7 ms·

Given that the batteries weigh a lot more than the motors, I would have thought that motor efficiency (which scales battery size) was much more important than m

by tlb 6y ago

Given that the batteries weigh a lot more than the motors, I would have thought that motor efficiency (which scales battery size) was much more important than motor weight. My back-of-the-envelope is: - Assuming 0.4 kWh/kg for batteries, and they have to run for 4 hours, then the total mass per kW is 10 kg (batteries) and 0.08 kg (motor). - A 1% increase in motor efficiency could eliminate 0.1 kg of batteries, which would let you double the weight of the motor. - (My analysis is invalid if you need much higher peak power than cruise power.) I'm curious how you optimize the entire system for such trade-offs.

audunw 6y ago
> Given that the batteries weigh a lot more than the motors, I would have thought that motor efficiency (which scales battery size) was much more important than motor weight. I guess that depends on what kind of airplane you are making. If you're just making the same kind of airplanes we've been making with ICE, but with electric motors and batteries instead, you're probably right. But if you're making an electric airplane from scratch, there's a lot you can do if you have a really light motor, which can drastically reduce drag. Look at Maxwell X-57 for instance: https://www.youtube.com/watch?v=-HvZ7c0F9ik https://www.youtube.com/watch?v=-HvZ7c0F9ik If you're going to have lots of motors on the wings, they better be as light as possible. I'm guessing the increased efficiency from a design like that can easily be as important as the efficiency of the motor itself.
- mliben 6y ago
  These are all great points- everything is very interconnected in these vehicles, and there is a lot of potential upside in high power density distributed propulsion (like on the Maxwell). In characterizing the vehicle-level benefit of power density, it is definitely important to consider the X kg of structure required to support 1 kg of motor/inverter/gearbox/etc.
  - toss1 6y ago
    Excellent points about associated structures! However for the motors, isn't it the case that the support structure design is dominated by the thrust loads, which should vastly exceed the motor mass? For sure, there is some non-thrust-related structure to react the motor's mass, e.g., inertia from a harsh landing, but how much extra is it? This is much more the case with power/mass optimized motors like yours. Consider an ideal case - you achieve the same power with negligible mass, say 1kg. How much structure in my aircraft using your motor could I really eliminate vs your current model? And the real case, switching from a competitor's similar-power motor to yours, how much additional structure weight can I save by switching, beyond the obvious great advantage of your motor's weight savings? (Obviously, these answers massively depend on other factors, but... )
    - mliben 6y ago
      Yeah, there are a lot of factors that play into this. A couple things come to mind: 1) Considering megawatt-class machines are necessary for many future applications, the mass of the motor+inverter+gearbox (especially using best current technology) definitely adds up. 2) With a very distributed propulsion system, motors that end up near the wing tips have a big moment arm compared to the ones typically tucked under the wing root
      - toss1 6y ago
        Excellent point about the moment arm and control issues! I'm definitely all about maximizing power-weight and strength-weight ratios... I am wondering about the ratios you are achieving, and about the issues of scale. Do things get better as you scale up? I notice you mentioning the state-of-the-art at 3-4 kW/kg, and you shooting for 12 kW/kg. This is even substantially better than small scal T-motor UAV motors at around 7w/g [1]. The chart shows them peaking at 3181W and weighing 453g. So, I'm wondering what scale factors may be working in your favor at your scale vs the single-digit kW scale. Also, any plans to scale slightly smaller (I'm involved in such a project)? [1] https://uav-en.tmotor.com/html/2021/Antigravity_0119/668.html https://uav-en.tmotor.com/html/2021/Antigravity_0119/668.htm...
        6y ago
jjsylvestre 6y ago
This is an excellent question. For narrow body aircraft we've studied, they require high propulsive power during the takeoff and climb phases, and a fraction of the peak propulsive power during the cruise phase. One aircraft we looked at required 30-35MW during takeoff and ~10MW during cruise. So, thrust power and system level power density (kW/kg) are critical during takeoff/climb and cruise efficiency is important for minimizing energy consumption. Like Audunw mentions, its very application dependent as well. It all boils down to the propulsion system mass fraction. For lower PSMF, efficiency matters more once you are above a certain power density. For higher PSMF, power density matters more. There is an optimal balance of efficiency vs. specific power for every aircraft. We can "tune" our technology relatively easily depending on what that balance is to maximize range. I'll let my cofounder Max chime in since he does a lot of vehicle-level architecture and optimization. He's been doing some studies for rotorcraft and planes to look at how specific power and efficiency impact range/endurance so I'm sure he can expand on my answer a bit.
