Unconventional fly-gen airborne wind energy system

Based on capturing wind energy at altitudes, known as Airborne Wind Energy Systems (AWES , and the system called Ram air turbine (RAT), we propose and study an Unconventional Fly-Gen Airborne Wind Energy System that converses mechanical energy into electrical energy by Faraday’s law of induction, using mass flow produced on the PT6 engine cowling operating at idle gear. We show that it is possible to obtain electromotive force by varying the angle between the air mass produced by the engine propeller and the axial fan. We found that the amplitude and frequency of this EMF are inverse functions of cosγ and the induced peak power is independent of the type of biofuel with which the PT6 is fed.


Introduction
The amount and type of energy available to meet various human needs are closely tied to the development of societies [1][2][3].Electrical energy is one of the most critical forms of energy in our society and is essential for its various and widespread applications, including food production, education, healthcare, and communication.Most of electrical energy is generated from non-renewable sources, such as fossil fuels, which have a negative impact on environmental sustainability through combustion.Nuclear fuel sources are facing several serious challenges, including economic affordability, security of supply, environmental sustainability, and disaster risks; in order to address then, nations are implementing energy policies focused on reducing greenhouse gas emissions and increasing the deployment of renewable energy technologies [4][5][6][7][8].The widespread use of wind energy as an intermittent renewable source has seen the installation of 730 GW of power capacity by the end of 2021, as reported in [9].However, the availability of suitable land for future installations is expected to decrease due to saturation.To address this, it is crucial to focus on developing single wind turbines with a nominal power of up to 15 MW, as suggested in [10].Off shore installation of wind generators can provide greater and more consistent wind resources compared to those on land, leading to a more stable utilization rate.It is estimated that offshore wind energy could reach a capacity of 80 GW, as reported in [11].
A new renewable energy sector has emerged in the scientific community, focused on capturing wind energy at altitudes between 200m and 10 km from the ground surface, known as Airborne Wind Energy (AWE) [3,[12][13][14][15].The basic principle was introduced by Loyd, who analyzed the maximum energy that could be theoretically extracted with tethered wings [16].AWE has received great interest both theoretically and experimentally [17][18][19][20], with hundreds of patents, prototypes, and demonstrators developed in the past decade [21][22][23][24].The field has also seen advances in control systems, electronic and mechanical design [25][26][27].
Archer and Caldeira were the first to evaluate the potential of Airborne Wind Energy (AWE) as a renewable energy resource [28,29].They found a huge availability of kinetic energy in wind, at altitudes between 0.5 km and 12 km above the ground.Their research suggested a great potential for technologies capable of harnessing energy from high altitude winds.The estimated kinetic power that could be extracted from winds ranges from 400 TW to 1800 TW with traditional turbines and high altitude wind energy converters [30][31][32][33].However, complex climate models predict that massive extraction of such magnitude could impact the global climate due to the distributed drag forces against wind flows [34].Thus, extraction of only 18 TW, which is comparable with the world's power demand, is suggested [32].
Airborne Wind Energy Systems (AWESs) are generally composed of two main components: a ground system and at least one aircraft that are mechanically or electrically connected by ropes [35][36][37][38].Systems in which the conversion of mechanical energy through of the traction force) into electrical energy takes place on the ground are known as Ground-Gen AWES (GG-AWES) [39,40], and if the conversion is done on the aircraft using wind turbines, they are known as Fly-Gen AWES (FG-AWES) [41,42].
In GG-AWES, the motion of an electrical generator on the ground is produced by exploiting aerodynamic forces through one or more ropes that transmit a traction force from the aircraft.These systems can be divided into fixed-ground station devices (Pumping Kite Generators) and moving-ground-station systems, which in turn are divided into Vertical Axis Generators and Rail Generators.In FG-AWES, electrical energy is transmitted to the ground via electrical cables and can be classified as crosswind or non-crosswind systems, depending on how they generate energy.These systems can also be distinguished based on their flying principles, such as Wings lift, Buoyancy and Static Lift, and Rotor Thrust.
Another system that uses wind flows in aircraft is called a Ram air turbine (RAT), which is deployed when most of the conventional power generation systems on the aircraft have failed or are unavailable.RATs are airdriven turbines, typically stowed in the aircraft's ventral or nose section, that are extended either automatically or manually.They have advantages like high reliability, light weight, small size, and long working time and are highly compatible with the emergency energy demands of Unmanned Aerial Vehicles (UAVs).However, this system only operates when the aircraft is in turbo fan mode and is in flight, taking advantage only of the mass of air produced by air drag on the fuselage [43][44][45][46].
In this framework, we propose the first theoretical approximation of an unconventional Fly-Gen Airborne Wind Energy System (U-FG-AWES) that converts mechanical energy into electrical energy and aims to minimize the effects on the global climate caused by the obstacles generated by the turbines on the natural drag forces.Our system leverages the mass flow produced by the propeller of the PT6 engine and the flow generated on the aircraft's fuselage to produce and store electrical energy.The energy generated by our U-FG-AWES always comes from the airplane engine (if running) or the mechanical energy of the aircraft itself, enabling the generation and storage of electrical energy to be used in any eventuality or for a specific system.
The calculations were performed assuming that the engine operates at the minimum RPM to allow for comparison with future experimental studies on land.Our system is distinct from conventional FG-AWES in that it doesn't incorporate a ground system.We expect future studies to emerge from this proposal, including an analysis of the increased drag the aircraft will experience from modifications to the fuselage, which can be considered as parasitic resistance, leading to an increase in both total air resistance and engine power, resulting in higher fuel consumption.The next step in our investigation is to conduct experimental calculations.
This paper is organized as follows.The methodology ispresented in section 2. The model used to convert mechanical energy into electrical energy and the effect of the different configurations of the prototype are investigated in section 3. The results are presented in section 4 and in section 5, we provide our conclusions and final remarks.

