generator, battery system and the control system for the four seat class of aircraft. The components were designed with the aim of functioning as a part of a complete airframe and hybrid drive system. The sizing of all components was performed by selecting a trade-off between their individual efficiency and the expected total system efficiency and performance. This involved balancing weight and efficiency aspects of components with the goal of overall platform efficiency, while considering the expected aircraft mission profiles. Furthermore, serial hybrid driven airplane presents an interesting trade-off case for system weight – on one side they feature a reduced weight of propulsion components in comparison to a piston powered aircraft, as the on board generator is significantly reduced in size, but on the other side the powertrain weight is increased by the batteries, with an amount directly proportional to the all-electric range envisioned. Another target when designing the hybrid system components was inherent increased safety and development of the ease of use of such propulsion. This is important both from the point of view of enhancing the levels of safety for small aircraft as well as for stimulating market acceptance of such new technology. A serial hybrid drive is powered by two independent sources of energy, namely the battery system and the generator system. In case of failure of either component, the other can supply reserve power. Furthermore, the electric motor, which is the only unit of a serial hybrid system driving the propeller, offers increased reliability and reduced maintenance compared to a piston-powered engine. Both of these aspects increase safety, especially because the electric motor and generator designed in HYPSTAIR feature two independent windings to enable these electric machines to operate even if one set of winding is damaged. In order to control the hybrid in a way that is both safe and energy efficient, a special human-machine interface was designed, developed and validated enabling the pilot to intuitively exploit all the benefits of hybrid propulsion while at the same time reducing the pilot workload as compared to managing a piston engine.

3. Objectives

The HYPSTAIR project follows several objectives, such as:
- Definition of hybrid propulsion aircraft concept and requirements which was achieved by optimizing and defining hybrid system and aircraft requirements; preparing a performance analysis based on an existing airframe platform; preparing the concept of a common platform for laboratory testing and validation of components; defining safety, standardization and certification requirements for safety of electrical components.
- Assessment of applicable regulations for certification which was achieved by analysing existing regulations in automotive and aerospace, and in absence of a dedicated standard, the design of the powertrain followed selected norms to form a base for powertrain certificability
- Development and design of the HMI for hybrid aircraft which was achieved with designing concepts of HMI for pilot-aircraft communication; executing HMI in the form of prototype components; researching and developing haptic interfaces in order to promote efficient use of energy.
- Development of hybrid system components which was achieved with designing and developing components of hybrid drive and by sub-system level integration and verification of components
- Integration of all the components of the hybrid system into integration platform which was achieved with developing testing methods for validation of the design of the components.
- Validation of serial hybrid drive components which was achieved with design and validation of serial hybrid drive components for the four seater segment of aircraft, together with human-machine interfaces for their use.

Project Results:
1. Hybrid propulsion aircraft concept and requirements

The performance of the hybrid airplane has been investigated at different levels of details, using different analysis tools and considering both the aircraft and the hybrid propulsion system characteristics. In the first step, a performance model of the hybrid airplane has been developed using the Matlab code, assuming a reference mission in order to define performance and limitations in terms of cruise speed altitude, endurance, climb ratio, etc. The calculation procedure has been conceived to estimate the consumption of both the energy sources, fuel and batteries, hence to find the best flight conditions which allows to maximize the flight range or, alternatively, the weight saving. In particular, two different types of analyses have been performed:
- maximum range analysis with given MTOW weight, in order to study the influence of mission parameters (cruise speed, cruise altitude, etc.);
- given range analysis (range (100 km, 400km and 1000 km), in order to define the minimum weight of fuel needed to complete the mission.

Results showed that a hybrid system, compared to an internal combustion (IC) one, has more limitations in terms of payload weight, i.e. number of passengers, since the low energy density of present batteries increased the empty weight fraction. Nevertheless, it was observed that the hybrid propulsion system makes the aircraft less sensitive to cruise altitude, since air density has weak influence on electric propulsion, improving take-off performance in hot and high conditions. In fact, the weight increase associated to a longer range request is lower for the system with hybrid propulsion than the IC one.

In addition, using batteries instead of fuel during take-off allows hybrid-electric aircraft to operate as zero-emission vehicle near ground, at the cost of lower cruise performance due to in-flight battery recharging. Such differences have more influence on global performance in case of short range missions, for which the charging time required for batteries may be a significant fraction of the overall flight duration. In this phase, special attention has been dedicated to batteries performance in order to assess the effects of normal utilization on their state of health (SoH). Such studies have been carried out through experimental activities in which Li-Ion batteries have been included in the hybrid power train testing platform of DESTEC laboratories.

Further experimental investigations have been dedicated to super capacitors and has been carried out in order to evaluate the suitability of the adoption of such devices as energy accumulators. Such tests have been conducted using the Electrochemical Impedance Spectroscopy (EIS) in order to fully characterize the frequency response of super capacitors, evaluate the ageing affects due to charge/discharge cycles and, hence, the possible failure regions in the frequency range between 1 mHz – 0.1 kHz.
In the second step, a more detailed analysis has been carried out developing a simulation software, called «HyPSim» (Hybrid Plane Simulator), consisting in a Performance Module in which the performance model routines have been implemented using Simulink, an in-house designed Mission Planner, which allows to define the flight mission through a set of waypoints with desired characteristics, a commercial Flight Simulator, used to provide the aircraft dynamics and for visualization purposes, and the Human Machine Interface developed within the project.

Thanks to the autonomous flight mode, «HyPSim» allows to perform pre-designed flight missions of any kind as well as to evaluate the effects of pilot behavior, since it is provided with a joystick for manual control. The accuracy of the simulator in evaluating the energy consumption has been verified by comparing the required power for flight with experimental results provided by the aircraft manufacturer. In addition, specific mission profiles have been given as input and positive results on the reliability of the power demand evaluation have been observed, although for some peculiar conditions, such as the case of fully discharged batteries, additional control logics must be implemented in order to avoid divergence phenomena. Power profiles obtained with «HyPSim» allowed further experimental tests on batteries, with additional experimental activities dedicated to analyze the batteries performance in conditions different to the nominal ones declared by manufacturers. Other tests have been additionally conducted using a climate chamber in order to evaluate the sensitivity of batteries performance to temperature.
Results showed a good accordance between batteries performance, related to the actual power profile obtained by «HyPSim», and the manufacturer data, with differences lower than 0.5% in terms of available energy. Analogue tests have been carried out at 0°C using a climatic chamber, finding a reduction of battery capacity of 12.5% if compared to 23°C condition, which is in good accordance with data provided by the manufacturer. In the final step of the project, the characteristics of the actual propeller have been implemented into «HyPSim» in order to obtain a more realistic power profile and to use it for the test of the hybrid power train prototype, built during the HYPSTAIR project. A short range and simultaneously power-demanding mission has been simulated in order to obtain thrust, power required and propeller speed as functions of time.