- mercurywells 6y ago
  > One aircraft we looked at required 30-35MW during takeoff and ~10MW during cruise. Do you envision some airframes to include assisted take-off technology? (JATO and the like, even catapults)
  - mliben 6y ago
    I think most forms of assisted takeoff technology would reduce electrical power requirements for takeoff, but not put much of a dent in the climb power (unless you can catapult/ JATO all the way up to cruising altitude, which would be challenging). Since climb power is still significantly higher than cruise power, and is effectively thermal steady state for these components (10-20min), it would still drive the propulsion system sizing.
    - hbrav 6y ago
      You could have another aircraft tow you, like gliders sometimes do. Doesn't seem terribly practical though.
serjester 6y ago
Keep in mind that an electric motor isn’t limited by the amount of oxygen in the air. As a result it can fly significantly higher where there is far less air resistance. Since air density is proportional to the square of the elevation this can lead to significant efficiency gains. Believe it or not, partly as a result of this, the SR-71 had it’s best mpg at peak speeds.
- mliben 6y ago
  A simple physics-based plane model (like the one we made to understand vehicle-level impact of our technology development) dictates that the range-optimal cruise speed is proportional to 1/sqrt(air density), so it makes sense that the blackbird was more efficient at high speed when at high altitudes (admittedly, this simple model is subsonic, and there are a lot of other factors for supersonic flight). Since having lower air density also means you need a higher lift coefficient (angle of attack) to produce the required lift, and then you have more lift-induced drag (which goes with the square of the lift coefficient). I think air density more or less washes out when it comes to its impact on range. That being said, you cover the full vehicle range at a higher velocity at higher altitudes, so it certainly seems like there would be significant benefit from a travel-time perspective. All that being said, there are significant high voltage insulation challenges at higher altitudes, which is something we are working on.
  - Retric 6y ago
    Unless I am missing something obvious, lift-induced drag is largely independent of altitude. However, parasitic drag is significantly reduced by lower air pressure. Thus the advantage from high altitude flight.
    - mliben 6y ago
      Less dense air -> higher angle of attack required to produce required lift -> true lift vector is more offset from vertical -> horizontal component of lift is actually producing drag Like I said in the other comment, if the plane is operating at the range-optimal speed, I think the air density does not impact the range capability (it cancels out) but it does increase the range-optimal speed, allowing for faster travel.
gnramires 6y ago
> - A 1% increase in motor efficiency could eliminate 0.1 kg of batteries, which would let you double the weight of the motor. Wouldn't it be much simpler to state that a 1% increase in motor efficiency could eliminate 1% of battery weight? (trying to get the theory clear) --- Obs: This is only approx. valid if efficiency is already high. If efficiency was very low, e.g. 2%, then 1% more (going to 3%) would enable eliminating 1/2 - 1/3 = 1/6 = 16.7% of the batteries. An equation to describe this situation, assuming constant energy need, is Eb = Em / n, where Eb is energy provided by batteries, Em the work of the motor, and n efficiency. Also, the energy need should indeed decrease with decreasing battery weight, amplifying this effect even more, but at high efficiency the correction isn't too large. Equations omitted because there are too many assumptions (acceptable battery mass fractions, energy usage vs weight, ...). (A starting model would be: Maircraft = Mbatteries + Mconst; Mb = aEb; Em ~ Ma^p ; Em = ( k(aEb+Mc) ) ^ 1/p; Is p~=1?; Eb = kMc/(n-a*k); ) So in principle an 1% increase in motor efficiency gives even more than 1% of less battery weight! A complication however is that batteries have power constraints as well as energy constraints (how power constrained . If the peak power only has to be sustained over a very small period, this would allow complementing energy-dense sources (batteries) with power-dense sources (capacitors). However, some power-dense sources do not last long enough to cover the peak-power intervals, so they would not fit. If the following diagram is to be trusted: https://commons.wikimedia.org/wiki/File:Power_vs_energy_density.png https://commons.wikimedia.org/wiki/File:Power_vs_energy_dens... Then for my guess of 5 minute take-off constant peak power time lithium-ion still has the greatest power density, which means other sources should not be combined. You can use variations in chemistry among Li-ion cells to achieve this tradeoff, but those limitations provide a slight negative correction (greater efficiency giving less mass gain). Those effects would need to be combined. Anyway, there is a lot of interesting performance and Operations Research (Linear programming) optimization here.

