Electric flight

kbd512 · Yesterday 19:42:07

The primary impediment to achieving useful electric flight is electric power generation. This problem applies to ALL electric aircraft, regardless of what produces the electricity.

My definition of a "useful electric aircraft" is any type of partially or completely electrically powered aircraft, through any phase of flight or the entire flight, which improves upon either the flight characteristics, such as improved performance margin under less-than-ideal weather conditions, or payload-to-distance figures achieved by modern aircraft using modern combustion engines, for a given dry mass, irrespective of configuration, through the use of electrification as a means toward that end.

By that definition / metric, which is deliberately broader than the traditional definition of "what makes a good aircraft", there are presently no useful electric aircraft. To the best of my knowledge, none of them improve upon flight characteristics over traditional aircraft designs during critical phases of flight (takeoff and landing) and all of them are absurdly overweight for the meager payload-to-distance figures they can achieve using Lithium-ion batteries as their electrical power source.

Practically speaking, only fuel cells have at least the potential to improve upon the payload-to-distance values achieved by modern combustion powered aircraft. No current or future projected electro-chemical battery technology provides greater gravimetric power density than chemical combustion or reaction within a fuel cell. That makes batteries non-starters for improving aviation cost and safety. The only people who seem to believe otherwise are either mathematically illiterate or willing to engage in deceptive business practices with people who are mathematically illiterate.

There are literally dozens of Rube Goldberg style electric VTOL machines that are multiples of the purchase and maintenance costs of comparably capable combustion engine helicopters. You might pay less for electricity than fuel in certain cases, which might offset the purchase and maintenance cost over time, but the electric flying machines themselves are far more expensive for the capabilities they offer.

The exemplar Rube Goldberg design concept furthest along in the certification process, quite possibly because they use a certified helicopter pilot, is Joby Aviation's S4 2.0 pre-production full scale prototype. This machine is powered by 6 electric motors driving 6 low-rpm 5-bladed propellers. Their "claim to fame" is that their S4 eVTOL machine is 100X quieter than a traditional helicopter, although their basis for that comparison is never stated, as is always the case with spurious marketing claims that have no basis in fact.

Specifications:
Aircraft type: eVTOL passenger air taxi pre-production prototype aircraft
Pilot: 1 pilot
Passengers: 4 passengers
Maximum speed: 200 mph (322 km/h)
Range: 150 miles (241.4 km)
Propellers: 6 tilt-prop, 4 propellers tilt vertically including the entire motor nacelle, 2 propellers tilt vertically with a linkage mechanism
Motors: 6 high performance electric motors
Power source; Lithium-nickel-cobalt-manganese-oxide batteries with a cutting-edge battery pack design
Wingspan: 35 ft (10.7 m)
Length: 24 ft (7.3 m)
Weight: 4,000 lb (1,815 kg)
Windows: Large windows for spectacular views for the passengers
Fuselage: Carbon fiber composite
Wing: 1 high main gull wing
Tail: 1 V tail
Landing gear: Tricycle wheeled retractable landing gear
Safety features: Distributed Electric Propulsion (DEP), provides safety through redundancy for its passengers and/or cargo. DEP means having multiple propellers (or ducted fans) and motors on the aircraft so if one or more propellers (ducted fans) or motors fail, the other working propellers (or ducted fans) and motors can safely land the aircraft. There are also redundancies of critical components in the sub-systems of the aircraft. The aircraft can also land like an airplane if necessary and has reserve battery power if there is an unexpected delayed when landing.

Total power loss from an electrical short or electronic control system failure would mean their airframe literally "falls out of the sky", because it has no auto-rotation capability whatsoever, with the most probable end result of killing everyone aboard, and possibly innocent bystanders on the ground.

Auto-rotation is the traditional helicopter safety feature in the event of total power loss, which requires the pilot to respond correctly immediately, but allows for a type of rapid controlled descent, a "controlled crash" if you will, that is survivable for both the helicopter and its occupants, when executed correctly, terrain features (such as a mountain top) that could still destroy the machine notwithstanding. This maneuver is deliberately and repeatedly practiced during helicopter pilot training, because it's the only method / mechanism to avoid death or serious injury from "falling out of the sky", following a loss-of-power incident.

