Glenair QwikConnect Magazine • July 2021 • Volume 25 • Number 3
GLENAIR • JULY 2021 • VOLUME 25 • NUMBER 3
Interconnect SOLUTIONS Air Taxi
LIGHTWEIGHT + RUGGED AVIATION-GRADE
Transitioning to renewable, green-energy fuel sources is an active, ongoing goal in virtually every industry. While the generation of low-carbon- footprint energy—from nuclear, natural gas, wind, and solar—might someday be adequate to meet our real-time energy requirements, the storage of such energy for future use is still a major hurdle limiting the wholesale shift to renewable power.
be harvested from 1 kilogram of an energy source. For kerosene—the fuel of choice for rockets and aircraft—the energy density is 43 MJ/Kg (Mega Joules per kilogram). The “energy density” of the lithium ion battery in the Tesla, on the other hand, is about 1 MJ/kg—or over 40 times heavier than jet fuel for the same output of work. And yet the battery on the Model 3, for all its weight and low “energy
Interconnect SOLUTIONS Air Taxi
LIGHTWEIGHT + RUGGED AVIATION-GRADE
density” is perfectly suited to meet that vehicle’s “concept of operation” i.e. short to medium length trips, at high speed, with a robust payload. The difference of course is the Model S is not flying but driving to its destination. A plane flies when the “lift” it generates equals the all-in weight of the aircraft. Lift of course can be achieved in various ways, including aerodynamically shaped wings, copter rotors, jet propulsion and so on. However it is achieved, the heavier the aircraft, the more lift is required to get it off the ground and accelerate it on its way. So, whenever we increase the weight of an aircraft (unlike a car) with a lower- density power source (such as a battery) we must increase the plane’s mechanism of lift accordingly. By way of reference, about 20% of the all-in weight of a commercial aircraft is its fuel. Given their relative energy densities, the transition to battery power from kerosene in an Airbus A320 would result in an additional 260,000 Kg of weight—or more than four times the total weight of the aircraft itself. The Falcon9—to return to our ridiculous comparison—
In aviation—an industry that universally relies on kerosene as its primary energy source—this dilemma is profound due to the basic thermodynamics of combustible fossil fuels versus battery-stored electrical power. Take, for example, the thermodynamics of the Tesla Model S all-electric battery-powered car versus the SpaceX Falcon 9 rocket. The high- performance 85 kWh Model S battery pack weighs in at 1,200 lbs. and delivers a range in excess of 300 miles for the 5,000 lb. vehicle. The Falcon9, on the other hand, burns 147 tons of rocket fuel to lift and vector its 50,000 lb. payload into low earth orbit. Apples and oranges, you say, as the scale of the missions and the “concepts of operation” are so different. True, but that in fact is the point. Different types of fuel can be measured for efficiency using a simple rule called “energy density.” Energy density is the measure of the energy that can
ENERGY DENSITY COMPARISON SpaceX Falcon 9: 43 MJ/Kg Tesla Model S: 1 MJ/kg
photo: NASA
photo: raneko via Wikimedia Commons
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would need in excess of 6000 Tons of battery power to replace its 147 Tons of rocket fuel. And one can only imagine the kind of lift design that would be required to get that baby off the ground. And therein lies the challenge for the nascent air taxi or Urban Air Mobility (UAM) industry. In fact, the only realistic circumstance in which eVTOL air taxis (electric vertical takeoff and landing)—flying in and out of urban air terminals—would be able to function solely with battery-powered propulsion is in small, ultralight aircraft with limited carrying capacity, limited flight duration, and limited range. Nevertheless, industry experts agree a market exists in high-density urban settings made up of individuals who will pay for fast air trips to popular destinations, rather than slog it out on congested city streets. This has every major aircraft manufacturer—as well as countless other entrepreneurs—actively engaged in eVTOL air taxi R and D. Incumbent OEMs including Boeing, Airbus, Embraer, and Bell all have active programs. And the many aerospace system manufacturers including Raytheon, GE, SAFRAN, Rolls-Royce, Honeywell and others are also hard at work developing new classes of electric and hybrid propulsion systems, fly-by-wire controls, electric motor controllers and more. The major automobile manufacturers are also fully engaged including Hyundai, Toyota, GM, and others. And not surprisingly,
much of this work includes all-electric as well as hybrid designs that leverage other sources of power such as small form-factor kerosene engines and hydrogen fuel cells. Indeed, it may turn out that the most viable air taxi designs are small jet engine configurations augmented with backup battery power, similar in concept to hybrid automobiles but with the fossil- fuel engine acting as the primary power source generating electrical power, or with electrical motors reduced to auxiliary roles, such as emergency backups in the event of engine failure. THE RISE OF THE AIR TAXI Vertical Take-Off and Landing aircraft with the ability to transport passengers or cargo short distances in the urban landscape may be either piloted or autonomous, and operate with environmental controls designed to moderate their impact on urban populations.
Artist’s conception of an eVTOL design with combined lift-and-cruise functions on articulating wings and horizontal NOTAR tail section, a system that uses a tail boom and fan to build a high volume of low-pressure air, utilizing the Coandă effect.