Methodology
To establish a comprehensive methodology for the development and assessment of the Unconventional Fly-Gen Airborne Wind Energy System, with a focus on its capability to convert mechanical energy into electrical energy using Faraday's law of induction and harnessing the mass flow generated by the PT6 engine cowling in idle gear mode, we consider: 1. Conceptual Design: defining the core components of the Unconventional Fly-Gen Airborne Wind Energy System, including the PT6 engine, wind turbine, and electrical generation components.Establishing the essential principles behind the conversion of mechanical energy into electrical energy using the system's components.
2. System Configuration: Determining the specific positioning of the Unconventional Fly-Gen system on the aircraft, with a particular focus on the PT6 engine cowling.Designing the interface and mechanical connection between the wind turbine, PT6 engine cowling, and electrical generator.
3. Airflow Analysis: analyzing the airflow patterns and conditions around the PT6 engine cowling during aircraft operation.Model and simulate the impact of variations in angle γ between the air mass produced by the engine propeller and the axial fan on the generation of electromotive force (EMF).
4. Electromotive Force Analysis: developing mathematical models to predict the amplitude and frequency of the EMF based on the variation in angle γ.Investigate the relationship between angle γ and EMF amplitude and frequency, with an emphasis on the inverse functions involving cosγ.
5. Safety and Reliability: address safety concerns regarding the system's integration with the aircraft, focusing on the risk of component detachment.Develop measures to enhance the system's reliability and safety, ensuring its operation does not jeopardize the aircraft, crew, or the environment.
6. Results and Conclusions: summarizing the findings from the analysis, modeling, and testing.Discuss the implications of the results and their significance for the development and practical application of the Unconventional Fly-Gen Airborne Wind Energy System.
7. Recommendations and Future Work:providing recommendations for further refinement and enhancement of the system.Suggest potential areas for future research and development in the field of Airborne Wind Energy Systems (AWES).By following this comprehensive methodology, a detailed understanding of the Unconventional Fly-Gen Airborne Wind Energy System can be achieved, paving the way for further advancements and applications in harnessing wind energy at high altitudes for sustainable power generation.