by tlb 6y ago

audunw 6y ago

> Given that the batteries weigh a lot more than the motors, I would have thought that motor efficiency (which scales battery size) was much more important than motor weight. I guess that depends on what kind of airplane you are making. If you're just making the same kind of airplanes we've been making with ICE, but with electric motors and batteries instead, you're probably right. But if you're making an electric airplane from scratch, there's a lot you can do if you have a really light motor, which can drastically reduce drag. Look at Maxwell X-57 for instance: https://www.youtube.com/watch?v=-HvZ7c0F9ik https://www.youtube.com/watch?v=-HvZ7c0F9ik If you're going to have lots of motors on the wings, they better be as light as possible. I'm guessing the increased efficiency from a design like that can easily be as important as the efficiency of the motor itself.

mliben 6y ago
These are all great points- everything is very interconnected in these vehicles, and there is a lot of potential upside in high power density distributed propulsion (like on the Maxwell). In characterizing the vehicle-level benefit of power density, it is definitely important to consider the X kg of structure required to support 1 kg of motor/inverter/gearbox/etc.
- toss1 6y ago
  Excellent points about associated structures! However for the motors, isn't it the case that the support structure design is dominated by the thrust loads, which should vastly exceed the motor mass? For sure, there is some non-thrust-related structure to react the motor's mass, e.g., inertia from a harsh landing, but how much extra is it? This is much more the case with power/mass optimized motors like yours. Consider an ideal case - you achieve the same power with negligible mass, say 1kg. How much structure in my aircraft using your motor could I really eliminate vs your current model? And the real case, switching from a competitor's similar-power motor to yours, how much additional structure weight can I save by switching, beyond the obvious great advantage of your motor's weight savings? (Obviously, these answers massively depend on other factors, but... )
  - mliben 6y ago
    Yeah, there are a lot of factors that play into this. A couple things come to mind: 1) Considering megawatt-class machines are necessary for many future applications, the mass of the motor+inverter+gearbox (especially using best current technology) definitely adds up. 2) With a very distributed propulsion system, motors that end up near the wing tips have a big moment arm compared to the ones typically tucked under the wing root
    - toss1 6y ago
      Excellent point about the moment arm and control issues! I'm definitely all about maximizing power-weight and strength-weight ratios... I am wondering about the ratios you are achieving, and about the issues of scale. Do things get better as you scale up? I notice you mentioning the state-of-the-art at 3-4 kW/kg, and you shooting for 12 kW/kg. This is even substantially better than small scal T-motor UAV motors at around 7w/g [1]. The chart shows them peaking at 3181W and weighing 453g. So, I'm wondering what scale factors may be working in your favor at your scale vs the single-digit kW scale. Also, any plans to scale slightly smaller (I'm involved in such a project)? [1] https://uav-en.tmotor.com/html/2021/Antigravity_0119/668.html https://uav-en.tmotor.com/html/2021/Antigravity_0119/668.htm...
      - 6y ago

jjsylvestre 6y ago

This is an excellent question. For narrow body aircraft we've studied, they require high propulsive power during the takeoff and climb phases, and a fraction of the peak propulsive power during the cruise phase. One aircraft we looked at required 30-35MW during takeoff and ~10MW during cruise. So, thrust power and system level power density (kW/kg) are critical during takeoff/climb and cruise efficiency is important for minimizing energy consumption. Like Audunw mentions, its very application dependent as well. It all boils down to the propulsion system mass fraction. For lower PSMF, efficiency matters more once you are above a certain power density. For higher PSMF, power density matters more. There is an optimal balance of efficiency vs. specific power for every aircraft. We can "tune" our technology relatively easily depending on what that balance is to maximize range. I'll let my cofounder Max chime in since he does a lot of vehicle-level architecture and optimization. He's been doing some studies for rotorcraft and planes to look at how specific power and efficiency impact range/endurance so I'm sure he can expand on my answer a bit.

