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I will start this article with an explanation of how the rankings are generated, then show the results of the rankings.

At the end I will give some advice based on my experience of working in the field for a number of years. 

Here we go…

Preamble
The rankings are based on a weighted score system that I have developed to represent the best of my knowledge and experience regarding technical, certification and commercial (market) risks.

This method has my own bias baked into it (despite my best efforts) and I welcome any challenge or disagreement. 

The aim of this effort is not to make people feel bad or harm any project. However, there are certain unavoidable realities regarding the certification process, the cost of developing and producing complex air vehicles and the unusual nature of the market regarding aircraft purchases and the flying public.

There are always winners and losers, and, in aerospace development there are no participation trophies. The failure of a program represents the loss of most, or all, of the value of the investment in that failed program. 

The purpose of these rankings is not to identify those who will fail. I do not claim to be able to tell the future. The rankings are all relative so all the programs may fail or all of them may succeed. However, the rankings represent my best guess at the relative risk of failure of all of the programs considered.

You are welcome to address any comments to me directly at rabbott@abbottaerospace.com

And a final word: I work with many of the eVTOL programs in this list. I have done paid work, pro bono and informal advising and technical work for many programs. It is a privilege to work with all of these programs. They are all doing their best to push the industry forward and develop new forms of transportation. I have done my best to avoid my relationships to any of these programs influencing my assessment.

The Assessment
The assessment is based on a modified set of weighted factors based on the white paper I authored in 2018: https://www.abbottaerospace.com/wpdm-package/aa-wp-2018-001/

I have updated the weighting and the method. If you are interested in the method please contact me for more information.
The assessment is broken down into the sub categories on the vertical flight society website. These are: Vectored Thrust, Lift + Cruise, Wingless Multicopter, Hoverbikes and Electric Rotorcraft.

These categories are all scored against the same metrics and so vehicles in these separate categories can be compared against each other.

A good number of the vehicles included in this assessment are pure technology demonstrators or early prototypes. For these immature designs, the outcome of my analysis and final placing is unkind and these projects are included for interest only.

The assessments are based on the state of current technology and the current regulations and interpretation. These rankings are not a criticism of the inherent value of the idea, it is my assessment of the likelihood of the design being brought to market and achieving commercial success in the current environment.

Categorical Comparative Assessment
All of the assessments are shown in a 2 dimensional space plotting a derived certification quotient (encompassing technical and regulatory risk – level of difficulty to develop and certify) and a similarly derived market quotient (encompassing all aspects of commercial risk – what is the likely market demand for the aircraft)

The 2D plots are all in the form shown below:

The top right corner of the graph represents a high score for both certification and and market quotients, the lower left hand corner of the graph represents a low score for both certification and market quotients

The comparative assessments within the separate categories are shown below:

Vectored Thrust

Lift + Cruise

Wingless Multicopter

Hover Bikes

Electric Rotorcraft

Summary and Combinations

The different types of VTOL aircraft when combined on the same graph give the following result:

And this is the same graph showing the projects with labels:

All projects considered and their scores are shown below in score order:

