2023 eVTOLs - Reality Check
It took VR about 4 decades to get from heavy, cumbersome, impractical gimmick - to the slick, usable, appealing (?) product soon to be available by Apple. During that time, there have been multiple rounds of hype, firing up hopeful enthusiasts prematurely, time and again - only to be disappointed by the shortcomings of the commercialized offering. When the technology isn't ready - it just. Isn't. Ready.
In the recent years, there has been endless new hype around eVTOLs, with every new player promising their "revolutionary" human-size multi-rotor to be available for the masses in "just about 2-3 years" and it will be everywhere and it will be awesome.
Let's start there: they are all lying.
According to the FAA - "the certification of a new aircraft type can take between 5 and 9 years". That's just for "traditional" aircrafts. You know - plain ol' fixed-wings and helis. Not even considering the multitude of new challenges introduced by multi-rotors irregular aircrafts intended to operate over dense urban population. It took 20 years (and counting) for the tilt-rotor Leonardo AW609. Is it possible to do it faster? Sure. But unlikely.
Now to the common myths and feel-good make-believe blurbs everyone parrot. I totally expect the typical objection "when automobiles came out there were skeptics that called it a fad and advocated for horses!". Cool - whatever makes you cosy and keep your dream alive. Yet - physics.
Myth #1: Multi-rotor VTOLs are sooo innovative.
The truth: It's worth appreciating the talent of engineers during the 1950s-70s, that worked with no CAD, Ansys, internet, McMaster or Excel, yet produced and tested numerous creative concepts in a fraction of the time (and costs?) it takes to bring complex new programs to life nowadays. Somehow, out of all this effort - humanity landed on helicopters as the common solution to balance maneuverability, range, performance and usability.
Myth #2: Efficiency. "Electric is so much more efficient than fuel"
The truth: No. Not really.
Energy density of state-of-the-art commercial batteries are barely 1MJ/kg.
Energy density of jet fuel is over 40MJ/kg.
"But electric motors are like 90% efficient! Combustion engines are like 30%" - cool - that still means jet-fueled turboshaft is ~15x more mass-efficient than battery electric propulsion - no matter how efficient your propellers / ducted fans / unicorn dust. That means you need to carry 15x more battery mass compared to fuel in order to carry same payload.
The comparison here may be a bit unfair, since most eVTOLs in development are designed to use propulsive takeoff/landing, while relying on fixed wings for more energy-efficient forward flight. Naturally - energy consumption during hover will be much higher compared to cruise. The same is partially true for helicopters as well, but due to different aerodynamics - for a typical helicopter the cruise:hover power usage ratio is about 1:2. For a state-of-the-art multi-rotor eVTOL, while every vehicle architecture is different, generally it's going to be in the 1:6. Good, right? Here's the catch: super high efficiency and tightly optimized margins make a double-edge sword - it also means high sensitivity to variations in energy consumption. Wanna take a spontaneous roundabout? Hover a bit longer to see the view? Follow a slow moving herd of antelopes for some nature photography? You pay precious milage in range that you can't spare. For gas-powered heli - the difference between cruise and hover are not as critical to be bothered with overall usable time/range. In multi-rotor eVTOLs like the ones currently hyped - the mission profile is highly limited and must be tightly planned and adhered to. Not very practical for, say, search and rescue missions. Unlike a car - you can't just stop in the middle of the road when you run out, if you're at 3,000 ft. Range anxiety gets whole new dimension.
Myth #3: Safety. "Multi-rotors are order of magnitude safer than helicopters"
The truth: It depends on the failure mode.
Are we talking about engine failure? Power loss? Sure - multiple rotors are more than one thus less chance of losing all engines. Ok. Let's discuss then what happens a moment after engine failure or loss of power.
A helicopter - ANY helicopter - can auto-rotate to safety. It's the most common maneuver during training. Multi-rotor? Not so much. Fixed-pitch blades and spread-out arrangements prevent any possibility of achieving controllable autorotation. So basically - multi-rotor still relies on available battery power to get to safety with however many rotors are still working. If eVTOL runs out of battery too high, the only way out is - you guessed it - emergency parachute. That indeed some said to have incorporated into the vehicles.
How about gliding? Most eVTOLs have also fixed wings. "The best of both worlds!" - Can't they just glide to safety? Yes and no. The more rotors you have in the flow path - the more parasitic drag they create when they aren't actively powered. The worst example is Lilium. If power runs out - those dozens of little fans would instantly become sails. No gliding for you. Too many rotors counter the effectiveness of the aircraft to glide smoothly to any stretch of potential landing ground.