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The first issue to tackle in a discussion of how air taxis will fit into the urban landscape, their “concept of operation” as the cool kids say, is that, like conventional helicopters, they will need to take off and land vertically. The infrastructure currently envisioned for such services is likely to take the form of rooftop air terminals, or skyports, with on-demand passenger flights departing to a range of fixed destinations such as airports, resorts, convention centers, adjacent city centers, and so on. In addition, and in order to reduce the environmental impact of the service, described operations are promised to be clean, green, quiet and safe. Concepts of OPERATION Air Taxi
Up in the sky, the aircraft are shaping up into five categories or types, each designed to meet the most common mission-profiles, from short duration flights to longer, inter-city jumps, and deliver zero local emission performance. To help provide a little more detail and understanding of these five types, we’ll take a look at some real-world examples, starting with perhaps the most power-intensive design, the Vectored Thrust.
Vectored Thrust (DEVT) Vertical takeoff and landing aircraft that use the same
propulsion system for hover and cruise operations are generally referred to as
“vectored thrust” aircraft. Like the well-known Harrier jet design, they use
the same propulsion system for Liftoff, landing and forward propulsion. Lilium, a German-based enterprise and design, is a prime example of a unique form of Vectored Thrust, called a “Ducted Electric Vectored Thrust (DEVT)” aircraft. In this implementation, two banks of propellers and electric jet engines—housed in rotatable flaps in the wings—are mechanically oriented for both hover and cruise functions. Additional vectored propulsion—for a total of 36 individual jet powered electric motors and propeller systems—is housed in a forward Canard. The eponymous Lilium flagship leverages its unique DEVT system to optimize aircraft versatility for both short duration flights and longer distance city-to-city jumps. And as designed, the Lilium is configured as a 6 passenger plus pilot aircraft, making it one of the larger form-factor eVTOLs in development. QwikConnect is not rooting for winners, but we have to observe that the design aesthetics of the Lilium flight control surfaces make it one of the most compelling jet designs in the long history of aviation. And the promised efficiency of the aircraft at cruising altitude and speed make it one of the most versatile in terms of mission-profiles.
The Lilium jet shown with wings rotated in cruise orientation (above) and on the tarmac with wings in their vertical orientation (below) ready to lift and hover once passengers are boarded. Images courtesy Lilium
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Transitioning Multicopter Powered with six tilting propellers for both lift and cruise functions, the five-seater plus pilot Joby S4—our featured example for a Transitioning Multicopter eVTOL—is designed for urban air transport with a range of up 150 miles. The aircraft is a mature design with over 1000 logged test flights, a production manufacturing arrangement in place with Toyota, and significant investment backing from Reinvent Technology Partners. On the operations side, Joby has acquired Uber Elevate to create a seamless urban transport system from rideshare car, to air taxi, and back again. In addition, a partnership with parking garage operator REEF will form
As shown in the photograph below, the four outboard propellers on the Joby S4 tilt with their nacelles, while the two inboard propellors actuate with a linkage mechanism Image courtesy Joby Aviation
the foundation for a network of sky ports. In terms of design, the outboard propellers on the S4 tilt with the nacelle while the inboard propellers actuate with a unique linkage mechanism. This design allows the flight control wings to remain fixed during all modes of flight. The V-Tail is also equipped with a pair of tilting nacelle rotors. For propulsion, each Joby S4 propeller is equipped with two 70 KV motors for complete redundancy. A low-noise signature is a hallmark of the Joby S4. The six amply-sized rotors on the aircraft are 2.9 meters in diameter, giving the aircraft just 46 Kg/m2 of disc loading and contributing to its low-
noise performance and long battery life expectations (less power intensive during hover). In addition, all rotors are equipped with an Anhedral Tip which contributes to both hovering and cruise- mode performance as well as noise reduction.
Anhedral Tip rotor design (above). The Joby S4 (left) utilizes an Anhedral Tip for performance and noise reduction. Aircraft photograph courtesy Joby Aviation
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Lift and Cruise For this category of eVTOL design we have chosen to highlight the six-passenger, piloted Beta Technologies Alia, a Lift and Cruise aircraft with separate, fixed-position propulsion systems for lift (hovering) and cruise (forward thrust). The Beta Technologies Alia is a versatile aircraft design with dual capabilities for short duration flights as well as longer mission-profiles, and an interesting focus on both passenger and cargo implementations of their flagship aircraft, the Alia 250 and 250c. And worthy of note, Beta Technologies has already inked a deal with UPS Flight Forward, a subsidiary of United Parcel Service for 150 Alia 250c aircraft. Flight control is managed by the speed and vector control of the propeller system, combined with conventional wing flaps. Beta Technologies, based in Vermont, has invested heavily in a unique, modular base-station and charging facility platform which promises to recharge the aircraft battery in just one hour, for fast turnaround
flight scheduling. The range of the Alia is described by the manufacturer as 250 nautical miles, making it suitable for longer-category missions. The design language of the aircraft (its flight surfaces especially) were, according to Beta, inspired by the wing profile of the Arctic Tern, perhaps
the most intrepid avian commuter in the world.
