Electric Powered - High Performance Design

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Tony Williams

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I would like to think out loud about an aircraft design:

1) All-electric power
2) Two tandem seats
3) 10,500 pounds MTOW
4) 700kWh batteries = 7000 pounds
5) 700kW takeoff power
6) 25,000 feet ceiling
7) Two engines / two propellers
8) Pressurized
9) Anti-icing
 

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What kind of range and useful load would you be anticipating? It looks like it would be a huge aircraft for a really modest payload. It may be better to hedge your bets on battery technology advances that are anticipated to happen in the next few years that would allow a smaller more conventional airframe that would be easier and cheaper to design and build with smaller lighter (although currently theoretical) solid state batteries. I do like the Piaggio design and your concept, but if you're trying to meet a specific mission with battery electric it may be a faster route to the finish line to allow the battery technology a few more years to mature. This is easy for me to say though as I cannot start anything for a couple of years anyways
 

Dan Thomas

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Harbour Air to resume electric-powered Beaver flights as certification work begins
An excerpt:

The demonstrator Beaver carried 135Wh/kg lithium batteries – a relatively low-density battery that, while close to aviation standard, generally lacks sufficient power-density for viable commercial operations, McDougall says.


Today’s better lithium batteries generate up to 235Wh/kg, but McDougall expects 400Wh/kg batteries will be available by the time Harbour starts passenger flights.


Harbour expects to deploy its electric aircraft on passenger flights of about 30min, with 30min of power reserve – sufficient for many of its routes from Vancouver to small communities.


It would seem that 700kWh batteries will be a fantasy for some time. Maybe I'm wrong.

Pressurization and anti-icing both consume considerable energy, especially anti-ice. And cabin heating must be provided as well. All of that eats into the range pretty seriously. Ambient temperatures at 25,000 feet can be in the -50°F range.


7000 pounds of batteries in a 10,500 lb gross airplane? Add the airframe and engine weight, and what is left?
 

Tony Williams

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=PIAGGIO (blue) is VERY LOUD ! (when it take-off in west site of Krakow,we hear them in ost site
of city !!!)
Yes, it may be loud! Those egg beaters have to be able to handle mid-altitudes at relatively high speed (maybe 300 kts).
 

Tony Williams

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What kind of range and useful load would you be anticipating? It looks like it would be a huge aircraft for a really modest payload.
Yes, it requires a very large and heavy battery to get anywhere. In the next ten years, I can only hope the battery weight drops significantly, but we have to work with "today" hardware. Range is wholly consistent on energy onboard and consumption rate. I predict a one hour endurance, with 30 minutes reserve.

So, I while I can't give you a nautical mile range (because I don't know how fast it will go), but I can guessimate that with a 700kWh (7000 pound) battery that consumes:

30 min reserve (175kWh required @ 350kW "cruise speed", 525kWh remaining)

CLIMB - 700kW on takeoff to the top of climb within 15 minutes (175kWh consumed, 350kWh remaining)

CRUISE - 350kW cruise for up to 60 minutes (350kWh consumed, 0kWh remaining)

0kW descent and landing within 15 minutes

90 minutes flying with this profile, leaving 30 minute reserve. If I can average 200 kts TAS, I might be able to travel 300 miles in 0 wind.

It may be better to hedge your bets on battery technology advances that are anticipated to happen in the next few years that would allow a smaller more conventional airframe that would be easier and cheaper to design and build with smaller lighter (although currently theoretical) solid state batteries.
Those future improved batteries will be used to increase range and endurance. The airframe won't get lighter.
 
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Tony Williams

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Today’s better lithium batteries generate up to 235Wh/kg, but McDougall expects 400Wh/kg batteries will be available by the time Harbour starts passenger flights.


Obviously, battery weight and size density will improve. Any improvement will be used to extend the duration and range of the existing airframe.

It would seem that 700kWh batteries will be a fantasy for some time. Maybe I'm wrong.
There is 100kWh underneath my Tesla car. The battery weighs over 1000 pounds, so I'm already pushing the technology to get 700kWh in 7000 pounds. The single biggest hurdle isn't going to be a 10,500 pound MTOW airframe, but instead the issue of how big does the wing and fuselage need to be to hold all the batteries. I estimate 4 cubic inches per kWh.

I think for a prototype, I'd only install about one third to one half the batteries (250-350kWh), and the rest would be ballast for the missing batteries.

Pressurization and anti-icing both consume considerable energy, especially anti-ice. And cabin heating must be provided as well. All of that eats into the range pretty seriously. Ambient temperatures at 25,000 feet can be in the -50°F range.
It will be a small cabin, so heating and cooling won't take much. A regular automotive style heat pump from a Prius type car should work. It would also require a purely resistance auxiliary heater. Maximum 2-3kW during a 60-90 flight. Easy peasy.

Anti- and de-ice are an entirely different issues, which I believe needs to be alcohol on the wing leading edges. A few gallons of alcohol, and a few small pumps that take very little power (air pressure might be the first power source). No de-ice would be needed below -40C.

7000 pounds of batteries in a 10,500 lb gross airplane? Add the airframe and engine weight, and what is left?
Payload is only 500 pounds, for two people. Enough for two normal dudes and backpacks for a 60-90 minute flight. It's perfect for me. If, in the future, I can get a 7000 pound battery with 1400kWh, it would just become a 2-3 hour endurance plane.

I'm probably being overly optimistic about the airframe weight, but the extremely heavy batteries need to be in the week (which lends more to the Starship type wing than the Avanti wing).
 