2. Human-machine interface for hybrid aircraft

At times during the flight, the situational awareness can become critical due to multiple concurrent events: replicated and simplified information can increase the safety and the quality of piloting. A hybrid aircraft, innovative and technically complex, needs a different man-machine interaction designed to increase the safety level and decreasing the possibility of human error. With that consideration in mind, the project developed a Human Machine Interface (HMI) for aircraft with hybrid engines including execution of GUI and interface components’ prototypes. Knowledge in the field of instruments for electric and hybrid engines was extended to (interaction / usability) creation of new standards in the design of HMI for aircrafts / vehicles with electric or hybrid engines.
The HYPSTAIR project introduces innovative concept of "common layer", trying to standardize the guidelines for interfaces. In the aeronautical field the need (as for the automotive field) for unification/standardization of the GUI of instruments is extremely high, especially for safety reasons. A good standardization means reduced instruction/training time for pilots, helping in case of dealing with different types of aircraft and different powertrains. This concept is more evident in hybrid airplanes as standardization regulating GUIs does not exist. This can increase the stress of the pilot, leading to lower general level of safety, especially in case of increased stress, caused by a fair/wrong interaction with interfaces overloading the pilot and lowering his/her threshold of awareness and compromising the safety of the flight.
Graphic User Interface provides the definition and design of strict guidelines that need to be followed throughout the planning of HMI. It is normed by a reference document that establishes the layout, colours, positioning, applications depending on different needs and mission profiles. As the hybrid aircraft represents the new concept and as there is lack of specific literature in this field, a great amount of information that must be controlled by the pilot have been studied in depth and designed from the point of view of ergonomics, laws of perception and cognitive psychology. To develop the most appropriate GUI for the HYPSTAIR project, several tests and simulations have been carried out capitalizing on the expertise gained in automotive and nautical field. The GUIs was designed with the everyday operations of professional pilots in mind. The design of GUI was inspired by the analogical instruments, but were developed to be more immediate in their perception, so not only to transmit numerical data, but also to represent and anticipate behavioural trends. This type of representation and layout is particularly helpful to pilots who are used to old generation aircrafts (low computerization), to those poorly trained on digital equipment, up to the sport, amateur pilot (the so-called "Sunday pilots"); they are often dealing with instruments of complex readability, which require interpretative ability and specific training to be used at their best.

The HYPSTAIR interface is divided into a spatial grid and defined to optimize the available space on the display and the different reading layers. In the Western world, the eye is used to read the information through a specific behaviour and scan of priorities (from left to right and from top to bottom). Moreover, the top of the screen is the closest to the line of sight that the pilot uses for scanning the external environment through the canopy of the aircraft. Following this approach, information has been put in a layout split in layers (levels) of priority. These layers facilitate the pilot in reading and can transmit useful and necessary information in the most appropriate moment and matter. The quality of HMI is evaluated by its main feature that it simplifies the access or readability of the information. This was achieved through immediately perceptible graphical encodings and chromatic codes, correct positioning within the viewing angle of the pilot and especially selecting and displaying the needed information in a specific moment or during a specific mission profile, avoiding to display all information at the same time.

Icons are an essential part of the GUI and use specific colours/symbols to display situations that would otherwise be indicated by words/information/phrases, complicated to read, causing possible distractions. When developing icons, the expertise gained in automotive field has been capitalized. The automotive field follows a strict normative, released by specific authorities and it is mandatory for manufactures to adopt them. In the project the similar approach was used when developing icons for light and non-conventional (hybrid) aircraft, where manufacturers, in the absence of specific constraints, often use different graphic representations for similar information.

The HMI background is black to increase the contrast of the information displayed. Through icon colours, information can be quickly read at a first glance; through the encodings of GUI, the colour scheme allows to understand the appropriate situation (normal, alert, warning). This homogeneity is specifically studied to highlight the most important information reported with noticeable colours (green, yellow, red). A complete palette of colours has been defined through simulations, to match both the existing standards within the field of aviation and the readability of the GUI in all light conditions.

In order to promote efficient use of energy and to produce an operating experimental prototype of the haptic interface, components of haptic interface for HMI of hybrid aircraft were also developed. Standard implementation of an aircraft cockpit HMI involves primary flight display, navigation display, flight controls, engine indicating display and throttle. However, in case of a hybrid propulsion system, which besides ICE includes also an electrical motor and generator as well as a battery pack and complex inverter power electronics along with a control system, a simple standard engine display may not be suitable anymore for communicating all the required information to the pilot. Providing all the relevant information of such complex powertrain system by conventional cockpit, displays may not be intuitive and the pilot may be overloaded with information. Energy may not be optimally used, and furthermore, safety may be significantly jeopardized. Hereupon, the idea was to enable simplified communication and exonerate pilot’s workload. This can be achieved with an enhanced information flow between the aircraft cockpit and a pilot, which will not rely solely on pilot visual attention of the HMI but can also effectively employ a more intuitive haptic human sense to communicate important system information in real time. The proposed concept of the enhanced HMI enables simultaneous power control and intuitive feedback information. Such interaction and information exchange is possible with a haptic interface. Thus, the proposed solution was that pilot operates the hybrid propulsion system of the electrical aircraft via the power lever that produces force feedback, or more precisely, operates as haptic power lever.
An experimental prototype of the haptic power control lever was designed in order to reliably provide relevant information about the hybrid propulsion system by force feel. The power lever can stimulate the pilot’s haptic sense by force feedback while he or she is holding the power lever handle in hand and operates the propulsion system. Thus, the power lever with haptic features, which is a part of the HMI, supplements visual information with more intuitive haptic information.
The power lever shall operate as a haptic interface that can produce numerous programmable effects, and thus stimulating human haptic sense by force. The haptic effects are designed in such manner that they can mostly produce force signals, similar to the ones we feel in nature and thus they can provide an intuitive human response. Possible feedbacks to be implemented in the haptic power lever in relation to the different sets of HMI behaviors were developed. Such haptic feedbacks are friction, bumps, constant return force, graininess, barrier, and vibrations. Obviously, these haptic effects can be further linked to higher-level system information and thus specify dedicated haptic cues. Such haptic cues, provided by the haptic power lever, involve all the relevant information of the powertrain system. The research works has analyzed general design principles of the haptic cues that should promote efficient use of energy, enhance level of safety, and provide intuitive control of complex powertrain system have been adopted. The set of haptic cues that link haptic effects with the status of the hybrid powertrain have been designed. The main powertrain components considered in the design of haptic cues are: i) battery pack, ii) internal combustion engine with electrical generator, and iii) electrical motor with propeller. We draw up a table with haptic cues which connect each specific piloting condition and the status of the hybrid system with a corresponding haptic feedback that makes instantly understandable the situation by the pilot.
The experimental testbed was developed in order to validate proposed algorithms for rendering the designed haptic effects and a dedicated haptic control algorithm that was derived during the project and verified by simulations. However, in the scope of human-in-the-loop tests, the performed simulations were quite limited since suitable mathematical models describing human neuromuscular system, human perception and cognition as well as human motor control are not available yet. During the design phase of the prototype a wide selection of mechatronic components and dedicated processing hardware was considered and different possible solutions for the technology of the drive concept of the haptic power lever were considered, resulting in a cost-effective solution for the prototype of electromechanical design of the experimental haptic power lever. A cogging-free linear motor with built-in Hall sensors for position measurements was chosen to allow fine and dynamic haptic effects that will enable smooth feel. A force sensor was mounted in order to provide human force information. The experimental system involves newly developed haptic controller based on DSP that enables fast processing and is suitable for applications of rapid control prototyping within Matlab/Simulink environment. The control board interface involves digital inputs and outputs, analog inputs, encoder inputs, and several communication ports USB, RS232 and CAN. Also a dedicated communication protocols for connection of the prototype system was developed to i) the development node via USB, ii) the cockpit HMI controller via CAN, and iii) the HYPSTAIR Hybrid Plane Simulator via RS232. Though the prototype has been designed mainly with the purpose to prove the concept of haptic-based information exchange in a more intuitive HMI such solution can also be employed in a design of a final product for installation in small or ultralight general aircraft. Here, the overall system configuration assumes link between haptic power lever and powertrain system with the control software running in the processing boards of the inverters. The experimental prototype system can operates alone as a demonstrator (as was showed on Symposium E2Fliegen, Stuttgart 18.-19.2.2016 and AERO EXPO 2016, Friedrichshafen) or as part of HMI and hybrid propulsion system.
Of particular importance for its innovation in aviation HMI is the predictor, the indication of the hybrid powertrain status and the haptic interaction with power controls.

