The following electric Vertical Takeoff and Landing (eVTOL) system architectures with differing numbers of Electric Propulsion Units (EPUs) have been assessed as part of our eVTOL optimum propulsion units safety analysis using publicly available information:
6 EPUs and 3 Battery Packs
System Architecture
- 1 + 6 = opposing pair A
- 2 + 5 = opposing pair B
- 3 + 4 = opposing pair C
- Battery 1 powers opposing pair A
- Battery 2 powers opposing pair B
- Battery 3 powers opposing pair C
- e.g. Elroy Air (old configuration)
Fault Tree Analysis: Loss of Vertical Thrust
The following fault tree represents a loss of vertical thrust from 2 separate opposing pairs:
The following branch is 1 of 3 total branches, it represents opposing pair A:
6 EPUs and 2 Battery Packs
System Architecture
- 2 + 4 + 5 = triplet A
- 1 + 3 + 6 = triplet B
- Battery 1 powers triplet A
- Battery 2 powers triplet B
- e.g. Joby with 1 winding per propulsion unit
- 2 batteries with no pseudo-redundancy at the propulsion unit level
6 EPUs and 4 Battery Packs
System Architecture
- 2 + 4 + 5 = triplet A
- 1 + 3 + 6 = triplet B
- Battery 1 powers half triplet A-1
- Battery 2 powers half triplet A-2
- Battery 3 powers half triplet B-1
- Battery 4 powers half triplet B-2
- e.g. Joby with 2 windings per propulsion unit
- 4 batteries with pseudo-redundancy at the propulsion unit level
Fault Tree Analysis: Loss of Vertical Thrust
The following fault tree represents a loss of vertical thrust from 2 separate half triplets:
The following branch is 1 of 4 total branches, it represents half triplet A-1:
8 EPUs and 4 Battery Packs
System Architecture
- 1 + 8 = opposing pair A
- 2 + 7 = opposing pair B
- 3 + 6 = opposing pair C
- 4 + 5 = opposing pair D
- Battery 1 powers opposing pair A
- Battery 2 powers opposing pair B
- Battery 3 powers opposing pair C
- Battery 4 powers opposing pair D
- e.g. A^3 by Airbus, Elroy Air (new configuration) and Vertical Aerospace
Fault Tree Analysis: Loss of Vertical Thrust
The following fault tree represents a loss of vertical thrust from 2 separate opposing pairs:
Fault Tree Analysis: Loss of Vertical Thrust (1 of 4 branches)
The following branch is 1 of 4 total branches, it represents opposing pair A:
12 EPUs and 6 Battery Packs
System Architecture
- 1 + 12 = opposing pair A
- 2 + 11 = opposing pair B
- 3 + 10 = opposing pair C
- 4 + 9 = opposing pair D
- 5 + 8 = opposing pair E
- 6 + 7 = opposing pair F
- Battery 1 powers opposing pair A
- Battery 2 powers opposing pair B
- Battery 3 powers opposing pair C
- Battery 4 powers opposing pair D
- Battery 5 powers opposing pair E
- Battery 6 powers opposing pair F
- e.g. Archer and Wisk
Fault Tree Analysis: Loss of Vertical Thrust
The following fault tree represents a loss of vertical thrust from 2 separate opposing pairs:
Fault Tree Analysis: Loss of Vertical Thrust (1 of 6 branches)
The following branch is 1 of 6 total branches, it represents opposing pair A:
Results of the eVTOL Optimum Propulsion Units Analysis
The following is a summary of the loss of vertical thrust probabilities for the different eVTOL system architectures:
| 12 EPU/ 6 battery packs | 8 EPU/ 4 battery packs | 6 EPU/ 3 battery packs | 6 EPU/ 4 battery packs | |
| Loss of vertical thrust | 1.6E-8 | 6.2E-9 | 3.1E-9 | 1.1E-8 |
The following is a summary of the analysis:
The assumptions of the analysis are as follows:
- Relative power is the requirement in the event of an opposing pair or triplet failure
- Relative probability is the relative probability of a failure of 2 separate opposing pairs or triplets (from 6 to 12 EPUs)
- Propulsion units are sized to tolerate the first failure and the second failure is assumed to prevent continued safe flight and landing
- First and second failures affect separate opposing pairs or triplets
- Valid for tilt or separate vertical and horizontal propulsion (assuming tilt is relatively reliable). This may or may not be the case for the A^3 by Airbus concept because of its tilt wing design. However, the tilt wing design can generate lift throughout its actuation profile. Therefore, it is a potentially certifiable design
- The system and sub-system failure rates between designs is normalized and they are assumed to be equally derated
The analysis shows that 8 propulsion units is optimum (which is less than Archer and Wisk but more than Joby). From a safety, cost, Size, Weight and Power (SWaP) perspective, Vertical Aerospace with 8 propulsion units is better optimized. This is because increasing the number of propulsion units actually increases as opposed to decreases the probability of a failure. However, as the number of propulsion units decreases, the relative power they are required to generate in the event of certain failures increases impacting their SWaP.
It is assumed loss of 50% of thrust would be catastrophic during vertical takeoff and vertical landing. Therefore, of the 6 propulsion unit configurations, the 2 battery as opposed to the 4 battery would be the only configuration that is certifiable. This configuration is dependent on there being independent power electronics, motor controllers and motor windings at the propulsion unit level.
Consequently, from a safety and systems optimization perspective, the system architectures are ranked as follows:
- 8 motors 4 batteries e.g. A^3 by Airbus, Elroy Air (new configuration) & Vertical Aerospace
- 12 motors 6 batteries e.g. Archer & Wisk
- 6 motors 4 batteries (propulsion unit pseudo redundancy) e.g. Joby v1
- 6 motors 3 batteries e.g. Elroy Air (old configuration)
- 6 motors 2 batteries (no propulsion unit pseudo redundancy) e.g. Joby v2
- 4 motors with IBC e.g. Overair
- https://www.beta.team/4 motors with no IBC e.g. BETA
Contact L-SYS for the complete safety analysis. Also, see our white papers.
Overair Addendum
Overair, founded by Abe Karem and Ben Tigne, has an unusual design that relies on Individual Blade Control (IBC) whereby the pitch of individual propeller blades is adjusted throughout their rotation. This prevents adverse torque in the event of certain failures. However, it adds some unique failure modes. Our conclusion is that with effort these failure modes can be controlled and it has better disk loading than the competition. Therefore, the Overair design is certifiable.
City Airbus Addendum
City Airbus has an interesting design that is closer to Vertical Aerospace, in that it is an 8 propulsion unit configuration, than is immediately apparent. Our conclusion is that the opposing pairs are not diametrically opposed. This is not optimal from a relative power and control perspective. However, the City Airbus design is certifiable.
BETA Addendum
BETA, initially funded by Martine Rothblatt, who also funded Tier 1 Engineering, have a simple forward + vertical propulsion design that others e.g. Elroy and Wisk have migrated away from. However, it only has 4 vertical propulsion units. Our conclusion is that, for the aircraft to be certifiable, it would need to instantaneously generate lift from its wings in the event of certain failures or be designed for short as opposed to vertical takeoff and landing for it to be certifiable. Hence its wheels.
Two things are currently pulling all of the eVTOLs down, one the unpredicted behaviour of the batteries in the event of the catastrophic failure and two the lack of simulations of the empirical calculations of the entire flight dynamics, even if we forget the disk loading aspects in the first place. These experiments are certainly welcome but except for the first guy trying to tilt the rotor others have just excelled at CGI.
Wanting to be proven terribly wrong, the full blown commercialisation of the eVTOLs will have to take a decade.