Tony Williams

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This design may require "tip tanks" of batteries. I just can't put thousands of pounds of batteries in a fuselage. If 1000 pounds could go in each tip tank, and 2000 pounds in each wing, only 1000 pounds needs to be in the fuselage.
 

Tony Williams

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1000kW = 1341.02 hp (two motors, less than 5 minutes)
700kW = 938.715 hp (two motors, max continuous)
500kW = 670.511 hp (one motor, less than 5 minutes)
350kW = 469.358 hp (one motor, max continuous)

How fast can this machine go at 15,000 feet with 470hp? It should climb quite well, and just as importantly, climb on one motor. It's relatively easy to overboost an electric motor for a short period, many 60 seconds at 500kW on one motor?

My hope is 200kts indicated, 250 TAS at 15,000 feet
 
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Tony Williams

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Pressurization seems like an modest achievement next to the power storage issues. Good luck I would like to see what the solution to some of these hurdles looks like.
Yes, energy storage and weight is number 1 and 2 issues that all other aspects of the design have to work around and with.

I think that there are three tricks to pressurization:

1) An extremely tight seal for the cabin.

2) Maybe an air tank with 3500 psi of air

3) An electric powered air pump
 

Tony Williams

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Because this plane is landing at MTOW every time, the landing gear needs to be capable of a 10,500 pound plane.
 

Mad MAC

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Airframe layout would be driven by how one deals to the issues relating the battery's (charge in place or removable, liquid cooling or air cooling, fire control). You are looking at 3000 lbs for the remaining empty weight (structure, engines, props and systems), so the structural weight is what 1500 lb.
It needs more engine power, for electrics range is greatly increased by altitude, 700 kw gives it a lower power loading than a stock Cessna Caravan, so climb will be poor (engine failure risk of current electric engines is about the same as IC without redundent battery control system).

Does it need a weeping deice system, where is all that heat from the batteries going to be dumped.

Carbon fibre structure in a typical configuration is probably not going to cut it weight wise. Maybe a pure delta config or something like the Avro Vulcan is worth consideration, fuselages and wings are heavy.
 

Dan Thomas

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Yes, energy storage and weight is number 1 and 2 issues that all other aspects of the design have to work around and with.

I think that there are three tricks to pressurization:

1) An extremely tight seal for the cabin.

2) Maybe an air tank with 3500 psi of air

3) An electric powered air pump
Pressurization doesn't work like that. Air is ducted into the cabin at a constant rate and let out via the control and emergency valving. You have to keep the occupants oxygenized, and simply pressurizing a closed tank will result in hypoxia.
And you need an oxygen tank and masks in case the pressurization fails at altitude.
 

Dan Thomas

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Does it need a weeping deice system, where is all that heat from the batteries going to be dumped.
Heated anti-ice consumes a LOT of heat. Think of the leading edges and windshield moving through air that's as much as 20°C below freezing, saturated with moisture, at 200 MPH or whatever, and trying to raise the surface temperates up to above freezing.
 

Tony Williams

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Airframe layout would be driven by how one deals to the issues relating the battery's (charge in place or removable, liquid cooling or air cooling, fire control).
To optimize things, all the extra hardware and weight from removable batteries is not in the cards. The battery will be charged in the airframe, just like an electric car. The cells need to be liquid cooled for safety. Fire control is cells in various locations between ribs and spars that are sealed and can be flooded with water. That’s the only safe way to put out a lithium fire.

You are looking at 3000 lbs for the remaining empty weight (structure, engines, props and systems), so the structural weight is what 1500 lb.
I estimate the motor with a 10:1 reduction gear box at 350 pounds each, plus 100 pounds for each prop. With miscellaneous hardware, about 500 pounds per motor prop assembly.... 1000 pounds per airframe.

1000 pounds = (2) 500kW liquid cooled motors + inverters + gear boxes + props
7000 pounds = battery at 700kWh
2000 pounds = airframe, with landing gear
500 pounds = crew + bags

It needs more engine power, for electrics range is greatly increased by altitude, 700 kw gives it a lower power loading than a stock Cessna Caravan, so climb will be poor (engine failure risk of current electric engines is about the same as IC without redundent battery control system).
I’ll have to go look at Caravan numbers. Yes, must get aircraft to altitude (15k-25k feet) to get any range.

Does it need a weeping deice system, where is all that heat from the batteries going to be dumped.
The cells aren’t going to make much heat, particularly on descent when they won’t make any at all. There just isn’t going to be much waste heat to consider. The motors and inverters will make some serious heat, however, but again, not in descent.

Carbon fibre structure in a typical configuration is probably not going to cut it weight wise. Maybe a pure delta config or something like the Avro Vulcan is worth consideration, fuselages and wings are heavy.
Yes, I’m probably GROSSLY underestimating fuselage weight. A delta wing would allow room for the battery cells. Obviously, any “extra” weight is bad, like all aircraft designs.
 

Tony Williams

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Pressurization doesn't work like that. Air is ducted into the cabin at a constant rate and let out via the control and emergency valving. You have to keep the occupants oxygenized, and simply pressurizing a closed tank will result in hypoxia.
And you need an oxygen tank and masks in case the pressurization fails at altitude.
I was initially thinking O2 tank for “mouth breathers”, and pressurized cabin for comfort. I honestly have no idea how much air needs to be exchanged. I need about 10 psi for a 25,000 foot altitude at sea level cabin pressure. Once I know how much air to move, and use 10 psi, we can calculate power required to maintain that with electric motor air pumps. Or, we can steel the power mechanically from the two main motors. That’s less hardware weight, and we don’t need pressurization for ANY takeoff (no loss of power, mostly for an engine failure past V1).
 
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