The predictor allows the pilot to know in advance the available power in each of the three modes that can be activated (echo, stealth, boost). In contrast to the classical indicator of the power in use, the predictor of the HYPSTAIR GUI displays the limit of the current available power (indicated through a graphic outline). To maintain uniformity among displayed information also the outline of predictor uses the same colour of the current active mode. In close connection with the "predictor" there is the “remaining flight time”. Especially for a hybrid aircraft, it is essential to understand when the battery is in charging mode (active generator) or discharge mode (electricity not generated). The “remaining flight time indicator” is a countdown of instantaneous available energy; the indicator takes into account the management of the hybrid powertrain and the flight conditions in real time, providing a continuously calculated forecast data. This countdown is important to make the pilot aware through an easily understandable information, managing the flight in its whole duration, and possibly to make safe decisions (e.g. preventive landing, consumption optimization, etc.).

The graphic area dedicated to the powertrain status is of immediate clarity. The clarity is achieved through the use of a simple info-graphic layer, reducing the complexity of a hybrid propulsion. In fact, within the plane silhouette it contains a set of icons representing the main components of the powertrain, following the scheme of the installed components. Outside the silhouette, at the potentially involved elements, lights are displayed, identifying potential issues in which the pilot may incur during the flight. This type of visualization simplifies the understanding of the hybrid powertrain, showing the information (by easily recognizable icons) only in case of effective need.

In a normal aircraft, the pilot usually relays on the noise of the engine to monitor and understand the proper functioning. In HYPSTAIR, taking-off in electric mode, or landing with the ICE engine under charge, could induce dangerous perceptual dystonia. These auditory information, which are part of the basic training of the pilots, may confuse the pilot introducing operational risks. For these reasons, the concept and design of a new throttle, now called “power lever” has been considered, which, through a “by wire” computer controlled interaction, communicates the status of the hybrid powertrain. The HMI graphic design considered also the possibility of an auxiliary display on the tunnel working as replication of the information that is shown in MFD but realized through a dedicated display and with a mixed mode, using percentage values (%), instead of numerical data. The integration of the tunnel displays and the power lever allows the pilot to know in real time the energy in use and to have a prediction of the one available. This power lever has a haptic feedback that transmits, through a force feedback, the various behaviors of the hybrid powertrain to the pilot during the piloting operations. This redundant system allows the pilot to keep under control a primary information such as the power in use, helping him to control hybrid aircraft.

3. Hybrid system components and functional safety requirements

To reach the ambitious goals set for the HYPSTAIR project, a powerful, efficient, yet lightweight electric engine had to be developed. The resulting E-Motor is a permanent magnetic synchronous machine with an inner rotor design. The electric part (stator) consists of two independent winding systems, which are galvanically and geometrically separated. They are fully redundant in case of failure of one power path, which ensures an availability of 50% of MTOP at the shaft. To ensure compatibility with existing propellers, the motor features a D-Flange according to ARP502. In order to further reduce weight, a bearing shield has been integrated into the motor, which supports the E-motor as well as the propeller, whose mechanical bearing forces are dominating.
The E-motor is electromagnetically and thermally optimized to be powerful enough to support maximum continuous power of 150kW and to be most efficient at a continuous power of 88kW required for a typical cruise flight with lower propeller RPMs. For one minute it is possible to boost the propeller power up to 200kW at 2250rpm corresponding with a maximum torque of 850Nm. The pilot can release this power by adjusting the throttle lever beyond 100% as long as the motor temperature is within the allowed range. To optimize the overall aircraft performance, the motor has been designed to operate with a coolant temperature as high as 90 to 105°C thus reducing colling drag. This feature distinguishes the HYPSTAIR electric motor from other electric motors. After investigating different options, the decision has been taken to integrate a Siemens automotive inverter into the HYPSTAIR propulsion System. The so-called NextGen-Inverter features a high power to weight ratio of 10.5 kW/kg. The inverter is liquid cooled and able to provide the necessary power.

The integration of an automotive inverter ensures cost-effectiveness and availability for a possible future application of a hybrid-electric propulsion system for general aviation. Driven by the design philosophy of a serial-hybrid propulsion system, a suitable Generator Set had to be developed. The Generator Set’s purpose is to provide the required continuous cruise power to the electric engine. The HYPSTAIR’s Generator Set combines a “Rotax 914 Turbo”, a four cylinder 4-stroke turbocharged engine with an electric generator, which is directly flanged via an adapter housing. The gearbox and the electric starter of the original Rotax 914 have been removed in order to save weight and space. To support the generator rotor, a new slide bearing in the adapter housing was developed by “BRP Powertrain” and “Siemens”.

The adapter housing consists of a newly designed stator adapter, which connects the ICE housing to the generator housing. For the rotor coupling a special crank shaft adapter was developed with a strengthened slide bearing, which is lubricated by the engine oil and the generator rotor flange, which transmits the engine torque to the electric generator. The E-Generator is a 16 pole permanent magnetic synchronous machine with an inner rotor design. The machine is electromagnetically and thermally optimized to be highest efficient at maximum continuous power (100kW) during cruise flight. Due to the two separated winding systems, the generator is redundant in case of a single system failure and can provide up to 75kW electric power with one system. The generator is capable of operating at a high coolant temperature of 90 to 105°C minimizing cooling drag.

The battery is the alternative source of energy for a hybrid-electric aircraft. Its role is to act as exclusive source of power when the airplane is operated in full electric mode or as supplementary source in hybrid mode when the Gen-Set is already spinning at maximum power. An extensive cells type search has been carried to select the optimal Lithium-Polymer cells capable of providing high discharge rates typical of all-electric power, yet maintaining high energy density to provide enough range – all within the weight and volume limits imposed by the envisioned four-seat airplane installation. This lead to the design and development of the 12kWh HYPSTAIR propulsion battery. It consists of 12 modules and a distributed master-slave architecture for the innovative Battery Management System (BMS). The purpose of the BMS is to manage the functional aspects of the battery: charging, power delivery, battery protection and status communication via CAN-bus. To avoid the arise of unbalanced charge, which can lead potentially to battery fire, an active balancing system was adopted to transfer charge between neighbouring cells. Of particular importance also for aviation propulsion applications is the “self-protection” concept developed for the battery – to avoid failures or fire, the BMS disconnects the battery when a deviation from normal operating conditions is sensed.

The HEPU system control software ensures that all components of the propulsion system jointly produce the behaviour commanded by the pilot while ensuring safe operation of the propulsion system. It is distributed over several control hardware units, which are connected by two redundant CAN busses to exchange the relevant information. Each inverter has its own internal Control Unit (CU). It contains a TriCore microcontroller and the necessary I/O connections to drive the inverter power electronics part. The inverters are grouped into two independent “Power Paths” A and B. Each group is supplied by an independent stabilized auxiliary power supply, so that in case of failure of one of those supplies, the other power path is still able to provide at least a minimum necessary amount of power to the propeller. A separate hardware box “ICE gateway” exists to control the ICE functions (throttle position, RPM, ignition). BMS and HMI components are also connected to the CAN busses.
Each of the four inverter sub-units needs its own local fast control algorithms called ICF “Inverter Control Function”. These local control units receive their torque setpoints from the CCF “Central Control Function”. All the functionality that is not local to only one inverter or the ICE / BMS is represented here. The execution cycle of this block can be much slower than the fast local control units, because no fast reaction time (like <1s) of the HEPU system is required and the CAN bus is not able to achieve shorter cycle times.