Position	Score	Project
1	0.662	Transcend Air Vy 400
2	0.655	Dufour aEro2
3	0.624	Flexcraft
4	0.612	DeLorean Aerospace DR-7
5	0.560	Samad Aerospace Starling Jet
6	0.554	Pegasus Universal Aerospace Vertical Business Jet
7	0.481	Cartivator SkyDrive
8	0.466	Ascendance Flight Technologies Atea
9	0.457	Lazzarini Hover Coupé
10	0.454	XTI Aircraft Trifan 600
11	0.407	Alaka’i Technologies Skai
12	0.388	Gizio EJ420 ElectroJet
13	0.386	Vertical Aerospace (unmanned)
14	0.378	EHang 184
15	0.371	Beta Technologies (prototype)
16	0.368	Lazzarini FD-One
17	0.365	Moller Skycar M200
18	0.359	EHang 216
19	0.354	Horus Hoverbike
20	0.345	EAC Whisper
21	0.335	Voyzon e-VOTO
22	0.333	AeroMobil 5.0
23	0.330	Frogs 282
24	0.327	Airbus Helicopters CityAirbus
25	0.318	Astro Elroy (“Passenger Drone”)
26	0.314	Imaginactive Ambular
27	0.304	Avioneo Robotics Avioneo 2345
28	0.300	Davies “Tesla Concept Model V”
29	0.290	Alauda Airspeeder
30	0.290	AirCar
31	0.283	Moog SureFly
32	0.283	Moog SureFly
33	0.280	Tier One Modified Robinson R44
34	0.267	China Helicopter Research and Development Institute Electric Helicopter (CHRDI AVIC)
35	0.266	Kármán XK-1
36	0.263	Airflight
37	0.263	Assen A1
38	0.261	Zuri
39	0.260	Bay Zoltán Flike
40	0.260	SkyDrive SD-XX
41	0.257	ALI Technologies Xturismo
42	0.244	Piasecki PA-890
43	0.234	HoverSurf Scorpion
44	0.213	Flyter PAC VTOL 720-200
45	0.210	ALI Technologies Hover Bike
46	0.210	AirspaceX MOBi
47	0.205	Eco’Trip
48	0.204	Edea 22/1 Jay
49	0.201	Jetson Aero Speeder
50	0.197	Air Transportation Technology Catapult One – I, II
51	0.195	Ambular 2.0
52	0.187	UAVOS SumoAir
53	0.174	Assen Aeronautics A2 Avenger
54	0.174	Wisk
55	0.172	Heitech Cruiser
56	0.170	Baykar Cezeri
57	0.166	Baaz Concept Design
58	0.165	SkyDrive SD-02
59	0.164	Aerodyne Vector
60	0.164	Gizio DDRH/DDVL
61	0.162	Ray Research VTOL Aircraft
62	0.162	Removed
63	0.159	Leap Vantage – I
64	0.156	Pop.Up Next
65	0.156	Malloy Aeronautics Hoverbike
66	0.147	Heitech Air F1
67	0.141	Archer
68	0.136	Davinci ZeroG
69	0.134	Zenith Altitude EOPA
70	0.133	Hemanth Sudhakaran AVEM
71	0.132	Aquinea ENAC Volta
72	0.131	PDRL AeroHans 2S
73	0.131	Kovacs Flike
74	0.125	Kronstadt Air Taxi
75	0.124	EVA Valkyr
76	0.122	Flyt Aerospace FlytCycle
77	0.112	Digi Robotics DroFire
78	0.103	Rolls-Royce EVTOL
79	0.101	Lazzarini Linux
80	0.098	Applied VTOL Concepts Epiphany™ FLYING CARpet
81	0.093	Deep Blue Aviation MX 18 Silhouette M
82	0.092	PAL-V International Liberty
83	0.085	Flexcraft Test Bench RPV
84	0.084	Lazzarini I.F.O.
85	0.084	Aston Martin Volante
86	0.083	Ambular 3.0
87	0.080	Overair (Karem) Butterfly
88	0.079	Joby Aviation S4
89	0.078	Gravity X Koncepto Millenya
90	0.074	Zeva Zero
91	0.072	Beta Technologies ALIA
92	0.071	Aurora Flight Sciences PAV
93	0.070	Neo Aeronautics Crimson S8
94	0.069	Vinati F-Helix
95	0.066	EVA X01
96	0.065	Hyundai S-A1
97	0.064	OVER LLC
98	0.062	Skyworks Global eGyro
99	0.061	CAPS
100	0.060	Samad Aerospace HUMA
101	0.058	NFT ASKA
102	0.057	Embraer Pulse Concept
103	0.052	AirisOne
104	0.050	Robathan Range Rover eVTOL
105	0.047	CHRDI AVIC Sylan
106	0.045	AutoGyro eCavalon
107	0.043	MyDraco
108	0.041	Flutr Motors Flutr
109	0.037	Fraundorfer Aeronautics Tensor
110	0.036	DragonAir Airboard 1 & 2 – II
111	0.030	Varon V210
112	0.023	Moscow Team AI Kamchatka
113	0.021	SKYLYS Aircraft AO
114	0.020	VerdeGo Aero PAT200
115	0.017	SkyCab
116	0.015	Georgia Tech HummingBuzz – I
117	0.012	Micor Technologies Variable Geometry VTOL aircraft (VAGEV)
