Question About Electric Aircraft Propulsion

[russ_watters]

I stand corrected. At the Paris Airshow (june 2019) an electric aircraft was offered for sale. 3 engines, propeller driven, 650 miles at 500 miles/hour. 9 passengers. $4 million each. Roughly a dozen orders.

Don't stand corrected until it happens. I flat-out don't believe those specs are possible.

Edit: searching finds some badly written articles giving the impression it could fly 500 mph, but the actual spec calls for 240kt. Still won't believe it until I see it, but it at least it passes the laugh test at that speed claim.
https://en.m.wikipedia.org/wiki/Eviation_Alice

 

[cjl]

Don't stand corrected until it happens. I flat-out don't believe those specs are possible.

Edit: searching finds some badly written articles giving the impression it could fly 500 mph, but the actual spec calls for 240kt. Still won't believe it until I see it, but it at least it passes the laugh test at that speed claim.
https://en.m.wikipedia.org/wiki/Eviation_Alice

That makes sense - I was about to express disbelief at that cruise speed myself. I don't believe a 500mph cruise is possible with current electric motors, at least not in a remotely economically viable design.

 

[boneh3ad]

The key to getting a flyable aircraft is the power/weight ratio of the engine. For Jets, the power outlet depends a lot on the speed, but roughly speaking a Boeing 777 engine (GE 90) puts out about 6 hp/lb (10 Kw/Kg). Very good electric motors can manage power/weight in this neighborhood. Viability depends on having light batteries that hold a lot of energy.

You're forgetting energy storage. Hydrocarbon fuels are currently more efficient than batteries for storing maximum energy in a small volume and weight.

 

[merriam]

You're forgetting energy storage. Hydrocarbon fuels are currently more efficient than batteries for storing maximum energy in a small volume and weight.

Energy storage affects range. Jet fuel has an energy density of about 43 MJ/kg. A fully charged lithium battery can manage about 1MJ/Kg on a good day (maybe half that on an average day). There is a big difference. That means that if your aircraft allocates 1000 Kg for "fuel", you can go farther if you use jet fuel. The electric aircraft referenced above has a range of 650 miles (compare to 777 range of 5000 to 8500 miles) limited strictly by battery capacity (and rules about amount of reserve that must be carried).

But to get off the ground you have to overcome drag (and inertia) and that takes engine thrust. Power is thrust times speed. A typical aircraft has lift = 10 x drag (gliders more, fighter planes less) and the lift has to at least equal the weight of the engine. That's why thrust/weight is so important. Even if the plane is made of super material that weighs nothing, the engines still have to get off the ground.

 

[boneh3ad]

That's all true but was not the point I was making. My point is that just having enough power to lift a plane off the ground is not all it takes to be flyable. So yes, you need electric engines with an appropriate thrust to weight ratio so that you can actually lift off, but you also need to be able to carry enough stored energy that the vehicle can fly a useful distance. If you don't solve both of these problems simultaneously, then what you have is essentially a new age Wright Flyer: intellectually interesting but not particularly useful without substantial continued technological development.

(Note: I am not trying to denigrate the Wright Flyer. I am merely pointing out that an electric airplane is not practically useful unless it can solve both problems, much like the Wright Flyer wasn't practically useful except for demonstrating that powered flight was possible.)

 

[gleem]

Two items have come to my attention that may expedite the development of larger (non hover) electric aircraft.

First, higher energy density LI-S batteries with energy densities of up to 500W/kg. They are cheaper, lighter than current Li batteries and non flammable. They are currently limited in discharge rate as well as cycle life.

Second, superconducting electric motors with power densities of 20kW/kg. Current test motor is 1 MW and scalable to at least 10MW.

 

[Anand Sivaram]

That is the reason why many people are doing research into renewable biofuel based aviation fuel.

That's all true but was not the point I was making. My point is that just having enough power to lift a plane off the ground is not all it takes to be flyable. So yes, you need electric engines with an appropriate thrust to weight ratio so that you can actually lift off, but you also need to be able to carry enough stored energy that the vehicle can fly a useful distance. If you don't solve both of these problems simultaneously, then what you have is essentially a new age Wright Flyer: intellectually interesting but not particularly useful without substantial continued technological development.

(Note: I am not trying to denigrate the Wright Flyer. I am merely pointing out that an electric airplane is not practically useful unless it can solve both problems, much like the Wright Flyer wasn't practically useful except for demonstrating that powered flight was possible.)

