Published November 30, 2021 | Version 2021-11-30
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Final Report - Multi-Disciplinary Design and Optimisation of a Long-Range eVTOL Aircraft

Description

Final Report of the Design, Synthesis and Exercise (DSE) carried on during the Spring DSE 2021, group 06, assignment "Multi-Disciplinary Design and Optimisation of a Long-Range eVTOL Aircraft".

Tutor: Saullo Giovani Pereira Castro
Coaches: Davide Biagini and Ali Nokhbatolfoghahai
Institution: Delft University of Technology
Place: Faculty of Aerospace Engineering, Delft
Submission Date: Tuesday 29th June, 2021

Summary

The issue of adequate transportation is widespread. Not only are many modes of transportation expensive, but they often require dedicated infrastructure and are prone to traffic and congestion. In an attempt to attenuate this costly problem, an electrical Vertical Takeoff and Landing (eVTOL) aircraft concept, the Wigeon, is proposed after a trade off of three such concepts. The purpose of the project is to provide sustainable, personal aerial transportation for inter-city travel that is competitive with the current transportation methods while requiring minimal infrastructure. In order to accomplish this goal, a multidisciplinary design approach was conducted.

In order to focus the design on a single idea, three possible configurations were analysed and put into a trade-off. It was found that the tandem configuration was least complex, cheapest and safest with regards to one engine inoperative condition. All configurations analysed carried 4 passengers and 1 pilot.

During the market analysis, primarily focused on central Europe, it was decided that the Wigeon should target daily commuters and short overnight trips, as these account up to 80% of regional travels and are mostly within 300 km, the target range of mission. Moreover, increasing the range by another 50 km allowed for an increase in customer base by 36%. The cost and price of one unit was estimated to be roughly 940 thousand euros and 1.8 million euros respectively, leading to the break-even point at 152 units.

The technical design phase began with an aerodynamic analysis. The wing planform was defined with the selected airfoil being NACA 44017 for both wings, a sweep quarter-chord angle of 0 deg and a taper ratio of 0.45. The wing performance was evaluated along with the tips which were all modelled with lifting line theory. Then the propeller wing interaction was determined, along with the drag polar which permitted the modelling of the transition to horizontal flight, showing that the latter assists in the reduction of the surface area. Moreover, given the span limits of the aircraft, it was decided to include blended winglets, with a total height of 0.4 m improving the aerodynamic efficiency of the Wigeon. The validation of the model was finally conducted using CFD shown to be in high agreement with the implemented models.

The propulsion and power subsystem design for the Wigeon project consisted in the design of propellers to maximise efficiency and ensure that the wide variety of needed thrust levels can be achieved. The selected number of engines was 12 (3 per half wing), ensuring sufficient redundancy and ground clearance. To avoid problems with wing positioning and rotation, it was decided to leave a clearance of 0.3 m between the propeller tip and the fuselage and other propellers. With the latter, the final radius of each propeller was 0.5029 m, based on blade element momentum theory (BEM). The thrust values were: 158 N, 2502 N and 3745 N for cruise, hover and full thrust respectively. The final stage was to estimate to size the power system and estimate noise which was found to be: 74.85 dB at a 100 m distance. The battery estimations yielded a value of 886.2 kg and a powertrain mass of 502.6 kg for a total of 16483 cells.

The design is consolidated with a flight performance analysis which detailed the performance related aspects of the vehicles. This began with a simulation of the Wigeon during transition from stand-still to cruise and vice versa. This proved the ability of the aircraft to perform these manoeuvres and showed that accelerations did not exceed 0.2 g. Next, an energy estimation of a 300 km mission, including 15 minutes of loiter was performed. Furthermore, a performance evaluation of the rate of climb in cruise and vertical configuration was done, along with a payload-range diagram. This was concluded with a sensitivity study for different parts of the model, as well as the validation of the models used. It was found that the Wigeon cruises most efficiently at 72.2 m/s. At this speed, it has a range of 400 km, and can loiter for 15 minutes. This high range resulted from a 10 % contingency applied to the MTOM to size all systems.