Curiously, no eVTOL that I'm aware of incorporates this critical flight safety feature into their vehicle design to prevent a loss-of-power incident from destroying the machine and killing its occupants, because none of them use traditional helicopter rotor blades capable of providing the necessary residual "free rotation" lift necessary to use it. This is concerning to me because it means there's only one highly probable outcome from a loss-of-power incident, which is not the least bit desirable.

It may be the case that an eVTOL aircraft is less susceptible to loss-of-power incidents than traditional helicopters and autogyros, but it absolutely is the case that none of these eVTOL machines possess the auto-rotation capabilities of traditional helicopters, because all of them eschew traditional helicopter rotor blades in favor of what could best be described as the kinds of propeller blades you might find on a purpose-built STOL aircraft, meaning broad high-lift blades, large diameter for a traditional aircraft propeller, and slow-turning. STOL aircraft use a modified traditional prop for propulsion, combined with large wings and control surfaces that provide high aerodynamic lift generated by forward flight, typically at speeds similar to the cruise speeds of a helicopter. This prop design reduces cruise flight efficiency at higher speeds but generates quite a bit of thrust for vertical flight. It should also provide reduced prop wash noise when operated well below the speed of sound.

During the critical phases of eVTOL flight, which necessarily involves vertical flight like a helicopter, using props vs rotor blades means you're literally "hanging off the prop". That phrase is one that STOL pilots frequently use to describe a situation wherein the aircraft is being "dragged through the air" below its normal stall speed, usually under significant or full power from the engine(s) or motor(s), and the only reason the STOL plane doesn't stall and crash is that it's generating enough thrust and blowing enough air over the wing to prevent a stall. That's another way of saying that pure thrust is what's keeping you suspended in the air, rather than aerodynamic lift from the wings. If the engine / motor loses power, even for a split second, you're quite likely to "fall out of the sky". That maneuver is most frequently done to dramatically reduce the landing roll distance, but it's dangerous and more frequently seen in STOL competitions than actual off-airport rough field landings with paying passengers. This particular flight regime is traditionally / colloquially referred to during pilot training as "slow flight", and is typically only done at significant altitude so that a pilot understands their aircraft's behavior and maneuvering characteristics near a stall. I would generally consider "slow flight / hanging off the prop" to be a "stunt flying" maneuver, not something I would intentionally do with four paying passengers aboard, unless there was no other choice to safely land in the space available. All eVTOL designs to date provide no other option for landing, so you're totally dependent on continuous thrust generation during a "normal" landing.

How is that not the same thing as "hanging off the main rotor"? If you lose engine power in a helicopter while you're already in a hover near or over the landing pad, you are still going down, no two ways about that, but you also have considerable "cushion" for that landing from residual lift provided by the main rotor blades. All that inertia means they don't immediately stop spinning. I've seen a helicopter lose all engine power while it was hovering about 50 feet above the ground, which only resulted in a normal landing that the occupants said was not noticeably firmer than landing with power. If the same thing were to occur with power only from multiple aircraft-like propeller blades, then you're going to fall at almost freefall vertical velocity. As an observer, what you'll see during the crash will look a lot like the handful of videos of Harriers that lost engine thrust during a hover- the machine "drops like a rock". Since there are no ejection seats in the S4, the only thing that might prevent serious injury or death are crash energy absorption capabilities built into the fuselage, if any, and very close proximity to the ground.

Since everything in the S4 is in fact tied to a computerized unified flight control system like the F-35B has, power loss to all motors at the same time is a very real possibility, regardless of how perfect the flight control software happen to be. This has already happened to a F-35B in level flight that was not, to the best of our knowledge, brought on by pilot error, bad weather, or engine damage. The computer suffered a major failure of some kind, precise reason still unknown, the pilot tried to reboot it while in a "dead stick" plunge to the ground, briefly regained control, and he eventually opted to eject after the problem resurfaced following the reboot attempt. He was then drummed out of the Marine Corps for refusing to stay with an obviously crippled aircraft which he had no power or control authority over following the computer malfunctions. Nobody wanted to admit that the software was faulty after a decade of flight operations, including combat flight operations, so it was easier to blame the pilot than to blame the team of engineers who failed to account for every potential problem. This same scenario would likely play out very differently in an eVTOL air taxi primarily operating over heavily populated areas, with the most likely outcome involving death and significant property damage.