Due to the high criticality of these central functions, the block needs to run on redundant hardware. This is achieved by running the software on each CCU, so theoretically it can be executed on four separate hardware units. How this redundancy is realized will be described in the section “Redundancy concept”. Because each of the redundant CCF “instances” needs to be connected to all local inverter controllers and to ensure consistency, it is necessary that the required information is always passed to the CAN bus from the (logical) CCF unit to each of the inverter controllers, even if they are physically running on the same CPU. Because the CCF control cycle is relatively slow, this can be accepted. Because of their high criticality most of the HEPU functions need to be realized with redundant hardware and the necessary control strategy to achieve high availability. Two independent drive-train parts (A and B) supplied by separate auxiliary power supplies are each able to provide enough power to keep the airplane in the air. Each of these two parts has its own independent local control units.
To achieve redundancy for the Central Control Functions (CCF), they will be executed on each of the four TriCore controllers of the inverter ControlUnits (CUs), as described above. Each CCF instance that is active can provide its setpoint values on the CAN busses using its own unique ID. At system startup, per default all four CCF instances are active and calculating setpoints. A receiving slave unit (inverter, ICE control, BMS) always has to use the one with lowest ID and ignore the others.
Alongside the components development, also the research on functional safety requirements was carried out. The research activities were focused on the development process for the purpose of development of hybrid electrical powertrain. In the first stage the process requirements were studied and compared with the requirements in the related field of industry, namely automotive industry. Industrial standards and guidelines were collected and studied, making a selection of relevant documents during the review process. Based on the requirements posed by the studied literature the risk & hazard evaluation was performed together with the safety assessment of the proposed powertrain implementation. The tools and methods were studied and presented. For the purpose of development, the design process requirements were studied and as a result recommendations were made for the requirements set by the relevant documents (standards and guidelines). A coding standard for programming language C (which is an important part of certification documentation) was proposed, based on MISRA C rules and several embedded systems coding documents. The existing documents for model-based software were collected and presented. For the purpose of verification and validation (for the purpose of certification), testing requirements were collected and evaluated. The testing processes and methods were developed in such a way, that the costs of required additional hardware and software were minimized. However, it was crucial to use tools enabling not just the evaluation of correct operation regarding the output, but also the timing measurements and event capturing. HiL (Hardware-in-the-Loop) is an excellent tool for testing. It is in fact strictly required for higher ASIL level products in automotive industry. As such it is gaining importance in the aerospace industry as well. The HiL system developed was a small version of load emulation hardware, which can be applied for testing of electrical drives. With only minor adaptations it can also be used for testing of ICE motors and power electronics components. The developed software is created with model-based tools, and it enables its simple reuse for other projects. Finally, testing cycles are required to verify and validate the operation in all required ranges, also considering events, introduced by user and environment, as well as faults of equipment.

Like in all safety critical applications also in the aerospace it is not enough to just demonstrate a safe operation, but it is also required to demonstrate and prove that the development of system, hardware and software was performed in a prescribed way. Prescribed processes are defined in the standards and guidelines (in aerospace we are dealing with the guidelines, which are actually required by customers, making them de-facto standards in the industry). They are based on classical V-model. The development needs to start with the definition of requirements, which have to be traced throughout the process. The system design and architecture are made based on requirements consisting out of functional and non-functional requirements (customer, standard-related ....). The high level requirements are further developed into low level requirements, which serve as hardware and software requirements – requirements for the software and hardware designers. The designed system has to be verified (correct operation need to be proven) and validated (the conformance with customer requirements and standards has to be demonstrated). In aerospace industry the main requirements for the system, software and hardware design are described in following documents:
• ARP- 4754A / ED-79A (System),
• DO-178C / ED-12C (Software) and
• DO-254 / ED-80 (Hardware).

For the safety assessment purposes ARP-4761 has been created.

The system development process, required by ARP-4754A / ED-79A, DO-178C / ED-12C and DO-254 / ED-80 (Hardware) shall follow the prescribed pattern, from the requirements to testing. The hardware and software requirement tracking is mandatory in functional safety related standards. The software and hardware integrity level of the product determines the way in which its development shall be traced, documented and tested. There are five levels, Level A, Level B, Level C, Level D, and Level E, where Level A is the highest and level E means that there are actually no functional safety requirements.

Programmer’s manual (for C code) comprises rules, based on the requirements by RTCA-DO178C / ED-12C and MISRA C guidelines. For the reason of producing the code, which would be also useful in the automotive industry, also requirements of ISO/IEC 26262, especially Part 6 have been taken into account. The guidelines (just as MISRA C Guidelines or any similar ones) present a subset of C programming language, in which the opportunity to make mistakes is removed or reduced. Namely, the ISO Standards for C don’t specify the language completely. There are areas of the language, for which the behaviour is undefined, unspecified or dependent on implementation. Compilation and build might result in undesired behaviour. Every guideline contains the description of rules, followed by the included code example of correct and incorrect code. A short explanation is included as a form of a rationale.

For the purpose of safety assessment the proposed power propulsion system was analysed by the use of fault three. Based on the structure and expertise in the field of power electronics a safe state was defined, representing the state to which the system needs to transfer in case of fault. Techniques like FMEA (Failure mode and effects analysis) were applied in the process. One of the important results in the project was also a proposal of a safer system structure.

A comparison of guidelines in aerospace with the related automotive standard ISO/IEC 26262 was an important part of the project. Several differences were identified. First important issue is in the used processes. The relevant support processes for ISO 26262 are distributed over several parts of the standard, DO-178C defines four integral processes. Two integral processes, Software Quality Assurance Process and Certification Liaison Process, target safety management of the software and two, software Configuration Management Process and Software Verification Process, can be identified to be supporting processes to Software Development Process in the sense of ISO 26262. A mapping of processes was presented and also the processes with no mapping were clearly identified. Likewise, the software development processes and artefacts of the process were compared.

Summing up, the good mapping artefacts between the standards was observed, taking into account the objectives and activities necessary to create them. In fact, it would simpler to transfer the product from automotive industry into the aerospace than to the opposite direction, if there would not be a crucial requirement by the certification authority to be involved into the development process from the start. Software testing is used to demonstrate that the software (specifically the executable object code) satisfies its requirements and to demonstrate with a high degree of confidence that errors that could lead to unacceptable failure conditions, as determined by the system safety assessment process, have been removed.

The objectives of software testing are to execute the software to confirm that:
• The Executable Object Code complies with the high-level requirements.
• The Executable Object Code is robust with the high-level requirements.
• The Executable Object Code complies with the low-level requirements.
• The Executable Object Code is robust with the low-level requirements.
• The Executable Object Code is compatible with the target computer.

Since it is strictly required to test the Executable Object Code, the classical software tools are not adequate. They can be applied to prove to some point the coverage, but this is not sufficient to prove the compliance with safety-related requirements. HiL is a good method to be applied in both, design and testing of systems. For that purpose a dedicated system was developed. The HiL simulator consists of tested drive and an active load, both with a complete control hardware. The testing hardware enables the evaluation of algorithms for a complete drivetrain, with generator, for ground testing of the drivetrain. It consists out of a load machine, emulating the load torque (force) on the propeller and another load machine to drive the generator instead of ICE. Additionally, through communication protocols the pilot and simulated environment can be included.

Control hardware consists out of two boards, which were developed in the frame of the project. The first one is based on the microcontroller (Texas Instruments digital signal controller TMS320F28335). The processor board itself was not developed, since the control board for this processor is available. The second card to be used for the purpose of fast system emulation, is based on an Altera FPGA, enabling parallel operation and thus performance of control algorithms with higher frequency. The testing hardware has been designed in a small scale version regarding the level of power, allowing the development of HiL simulators. The dynamic emulation of loads was chosen as the emulation method in the HiL system. An aeroplane model was created and parametrized based on provided data, to be used as the emulation model in the load emulation method. Testing for the aerospace products shall be performed in such a way, that a coverage of requirements can be evaluated, for operation in normal range, at border values and safety-critical events. For that purpose, a testing profile (speed / height / power) was created. It is designed in such a way, that it covers maximal part of the operational range possible.