The Lift and Cruise Alia (above) will quickly charge at a modular base station (left) for quick-turnaround
flight scheduling. Images courtesy Beta Technologies
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Wingless Multirotor Wingless eVTOL aircraft have multiple rotors which handle all aspects of the aircraft operation, including vertical liftoff and landing, forward cruising, and flight control. Flight control is managed by both vectoring the rotors and by modulating rotor speed and torque. The EHang 116, manufactured in China, is the poster child for this design and stands out as one of the farthest along in actual aircraft production and deployment. Unlike the Lilium and the Beta Technologies Alia, the EHang 116 is optimized for short-distance flights only. The massive energy cost to power its 16 rotors limits the duration of flights to just intra-city destinations in which a significant portion of flight time is consumed by liftoff and landing. EHang aircraft are likewise limited in passenger capacity. The 116 version is optimized for autonomous (pilotless) flight with a single passenger. Readers interested in the EHang success story should check out their partnership with the Dubai RTA that is already implementing a forward-thinking air taxi program based around the EHang 116 and 184 wingless multirotor aircraft.
Top: The EHang 216 AAV on a trial flight in Japan. Middle: conceptual art of an Eco-sustainable Vertiport Bottom: EH216 conducting passenger-carrying trial flights at the Digital China summit Images courtesy EHang
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Gyrodyne A Gyrodyne is a proven category of VTOL aircraft with a long qualification history under FAA Part 29 rotorcraft requirements. In operation, a single top-mount rotor is employed for both takeoff and landing. Forward thrust during flight is provided by conventional propeller or jet engines. As mentioned, Part 29 regulations are well understood, both by aviation engineers and by the regulators themselves, making this an attractive design concept—especially given the high cost of certification efforts for new, innovative designs.
While there are several viable eVTOL Gyrodynes now in development, we have selected Jaunt Air Mobility’s “Journey” for our featured example in this category. The Jaunt Journey features a single, large main rotor and four forward-positioned rotors. The battery powered electric main rotor should be both efficient and quiet during take off and landing given its relatively slow spin rate and large form factor. The helicopter-like aircraft is not equipped with a tail rotor to offset the main rotor’s torque. Rather, the four large electric props mounted on the wing prevent the aircraft from tailspining during hover-mode, and as mentioned, provide the necessary forward thrust to accelerate the aircraft to its cruising speed of 175 MPH. Interestingly, the large central rotor offers a significant safety premium compared to multirotor systems. This is due to the patented Jaunt Air Mobility technology called SRC (Slowed Rotor Compound) which modulates the main rotor in accordance with aircraft speed, and provides the abiloity for the aircraft to autorotate down in the event of power system failure. Jaunt Air Mobility is privately held and self-funded. The Dallas, Texas based company has plans to achieve FAA certification by 2023 and commence commercial services by 2025. Schematic drawing of a prototype Fairey Gyrodyne rotorcraft from 1946
The Jaunt Air Mobility Journey Image courtesy Jaunt Air Mobility
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DISTRIBUTED Electric PROPULSION
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NOTE: This is an excerpt from a technical Glenair whitepaper on the topic of all-electric distributed power in eVTOL systems. For the complete dissertation, including, charts, tables, footnotes, etc., please consult the whitepaper available at glenair.com/eVTOL-air- taxi-interconnect-solutions. Distributed Electric Propulsion (DEP) is a key element of all eVTOL aircraft. A basic description of a DEP design is a power transmission system whose electrical energy sources are interconnected, via EWIS cabling, to multiple electric-motor-driven propellers or rotors. The native power sources in a DEP can be as simple as a battery-plus-inverter design, or as complex as a hybrid system made up of gas combustion engines, storage cells, electric generators, inverters, power feeder cables, interconnect harnessing, and more. The DEP is designed to feed aircraft “propulsors,” or thrust producing devices including propellers and fans, with adequate power for vertical takeoff, landing, and cruise operations. An all-electric DEP system may incorporate high- voltage elements (greater than 3kV) as well as high kW power for peak output to electric propulsion motors, inverters, controllers, and batteries during takeoff and landing. Shared components may be grouped together or distributed throughout the airframe for a redundant distributed thrust system. The safety hazards inherent in such distributed electric systems requires the platform be designed and configured with robust technologies qualified for high-voltage, high-current, and high-frequency aviation applications. The following guidelines explore these critical issues in greater detail. Working Voltage vs Dielectric Withstanding Voltage All DEP designs begin with a definition of the operational voltage, or maximum continuous working voltage, of the equipment. For all DEP applications, a certain safety factor is required between equipment operational voltage (OpV) and the proof-test voltage (DWV) of interconnects and other EWIS components. However, the magnitude of this safety factor varies greatly depending on the exact implementation of the distributed power system.
Here is a useful metaphor for why aviation systems take “derating” so seriously: if a
lifeline rope needs to support a 200lb individual, it would be unsafe to use a rope that has only been proof-tested to 200lb., as this would provide no margin for error, nor allow for any aging
or degradation over time. Instead, emergency teams use a 2000lb proof-tested rope, providing a 10x safety factor to guarantee performance in critical situations. However, if the goal is to hang a 20lb bicycle in a garage—where failure could hardly lead to loss of life—a 50lb proof-tested rope (2.5x) would likely be sufficient. For this reason, high safety derating factors are always used in aviation interconnect systems where failure could result in loss of life. FAA or other national agency qualification of eVTOL electric propulsion systems will absolutely require adherence to higher levels of safety and testing in electrical wire interconnect systems. This table illustrates the relationship between the actual Working Voltage of an aviation-grade system to the tested Dielectric Withstanding Voltage of its component parts: Suggested DWV Based on Referenced Industry Standard Working Voltage (OpV) Suggested Dielectric Withstanding Voltage (DWV) 250 1,500 500 2,000 750 2,500 1,000 3,000 1,250 3,500 1,500 4,000 1,750 4,500 2,000 5,000 By way of example, while it is common practice for airframe harnesses to operate at 115 VAC, these cables would need to perform and be tested at 1500 VAC DWV for flight qualification.