 

[cjl]

Energy storage affects range. Jet fuel has an energy density of about 43 MJ/kg. A fully charged lithium battery can manage about 1MJ/Kg on a good day (maybe half that on an average day). There is a big difference. That means that if your aircraft allocates 1000 Kg for "fuel", you can go farther if you use jet fuel. The electric aircraft referenced above has a range of 650 miles (compare to 777 range of 5000 to 8500 miles) limited strictly by battery capacity (and rules about amount of reserve that must be carried).

But to get off the ground you have to overcome drag (and inertia) and that takes engine thrust. Power is thrust times speed. A typical aircraft has lift = 10 x drag (gliders more, fighter planes less) and the lift has to at least equal the weight of the engine. That's why thrust/weight is so important. Even if the plane is made of super material that weighs nothing, the engines still have to get off the ground.

It's worth noting that engine thrust and power are not directly interchangeable concepts. Modern jetliners have vastly more power than they need to get off the ground safely, and the reason for this is the need to fly at a high cruise speed. If you eliminate the high cruising speed requirement, you can change over to propellers with a significantly larger disk area than the exit area of the jet engines, and by doing so, you substantially improve the power to thrust ratio.

This is also why all the electric concepts have multiple motors driving fairly large props along with a relatively low cruising speed - all of those things reduce the total power requirement.

 

[sophiecentaur]

Sophie . . . I don't think that there's no hope for us, just that it mostly lies with the thinking people coming up with technical solutions.

Present experience of the Politics of the world do not support the theory that "thinking people" will be allowed the power to affect things. Extremist rulers tend not to think very far into the future and there will always (however bad things get) be some group of people (robber barons warlords etc. ) who will step in and profit at the expense of a defenceless population. Those kinds of régimes think in terms of just one lifetime.
Even the second amendment would not help in that respect - just speeding up the process of decline. But I do not need to point that out to the majority 'thinking' members of PF.

 

[sophiecentaur]

You're forgetting energy storage. Hydrocarbon fuels are currently more efficient than batteries for storing maximum energy in a small volume and weight.

In the overall picture, this is one of the most relevant facts. It's only when all electric transport energy produces almost no climate effect that the quoted massive ratio can be ignored. An intermediate solution would be to store energy in the form of Hydrogen, which sits somewhere in between. Hydrogen is something that seems to vary in popularity over the years.

 

[boneh3ad]

In the overall picture, this is one of the most relevant facts. It's only when all electric transport energy produces almost no climate effect that the quoted massive ratio can be ignored. An intermediate solution would be to store energy in the form of Hydrogen, which sits somewhere in between. Hydrogen is something that seems to vary in popularity over the years.

Hydrogen and manned aviation have a fraught history, as it turns out.

 

[sophiecentaur]

Hydrogen and manned aviation have a fraught history, as it turns out.

Yes. The shadow of Hindenburg doesn't fade.

 

[cjl]

It also has a very poor density, so even though it has the advantage of being very light, you need massive tanks (and the associated drag and weight penalty) to carry very much of it.

 

[sophiecentaur]

It also has a very poor density, so even though it has the advantage of being very light, you need massive tanks (and the associated drag and weight penalty) to carry very much of it.

The timescale is a bit different for rockets but why not store it cryogenically? That must have ben considered for planes.

 

[cjl]

Even cryogenically the density sucks. LH2 has a density of 71kg/m^3, while kerosene (or Jet A) is around 810 kg/m^3. Jet fuel has 42.8 MJ/kg, so the volumetric energy content of jet fuel is 34.7 GJ/m^3. Hydrogen has 130MJ/kg, but combine this with the low density and it only has 9.2 GJ/m^3, so you need nearly 4 times the tank volume to store identical energy compared to jet fuel.

 

[Klystron]

I'd really like to see a calculation on the volume of lifting gas required to carry 100,000 tons, to replace a cargo ship.

Then I'd like to see the kinetic energy of impact if one breaks apart at 10,000 ft.

My post stipulated an upper limit of one cargo container built from light weight materials similar to cargo space for a typical delivery truck that carry a few tons at most or equivalent 10 passengers with luggage. I fail to see the humor in citing a ridiculous 100,000 tons cargo criteria for a single aircraft. Unless you meant kilograms?

The largest civilian aircraft I have entered a C-5A Galaxy could possibly lift 10E5 Kg under stringent take-off and landing conditions, minimum fuel, minimum crew, perfect weather conditions but so what?