4. System level integration and verification of components

The first activity for the integration work was the construction of the test platform, which in HYPSTAIR is a composite structure which resembles a typical CS-23 four-seater fuselage. The motivation behind adopting a fuselage and not a classical test bench was to assess which installation challenges will be faced by general aviation aircraft manufacturers switching from classical piston engine to a greener hybrid-electric powertrain. Due to the non-flying nature of the test bed, the central spar was built as dummy structure without load carrying capability. The bulkheads and ribs in the fuselage were adopted to fit the additional cabling for the battery system. Provision was made for mounting the dashboard and the haptic throttle lever by designing a new central tunnel between the front seats.
With the test platform ready, the integration work consisted of engineering the powertrain installation concept. The starting point was the mechanical installation of components, where a single engine mount fixed on only five attachment points accommodates all components of the powertrain, with dampers positioned on the Gen-Set and rigid attachment points for the E-motor and inverters. The GenSet is located inside the engine mount, below the propeller axis with the shaft facing backwards – an unusual installation enabled by the serial hybrid solution. Above the GenSet are positioned the four inverters, with the electric motor positioned frontally on the upper position to accommodate a large diameter propeller to increase the overall efficiency.

The component installation was followed by the electric installation, which covers the routing of data busses and power lines connecting the components. With the installation of the cooling ducts and radiators, the functional powertrain integration in the firewall forward section was completed. Behind the firewall, the integration activity consisted of the installation of avionics, HMI, seats and covers defined the cabin space of the hybrid demonstrator. The battery, located externally, was connected through the designed service channels to the powertrain. Finally, the cowling preparation was made. With a big intake at the bottom for the ICE cooling and two small side intakes for the inverter cooling and the electric machines, the cowling defines the characteristic aspect of the HYPSTAIR test platform.

After the hybrid powertrain components have been installed, the beginning of the testing phase was the most awaited phase of the project. The first power-up involved the electric motor and the twin inverters using battery power only and without the installation of the propeller. This testing allowed the tuning of specific system parameters and cleared the system for propeller installation to provide the necessary load for the powertrain. Testing the system with the propeller was performed gradually, starting first with short sequences on low power (up to 100 kW) and slowly expanding the testing envelope up to 200 kW. With the initial testing performed on battery power only, the successive phase involved power generation using the generator/ICE only. The operation of the ICE required an extensive tuning of the control system, as the HMI operation of the hybrid propulsion does not provide to the pilot specific actuators for starting and tuning the internal combustion engine operation regime – thus the control system needs to handle seamlessly and autonomously all phases of operations from starting, to idling, regime tuning and switching off the ICE. With the system tuning completed, the next stage of testing focused on replicating simulator missions, as well as on specific flight segments power profiles. All system parameters from sensory inputs, can bus data, temperatures, etc. were monitored during operations to analyse the system and provide inputs for future system operation. The system was tested under different ambient conditions, from cold winter days to hot & humid summer days operating within specified parameters. Alongside the functional testing, also the testing of Human Machine Interface was conducted by manually operating the powertrain with the haptic lever and monitoring the system status on the HMI display. Eventually testing was concluded with safety tests, where simulated failures where used to validate the system capability to reach the safe state.

Potential Impact:
1. Overall project impact

The HYPSTAIR project contributes to three main socio-economic challenges, addressed also by Cooperation Work Programme 2013 of European Commission, particularly competitiveness and growth through innovation, eco-innovation and safe and seamless mobility. The HYPSTAIR project was based on broadening the horizons of knowledge in the field of hybrid propulsion systems of light aircraft and creating novelties in field such as human-machine interface, haptics, hybrid components and battery systems. Lightweight design, efficiency and safety were the main project objectives; moreover, a great share of research and development in the project was a pioneer work and as such paved the way to the new generation of aircraft. By developing electrical components, comprising a serial hybrid drive system for general aviation segment, the project is introducing the advanced technologies for all-electric aircraft and thus stimulating innovation. The hybrid solution, due to range restriction of fully electric propulsion, mainly the poor energy density of batteries, enables market penetration of electric propulsion technology into the market of general aviation aircraft before 2020.
Several advanced concepts and technologies have been applied to achieve the lightweight design of hybrid propulsion system, which lead to reduction of weight of mechanical systems and maintenance costs and at the same time enhancement of safety. The project presents therefore a significant step towards reduction of aircraft costs through reduction in fuel consumption and maintenance. The reduction of fuel consumption is a result of increased drive efficiency as well as the lower price of electric energy as compared to aviation fuel. The hybrid generator runs on automotive fuel instead of aviation fuel, reducing fuel costs even further. Alongside, the maintenance charges are reduced due to simpler construction and increased reliability of an electric motor as compared to a piston one. The enhanced level of safety derives from integration of electric propulsion motor which is typically more reliable than a petrol powered engine and from relying on two separate power sources. In case of failure of either power source, the system still allows the aircraft to land safely with functioning propulsion. New advance concept was also applied when considering the complexity of the hybrid propulsion system for pilot. The integration of an intuitive HMI and haptic effects enables the pilot a friendlier user experience of hybrid systems in comparison to typical piston powered engine. Therefore, the pilot is able to safely utilize the hybrid propulsion system. The hybrid propulsion system presents not only the reduction of greenhouse gasses emissions and pollutants but also reduction of noise emissions. The serial hybrid propulsion allows completely electric take offs and landings, thus making a serial hybrid-electric aircraft a zero emission vehicle near ground. Beyond the environmental impacts that are evident from the project, HYPSTAIR technology could massively effect the wholesome transport system in the long-run. Since electric propulsion in the aircraft provides silent take-off, smaller airports in the vicinity of city centres could be reopened without concerns of emissions and pollutants from transport. In that case, hybrid aircrafts could be used not only for recreational flying but also for commercial flights, contributing to safe and seamless mobility.
The project developed, validated and demonstrated several advanced concept and technologies, and at the same time acknowledged also the aspect of future commercialization. Due the absence of any kind of regulations and standards for hybrid and electric aviation drives, the commercialisation of hybrid aircraft remains a challenge. Establishment of appropriate standards for certification is therefore necessary and will significantly reduce the time to market of hybrid and electric propulsion solutions, not only for the project further development, but for general certification of such drives. The project presents one of the many but important steps toward the commercialisation of a hybrid drive airplane.
While the adoption of hybrid-electric powertrain for large passenger aircraft will require several years of research, its application on small and general aviation aircraft can provide the necessary public acceptance and field experience. As the hybrid propulsion is new and complex in terms of managing the aircraft, special efforts needed to be made to introduce new technologies to scientific and wider audience and gain their understanding and trust in the new technology. During the lifetime of HYPSTAIR project, we successfully managed to present the benefits of hybrid aircrafts and diminished the concerns about safety and complexity of use. HYPSTAIR project was presented at major European and global events, (AERO Friedrichshafen 2014, 2015 and 2016, European flagship event Aerodays2015 in London, Emission free & Electric Flying Symposium, held by the DLR, Cleansky2 Small Air Transport conference etc.), which were also attended by crucial local, national and European stakeholders and policy-makers. Both of the latter supported the general concept of the project and expressed great interest for its final results.
Although there is still a long way until the HYPSTAIR hybrid aircraft will fly, the consortium successfully powered up the world’s most powerful hybrid electric powertrain for aviation and showed the way to a new light hybrid aviation generation. This achievement reminded crucial policy-makers that legislation, safety measures and regulations must immediately follow the industry development in order to avoid bureaucratic barriers which inhibit innovation. However, the project seemed rather interesting also for big general aviation players, since all segments of technological innovations with additional adaption could be used in general aviation. To arrive at conclusion upon gathered feedback from professional sphere, HYPSTAIR strengthened the cooperation among both the small private companies and big players in general aviation and initiated research and development in the field of hybrid propulsion concept, human-machine interface, haptics and energy efficient electric motors.