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DISTRIBUTED ELECTRIC PROPULSION [CONTINUED]
• Void Discharge – partial discharges that occur in air voids within an insulation system. The voids can be created either by defect (an air bubble in a molded insulator for example) or by design (clearance zone between contact and insulator cavity). Internal discharges ionize the air, creating byproducts such as ozone, and release UV energy, which causes chemical breakdown of the insulator. • Surface Discharge – partial discharges on the surface of an insulator. Naturally occurring contaminants deposited on the surface of an insulator create a conductive path. Upon conduction, the insulator carbonizes (or “burns”), creating a permanently conductive path. • Corona Discharge – partial discharges which occur in air when the electric field created by a high-voltage conductor is higher than the strength of the air. Similar to void discharges, corona discharge creates byproducts, such as ozone, and in turn releases UV energy, which cause chemical breakdown of nearby insulators. • Treeing – a secondary effect of internal discharges created by void discharge, conductive impurities, or defects, which lead to insulation damage. This is similar to surface discharges, but internal to the insulator. As treeing progresses, a branched network of conductive paths is permanently created within the insulation. Partial discharge testing is an essential performance requirement for airborne applications that, again,
Partial Discharge Partial discharge (PD) in electrical wire interconnect systems refers to a localized breakdown of insulation materials, which does not result in catastrophic failure degrading the insulation between conductors. Simply put, PD is small yet measurable micro-failures that leave the insulation intact, but over time may age and reduce the life of the interconnect system. Partial discharge is a critical issue in advanced eVTOL airborne power applications, given the role of the cabling in distributed electrical propulsion applications, and the higher voltages, current levels, and frequencies typically carried by such transmission lines. Here’s a useful metaphor to illustrate this point, once again using a rope: for a fibrous load-bearing rope, when the load is near breaking strength, individual fibers may fail. Initially, loss of one or two fibers would be negligible, and the rope would continue to support the load. However, over time, as more fibers fail, the accumulative effect would eventually compromise the overall integrity of the rope, and total failure would occur. Similarly, individual partial discharges do not cause immediate failure, but will erode the insulation and eventually lead to failure—a condition that is absolutely intolerable for any airborne system. In terms of FAA and other agency qualification of eVTOL aircraft with distributed electrical power, there are several types of partial discharge to consider:
handle high voltage and/ or high current electrical energy. Frequency Effects Airborne power distribution systems use a wide range of frequencies for various applications. Battery banks provide DC power. In more-electric aircraft (MEA), generators produce power at 400 Hz AC. In all-electric aircraft (AEA), variable- frequency drives (VFD) powering electric motors
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Source: “The Effect of Frequency on The Dielectric Breakdown of Insulation Materials in HV Cable Systems.” 8
produce fundamental frequencies from 1 kHz to 4 kHz, using pulse-width modulated (PWM) switching frequencies up to 30 kHz . Due to several physical phenomena, including dielectric heating and ion accumulation, source frequency affects the dielectric strength of the insulation. Simply put, high frequency loads put additional stress on the insulation, leading to lower breakdown voltages. While most materials degrade with increased frequency, the sensitivity and degree of degradation vary between materials. Let’s take a look at the impact of higher frequencies on XLPE (a common insulating material in high-voltage cables) Most airborne interconnect test regimens do not include electrical tests using frequencies other than 60 Hz. For example, interconnect manufacturers perform DWV at 60 Hz in accordance with EIA-364- 20. In the case of distributed electric propulsion systems, however, Glenair considers it far more XLPE Breakdown Voltage at Various Source Frequencies Source Frequency Breakdown Strength (kV RMS /mm) Relative Change (vs 50 Hz) DC 438.4 ↑ 406.06% 50 Hz 86.63 - 300 Hz 74.95 ↓ 13.49% 500 Hz 73.02 ↓ 15.71% 1000 Hz 64.48 ↓ 25.57% 1500 Hz 57.36 ↓ 33.79% 2000 Hz 56.87 ↓ 34.35% 2500 Hz 51.96 ↓ 40.02%
Jiayang Wu, Huifei Jin, Armando Rodrigo Mor, Johan Smit; Delft University of Technology; 2017
interconnects, such as PowerLoad, are well-suited to support higher frequency generators, variable frequency drives, and other such equipment. In fact, for AEA applications operating at substantially higher frequencies, ignoring frequency derating could have dire consequences on performance and reliability. Similarly, using DC-rated products in AC applications is inadvisable given the need to optimize passenger safety. Operational Stress Factors In application, there are numerous operational factors that may affect long-term reliability of the electrical wire interconnect system including: • Increased temperature = decreased insulation effectiveness resulting in thermal aging and reduced dielectric strength. • Increased humidity = Increased leakage current, surface conductivity, and reduced dielectric and creepage (tracking) strength. • Increased mechanical stress = contact fretting, material embrittlement, and reduced dielectric strength. • Chemical exposure = surface corrosion, material breakdown, insulation aging, and reduced dielectric strength. • Increased voltage = increased insulation failure, electrochemical erosion and intrinsic breakdown. • Increased source frequency = increased dielectric heating, leakage current, and reduced dielectric strength. • Increased partial discharge = increased carbonization/ resistance, chemical degradation, and eroded insulation integrity