...but the maximum allowable payload was reduced from 220,000 to 190,000 lb (100,000 to 86,000 kg). At the time, a 90% probability was predicted that no more than 10% of the fleet of 79 airframes would reach their fatigue life of 19,000 hours without cracking of the wing.[15]

 

[cjl]

The point of the 100k tons is because that would make it similar to modern cargo ships in capacity, since the suggestion was to replace earthbound transportation with airships (not to replace existing cargo aircraft). You're right that that would be a pretty crazy amount to move by air though, since the largest existing cargo plane can only carry around 250,000 kg of payload.

 

[Klystron]

The point of the 100k tons is because that would make it similar to modern cargo ships in capacity, since the suggestion was to replace earthbound transportation with airships (not to replace existing cargo aircraft). You're right that that would be a pretty crazy amount to move by air though, since the largest existing cargo plane can only carry around 250,000 kg of payload.

Negative. I made no such suggestion. Read the post. While unclear, perhaps, the reference to marine freight was only to compare the requirement of time in transit. Ships are slow relative to jet aircraft. The counter-argument devoid of sarcasm would be that ships carry large tonnage as the weight is supported by water. This argument could lead to comparison of transit times versus payload. Can we state that generally the faster the transportation, the lighter the payload?

Earthbound transportation is unclear as the atmosphere is earthbound.

My reference to LTA and airships was a carry-over from a parallel thread on NASA airships for exploring solar system objects such as Titan and Mars. No 'giant gasbags' mentioned. Amazon, among others, is diverting cargo from trucks to aerial delivery systems, all electric powered AFAIK.

 

[russ_watters]

My post stipulated an upper limit of one cargo container built from light weight materials similar to cargo space for a typical delivery truck that carry a few tons at most or equivalent 10 passengers with luggage. I fail to see the humor in citing a ridiculous 100,000 tons cargo criteria for a single aircraft. Unless you meant kilograms?

The largest civilian aircraft I have entered a C-5A Galaxy could possibly lift 10E5 Kg under stringent take-off and landing conditions, minimum fuel, minimum crew, perfect weather conditions but so what?

It was 6 weeks ago, so I don't recall if I read the post correctly (whether you intended an exact container ship replacement), but I do know my post was way less than 50% sarcastic. Whether what you suggest would take a large number of obscenely enormous airships or an obscenely enormous number of large airships is just two sides of the same coin. So, some related numbers I'd like to see for the other side:

1. How many airplanes are in flight over the US at any given time?
2. How many single container trucks are on the road?
3. What is the global annual helium production volume?
4. What volume of helium would be required to lift that many containers simultaneously?

A quick google tells me that replacing one large container ship with single container airships would roughly double the number of aircraft in the air at one time, globally.

If you're going to propose an outside-the-box idea, you really should do a little work to check on feasibility.

Can we state that generally the faster the transportation, the lighter the payload?

No, I wouldn't say that. There are several constraints affecting payload and transit speed. The constraints tend to be more fundamental to the mode of transport than related to speed/capacity.

 

[Klystron]

OK. Leave it at 6 weeks, sleepless night, mixed threads on my part. The LTA concept was asked on a speculative forum. I must have merged airship with electric propulsion. An interesting concept but I never mentioned helium or gas bags. You are using a form of argument of going to extremes in order to ridicule. ALL ground vehicles. ALL marine tonnage. Largest bulk cargo aircraft ever built.

Other engineers on this thread got it right that current e-propelled aircraft frames are designed for minimum weight. You are stuck in Leviathan. Current electric aerial delivery systems mimic hummingbird.

 

[sophiecentaur]

Current electric aerial delivery systems mimic hummingbird.

Local deliveries by drone could work - except that there WILL be accidents. Get indoors if you ever hear one!

 

[russ_watters]

OK. Leave it at 6 weeks, sleepless night, mixed threads on my part...

Ok...yeah, looking back at the original discussion, you already acknowledged the original description was "inapt". So I'm not even sure why you are circling back to it (I was letting it drop at that).

[first post]
...canny aero engineers should reconsider LTA ships for practical transport including cargo. Time is not a prohibitive factor for tons of cargo and passengers on the water. LTA ships could replace much ground transportation dependent on fossil fuels, even using similar modular containers.

[second post]
The reference to water-borne cargo was meant as analogy, perhaps inapt. I have seen an articulated truck loading three standard containers. Most ground vehicles carry only one or two containers.

I didn't reply to the second, but my reply would have been the same as the one I gave today; a lot of one-container airships or fewer 10,000 container airships are two sides of the same coin/problem.

The LTA concept was asked on a speculative forum. I must have merged airship with electric propulsion. An interesting concept but I never mentioned helium or gas bags.