2. Project impact per activity

In scope of WP2: Hybrid propulsion aircraft concept and requirements, coordinated by University of Pisa DICI and DESTEC department, the HYPSTAIR project underlined the strengths as well as the weakness of a serial hybrid general aviation aircraft. Such conclusions can have a significant impact on both academic research and industrial development concerning such technology. In particular, the implementation of performance models and experimental results on power train components (batteries, propeller, etc.) has led to the set-up of a simulation tool, which allows to study the system behaviour for different parties who want to approach hybrid airplanes, such as:
• Pilots, whose perspective is most important since a hybrid airplane is a more complex system and the capability of managing all the additional variables related to power management can only be achieved through a specific and “unconventional” training;
• Aircraft designers, who can derive new constraints and objective functions for the aircraft optimization just analysing the system in operating conditions through the simulator.
• Powertrain designers, who can obtain useful information about power requirements in every flight condition, including the off-design and failure ones.
• Avionic designers, whose perspective is strictly related to pilots’ one since the instrument panel is the human-machine interface (HMI). In the simulator design and creation, a significant part of effort has been put in the connection between system output and information/communication functions of the HMI developed within the project.

Results achieved, in particular requirements and expected performance, could be exploited for the preliminary design of hybrid airplane components, whereas the achievements concerning the simulation tools can have interesting additional applications in pilots training, marketing activities and dissemination of knowledge.

In scope of WP3: Human-machine interface for hybrid aircraft, coordinated by MBVision, main results achieved were the development of human-machine interfaces and haptic power lever. MBVision carried out a multidisciplinary research on human-machine interfaces (HMI) and innovative technologies, expanding its knowledge and know-how in new interaction a usability instruments especially designed for electric and hybrid powertrain. Throughout the project, MBVision created new standards for the design of HMI for aircraft/vehicles with electric/hybrid engines, pioneering this specific field in aviation. A great share of performed studies are reusable and represent an excellent starting point for other types of HMI projects and an overview of the last trends in interaction. Performed studies and the GUI layouts were designed to be usable and scalable also on other devices. This gives the possibility to adapt components and processes to other aircraft instruments and interfaces, which could be market-ready in a short time. The second part of the design process done for HYPSTAIR project is the introduction of the innovative concept of "common layer", trying to standardize the guidelines for interfaces. In the aeronautical field the need (as for the automotive field) to unify/standardize parts of the GUI of instruments in extremely high, especially for safety reasons. A good standardization also reduces instruction/training time for pilots, helping when dealing with different types of aircraft and different powertrains. The project was a great opportunity to monitor and research the state-of-the-art cutting edge technology in the interaction design, but also to understand the trends and the potential future opportunities and planning the possibility of transferring the project results in other areas such as nautical, automotive, or complex machines. The impact of this research section is of great importance for the general development of aerospace and automotive industry and as such raised a lot of interest of scholars and professionals.
While hybrid propulsion system can extend flying range of the electrical aircraft, however, the complex structure of the hybrid powertrain complicates the control. Haptic power lever, developed by the University of Maribor, will enable the pilot not only to control power delivered by the hybrid powertrain but also to communicate actual state of the complex hybrid propulsion system to the pilot via intuitive haptic human sense. The designed haptic power lever will therefore not only enhance the information flow between the pilot and cockpit and thus reduce pilot workload, but will also provide energy efficient control of a hybrid powertrain system. The experimental prototype together with a proper flight simulator will enable ground tests with pilot-in-the-loop and evaluation of haptic based control in simulated flight conditions in a cheaper and more flexible way. However, it is highly required to experimentally evaluate the real efficiency of the haptic-based power control in all-electric or hybrid propulsion system aircrafts also in the air, when possible, since the proposed haptic effects were designed by a certain degree of speculation. The experiments with pilot-in-the-loop, including proper psychophysical tests, will show and asses more deeply how the haptic information really help the pilot from the performance and safety point of view.
In scope of WP4: Hybrid system components, coordinated by Siemens, innovative and powerful hybrid serial propulsion system components were developed. It is the world’s first hybrid-electric aerospace propulsions system in the 200 kW class, which most certainly left a mark in the research and development. The HYPSTAIR propulsion system represents the most complex hybrid-electric aerospace propulsion system developed so far by Siemens. Valuable experience has been gained during the development and the testing of the HYPSTAIR propulsion system. The integration of the Rotax 914 combustion engine into the control cycle via the ICE gateway translating control signals to physical control inputs via servos proved to be successful. The results of the HYPSTAIR ground test campaign gave valuable feedback on the behaviour of the Siemens-designed components in a typical general aviation installation. This will benefit future developments as the knowledge gained greatly increased the confidence in the design methods and design tools used reducing risk for future projects. The thermal behaviour of the electrical machines unveiled further growth potential for those components as they did not reach their thermal limits during testing. This knowledge will help to further improve the power to weight ratio of future electric or hybrid-electric propulsion units.
Another major research work, performed by Pipistrel, was done in the scope of the battery. It was developed to be powerful enough to support silent take-off, but also light and small enough to fit inside the aircraft wings. A prototype battery along with a dedicated battery management system has been built and tested for performance. In order to guarantee pilot and passenger safety, a self-protection monitoring has been implemented to prevent the case of battery overheating.
In the field of testing methods, especially the Hardware-in-the-Loop (HiL) methods, studied by University of Maribor, several important considerations were derived. One of the challenges was the lack of the requirements of system developed in the project. However, based on the system created by the University, the requirements can be different. Even when something is not strictly demanded (as it is the case in aerospace), the safe operation of the system can be presented with the use of the HiL system. It is important to mention, that the use of HiL systems in testing makes it possible to reuse the big part of the testing equipment, especially software, which reduces the costs of the development process. HiL system is a useful tool in Rapid Control Prototyping (RCP), so not only testing costs are reduced, but also costs of other development phases (design, integration, etc.). Because of the simple handling the time to product is shortened.

In scope of WP5: System level integration and verification of components, coordinated by Pipistrel, the major engineering breakthrough was the integration and testing of the hybrid components in a test platform. This development work validated the concept of a single engine mount bearing all components of the powertrain, a mounting method which simplifies maintenance and retrofit to existing general aviation aircraft. Also the multi-temperature cooling principle was tested, showing a compact installation suitable for operating components on various operating temperatures, namely the ICE, electrical machines and power electronics.
Powertrain testing validated the knowledge in electric propulsion integration principles and component dimensioning. With the powertrain test platform resembling a four seat aircraft fuselage, the successful functional tests of HYPSTAIR powertrain paved the way towards an early market introduction in general aviation.

3. Exploitation activities

The project partner’s knowledge has been efficiently exploited among partners and external stakeholders and contributed to successful closing of the project. Although the project ended, several forthcoming research opportunities have been identified.

In field of hybrid propulsion aircraft concept and requirements, the aim is that the collaboration among partners continues and that the project knowledge is further exploited, especially to:
• integrate haptic interfaces in “HyPSim” (University of Pisa and University of Maribor);
• optimize aerodynamic characteristics of the aircraft (University of Pisa and Pipistrel);
• implement detailed models of power train component in “HyPSim” (University of Pisa and Siemens);
• implement more information and off-design/failure condition in the HMI (University of Pisa and MB Vision).