Continuing with stability and control, in which the criteria for stability and control were defined for both flight conditions hover and cruise. The wings were sized and positioned at the front and the very rear of the vehicle in order to ensure optimal control authority at stall and positive stability margin at cruise. The optimal size for stability is determined to be uneven, although this was not selected. The landing gears were then positioned to not be below passengers and sized to have a height of 0.6624 m and a track width of 1.850 m. The vertical tail and rudder sizing followed to ensure lateral controllability (for an OEI condition) and stability which lead it to be 1.503 m tall. In terms of pitch and roll controllability, an elevon was sized in order to provide sufficient positive pitching moment at stall and ensure a roll rate specified by regulations. The elevon’s chord ratio was found to be 0.25 with a span ratio of 0.868, with the aileron placed at 47.03% of half-wing span.

The aircraft’s dynamic behaviour was analysed using a combined method of derived analytical equations and semi-empirical relations in order to compute the stability and control derivatives. A special design consideration was given to the unstable Dutch Roll and Spiral modes for lateral motion which led to a necessary dihedral of the wings of –0.5 deg and –4.0 deg for the forward and rear wings respectively. The stability and control design ends showing how to improve the damping of the longitudinal dynamics and ensure lateral stability with a feedback loop controller.

The structural design began with an a flight envelope where a maximum load factor during a cruise gust was discovered, equation to 3.4. The structure thereafter is designed for an ultimate load factor of 5.1. The wingbox was primarily designed against buckling and yielding using the Von Mises failure criterion. The wingbox resulted in 1.3 mm skin and 18.7 mm vertical spars, placed at 0.15% and 0.75% of the chord, caused by large vertical shear forces. With an addition of only 3 stringers, the buckling could be fully prevented. Fatigue and aeroelasticity effects were also taken into account, with fatigue mainly focusing on the lug design and aeroelasticity focusing on flutter, leading to a design of a propeller support with stiffness of 3875 kN/m and a lug that is safe-life for 46,000 flights. Crash-worthiness was thoroughly analysed, deeming the aircraft safe after an optimised landing gear design as well as the energy absorbing honeycomb structure made out of aramid composite that was introduced. The Maximum Take-Off Mass was estimated to be 2.8 tons after a Class II weight estimation iteration. The structure is almost entirely made out Aluminium 2024-T3 and Aluminium 7075-T6, further aiding sustainability and ease of manufacturing.

The final design outlined the integrated design program which was used to come up with a convergent design, the specifications of which can be found below. Next, an local optimisation with respect to energy consumption was performed using this program. The result was an aircraft with an MTOM of 2125 kg, a cruise speed of 65 m/s and an energy consumption of 159 kWh for a 300 km mission including 15 minutes of loiter. This is the result of a purely numerical simulation, and should still be analysed further in a later design stage. This is followed by the sustainability approach taken. There, the three pillars of sustainability (social, environmental and economic) were used to plan efforts towards obtaining a sustainable design.

Lastly, the compliance of the final design to the requirements that were previously set was assessed, showing that most of the requirements were respected with a small number that will be investigated in the future. With all the aforementioned, the final values of the design can be found in the table below.

Final design parameters of Widgeon
Parameter Value Parameter Value
MTOM [kg]

2790.1

Wing span [m] 8.2
OEM [kg] 1428.9 Total wing area [m2] 19.8
Range [km] 400 Fuselage length [m] 7.3
Cruise speed [m/s] 72.2 Lift to Drag ratio [-] 16.3
Stall speed [m/s] 40 No. of engines [-] 12
Battery capacity [kWh] 301.1 Maximum Thrust [kN] 34.3
Battery recharge time [min] 25 Payload mass [kg] 440
No. passengers and pilot [-] 5 Cost [€] 938700

 

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Related works

Is supplemented by
Report: 10.5281/zenodo.5575953 (DOI)
Report: 10.5281/zenodo.5576027 (DOI)