The idea of deliberately doing something like this in a vertical flight regime in a machine that's so heavy that there's no ability whatsoever to "push the nose over" in an attempt to regain aerodynamic lift from the wings, ought to give any knowledgeable pilot pause to reconsider what they're doing, even in a machine that's fully capable of vertical flight.

The question lingering in my mind is, "If electric flight is truly ready for prime time, regardless of any range and payload considerations, then why are there no electrically powered helicopters that possess the traditional power loss safety features of helicopters, and why do they need a half dozen or more electric motors driving dozens of blades?"

These eVTOL machines are not significantly smaller and unquestionably much heavier than traditional helicopters for the payloads they carry. On top of that, they're no less costly to purchase. Under certain scenarios, which seem unrealistic to put it charitably, they might be more cost effective to operate. If actual operational experience proves that to be true, then that one objection is removed. I'm happy to be wrong about this, and I hope I am.

This is the S4's real world competition:
Robinson R66 Helicopter
Seating Capacity: 1 pilot and 4 passengers (same as the Joby Aviation S4)
Length: 29ft 6in
Width: 4ft 10in
Height: 11ft 5in
Main Rotor Diameter: 33ft
Tail Rotor Diameter: 5ft
Weight: 1,300lbs (empty); 2,700lbs (max takeoff weight)
Powerplant: 1X 270hp Rolls-Royce RR300 gas turbine engine driving 2-blade main and tail rotors
Range: 400 miles
Speed: 130mph (max cruise); 160mph (never exceed)
Purchase Price (new): $1M to $1.4M (varies with avionics and interior options)
TBO (for flight critical components): 4,000hrs (FAA increased this from 2,000hrs, following considerable flight experience)

Across the fleet of over 1,000 R66 models produced, more than a million total operating hours were accumulated by installed RR300 engines, as of September of 2019.

Joby Aviation S4 (top right) and Robinson R44/R66 (top left; the R66 is a R44 with a gas turbine vs piston engine):

The Robinson R22/R44/R66, Bell 47, and Bell 206 Jet Ranger are some of the most mass-produced civil application helicopters of all time, which means virtually all helipads within cities are already designed and built to accommodate them. While these traditional helicopters are significantly lighter than the S4, helipads tend to be very sturdy so supporting the S4's weight ought not be a problem. Most traditional helicopters are a touch longer than the S4 because they use tail rotors to counteract torque and gyroscopic precession from the main rotor. This minor length difference is relatively meaningless since helipads tend to be circular and the main rotor diameter of the R44/R66 is smaller than the width of the S4. If you require a "clear area" where you can rotate either the S4 or R44/R66 to arrive or depart in the desired direction of travel, and you absolutely do, then the space claim is functionally identical. There are not very many places that a S4 could land, whereas a R44/R66 could not. How or why this was ever construed as a "selling point" in favor of the S4 is beyond my understanding. I guess marketing gimmicks never cease. When it comes time to pay hangar fees, I can tell you which aircraft I want to own, and it's not the S4.

There's nothing "small" about the S4. I think it's aesthetically pleasing. It looks like a proper aircraft to me, a product of serious design effort. That's a compliment I cannot extend to any other eVTOL design I've seen. It's a miniature electric V-22 after a fashion. Unfortunately, it's also an overweight and over-complicated beast of a machine for what it does, unlikely to ever meet production or operational cost figures. It's been under development for 10 years now. As "first attempts" at new design concepts go, it's far better than most.

Speaking of absurd marketing gimmicks, let's apply some cursory evaluation to the claimed operating hours per year:

Each aircraft is projected to cost $1.3 million to manufacture and generate $2.2 million in annual revenue, resulting in a payback period of 1.3 years per plane based on an assumed passenger load factor of 2.3 and approximately 4,500 operating hours per year. - eVTOL Magazine, Feb. 24, 2021