prudent to broaden the range of high- frequency testing to ensure our native airborne
While it is certainly possible that wire interconnect technologies qualified for use in automotive or
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DISTRIBUTED ELECTRIC PROPULSION [CONTINUED]
general industrial applications may be successfully employed in eVTOL air taxi applications, Glenair’s expectation is that the FAA and other qualifying agencies will take a dim view of interconnect technology that has not been tested and approved for its proven resistance to common airborne operational stress factors. Current Rating When electrical current travels across a conductor, inefficiencies and resistance in the conductor cause some of the electrical power to convert into heat. This can be seen in the glowing filament of an incandescent light bulb or the red-hot coils in a toaster oven. The total heat produced is dependent on the current moving across the conductor (measured in amps, “A”) and the inefficiency of the conductor, or resistance (measured in Ohms, Ω). For any given conductor, as more current is applied, more energy is converted into heat. The heat generated causes the conductor to reach temperatures above ambient, a process known as “temperature rise.” For any given current, less efficient (or more resistive) conductors will lose more energy to heat, and experience greater temperature rise. Large conductors (measured in “gauge,” typically American Wire Gauge or “AWG” in North America) have more material to conduct the current, and therefore lower resistances. Similarly, different conductor materials have inherently lower resistances (copper versus aluminum, for example). The lower the resistance, the lower the temperature rise. By extension, the lower the resistance, the greater the allowable current for a specified allowable temperature rise. This is the foundation for current rating. For any given conductor and current, the temperature rise is a balance between heat produced and heat lost through conduction or convection. For eVTOL airborne applications, low- density, high-altitude air is a very poor coolant, keeping more heat in the conductor and leading to higher temperature rise. As a result, current ratings for eVTOL interconnect products are generally lower
than current ratings for land and sea interconnect products, even if the underlying interconnect is essentially identical. Here’s another handy metaphor: it is common knowledge that water feels colder than air. 60°F air would feel like a nice fall afternoon. However, 60°F water would lead to hypothermia within an hour. Compared to air, water has a high heat-capacity and conductivity. Water can pull heat from the human body faster than it is metabolically produced. Similarly, water effectively pulls heat from conductors, allowing higher currents to be obtained while maintaining a low temperature rise. In eVTOL airborne power distribution systems, there are practical limitations to the maximum temperature rise. Most electrical interconnects are only rated to 200°C, some a bit higher (230-260°C), some a bit lower (150-175°C). Often the electrical equipment utilizing the interconnect could have an even lower temperature rating. Also, ambient temperature will vary between applications, depending on location within the aircraft and proximity to other equipment. The difference between the maximum permissible temperature of the interconnect and the ambient temperature is the maximum allowable temperature rise . Application conditions dictate maximum allowable temperature rise, and by extension the maximum current permissible on any given conductor. Current Derating With baseline current rating established by allowable temperature rise under ideal conditions, conductor performance can then be derated to conform with estimated performance under actual application. As just discussed, the first current derating element is allowable temperature rise , determined by the difference between ambient temperature and maximum allowable temperature of the interconnect. Since temperature rise under
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load will vary based on gauge, conductor material, and insulation, it is best to evaluate the temperature rise data for each unique assembly. However, SAE AS50881 does provides a conservative estimate which may be used as a universal reference point. The second primary current derating factor is the number of wires in a harness bundle . A single wire can freely dissipate heat through convection since all outer surfaces are exposed to ambient air. However, as additional wires are added to a bundle, wire surfaces are in contact with each other and therefore lose the ability to dissipate heat. This is especially true for wires at the center of a large bundle. Since the wires are no longer able to dissipate heat efficiently, current must be limited to reduce the aggregate heat produced. The offset of heat produced versus heat dissipated will ensure temperature rise does not exceed the maximum allowable. While “bundle derating” is ideally evaluated for the unique assembly, again AS50881 provides a conservative estimate which may be used as a universal reference point. The third primary current derating factor is altitude . At sea-level (standard pressure), air is dense and able to convect substantial heat away from the wire. However, at higher altitudes (low pressure), air density is reduced. Reduced air density means there are fewer air molecules available (per unit volume) to convect heat away from the wire. Since the air is no longer able to convect heat as efficiently, the current must be limited to reduce the heat produced. The offset of heat produced versus heat dissipated will ensure temperature rise does not exceed the maximum allowable. While “altitude derating” is ideally evaluated for the unique assembly, again AS50881 provides a conservative estimate to be used as a universal reference point. The fourth current derating factor is source frequency . In an AC system, the current alternates direction along the conductor. When current travels one direction, a magnetic field is generated which supports the current flow through inductance. As current reverses, the magnetic field collapses, and the inductance opposes the current reversal. This is known as counter-electromotive force (CEMF), or “back EMF.” The CEMF is strongest at the core of the conductor, leaving the outer surface less effected and more