118	0.007	FanFlyer
119	0.006	Avianovations Hepard
120	0.003	Trek Aerospace FlyKart 2 – I, II
121	-0.004	Grug Group Business eVTOL Jet
122	-0.006	Uber Elevate eCRM-001
123	-0.009	Athena Aero
124	-0.012	Aufeer Design Flying Taxi
125	-0.014	Tecnalia
126	-0.015	Moller Skycar M400
127	-0.020	Prime Design Consultancy Services Haricopter X1-B
128	-0.021	Flyter PAC VTOL 420-120
129	-0.023	Embraer DreamMaker
130	-0.023	ElectraFly ElectraFlyer
131	-0.026	Manta Aircraft ANN1
132	-0.026	Heitech Magic Cloud
133	-0.034	Leap Aeronautics
134	-0.043	Colugo Systems
135	-0.044	Macchina Volontis Flying Car
136	-0.051	Terrafugia TF-X
137	-0.051	PFV Personal Flying Vehicle #1
138	-0.052	PteroDynamics Transwing
139	-0.053	Aliptera ADR-1 Dragon Rider
140	-0.054	Napoleon Aero VTOL
141	-0.056	Edea 22/2 Squid – II
142	-0.062	COMAC eVTOL
143	-0.064	CycloTech Compound Helicopter
144	-0.065	Micor Technologies Advanced Individual VTOL Aircraft (AIVA)
145	-0.066	KARI PAV
146	-0.067	Swallow VTOL
147	-0.080	aeroG Aviation aG-4 Liberty
148	-0.080	Opener BlackFly V3
149	-0.081	Vickers WAVE eVTOL
150	-0.082	Ryerson Helium – II, III
151	-0.087	Esprit Aeronautics Lancer ePAV
152	-0.088	Skyworks Vertijet
153	-0.096	Carter Aviation Air Taxi/Jaunt
154	-0.097	Detroit Flying Cars WD-1
155	-0.098	LIFT Hexa
156	-0.099	Doroni Carbon One
157	-0.100	rFlight rWing
158	-0.104	Stuttgart Aerospace Apollo
159	-0.110	Happy Takeoff Prism
160	-0.110	Jetoptera J2000
161	-0.122	VRCO NeoXCraft
162	-0.124	JAXA Hornisse 2B
163	-0.125	Gizio CellCraft G150
164	-0.127	AutoFlightX V600
165	-0.128	Aeroxo ERA Aviabike – I, II
166	-0.130	VOX Aircraft M400
167	-0.132	Ray Research Dart Flyer
168	-0.134	Neva Aerospace AirQuadOne
169	-0.140	Innowings Aerospace PKOK
170	-0.143	Horizon AutoCopters Auto-Copter
171	-0.145	Jaunt Air Mobility Journey
172	-0.153	Kitty Hawk Heaviside
173	-0.161	GroundAero Sport Utility Aircraft
174	-0.163	Electric Jet Aircraft EJ-1S
175	-0.174	VTOL Aviation Abhiyaan
176	-0.175	Gizio CellCraft G450
177	-0.182	SkyBoom eVTOL Automobiles
178	-0.183	Raven – III
179	-0.184	B-Technology Beccarii
180	-0.184	Collaborative Mini-Bee
181	-0.185	Bell Nexus
182	-0.186	VTOL Aviation India Abhigyaan NX
183	-0.197	Lilium Jet
184	-0.203	Industry Network Cocooon X-1
185	-0.204	Bell Nexus 6HX
186	-0.205	EXA Air Car
187	-0.212	Terrafugia TF-2 Tiltrotor
188	-0.216	Autonomous Flight Y6S
189	-0.228	KineticCo Aerospace and Advanced Technologies
190	-0.234	CFC AirCAR
191	-0.235	NEAE-GSI eVTOL-BUS
192	-0.235	Prime Design Consultancy Services Haricopter X1-Q
193	-0.238	Scoop Pegasus 1 – I
194	-0.239	HoverSurf Formula
195	-0.242	chAIR Multicopter
196	-0.243	Hirobo Bit
197	-0.250	Urban Aeronautics CityHawk
198	-0.251	Paragon VTOL Aerospace T21 Raptor
199	-0.254	Grug Group Ghost X V1
200	-0.254	Grug Group Ghost X V1
201	-0.257	Grug Group Ghost X V3
202	-0.264	AMSL Aero Vertiia
203	-0.271	Aliptera APV-1
204	-0.271	GyroBike
205	-0.276	A³ Vahana
206	-0.279	Hoversurf Formula (No wing)
207	-0.282	ACS Aviation
208	-0.299	Aeronext Flying Gondola
209	-0.302	Gizio EJ11 ElectroJet
210	-0.302	CycloTech Passenger Demonstrator
211	-0.338	Imaginactive Transvolution
212	-0.345	Bartini Flying Car
213	-0.354	Sharifzadeh Zero G eCruzer
214	-0.358	Neoptera eOpter
215	-0.374	Vertiia
216	-0.385	HoverSurf Drone Taxi R-1
217	-0.417	Vision VTOL
218	-0.467	Advanced System Engineering – FIPSI WX4
219	-0.507	Advanced System Engineering – FIPSI BX4
220	-0.536	HopFlyt Venturi