Additionally, the University of Pisa will continue the development of the simulation tool in future research activities, aiming at implementing DEP solutions, different propulsion systems and novel aircraft configurations (e.g. box-wing). Further research will be focused on the implementation of more detailed models and/or experimental data concerning the hybrid airplane components, with a particular attention to power train components such as the electric motor, the inverter and the internal combustion engine. An optimization study concerning the aircraft design, in which Distributed Electric Propulsion (DEP) concepts as well as novel aircraft configurations could be taken into account, would be of great interest from the research standpoint.

In field of human-machine interface for hybrid aircrafts, the studies performed within HYPSTAIR project can be reused in large part as an excellent starting point for development of other types of HMIs and as an overview of the last trends in interaction. The GUI layouts were designed to be usable and scalable on other devices. This gives the possibility to adapt components and processes to other aircraft instruments and interfaces, theoretically ready for the market in a short time. An adapted graphical user interface derived from HYPSTAIR has been licensed to NASA (National Aeronautics and Space Administration, USA), and will be used by American Space Agency in the scope of the electric flight research program. The experiences gained in the project are pursuing collaborations for interface design with new and old customers, and of course putting to use, in a scientific context, the results achieved in research, through a series of interventions. Several scientific and technical publications and journals specialized within aeronautics and applied research in industrial design (Creactivity Magazine 2016, German journal Fligermagazin, Italian journal JP4 ect.) will be addressed. Additionally, papers and technical assessments will be presented at conferences such as Creactivity 2016 (Piaggio Museum, Pontedera, Italy) and at the forum space of AERO2017 fair in Friedrichshafen (Germany). Academically, collaborations for research in interface design will be developed with project partners (engineering department of the University of Pisa and University of Maribor), but also with university and research institutes, such as ISIA Firenze (Institute for Industrial and Communication Design, Florence), Sant'Anna Superior School (Industrial and Information Engineering, Pontedera, Italy), Research Center E. Piaggio (Bioengineering and Robotics Research Center, University of Pisa). The underway commercial collaborations build on the development of specific interfaces for aircrafts, robotics vehicle (in aeronautics, automotive, automation) with electric and hybrid propulsion together with project partners such as Pipistrel and new local and national (QDesign, Evantra, Piaggio) and international (DLR, Germany, Dynon, USA) clients. The gained knowledge and experience in field of human-machine interface will be capitalized in the upcoming research projects more extensively in the Mobility and Creative Engineering.
The ultimate goal of the upcoming research in field of haptic power lever will be to prove whether the combination of the visual and the haptic cues in the cockpit improves the control or results in a poorer performance. Studies in cars have shown that the relationship between the visual and the haptic cues is an intricate one and therefore must be carefully designed to avoid a perceptual inconsistency in visual and haptic stimuli. The project has also shown that fine and vivid haptic patterns may be challengeable and need further research to improve performance if necessary. It will also require further R&D activities to simplify the system configuration and thus provide even more cost effective solution whereas maintaining a required performance. The knowledge gained regarding haptic interfaces in vehicle cockpits will make possible continue the research in higher technological level with industrial and academic partners. It is also important to mention, that developed hardware and software is possible to reuse in the big part in laboratory for robotics. This reduces the cost of the development process in similar future projects. With presenting the research results from this project on scientific conferences and publishing in scientific journals, the electric vehicle technology will be promoted. Haptic interface in aircraft is undoubtedly innovation in the field and as such paving the way to safer and user-friendlier aircraft.
In field of hybrid system components, the lessons learned concerning the performance of the individual components as well as the overall system behaviour have become part of the knowledge base at Siemens and are of great value for present and future projects. The work done in the HYPSTAIR project greatly improved experience on controlling a hybrid-electric propulsion system. Amongst other projects knowledge gained during the HYPSTAIR project is of much help in the Airbus-Siemens-Collaboration.
The development work on the HYPSTAIR high power propulsion battery required to implement and test the same functional and protection principles required for a flying prototype. The advancement of knowledge in the battery management system and the battery operational principles will be directly exploited to existing electric aircraft, but also pave the way toward future, safer hybrid-electric aircraft.
The knowledge gained on testing methods will be transferred into the class programme at the University of Maribor, enabling the students to obtain a good insight not only into the technical background of the aviation, but also to data regarding regulations and certification requirements. Especially knowledge regarding functional safety in aerospace will be very welcome. The understanding of development and production processes in aerospace industry will be an advantage of our students when compared to their peers from other universities. This will give an advantage not only to them but also to the complete local industry towards competition. The knowledge regarding functional safety will extend the possibility to be included in new research projects, funded by the EU, local government, interstate or direct industrial. Functional safety is gaining importance in the industry, where the forerunner was a nuclear industry, but currently the most of activities are in the automotive. This will be even more important, when the cars of the future become more automatized, with features like autonomous driving. The new possibilities also opened for us to be involved in activities related to flying car, which might be an interesting product for the future.
In the field of system level integration and verification of components the project provided valuable know-how on integration and operations of hybrid-electric powertrain. With the ambition of introducing before year 2020 a hybrid-electric Panthera on the general aviation market, HYPSTAIR served as a benchmark project to gain expertise on novel powertrain integration. The project also led to several breakthrough solutions, from the algorithm developed to estimate available range on a dual power source aircraft to the concept of integrating accessories drives to the electric motor bearing case.

4. Dissemination activities

Throughout dissemination activities and strategy, the efficient and effective dissemination and exploitation of knowledge among the project partners and knowledge transfer to the interested stakeholders in aeronautics were achieved. The dissemination strategy identified crucial project milestones suitable for dissemination, main audience and target groups, dissemination tools and provide efficient implementation of dissemination activities. The Final dissemination plan also outlined forthcoming activities that should be implemented after the project end, such as updating official project website with project activities and events; increasing the links to project website; disseminating of promotional materials; submitting articles on the HYPSTAIR project in national and international magazines; presenting at relevant conferences, workshops and fairs and participating at relevant fairs and exhibitions, which have the potential to significantly raise awareness. Since there is a sensitive issue on how to use the materials and foreground after the project, the consortium signed the Agreement on use of materials during and after the project HYPSTAR. The latter comprises clear identification of project foreground, summary of all the rules and procedures, defined confidentiality issues and the list of materials that could be freely used after the project. The main dissemination tools used in project were:
- The official project website:
The project website is the main tools for public dissemination, internal project and knowledge management. The aim of the website is to reach a wide range of interested audience and target bodies. It contains overview of project, activities and progress. The project website is frequently updated with news, project materials and deliverables and will remain active also after the project ends.

- Workshops and conferences
Throughout project events, such as scientific workshop and final conference, the knowledge in field of electric and hybrid concept of aircraft was exchanged and further exploited.

The first project workshop “Certification requirements of components for electric aircrafts” outlined a pathway to first milestones of certification requirements for electric aircraft. The workshop successfully gathered stakeholders from electric aircraft industry together, from regulators, researchers, and manufacturers/designers to discuss the development and validation of hybrid propulsion system components and sub-systems for electrical aircraft. The debate on certification process was supported by presentation of state-of-the-art in certification of electric and hybrid-electric aircraft, delivered by representative of certifying authority (Mr. Ronig from EASA). Additionally, the regulatory overview of electric flying and HEPU regulatory provided experience which were taken into consideration in the project. The necessity of authority support for certification of innovative components, such as developed in the project, were presented to certifying authorities. The workshop provided in depth understanding of certification process, and especially exposed previous experience with certification processes, especially challenging nature of certifying innovative technology for commercialization use.

The second workshop “Current State of Art in Hybrid Propulsion Components and Future Developments” outlined the current state of art in hybrid propulsion components and addressed several interesting aspect regarding the future developments. Professionals and stakeholders in aviation scientific community which, attending the workshop, exchanged several interesting approaches of quantifying the impact of future technologies in aviation. In this context also the project advance technological concept was re-evaluated and again the future challenges of hybrid electric propulsion system for aviation due to certification processes were exposed.