available to conduct the source current. The “skin effect” is where AC current tends to
only conduct on the outer surface of the conductor, leaving the core useless. At higher frequencies, the period of reversal is reduced, and CEMF has less time to dissipate. As a result, higher frequencies lead to more core left unutilized, pushing more current to the outer surface. We can calculate the “skin thickness” (δ) to estimate how much of the conductor is in fact utilized. If the skin thickness is greater than the conductor radius, then we would expect the conductor to be fully utilized. However, if the skin thickness is less than the conductor radius, we would expect core loss. As shown in the table below, standard power frequency (60 Hz) will utilize 100% of all conductors up to 4/0. However, 400 Hz MEA power will only utilize 100% of conductors up to 2AWG, suffering loss on larger conductors. For AEA applications with high frequency and shallow skin depth, the alternating current will utilize only a small portion of large conductors. With the current only traveling across a small percentage of the conductor, conductor efficiency is lost, and conductor performance may be estimated using an elevated current at lower frequency. Glenair engineering routinely analyzes any application where the conductor may experience skin effect loss to more exactly estimate current rating and temperature rise. Conductor Percent Utilization due to Skin Effect at High Frequency Conductor Source Frequency (Skin Thickness, inch)
60 Hz (0.372)
400 Hz (0.131)
1000 Hz (0.083)
1500 Hz (0.068)
2000 Hz (0.059)
2500 Hz (0.053)
AWG Radius
8 4
0.064 100% 100% 100% 100% 99% 97% 0.102 100% 100% 97% 89% 82% 76%
2 0.129 100% 100% 87% 78% 70% 65% 1/0 0.162 100% 96% 76% 66% 59% 54% 2/0 0.182 100% 92% 70% 61% 54% 49% 4/0 0.230 100% 82% 59% 50% 45% 41% Calculated estimates for copper (ρ = 1.72×10 -8 Ω-m, α = 4.29×10 -3 °C -1 ) at 25°C
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DISTRIBUTED Electric PROPULSION
FREQUENTLY-ASKED QUESTIONS
What is the difference between “breakdown voltage,” “DWV,” and “voltage rating?” Breakdown voltage is the voltage required to cause failure and permanent damage to electrical equipment. Dielectric withstanding voltage, or DWV, is a proof- test voltage performed to ensure the electrical equipment is free from defects. Voltage rating, AKA working voltage, is the operating voltage of the equipment. To provide safety margin, there is always a difference between operating voltage and DWV test voltages. In practice, Glenair provides DWV test data for our interconnect technologies to ensure defect- free material makeup, but it is understood to be the customer’s ultimate responsibility to derate DWV according to their exact requirements. For situations in which an actual qualified voltage rating is essential—again as opposed to a DWV test voltage—additional testing and evaluation may be completed by Glenair. What does “current rating” mean? Current rating is the maximum allowed current before failure, often defined as 42.8°C temperature rise for a single conductor at sea-level. It is acceptable to use a conductor for currents greater
than the current rating if the system can tolerate the corresponding temperature rise. However, increased altitude and wire bundle size will reduce performance, and should be derated accordingly. Any application requiring currents in excess of the standard current rating should be carefully evaluated on an application-by-application basis. Glenair engineering may assist evaluation in accordance with SAE AS50881. What is partial discharge, and how does it affect performance? Partial discharge is localized breakdown within the insulation which does not fully bridge between conductors. Small defects within the insulation, such as air voids or contaminants, are the primary cause. While partial discharge does not result in immediate failure, it will accelerate insulation aging and shorten the interconnect life expectancy. Partial discharge takes many forms, including void discharge, surface discharge, corona discharge, and treeing.
How does altitude affect voltage performance?
In general, clearances between components in connector insulation are filled with air. The air pockets are electrically insulating, acting as virtual “components” within the insulation system. Increased altitude reduces air pressure, which reduces the electrical (dielectric) strength. As the air’s dielectric strength is reduced, the total combined strength of the insulation system is reduced as well. How does source frequency affect voltage performance? Increasing the frequency of the applied power source increases stress on the insulation, reducing the breakdown voltage (and therefore DWV and voltage rating) of the interconnect. Direct current (DC) sources are the least stressful on insulation, allowing much higher voltages than alternating current (AC). The interconnect testing standard is power-frequency AC (50/60 Hz). Increasing frequency will reduce strength and the interconnect should be derated accordingly. Any application requiring high frequencies (>800 Hz) or optimized DC performance when only AC DWV is known should be carefully analyzed for safe performance.