Now I don't follow: if you didn't mean helium or gas bags, what does "LTA" mean? As far as I know, that's the only way to achieve it.

You are using a form of argument of going to extremes in order to ridicule. ALL ground vehicles. ALL marine tonnage. Largest bulk cargo aircraft ever built.

I am not intending that, nor did I say most of those -- indeed, I think my examples substantially understate the scale problem in what you are suggesting. Most of my examples were for replacement of a tiny fraction of our shipping (one container ship). Asking how many trucks are on the road was more for scale -- I have no idea how many shipping containers are in transit at anyone time. And on the other end, saying how many planes were in the air wasn't to say I think you're talking about replacing all planes, but using that number to show how unworkable what you propose is, to replace even one container ship. But if you have a more specific idea in mind as to what "much ground transportation" means, please say so. 1%? 10%? 40%? If you don't say what you mean, what choice do I have but to guess what you mean?

 

[Klystron]

Thanks. I finally remember my point in returning to this thread. Not as important or profound as I thought but here is my reasoning.

Around the time this type of electric aircraft was reported in the media, my family noticed a grocery delivery truck with an enormous fairing stretching from the tractor cab over part of the top of the refrigerated cargo section high balling through the Mojave desert on a lower section of highway.

My artist granddaughter commented on the "decorative panels" and thick coils running under the fairing into the refrigeration unit. My engineer granddaughter recognized solar panels on the fairing and uncovered area of the roof of the cargo container from school projects and working with her dad. Were the panels providing electricity from solar to help power the refrigerators?

Discussion led to comparing the super-light solar powered aircraft with broad wing geometry carrying two passengers to a speculative electric delivery truck with batteries recharged by the sun between roadside charging stations.

What was the optimum "solar area" for an e-truck? An e-train? E-car?
Should the truck "follow the Sun" as the solar aircraft was reported to have done?
How efficient were the solar panels versus weight and size?
Instead of frozen groceries, should our hypothetical E-truck just carry solar panels to market...?

 

[sophiecentaur]

"much ground transportation"

much ground transport can be achieved on the ground on rails. Very efficient and nothing to drop out of the sky. Sea transport costs more to run and is slow but, as in the days of canal transport, a delay is not necessarily a problem if the arrival rate is high enough. The safety aspect makes sea transport attractive, too. Except when approaching land, engine failure just means you stop. Near land, you may need rescuing but that probably has a timescale measurable in hours.
Passenger transport is a different matter but, as I have remarked several times in the past, there are alternatives to many journeys by plane. Modifying lifestyles is not an unthinkable concept if the result is to reduce 'carbon' footprint.

 

[merriam]

In the overall picture, this is one of the most relevant facts. It's only when all electric transport energy produces almost no climate effect that the quoted massive ratio can be ignored. An intermediate solution would be to store energy in the form of Hydrogen, which sits somewhere in between. Hydrogen is something that seems to vary in popularity over the years.

Actually, Hydrogen has an energy density (per kg) of approximately 3 times that of jet fuel (varying slightly with temperature) and at least 100 times that of Lithium polymer batteries. However, it is tough to use. Among its drawbacks: Takes a lot of volume store as a gas, though hydrogen fuel cell cars (like Toyota Mirai) have a range comparable to that of gasoline vehicles 2) hydrogen tends to degrade any metal it comes in contact with (see hydrogen embrittlement) though this may be overcome with coatings and liners. 3) it is explosive at all concentrations between 5% and 95% (safety issue). The most serious however is that hydrogen gas contains some molecules traveling at escape velocity, so it might depart the Earth's atmosphere (as helium does). Normally it is attached to oxygen (or carbon) so this isn't a problem, but in a hydrogen economy it could be a problem.

 

[cjl]

The most serious however is that hydrogen gas contains some molecules traveling at escape velocity, so it might depart the Earth's atmosphere (as helium does). Normally it is attached to oxygen (or carbon) so this isn't a problem, but in a hydrogen economy it could be a problem.

Nah, that isn't an issue at all. We have so much hydrogen available that the small amount lost to atmospheric escape is completely irrelevant.

 

[256bits]

What was the optimum "solar area" for an e-truck? An e-train? E-car?
Should the truck "follow the Sun" as the solar aircraft was reported to have done?
How efficient were the solar panels versus weight and size?
Instead of frozen groceries, should our hypothetical E-truck just carry solar panels to market...?