The S4 appears to use 6X 5-bladed MT composite propellers with modified tips to reduce noise. MT composite propellers have a 4,500hr TBO, as a general rule. I happen to know this because I own a pair of 5-bladed MT composite propellers. Overhaul cost is in the range of $10,000 to $20,000 per 5-bladed prop. You get charged for every inspection and service they perform. They have to reseal the bearings a bit more often than that, maybe every 2,000hrs or so. When they take the blades in, they disassemble the prop, check for impact and moisture ingress damage, remove the CFRP covering the wood core if required, replace the Nickel leading edge protection strips, re-balance the prop (absolutely necessary if they have to strip and recover the CFRP over the wooden core), etc. All told, the operator is probably looking at $100K per year if they're really flying 4,500hrs per year and servicing 6X 5-bladed props at MT's recommended TBO interval. I don't know what the rules are regarding engine / motor / prop TBO for Part 135 operations, but I suspect operators who wish to retain their insurance coverage treat the manufacturer's TBO recommendations as a hard stop. As expensive as they are, retaining a few on-hand spares would be required to avoid interrupting operations. Inspection intervals are mandatory, though. I at least know that much.

To fly 4,500hrs per year, you're flying at least 12 hours per day. Commercial jet aircraft that perform long-haul flights might see 3,000 to 4,000hrs per year, but the rule of thumb for "hard working" commercial jets is around 3,000hrs per year. Most commercial pilots fly no more than 1,000hrs per year, which means each S4 will need at least 4 pilots assigned per airframe to generate that kind of annual revenue stream. In the actual airline industry, you have 1 to 1.5 crews available per working airframe, at most. That places the S4's operating tempo to achieve the purported annual revenue figure deep within the realm of fantasy. There aren't enough qualified pilots, for starters. I'd have to check, but I think Part 135 operations also have mandatory crew rest periods, same as airline transport pilots. No existing kind of vertical lift aircraft I'm aware of flies nearly as many hours per year, so if they really do intend to push these pilots and machines that hard, I'd like to see how they hold up over 5 years. The S4 purportedly has a 125kWh battery pack, which is on-par with the highest capacity BEV packs, so that's fairly realistic. Maximum charge time has to be less than 12 hours per day to fly that many hours per year, which implies "fast charging" on a daily basis. Fast charging will inevitably degrade the battery pack's useful service life faster than slow charging. I suppose "charge to 80% capacity" could be used to reduce stress on the cells. I could see the batteries lasting 2 or maybe 3 years at most.

EAA AirVenture 2024: Pioneering Electric Seaplane Aviation — The Harbour Air eBeaver Case Study

In 2024 at EAA AirVenture in Oshkosh, Lead Engineer (she's a structures engineer by training, not an electrical engineer), Erika Holtz, speaking about the development experience of the Canadian airline / development company, Harbour Air, as overall project manager for Harbour Air's "eBeaver" project responded to a question about battery life, wherein she stated that to get any kind of decent battery life out of their electric converted DHC-2 Beaver seaplanes, they required a liquid thermal regulation system that's pumped into the Lithium-ion battery packs on the ground while charging the battery, then pumped out, because they only have 30 minutes to recharge the eBeaver's battery pack between sight-seeing flights in and around Vancouver Harbor. A Beaver is a known good airframe, a 5,100lb bush plane / 5,600lb seaplane, nominally powered by a 450hp Pratt & Whitney radial, or less frequently PT-6A turboprops, with many decades of operational use. Furthermore, she said that maintaining the aircraft's original CG, cockpit instrumentation / layout, weight distribution, and engine power output was critical to achieving certification. They did get more speed and faster acceleration from better aerodynamics by using Magnix Magni650 850hp electric motor, de-rated to 450hp, so that's good. The motor weight was fine, but she said every single electric component weight figure given by their electrification components suppliers, without exception, was 10% overweight (false advertising). They needed 3 different kinds of electric pumps, one for the prop that ran on turbine oil, one for electric motor cooling to pump dielectric coolant, and one for the instruments and some kind of hydraulic system. That added a lot of weight and parasitic power draw which required adding even more batteries, of the 12V variety, for powering the pumps.

In her talk on YouTube, she said the difference between a 2-3 person aircraft and a 4-6 person aircraft was 225Wh/kg vs 350Wh/kg batteries, so battery weight remains a critical issue. 6pax is max pax for a Beaver, I think. Their electrical engineer is still trying to figure out the best battery configuration for overall reliability and maintainability vs safety. There is also (obviously) zero ability to "burn off fuel" to reduce landing weight to within limits for aircraft spec'd for higher takeoff vs landing weights. She said leaving Beavers tied to the dock at or near max gross weight is undesirable if the seas become heavy, as she indicated that the aircraft could capsize relatively quickly. They're still working with Fire and EMS on emergency rescue procedures with electric aircraft. Harbour Air requires 480V 100A power for fast charging their electric seaplanes, which they still do not have at any of their sea port locations around Vancouver. They still do not have an aviation-grade fast charger system worked out, either. They're presently comfortable with 30 minute to 45 minute flights, but mostly do 15 minute hops across the harbor. They had 30 flight hours accumulated over 90 flights at the time of the talk. She said if the aircraft cannot be flown at least 8 times per day, then Harbour Air (the airline transport side of the business) doesn't want any electric aircraft.