Unconventional FG-AWES (U-FG-AWES)
The system proposed in this study differs from the conventional FG-AWES system in that our U-FG-AWES is not composed of the ground system and an aircraft connected by ropes, which allows simultaneous generation and storage of electrical power in aircraft of the turboprop type.This system differs from the RAT in that it can work on the ground, taking advantage of the mass flow coming from the engine propeller and using air drag when the aircraft is in the air.The basic electrical diagram for our unconventional FG-AWES is shown in figure 1.The diagram represents a system for generating electrical energy using the mass airflow produced by the PT6 engine blades and a battery for storing the energy produced.
The renewable energy system under discussion comprises several critical components that collaborate harmoniously to deliver secure and dependable power to the aircraft's flight instruments.The system's energy generation is spearheaded by a wind turbine, responsible for producing electrical energy.This energy output undergoes regulation via a voltage regulator, ensuring it falls within safe and suitable limits for powering other critical components.
An integral charge controller is seamlessly integrated into the system to maintain the battery.This essential component continuously monitors the battery' charge status and efficiently allocates the energy generated by the wind turbine to recharge the battery when it registers a low charge level.If the battery is already fully charged, the charge controller redirects surplus energy to the load connected to the system.
The battery' inclusion in the system assumes a pivotal role in mitigating fluctuations and enhancing the overall stability of the power supply.Its primary mission is to provide a reliable and adaptable power source for the aircraft's instruments.When the primary power source faces interruptions or failures, the battery smoothly transitions into emergency power mode, ensuring the uninterrupted operation of critical instruments.
More specifically, the battery functions as a reserve of emergency power, capable of energizing the instruments for a predefined duration until the primary power source is restored to normalcy.The selected battery for this critical role is a nickel-cadmium (NiCd) battery, distinguished by its nominal voltage rating of 24 V and an impressive capacity of 10Ah.
It's important to underscore that the decision regarding battery voltage and capacity is a well-thought-out choice influenced by several factors.These considerations encompass the anticipated duration of operation during power outages, as well as the constraints imposed by the weight and spatial limitations intrinsic to the aircraft environment.Consequently, tailoring the battery specifications to the distinctive requirements of each aircraft is pivotal to ensuring reliable and uninterrupted instrument operation.
To facilitate the seamless transition between the energy stored in the battery and that generated by the wind turbine, a commutator switch is seamlessly integrated into the system; this switch serves as a versatile means for selecting the power source, allowing effortless switching between them to align with the aircraft's prevailing power requirements.
The basic principle of operation of our device is Faraday's law of induction which states that the electromotive force is given by the rate of change of the magnetic flux and is expressed as where ξ is the electromotive force in volts (V), Φ is the magnetic flux in webers (Wb), and t is time in seconds (s).
Faraday's law (equation 1) states that an electromotive force (EMF) is generated in a conductor when the magnetic field varies with time.In order to generate an EMF, we aim to utilize the rotation of the coils in an electric motor by using an axial fan driven by the mass flow produced by propeller wind, that wraps around the fuselage of PT6 engine.The energy produced by the EMF can then be stored in a battery or other energy storage device.The effectiveness of this system will depend on various factors, such as the design and performance of the electric motor, as well as the wind speed and direction.
According to Bernoulli's principle, the high fluid velocities associated with low pressure are present on the engine cowling, and low fluid velocity high pressure fluids are in the bottom.This indicate that the best position to locate the U-FG-AWES, is on the engine cowling and away from the blades.Our unconventional U-FG-AWES incorporates an axial-flow fan.This choice is rooted in the design's intent to optimize the utilization of the air mass produced within the engine cowling.The axial-flow fan offers distinct advantages in this context, as it enables a substantial flow rate increase while incurring only a modest pressure gain.This feature aligns well with our objectives, as opposed to a centrifugal fan which handles a comparatively smaller flow and necessitates a larger pressure rise.The reasoning behind this choiceis the following: Bernoulli's principle is a foundational concept in fluid dynamics and is frequently applied to the airflow around objects, such as aircraft.In the context of our proposal, the PT6 engine cowling is exposed to the airflow generated by the propeller, leading to variations in fluid velocity and pressure around the cowling.On the upper surfaces of the engine cowling (the top and sides), the propeller accelerates the airflow.According to Bernoulli's principle, this higher fluid velocity corresponds to lower pressure.This phenomenon is employed in various aviation components, including wings and airfoils, to generate lift.In our proposal, this low-pressure region presents an opportunity for energy extraction.
Conversely, the lower part of the engine cowling experiences reduced fluid velocities due to the obstruction posed by the aircraft structure and the ground.Consequently, the pressure in this area is higher.While this highpressure region is less suitable for energy extraction, it contributes to creating the pressure differential required for the airflow around the aircraft.
On their side, an axial-flow fan is characterized by its ability to move a large volume of air in a linear direction along the axis of rotation.This design is highly suited to exploiting the mass flow of air generated within the engine cowling.Here's why this feature is advantageous: 1. Mass Flow Utilization: the engine cowling produces a significant mass flow of air due to the propeller operation.An axial-flow fan excels at efficiently capturing and directing this mass flow, optimizing the potential for energy extraction.
2. Relatively Small Pressure Gain: the axial-flow fan design results in a relatively minor increase in air pressure as it accelerates the airflow.This aligns well with our purpose of generating energy, as we aim to convert the kinetic energy of the air mass into electrical energy via Faraday's law of induction.A small pressure gain is ideal for maintaining efficient energy conversion.
3. Suitability for the Purpose: Given the focus on converting mechanical energy into electrical energy using Faraday's law, an axial-flow fan's characteristics match our requirements.The energy extraction process benefits from the large flow rate and modest pressure gain associated with this type of fan.