Corona Discharge on a 500 kV power line Photograph by Nitromethane via Wikipedia
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What are the key terms and definitions of Distributed Electrical Propulsion? AC Alternating Current, supply current cyclically changes direction
Breakdown
Catastrophic failure in electrical insulation resulting in instantaneous and complete loss in electrical performance Rate of flow of electrical charge, usually measured in amperes (A) Direct Current, supply current flows in one continuous direction Dielectric Withstand Voltage, proof- test voltage to ensure interconnect is free of manufacturing defect Cyclical rate of change in current direction for AC systems, usually measured in Hz (cycles per second) Operational Voltage, safe working voltage for continuous use based on application environment and reliability requirements Partial Discharge, localized failure in electrical insulation resulting in slow aging of electrical insulation , usually measured in picocoulombs (pC)
DWV testing of a Glenair high-voltage interconnect assembly at our independently-certified test laboratory
Current
DC
DWV
How does source frequency affect current performance? Increasing frequency reduces current rating. Two critical factors, skin and proximity effect, combined with high frequencies, prevent full utilization of the conductor. High frequencies are particularly detrimental to the performance of larger conductors. Again, for applications in frequency ranges greater than 800 Hz, additional analysis must be performed to ensure safe and reliable performance. How do we estimate reliability? Various methods may be employed to estimate interconnect long-term reliability by applying elevated stress-factors to induce failure, then deriving life-expectancy at intended usage through complex calculations. Accelerating factors include elevated voltage, elevated temperature, temperature cycling, and increased frequency. Accelerated aging tests may be useful in some instances, but the gold standard of reliability testing is Weibull analysis. For mission-critical systems utilizing high-voltage, high-current, and/or high- frequency interconnects, it is Glenair’s preference to determine reliability through this methodology to ensure safe and reliable performance of the electrical wire interconnect system.
Frequency
OpV
PD
Safety Factor Difference between OpV and DWV, required to ensure proper operation and varies based on application criticality
Skin Effect
Loss in conductor utilization and efficiency due to counter- electromotive force (CEMF) in high- frequency alternating currents Electrical potential difference between two conductors, usually measured in volts (V)
Voltage
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A BRIEF HISTORY ~ o f t h e ~ Flying Car
The dream and promise of the flying car is more than a century old. This brief history of some of the more audacious attempts demonstrates how long and arduous the journey has been.
1917 Renowned aviator Glenn Curtiss, rival of the Wright Brothers and a founder of the U.S. aircraft industry, could also be called the father of the flying car. In 1917, he unveiled the Curtiss Autoplane at New York’s Pan-American Aeronautic Exposition. It featured an aluminum Model T Ford-like body, four wheels, a 40-feet wingspan, and a giant 4-blade propeller mounted in the back, which unfortunately was unable to generate adequate lift to propel the aircraft beyond just a few hops down the runway.
1935 Frank Skroback, a retired industrial technician and electrician from Syracuse,
studied the concepts of French furniture-maker- turned-aircraft designer Henri Mignet, and modified his tandem wing monoplane design into a multi-purpose, 6-wing, 21 foot long Flying Car.
1953 Leland Bryan of Buick flew his Autoplane, which used a rear propeller for forward propulsion. Bryan died in 1974 when he crashed an Autoplane at an air show.
1947 Henry Dreyfuss combined a lightweight fiberglass automobile body with a wing-and-propeller module to create the ConvAirCar. Unfortunately, it crashed during a test flight, killing its operator, and ending enterprise.
1973 Aerospace engineer Henry Smolinski unveiled the AVE Mizar “Flying Pinto,” in which the back half of a Cessna Skymaster was mated with a stripped-down Ford Pinto body. The car engine was used for surface travel and runway boost on takeoff. In flight, the craft depended
on Skymaster wings, a twin-boom tail and pusher propeller. All flight equipment was detachable to convert
the vehicle for street travel. Sadly, Smolinski and pilot Harold Blake died when a wing folded in a test-flight crash.
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1989 Aeronautics engineer and inventor Paul Moller worked to bring a flying vehicle to the mass market for four decades. In 1989 he launched the M200X prototype, now known as the Moller M200G Volantor, a multi-rotor vertical take-off and landing aircraft. The unique vehicle was designed to take advantage of aerodynamic “ground effect” by limiting maximum altitude to 10 feet. Interestingly, the FAA does not regulate vehicles that operate below 10 feet as “aircraft.”
2009 Steve Saint was awarded Popular Mechanics’ Breakthrough Award for the Maverick, a flying dune buggy he invented to deliver supplies and medical care to remote areas. The Maverick’s short take off and its ability to both fly and drive, makes it one of the more interesting and successful attempts at a flying car.
2012 Terrafugia Corporation’s Transition “street legal” production prototype completed its first flight, and multiple phases of testing. The one-pilot, one passenger vehicle can reportedly go 70 mph on the road—and fit in the garage with its wings folded. In flight, the pusher propeller can attain a cruising speed of 107 mph. Equipment includes a Dynon Skyview glass panel avionics system, an airframe parachute, and an optional autopilot.
2015 The DARPA TX Transformer was, believe it or not, a proposed lift-and-cruise flying car for the U.S. Military. The objective of the Transformer program was to demonstrate a four-person road vehicle that could provide enhanced logistics and mobility by transforming into an aircraft.