For heli, aside from auto-rotation, there's another maneuver that can be handy in tight situations: Running Landing. Often intended for situation like miscalculated destination having too thin air (like a high mountain in a hot day) and not enough power to hover, but also useful in near-fatal situations like loss of tail rotor. With enough speed - the tail fin can still keep the heli moving straight and the pilot can basically land like a fixed-wing airplane using the skids until friction does its thing and brings the heli to a full stop - safe and happy.
Bottom line is: the claims that redundancy = safety multiplier - are overblown and misleading. However many rotors left functioning - they rely on having fully operational and accessible battery power. Helicopters have a chance to touchdown safely with zero power.
#4 (not myth): Noise
领英推è
Yes, helis are noisy. Trucks are noisy too. Most eVTOLs will certainly be not as noisy. Is that their saving grace? Maybe. But...
Most eVTOL companies, limited by the usable range, mission profile, and unavoidable hefty price tag - naturally aim to commercialize as taxi service for short hops across urban centers. If you're in downtown NYC, Boston or LA - you'd likely not even tell the difference between one more or less truck, helicopter, or jackhammer. But sure you'd be happy to know you can't hear that eVTOL on the rooftop of the tower around the corner.
#5 (math): Not all propellers made equal
Let's assume during hover most lift comes from pushing air down fast enough. Let's assume the effective downwash area is from about half the blade to the edge. You can instantly see that one large prop has much larger doughnut than multiple small rotors:
Which means - smaller area need to push air faster in order to achieve same mass flow rate. Worse yet - each and every one of the eVTOLs weighs 2x or more than comparable gas-powered heli.
For comparison: R66 heli has blade diameter of about 10m. In sea level on a nice chill spring day - air density is ~1.2 kg/m3. For a total mass of 2700lb (1230kg) - the avg air speed of the downwash is at least 17m/s, or 38mph. That's bearable, but can still be irritating if there are a lot of debris for the flow to pick.
Now, let's take Joby for example. Eyeballing it, lets say each of the 6 rotors has a 2m diameter. The much smaller total effective propeller surface area, and the much heavier vehicle (4800lb) means an avg air flow right under the rotors of 128m/s, or 286mph. Quite a bit higher than what a heli generates.
For reference - a commercial leaf blower can output 260mph, from a 3" nozzle. Downwash is enough of an issue for commercial and military operations. It's going to be much stronger under multi-rotor eVTOLs. All eVTOLs in development are targeting urban or near-urban operations. Let's just say - those rooftops and parking-lots-converted-to-vertiports better be darn clean of gravel and debris, or it won't be good for bystanders or nearby parked cars...
Conclusion: If you had the money - which would you buy?
The most common helicopter available to rent in the US is Robinson R44 with 4 seats, and it has a bigger brother R66 with 5 seats. The contenders for the 5-6 sitter air vehicle throne with most headlines have been Joby, Lilium, Archer and Beta:
Naturally, not all companies advertise complete data and costs, so some details are speculated (sources linked below). But enough data has been made public to get the picture.
Vehicle specs references:
Entrepreneur
1 å¹´https://m.youtube.com/watch?v=cvMHvJhXRq0
Entrepreneur
1 å¹´High-power, high-energy, fast-charging battery is the enabling technology for eVTOL. Expect more collaboration and progress in the electric aviation industry in 2024.
Engineer Sr. Specialist and ODA UM at Textron Aviation
1 å¹´Intersting article. One small comment about the leaf blower "output 260mph, from a 3" nozzle." A while back I had a co-worker who brought in his leaf blower so we could test it on our fan testing chamber. Surely a blower with a 260 mph airspeed would have a powerful blower. In reality the velocity stated did not come from a 3" outlet but rather the outlet necked down to a smaller outlet area which provide the high velocity air. The air in the 3" portion of the duct had a much lower air velocity. The blower actually turned out to be pretty weak compartively to other fans. Marketing ratings are not always what you expect. I understand your point though.
boss at ENF Savunma Sanayi ltd.?ti
1 å¹´Great analysis.. A lot of money was spent on these romantic environmentalist dreams. I think the table I prepared explains well enough that it is impossible for batteries to compete with gasoline. This eVTOL heist must end now
I agree most of the statements but not all. Nevertheless, I understand, and also agree, the main idea of the article. By the way, 54 M USD price for Beta can not be true. Lillium, Joby, Archer, Beta all have price tag at around 5 M USD, as far as I know.