Contrast with Centrifugal
Fan: A centrifugal fan, in contrast, operates by directing air radially outward from the center of rotation.While it can generate higher pressure differences, it is better suited for scenarios where pressure gain rather than efficient mass flow utilization is a primary concern, making it less suitable for our specific purpose.
In summary, the choice of an axial-flow fan for our U-FG-AWES design is deliberate and advantageous.It capitalizes on the unique characteristics of the engine cowling's air mass flow, efficiently converts kinetic energy into electrical energy, and ensures optimal performance for our energy generation goals.
Since the magnetic flux variations in our device are produced by variations of the angle between the magnetic field and the plane of the coil from the equation (1) we have: where N is the number of turns of the loop, B is the magnitude of the magnetic field, ω is the angular velocity of the blades of axial fan, A is the solenoid area, and t is time.Equation (2) indicates the necessity of calculating the angular velocity at which the motor shaft rotates.This velocity is generated by the force supplied by the PT6 engine propeller, where the exhaust gases expand into the atmosphere, producing power P on the axial fan.

P ( )
where r is the distance between the axis of the blades and the point of action of the air force F supplied by the PT6 engine's paddles, and γ is the angle between the displacement vector and the force vector.
This study considers the propeller thrust force due to Jet A1 fuel and biofuels B10, B30, B50, and B100, as illustrated in table 1.
The equation (3) indicates that the power in the axial fan is a function of the angle γ and the air force F. A particular extreme case is obtained when we consider γ = 0°; for this case, we find that the power is P Frw = , where Fr is the physical quantity known as torque τ.In this particular case, τ represents the maximum torque generated by a perpendicular force acting on the axial fan.The other extreme case is generated when γ = 90°, and for this case, the force is perpendicular to the displacement, resulting in a power of P 0 = .The preceding analysis indicates that the power can vary for 0° γ < 90°.
From equation (3), it is possible to calculate the angular velocity.

Results
The equation (5) indicates that when γ = 0°, the electromotive force is at a minimum, as it is inversely proportional to cos g.Taking this extreme value and considering the propeller power produced by Jet A1 fuel (table 1), we can find analytically using equation (6) and graphically (figure 2) that ξ has an amplitude of 12.25 Volts and a frequency of 0.040Hz.
When γ = 90°, ξ is undetermined, however, for values close to it, for example, γ = 89°, the electromotive force is at its maximum with an amplitude of 698.422Volts and a frequency of 2.327 Hz, as illustrated in figure 3. The above indicates that it is possible to obtain electromotive force for 0° γ < 90°, as indicated in figure 4, where we investigate electromotive force for γ = 15°(red squares), γ = 30°(blue circles), and γ = 45°(green diamonds).This graph confirms that both the amplitude and frequency increase with γ.We extend our investigation to the case in which the PT6 engine runs with biofuels, using the values detailed in table 1 for biofuels B10, B30, B50, and B100, and for different values of the angle between the propeller displacement and the force vector γ, as illustrated in figure 5 for γ = 15°(red squares), 45°(blue circles), and 75°( green diamonds).
The figure shows that for the same γ value and different biofuels, the same peak value is obtained, so the slope of each of the lines (red, blue, and green) is zero.The peak potential for each angle is as follows: for γ = 15°, the peak potential is 12.619 Volts; for γ = 45°, it is 17.238 Volts, and for γ = 75°, it is 47.095 Volts.These results indicate that as the value of the γ angle increases, the peak potential also increases.This behavior is mainly due to the fact that the differences in power and thrust force produced by the engine propeller with biofuels are very close, as shown in table 1.