The spread of scores were assessed to determine if the distribution included any anomalies that would indicate an error in the application of the scoring algorithm:

It appears that the application of the scoring method is free of obvious errors.

Summary of Results and ‘Awards’
The top scoring projects are unchanged from the initial assessment made in 2018. The top five eVTOL projects that exhibit the most favourable characteristics for certification and commercial success are:

Position	Score	Project
1	0.662	Transcend Air Vy 400
2	0.655	Dufour aEro2
3	0.624	Flexcraft
4	0.612	DeLorean Aerospace DR-7
5	0.560	Samad Aerospace Starling Jet

Below are high scoring projects in some of the categories that I use for the assessment

Most Beautiful (In no particular order and according to my taste)

Davies “Tesla Concept Model V”

DeLorean Aerospace DR-7

Lazzarini Hover Coupé 

ALI Technologies Xturismo

Most Unusual

Industry Network Cocoon X-1

Robathan Range Rover eVTOL

Volerian

Zeva Zero

Avianovations Hepard

Advice

This is where I sound off on what I think may help improve a project’s chances of getting to market and being successful once you get there.

This section is as populated with my personal bias as the rest of this article, these opinions are all my own. 

1.  Gatekeepers: You have to get through the certification process. The award of a type certificate depends on a succession of findings of compliance for each paragraph of the regulations. Achieving a finding of compliance relies on the person awarding that finding. Develop your relationships with those individuals. Inspire confidence and over achieve with presenting good quality data in all circumstances. Management of the perception of risk is paramount.
    
2.  Money: The total cost of development is directly related to the time spent in development and certification. Keep your team small (minimize your $ per hour), expedite your certification program (minimize the total number of hours), reduce risks (minimize the chance of the number of hours increasing). Do not expand your team beyond the minimum size until you have a frozen detailed product specification and an approved certification plan. It is very easy to get into certification, very difficult to get out of it.
    
3. Batteries: Assumptions of future battery energy density are unwise. Overestimation of present battery energy density is unwise. 250Kwh/Kg may be presently theoretically achievable. For the installed system it never is. Battery charge times and battery lifetimes have to be practically addressed. Thermal failure modes of Li-Po batteries have to be addressed and mitigated. Current installed battery/electrical drive systems are approximately 14 times less weight efficient than a turboprop system burning JetA. 
    
4. Hydrogen: Great energy density, lousy density. Expensive. No infrastructure. Hydrogen is a form of displaced NIMBY carbon dioxide generation. Your aircraft may be carbon neutral, the supply chain for your fuel is not.
    
5. Mistakes: Everyone makes mistakes. It is very cost effective to learn from other people’s mistakes, it is very expensive to learn from your own. 

In general, the air vehicle market has proven to be historically very conservative. It is very risk averse. In part this is because of the cost of insuring aircraft and in part because of the flying public’s general low threshold of perceived personal risk.

Just as you have to manage the perception of risk in the mind of the regulator you have to do the same for the insurance market and the minds of your future customers and users. We engineers find innovation very attractive and exciting. Other institutions and individuals do not feel the same way we do. There is some good data on the fear of flying and anxiety regarding flying here: Fear of Flying Statistics (2020 Data)

There is a trade off between promoting the new and exciting and reinforcing the dependable and reliable.

I fear that most eVTOL programs concentrate on the former and ignore the latter and this will harm them both in the certification process and with the customers.