eVTOL Air Taxi Design
Having the ability to operate in tightly constrained city spaces, eVTOL aircraft may be able to allow a greater freedom in travel using electric power that may be generated using green technologies. Our team developed a short range, hybrid tiltrotor and lift plus cruise eVTOL aircraft concept that can carry one pilot and three passengers on multiple trips within a city.
Mission Description
We chose to design an eVTOL UAM vehicle that can carry one pilot and three passengers on short hops within average city limits. Our requirements were selected to accomplish this mission. Furthermore, a mission profile for a characteristic intracity hop was generated.
Crew: 1 pilot + 3 passengers
Assume 200lb per person; compromise between compact aircraft and economies of scale
Battery Density: 270 Wh/kg
Expected battery energy density by 2028
Power: replaceable batteries
Take advantage of existing electrical infrastructure, safer, allows batteries to charge as aircraft are flying elsewhere
Size: Less than 50’ in width, length
According to FAA Engineering Brief #105 on Vertiport Design reference UAM dimensions
Sizing
The initial battery weight fraction estimate was considered low, and a greater allotment of weight was given to the batteries in the breakdown. The weights of the components summed to about 3900 lb, leaving about 10% of the gross weight to be used for additions during or after design.
Empty Weight Fraction and Passenger Count Effect on Gross Weight
Payload-Range Diagram
To better understand the optimal power-to-weight and wing loading parameters, several constraints were selected and plotted. The primary constraining factor was the power required for a vertical takeoff at 100 FPM. We selected the point that minimized the climb power-to-weight ratio while satisfying the power requirement for takeoff. This resulted in a selected wing loading of 23.87 lb/ft2 and a minimum power-to-weight ratio of 0.1502 hp/lb. To approximately achieve the specified wing loading, the wing area was set to be 178 ft2.
Constraint Analysis Sizing
Propulsion
For the configuration of the propulsion system, four tilt rotors are mounted forward of the wing and four fixed rotors are mounted aft of the wing. The four front rotors end of the wing consist of five five-foot long blades that will be used in both the take-off and cruise portion of flight. The four aft rotors consist of two five-foot long blades each and will only be used during take-off, after which, they will lock in position parallel to airflow.
In order to find the proper propulsion system for this aircraft, the power needed for each disk was first determined by the following equation:
We decided that the best option for our motor would be the EMRAX 228 motors for each of the eight rotors. This option will provide a peak of 166hp, which would contribute to a one engine inoperable scenario, where each motor would need to operate at approximately 71hp. When considering possible weather emergencies, maneuvers, or even multiple engine failures, we decided that the having additional power would be the safest option for our clientele.
When deciding on an airfoil for our rotor blades, NACA 4412 was chosen to be optimal due to its low drag coefficient, high lift coefficient and the simplicity of its design.
Rotor Blade Chord Distribution
Aerodynamics
With Reynolds number and thickness bounds, we sorted through airfoils that perform best at these parameters. Using the website www.airfoiltools.com, we created a shortlist of three airfoils: DAE 31, GOE 405, and MH 115.
We analyzed these airfoils using XFLR 5 to compile a collection of 2-dimensional (2-D) analyses. 2-D analysis of each airfoil will allow us to create and simulate wings for each airfoil and perform 3-D analysis and comparison. With the 3-D analysis, we can then select the airfoil that has the best performance parameters.
According to the performances of airfoils, we selected MH 115 as our airfoil of choice for the main wing and NACA 0012 as our airfoil for both the horizontal and vertical tails
2-D Analysis of Candidate Airfoils at Re 4∗10E6
3-D Analysis at Fixed Speed (Cruise)
We performed fixed lift and fixed speed analyses in XFLR 5. Using the fixed lift analysis, we can investigate what angle-of-attack we will operate at our cruise speed. We found that at 120 MPH, we will have to fly at an alpha of 2 degrees. We also noticed that this operating point is very close to the max CL/CD of the airfoil, thus validating our airfoil selection from before. XFLR 5 produces poor drag estimates, so the L/D and drag polar axes in the fixed lift analysis figure are not to scale. An accurate drag polar graph was generated in the Drag Polar Figure.
Fixed Lift Analysis (blue) at Gross Weight with Cruise Operating Point Highlighted (red)
Drag Polar Figure
Stability
Static Margin and Center of Gravity
Most aircraft have a neutral point at about 40% to 50% of the mean aerodynamic chord, so we estimated our aircraft’s neutral point to be at 45% MAC. Most general aviation aircraft keep their static margins between 5% and 25% of the mean aerodynamic chord. In order to maintain this, it is necessary to keep the heavy and variable loads close to the center of gravity. Thus, the battery, pilot, and passengers are all within a few feet of each other in terms of longitudinal location.
Through trial and error, we tested different fuselage relative to wing locations, pilot plus passenger arrangements, and battery placement. An optimal arrangement we found puts the start of the fuselage about 8.25 ft in front of the leading edge of the wing, and the battery located directly underneath the pilot. This arrangement allows the primary load conditions to have a static margin between 10% and 20%, which places the center of gravity between 25% and 35% of the mean aerodynamic chord.
Control Surface Sizing
The control surfaces of the aircraft were chosen based on empirical data from general aviation aircraft. The flap was sized to be 20% of the chord of the untapered, inboard section of the wing. The aileron was set at 15% of the chord of the remaining outboard section of the wing. Due to the relatively high aspect ratio of the wing, we expect the aileron to have a larger amount of control authority from the increased lever arm length. Additionally, we recognized that a large flap surface was especially important due to the dynamics of the VTOL transition from vertical to horizontal flight and vice versa. The elevator was placed along 30% of the chord of the entire span of the horizontal stabilizer and the rudder was chosen to be 50% of the chord of the vertical stabilizer. We considered a large rudder surface to be a priority to increase yaw authority during vertical flight.
Dynamic Stability
In order to analyze the dynamic stability of the aircraft, a simplified model was modeled in Athena Vortex Lattice (AVL) and constraints related to level flight cruise conditions were applied.
We found that a wing incidence angle of 2 degrees and a horizontal stabilizer incidence angle of -3 degrees produced a level flight trim of -2.7 degrees which was acceptable. The calculations also resulted in a pole-zero plot of the various stability modes.
The aircraft is stable to various degrees in the roll, short-period, Dutch roll, and phugoid modes. However, it displays spiral divergence, but this mode is of least concern because it is easily rectifiable with a simple aileron input.
AVL Geometry
Dynamic Stability Eigenvalue Analysis
Structural Design
We expect our aircraft to spend a relatively large amount of time in vertical flight for a VTOL aircraft when considering our mission profile consisting of multiple short hops. Additionally, our aircraft is dimensionally constrained by the expected size of vertistops to 50 ft in length and width. Finally, since we expect to serve as a public transit option, especially one that might act as a shuttle to airports, we decided that we needed a relatively large interior and cargo space to improve passenger experience. With these considerations in mind, we modeled a fuselage closer to a helicopter in terms of ergonomics and construction.
Fuselage Cross Section in Rear Passenger Area
CAD with weight locations
CAD planform images
Cost Analysis
Using the Modified DAPCA IV model [5], we estimated the cost of 2 test aircraft and 100 production aircraft in five years. The model was adjusted by a factor of 1.31 to account for inflation from 2012 dollars to 2023 dollars.
The learning curve effects show the production cost (in labor hours) per aircraft. We estimate 20% lower cost each time the quantity doubles. Thus, from Figure below, where the production quantity is at 100 aircraft, the production cost will be $26,640,400.