Conclusions
We propose and study an U-FG-AWES, that converses mechanical energy into electrical energy by Faraday's law of induction, using mass flow produced on the PT6 engine cowling.Electrical energy istransposed to a storage unit, from which it can be given the desired use.The research is in the theoretical analysis phase, which will be extended by experimental tests on a PT6 engine.
We found that it is possible to obtain electromotive force varying the angle γ between the air mass coming from the propeller of PT6 and the blades of the axial-flow in the interval 0°γ < 90°.It is important to highlight that both the amplitude and frequency of the EMF are inverse functions of cos g so the greater the γ, the smaller the cos g, and the greater the EMF.
We also found that as γ increases, the peak potential and the oscillation frequency of the electromotive force also increase.We also found that when using different bio fuels in PT6, both the thrust force and the power of the motor propeller have very close values, so the peak voltages remain constant.
It should be noted that this is the only proposal that exists so far for the production of electrical energy from the mass flow of air produced by an airplane's engine.The research is currently in the theoretical analysis phase and aims to be extended to experimental tests on a PT6 engine.Here are some tests, challenges, and potential benefits related to this proposal: 1. Tests: (a) Experimental Validation: conducting tests on an actual PT6 engine to validate the theoretical findings is essential.Experimental setups could involve measuring the induced electromotive force (EMF) at different angles between the air mass from the propeller and the axial-flow fan.These tests will help verify the feasibility of the proposed energy conversion process.
(b) EMF Amplitude and Frequency: performing tests to measure the amplitude and frequency of the induced EMF at different angles.This will help establish a relationship between the angle and the resulting electrical output.(c) Effect of Biofuels: testing the system using different biofuels in the PT6 engine to verify the consistent performance of the proposed energy conversion method.Comparing the peak voltages and other relevant parameters for different biofuels.
(d) Stability and Control: evaluating the stability of the system at different angles and under varying wind conditions.Ensuring stable operation and control mechanisms will be crucial for practical implementation.

Challenges:
(a) Efficiency Optimization: maximizing the efficiency of the energy conversion process is a significant challenge.Minimizing losses at various stages, such as conversion and storage, will be crucial to ensure a viable energy generation system.
(b) Angle Control: maintaining a precise and controlled angle between the air masses from the propeller and the axial fan can be challenging, particularly in dynamic wind conditions.
(c) Practical Implementation: transitioning from theoretical analysis to practical implementation involves numerous engineering challenges, including designing a robust and safe system for real-world applications.
(a) Novel Approach: our proposal introduces a novel method of generating electrical energy from the mass flow of air produced by an airplane engine.This innovative approach could lead to unique benefits in terms of energy generation and application.
(b) Renewable Energy Source: if successful, the U-FG-AWES could be a useful new source of renewable energy, utilizing the strong and consistent wind currents at higher altitudes.
(c) Mobility and Accessibility: incorporating this system into aircraft or drones could provide mobile energy generation capabilities, making renewable energy accessible in remote or disaster-stricken areas.
(d) Reduced Emissions: by generating energy from the air flow produced by the engine, the system could potentially contribute to reducing overall emissions and environmental impact.
(e) Contributions to Aviation Industry: the integration of innovative energy generation methods into aircraft could have broader implications for the aviation industry sustainability efforts.
In summary, our proposal presents an intriguing concept for airborne wind energy conversion using an unconventional approach.While the theoretical analysis is promising, the challenges of practical implementation and efficiency optimization must be addressed through experimental testing and engineering advancements.If successful, this system could contribute to the renewable energy landscape and showcase the potential of unconventional energy sources.
is substituted in equation (2) to obtain the electromotive force.Since the function for electromotive force (5) is known, it is possible to find the maximum and minimum values of the function by taking the derivative with respect to time and setting it equal to zero.

Figure 2 .
Figure 2. Electromotive Force at γ = 0°.The maximum value of the potential generated is 12.189 Volts, with a frequency of 0.040 Hz, considering Jet A1 fuel.

Figure 3 .
Figure 3. Electromotive Force at γ = 89°.The maximum value of the potential generated is 698.422Volts, with a frequency of 2.327 Hz, considering Jet A1 fuel.

Table 1 .
Comparative table for the power and thrust force of the propeller, for Jet A1 and different biofuels.