Within the final conference, organized in cooperation with Symposium E2 Flying, a wide range of topic in the field of electric flying such as electric drives, battery research and battery safety, electric aircraft projects, amphibious hybrid electric LSA and high-temperature superconductors in the electric plane were covered. Beside mentioned, the symposium addressed also the hybrid-electric retrofit for existing aircraft, fuel cell-battery hybrid concepts and fuel cell technology in general. In this promising area of research, special attention has been given to progress made within HYPSTAIR project.

- Fairs and exhibitions (HYPSTAIR stand)
The project has been disseminated also through exhibitions in framework of several well-known fairs and events in field of aeronautics, such as AERO Friedrichshafen 2015 and 2016, Aerodays 2015 and E2 Flying Symposium 2016. The most visible results of dissemination in the HYPSTAIR project were adaptable stands that consortium provided for the biggest European events in the field of aviation. They were designed (by MBVision) in context of overall project visual identity, by considering the HYPSTAIR colours and accompanied with logotype and key information of the project. Throughout the events people started recognizing HYPSTAIR based on the colours and logo and were continually attracted to the stand where they were provided with thorough information and details about the project.

- Project demonstration instruments
During the project several deliverables primarily used for research activities were appropriately adopted for dissemination activities. This allow much more interactive presentation of project results, especially at fairs were wider public had opportunity to examine developed technology face to face. It is much easier to understand basic idea about the project if there is a possibility to personally test something. For that reason, the HyPSim simulator and the Haptic power lever were presented at all major event, with the HYPSTAIR test platform publicly unveiled at the “electric flight” section of AERO 2016.

- Articles and presentation at conferences
Several articles has been published during the project to present the project context and progress. In addition, the following scientific publications have been produced:
• Cipolla V., Oliviero F., “HyPSim: A simulation tool for hybrid aircraft performance analysis”, Contribution for the book “Variational Analysis and Aerospace Engineering III: Mathematical Challenges for the Aerospace of the Future” (Springer U.S. to be published in 2017).

The project results have been widely disseminated on several conferences, among others:
• Creativity 2013, Pisa, 22th November 2013: Innovation in light energy efficient aircraft design (Pipistrel)
• International Electrotechnical and Computer Science Conference ERK, Portoroz, 23th September 2014: Technology Enabling Hybrid Powered Flight (Pipistrel)
Electrisch und Emmissionsfrei Fliegen Symposium, Stuttgart, 26- 27th February 2015 (Pipistrel)
• EWADE/CEAS Conference, Delft, 9th – 11th September 2015 (University of Pisa)
• Symposium on Collaboration in Aircraft Design, Naples, 12th −14th October 2015 (University of Pisa)
• Aerodays 2015: "Towards Certifiable Hybrid Powertrains for Electric Aircraft", London, 20th –23th October 2015: (Pipistrel)
• XXIII AIDAA Conference + Aerospace & Defence Meeting, Turin, 17th −19th November 2015, (University of Pisa)
• Cleansky 2 Small Aircraft Initiative Workshop, Warsaw ,5th February 2016 (Pipistrel)
• Sustainable Aviation Symposium: Dr. Frank Anton: Siemens Purpose-built Electric Aircraft Propulsion System, , San Francisco, 6th - 7th May 2016: (Siemens)

- Project video
The HYPSTAIR project video demonstrating the first power up hybrid electric powertrain was produced and disseminated through several channels over World Wide Web. Immediately after the final conference, video was uploaded to the website and Youtube. Pipistrel and Siemens published their press releases about the achievement on the internet and send them to major news houses, which resulted in immense media coverage. Average number of visits on HYPSTAIR website tripled just in few days. The project is currently still available at https://www.youtube.com/watch?v=x1Xrr82Hbkc

- HYPSTAIR book
HYPSTAIR book summarize the whole HYPSTAIR story with struggles and achievements, not leaving out one milestone or technical detail. Book consists of final description of each topic accompanied by images, schemes and tables to supplement the text.

- Promotion material: Newsletters, brochures, leaflets and posters
The Newsletters target at raising awareness about the project, informing the target groups about technical and substantive progress of the project and inviting target groups to participate at project events.

The first edition of Newsletter presented project in general, underlining the project objectives, context and consortium. The second Newsletter was more extensive as first addition, addressing the project progress in more detail, especially the flight simulator model (HyPSim) used for a hybrid aircraft, human machine interface (HMI) components with haptics effects, battery system, design and manufacturing of the component installation platform and technical characteristic of electric components of hybrid propulsion system. The newsletters were distributed in hard copy and electronic version. The hard copy was mainly distributed on project workshops and events (such as AERO 2014, 2015, 20162016, International Paris Air Show 2015, E2 Symposium 2016, etc). Electronic version was distributed to the interested stakeholders in aviation based on gained databased from previous projects and enriched during the project workshops and events.

The HYPSTAIR brochures present printed and electronic dissemination tool for awareness raising at the all levels. The first edition of brochures contain general description of the project, objectives and partners. In line with project development also the need for more extensive brochures containing the project progress emerged. Therefore, new and updated brochures were prepared containing more detailed description of project’s work and progress.

The HYPSTAIR leaflets are additional printed dissemination tool for awareness raising at national and EU level. Leaflets contain project description distributed in all partner countries and at all dissemination events.

The HYPSTAIR Posters were design in accordance to project development stage. At beginning very general poster were designed, containing only basic information about projects. After several project milestones were achieved, additional posters were designed containing in depth presentation of project results such as Human Machine Interface for Hybrid Aircraft, Serial Hybrid Propulsion System Architecture, and HYPSTAIR Mission Profile. The printed version of posters was used at occasional dissemination events, especially fairs, exhibitions and conferences, while the digital version of posters are available on the project website.

Dissemination in numbers:
- more than 30.000 visits to the website with more than 250.000 page views
- 3 external events organised by HYPSTAIR
• First Workshop: 28 participants from 7 countries world wide
• Second Workshop: 30 participants from 6 countries world wide
• Final conference: 150 participants from world wide

- 4 exhibitions:
• AERO 2015: 33.900 participants
• Electric Aircraft Symposium CAFÉ 2015: 100.000 participants
• AERO 2016: 30.800 participants
• AERODAYS 2015: 1.000 participants

- 3 project dissemination demonstrators:
• HyPSim simulator
• Haptic power lever
• Integration platform

- 39 Articles in popular press
- 1 scientific papers to be published
- 12 conferences, workshops and other events attended with oral presentation of project
- 27.400 views of project video
- 300 project book printed
- 300 Newsletter Nr.1 (first edition) distributed
- 500 Newsletter Nr.2 (second edition) distributed
- 1.500 Brochures Nr.1 (first edition) distributed
- 600 Brochures Nr.2 (second edition) distributed
- 6.000 Leaflets distributed
- 94.000 audience reach with radio interviews
- 94.000 audience reached by TV clips of project HYPSTAIR
- 10 Press releases produced

List of Websites:
http://www.hypstair.eu/

Pipistrel d.o.o. Ajdovscina - COORDINATOR
Igor Perkon
igor.perkon@pipistrel.si
tel: +386 5 365 81 60

Siemens A.G.
Marco Schramm
schramm.marco@siemens.com;
tel: +49 911 433-9170

University of Maribor
Maršenka Marksel
Email: marsenka.marksel@um.si
tel.: +386 2 22 94 393

University of Pisa
Aldo Frediani
Email: a.frediani@dia.unipi.it
tel.: +39 0502217 26

M.B. Vision
Massimiliano Pinucci
Email: maxpinucci@mbvision.it
tel.: +39 0587 636435

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Final Report Summary - HYPSTAIR (Development and validation of hybrid propulsion system components and sub-systems for electrical aircraft)

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