There's lots of enthusiasm for electric aircraft, but clearly very little understanding of how to develop, test, and operate them. Major electric component suppliers are lying about the true weight of their products as well, apparently.

The Pipistrel Velis Electro electric trainer aircraft, which is a type certificated electric aircraft able to be used for flight training and carry paying passengers, has a 500hr TBO on its battery pack and a 300hr TBO on its liquid-cooled electric motor. The Alpha Electro's air-cooled electric motor has a 2,000hr TBO. TBOs may be adjusted upwards or downwards by manufacturers and regulators as operational experience accumulates. Maybe Joby Aviation's motors, batteries, and propellers are large improvements upon whatever else is available from European electric aircraft component manufacturers, but I kinda doubt it. By their very nature, vertical lift aircraft are likely to put significantly more stress and vibration into their airframe and power plant components.

Pipistrel Alpha/Velis Electro Specifications

The initial cost doesn't look that bad, to be honest, but this fanciful notion that electric aircraft are mostly "maintenance free", simply because they use electric motors and batteries, so don't require aviation gasoline or oil changes, doesn't hold up very well to cursory scrutiny. In point of fact, it appears that many of these new electric aircraft require more frequent maintenance. If the maintenance is simple and easy to do, that may not be a significant problem. However, maxing out the performance of anything always has a high price tag attached to it. The Bye Aerospace eFlyer-2 (Arion Lightning derived airframe) and eFlyer-4, Liaoning Ruixiang RX1E, and Yuneec International E430 have also been in development for multiple years now. Following the initial over-enthusiasm about the cost of electricity, it's now apparent to everyone in the electric aircraft business that all aircraft of a given type / mission have broadly similar fabrication, maintenance, and operating costs. Any hard-use flying machine comes with substantial operating costs and maintenance requirements to continue flying. Electricity is less expensive than gasoline or jet fuel in terms of energy units converted into thrust, but the primary reason electric aircraft appear superficially cheaper to operate is that you're not delivering very many units of energy, hence the severe payload-to-distance limitations of electric aircraft. For that reason, merely delivering these machines cross-country to their owners, or back to the factory for refurbishment, will come with unique logistical challenges. Commercial aircraft are typically flown to maintenance depots, but that may not be an option with electric machines that can only fly for an hour or so before recharging is necessary. The S4 cannot fly more than 150 miles and I presume that even a fast charge takes an hour to complete. That means delivery by a semi-truck might be the fastest option when the time comes to take it back for repairs.

The easiest alternative explanation to where all this promotional marketing happy talk is ultimately headed, is either dramatically reduced expectations for a reality-based Part 135 commercial electric air taxi service operation, or potential bankruptcy if the performance of the product fails to match customer expectations for operating cost reductions.

Flying any kind of aircraft 4,500hrs per year, irrespective of what powers it, is a little ridiculous to start with. There's a single example of a long haul Boeing 767 cargo freighter (C-FCAE tail number- an EASA registration number, hence the lack of the "N" prefixed tail number that the FAA assigns to American civil aircraft) that's racked up an average of 4,149.5hrs per year over 34.82 years of flight operations. Even for long haul flying, which typically accumulates more flight hours per year than short haul / commuter flying, 4,000hrs per year is pushing limits. Taxi cab drivers spend an average of 2,000 to 2,600 hours per year driving passengers around, for example. That means a pilot flying 1,000hrs per year is a very busy worker.

Who are these electric aviation companies actually fooling, besides themselves?

They provide a list of astonishing numbers backed by little to nothing. They could be absolutely correct, but without a single working example to compare them with, it's not possible to perform a sanity check on their validity of their outlandish claims, or lack thereof.

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#26 Yesterday 19:42:07

Re: Electric flight

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