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TYPE CERTIFICATION OF Air Taxi INTERCONNECT SYSTEMS
Electrical Vertical Take-Off and Landing Aircraft (eVTOLs) and hybrids are being rolled out now for use in short-distance urban air transportation missions. Goals range from increased green energy utilization and improved travel time, to reduced ground traffic congestion. This article presents the principal electrical wire interconnect (EWIS) requirements for FAA / EASA type certification. The intent of this article is to highlight electrical wire interconnect solutions for the emerging Urban Air Mobility (UAM) market, specifically targeting power distribution, avionic and sensor connections, and finally wire and cable management. The UAM, or Air Taxi industry, is focused on highly congested cities and population segments that require alternative solutions to ground transportation congestion. Many UAM solutions are focused on efficiently transporting small groups of people or single individuals in a node-to-node model, typically from high-traffic destinations such as airports, to hubs in city centers. The activity of autonomously transporting people through a controlled airspace and overflying urban areas with unique designs of eVTOL aircraft will require type certification from both the Federal Aviation Administration and the European Aviation Safety Agency including compliance with Federal Aviation Regulations part 23, 25, and 29.
Glenair offers a broad range of interconnect technologies that have been successfully implemented in aircraft required to meet FAR 25.1701 Electrical Wiring Interconnect System. These EWIS-compliant technologies were developed— using current-day materials and design principles— to ensure reliable and safe air transportation, free of electrical safety hazards. Qualification authorities will apply these or similar regulatory standards throughout the program development cycle (concept, preliminary design, critical design, initial build, power on, fight test and entry into service). In fact, while some industry analysts anticipate the emerging UAM market, over the next 10–15 years, will enjoy some latitude in performance and safety requirements—in line with the unique low-altitude/ autonomous operation nature of the technology— others argue that when the reality of transporting people over dense urban environments and the safety of both passengers and those living below the air space are fully considered, the safe and reliable operation of UAMs will require the use of electrical components that absolutely meet the stringent requirements associated with FAR 25.1701. The most likely UAM operations scenario will be to limit the vehicles to operation in a lower-altitude airspace than larger commercial aircraft, with a likely 10,000 foot AGL (Above Ground Level) limitation. Lower-altitude operation (say, 1500 to 5000 feet) has the advantage of simplifying the insulation design requirements for distributing high-voltage power as well as reducing atmospheric thermal extremes to a range of -40° C to +60° C. Some air taxi designs are already in production and rollout in
countries that are decidedly not in compliance with RTCA DO-160, and other FAA environmental conditions and test procedures for airborne equipment. However the likelihood that these requirements will be instituted as the baseline foundation for defining ongoing and future UAM flight requirements in North America and Europe goes without
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question—including test methods to meet the unique environmental challenges associated with these high-cycle, dynamic air taxi missions—from basic considerations of galvanic corrosion and dissimilar metal design principles, to a wide range of other environmental constraints. In this regard, the UAM environment may ultimately be considered as basically a modification to the DO- 160 Category A4, which states: Equipment intended for installation in a controlled temperature and pressurized location on an aircraft within which pressures are normally no lower than the altitude equivalent of 15,000 ft Mean Sea Level. The category may also be applicable to equipment installed in temperature controlled but unpressurized locations on an aircraft that operate at altitudes no higher than 15,000 ft MSL.
Why is understanding the power system voltage so important? Higher voltages in aircraft operating environments characterized by broad temperature ranges, altitude, and pressure define wire insulation thickness and electrical connector geometry creepage and clearance dimensions. Per AS50881, paragraph 6.6, “For DC, electrical cables can be used without ionization to a maximum voltage of 340 volts independent of the usual practical range of wire covering thicknesses. Under certain conditions (notably at high ambient temperatures and/or high altitude) some wire types may not be free from corona at rated voltage.” In certain applications of this type, Power Feeders (as opposed to mateable interconnects) combined with highly engineered power cabling, offer a viable solution which mitigates certain challenges associated with the use of conventional mil-spec insulated cables and connectors in high-power applications. Glenair Duralectric Power Feeders are in development for UAM applications with variable dielectric wall thicknesses IAW insulated conductor size and material and addressing the overall voltage / weight / flexibility requirements of the emerging UAM market. Glenair has several methods of terminating our aircraft-grade TurboFlex / Duralectric cables for use in applications of this type. For applications that do require the ability to mate and un-mate—such as for electric motors— purpose-designed interconnects with proven performance in rigorous commercial airplane environments are preferred for compliance to FAA part regulations. Interconnects of this type typically incorporate design features that enable connectorization of high voltage, high current, as well as high-frequency power distribution systems providing safe and reliable protection of power lines from arcing or other safety hazards.
Environment Constraints: • Operating Low Temperature: • Operating High Temperature: • Ground Survival Low Temperature: • Ground Survival High Temperature:
-15° C +70°C -55°C +85°C
• Altitude:
4572 m (15,000 ft)
• Absolute Pressure (at 15kft):
57.18kPa (751.8mbars, 16.89 inHg, 429 mmHg)
Electrical Power Distribution System Voltage Requirements The core technology for the electrical power distribution system in autonomous air taxis is based on the lithium-ion battery series/parallel design used in electric automobiles which, depending on the vehicle performance factors, produces a voltage of between 375Vdc to 800Vdc. An important benchmark in evaluating power distribution interconnect requirements can also be drawn from past successful NASA-instituted programs in which nominal voltage of 461Vdc (416Vdc – 525Vdc) with a maximum operating altitude of 15,000 ft. was achieved. Both of these power distribution models rely on lithium battery technology and are useful as a basis for selection of EWIS interconnect components suitable for the current generation of UAM lithium-ion HVDC power distribution systems.
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