If the top roof area of an 18 wheeler was covered with solar panels, expect about 5 kW production bright sun overhead, and then decreasing to 0 at night. Considering that a reefer ( the refrigeration container ) will produce about 10kW of refrigeration, and that depends on ambient / interior unit temperature difference, one comes up a bit short even at peak sunlight. One could thicken the insulation to reduce the cooling load, if one live and make a profit with reduced cargo carrying capacity compared to the competitors running on current technology. People are probably working on it. Perhaps you saw a test model.

Are you sure the fairing was not just there for reduced aerodynamic drag - very common these days.
I don't know what the hoses would be except the hookup from the tractor to trailer for lighting and braking for an 18 wheeler if that is what it was.

 

[Klystron]

If the top roof area of an 18 wheeler was covered with solar panels, expect about 5 kW production bright sun overhead, and then decreasing to 0 at night. Considering that a reefer ( the refrigeration container ) will produce about 10kW of refrigeration, and that depends on ambient / interior unit temperature difference, one comes up a bit short even at peak sunlight. One could thicken the insulation to reduce the cooling load, if one live and make a profit with reduced cargo carrying capacity compared to the competitors running on current technology. People are probably working on it. Perhaps you saw a test model.

Are you sure the fairing was not just there for reduced aerodynamic drag - very common these days.
I don't know what the hoses would be except the hookup from the tractor to trailer for lighting and braking for an 18 wheeler if that is what it was.

Right you are about each detail. +10.
As driver I only glanced at the grocery truck. The side I saw was encased in removable film announcing experimental delivery technology for regional stores (now owned by Kroger). Most likely a test model, as you say.

The front fairing looked enormous, certainly to reduce aerodynamic drag and protect the film. In retrospect the cable bundles and coils were probably part of the reefer-truck mechanism but also for radio communication. If memory serves, I saw a tall whip antenna for citizens band (CB), an AM/FM antenna and possibly (?) some kind of compact microwave rig -- like a Lewis antenna but smaller.

My (artist) granddaughter corrected my recollection. She liked the irredescent colors of the truck. My daughter and her daughter, who had some experience with home solar panels, noticed what they thought were solar cells on the trailer but not on the cab. Certainly that section of the Mojave high desert would be ideal for testing solar-augmented reefer trucks. The road alert signs, some lights and all emergency phones have solar panels that charge during the day. Thanks.

 

[zekise]

Pretty much all modern turbofans run with a blade tip speed at full throttle of mach 1.5 or so.
4MW isn't anywhere close to enough. Propulsive power at cruise for a modern jetliner the size of a 787 is more on the order of 40MW. At takeoff, each engine has to be making more like 50-60MW of shaft power just to run the front fan. Rerun the electric numbers knowing that and you'll see why running jetliners on electric power is a pipedream without a massive breakthrough in technology.

I think you need about 10 tons of thrust per engine during cruise for a 737. If the thrust to power ratio of an electric fan is 6 kg/kW, then you need 10,000 / 6 = 1,660 kW or 1.7 MW per engine. This assumes 95% motor efficiency and 90% propulser efficiency. Developers are building 0.75 MW motors. So four of these will do.

 

[cjl]

I think you need about 10 tons of thrust per engine during cruise for a 737. If the thrust to power ratio of an electric fan is 6 kg/kW, then you need 10,000 / 6 = 1,660 kW or 1.7 MW per engine. This assumes 95% motor efficiency and 90% propulser efficiency. Developers are building 0.75 MW motors. So four of these will do.

You actually don't need anywhere close to 40klb of thrust at cruise for a 737 - that's about a factor of 4 or 5 high. In reality, a 737 might have between 7000 and 10,000 pounds of drag (and thus thrust) during cruise, depending on gross weight, which 737 variant it is, etc.

Unfortunately, though, your thrust to power ratio is also way, way too high. At cruise, a 737 engine might be ingesting around 300kg/s of air at 250m/s. To generate 20kN (4500 lb) of thrust (since it has 2 engines), this air will need to be accelerated to 317m/s by the engine. This involves adding about 11MW of power to the air jet, so using your propulsor efficiency value of 90%, we need 12.5MW of shaft power per engine to generate 4500lb of thrust, for a thrust to power ratio of less than 2N/w.

The reason the power level needs to be so high is because of the high speed and the low density of air at altitude, resulting in a fairly low massflow. With a higher massflow (larger prop disk size, lower altitude) and lower airspeed, you probably could get a 737 to fly on only 3 or 4MW, but you'll never get anywhere close to the altitude or airspeed without 20-30MW.