mercurywells 6y ago
> One aircraft we looked at required 30-35MW during takeoff and ~10MW during cruise. Do you envision some airframes to include assisted take-off technology? (JATO and the like, even catapults)
- mliben 6y ago
  I think most forms of assisted takeoff technology would reduce electrical power requirements for takeoff, but not put much of a dent in the climb power (unless you can catapult/ JATO all the way up to cruising altitude, which would be challenging). Since climb power is still significantly higher than cruise power, and is effectively thermal steady state for these components (10-20min), it would still drive the propulsion system sizing.
  - hbrav 6y ago
    You could have another aircraft tow you, like gliders sometimes do. Doesn't seem terribly practical though.

serjester 6y ago

Keep in mind that an electric motor isn’t limited by the amount of oxygen in the air. As a result it can fly significantly higher where there is far less air resistance. Since air density is proportional to the square of the elevation this can lead to significant efficiency gains. Believe it or not, partly as a result of this, the SR-71 had it’s best mpg at peak speeds.

mliben 6y ago
A simple physics-based plane model (like the one we made to understand vehicle-level impact of our technology development) dictates that the range-optimal cruise speed is proportional to 1/sqrt(air density), so it makes sense that the blackbird was more efficient at high speed when at high altitudes (admittedly, this simple model is subsonic, and there are a lot of other factors for supersonic flight). Since having lower air density also means you need a higher lift coefficient (angle of attack) to produce the required lift, and then you have more lift-induced drag (which goes with the square of the lift coefficient). I think air density more or less washes out when it comes to its impact on range. That being said, you cover the full vehicle range at a higher velocity at higher altitudes, so it certainly seems like there would be significant benefit from a travel-time perspective. All that being said, there are significant high voltage insulation challenges at higher altitudes, which is something we are working on.
- Retric 6y ago
  Unless I am missing something obvious, lift-induced drag is largely independent of altitude. However, parasitic drag is significantly reduced by lower air pressure. Thus the advantage from high altitude flight.
  - mliben 6y ago
    Less dense air -> higher angle of attack required to produce required lift -> true lift vector is more offset from vertical -> horizontal component of lift is actually producing drag Like I said in the other comment, if the plane is operating at the range-optimal speed, I think the air density does not impact the range capability (it cancels out) but it does increase the range-optimal speed, allowing for faster travel.

gnramires 6y ago

> - A 1% increase in motor efficiency could eliminate 0.1 kg of batteries, which would let you double the weight of the motor. Wouldn't it be much simpler to state that a 1% increase in motor efficiency could eliminate 1% of battery weight? (trying to get the theory clear) --- Obs: This is only approx. valid if efficiency is already high. If efficiency was very low, e.g. 2%, then 1% more (going to 3%) would enable eliminating 1/2 - 1/3 = 1/6 = 16.7% of the batteries. An equation to describe this situation, assuming constant energy need, is Eb = Em / n, where Eb is energy provided by batteries, Em the work of the motor, and n efficiency. Also, the energy need should indeed decrease with decreasing battery weight, amplifying this effect even more, but at high efficiency the correction isn't too large. Equations omitted because there are too many assumptions (acceptable battery mass fractions, energy usage vs weight, ...). (A starting model would be: Maircraft = Mbatteries + Mconst; Mb = aEb; Em ~ Ma^p ; Em = ( k(aEb+Mc) ) ^ 1/p; Is p~=1?; Eb = kMc/(n-a*k); ) So in principle an 1% increase in motor efficiency gives even more than 1% of less battery weight! A complication however is that batteries have power constraints as well as energy constraints (how power constrained . If the peak power only has to be sustained over a very small period, this would allow complementing energy-dense sources (batteries) with power-dense sources (capacitors). However, some power-dense sources do not last long enough to cover the peak-power intervals, so they would not fit. If the following diagram is to be trusted: https://commons.wikimedia.org/wiki/File:Power_vs_energy_density.png https://commons.wikimedia.org/wiki/File:Power_vs_energy_dens... Then for my guess of 5 minute take-off constant peak power time lithium-ion still has the greatest power density, which means other sources should not be combined. You can use variations in chemistry among Li-ion cells to achieve this tradeoff, but those limitations provide a slight negative correction (greater efficiency giving less mass gain). Those effects would need to be combined. Anyway, there is a lot of interesting performance and Operations Research (Linear programming) optimization here.