# Question About Electric Aircraft Propulsion

#### cjl

The compressor /burner behind the duct ed fan spin at high rate. The fan must keep the tip of blades below sound speed.
Pretty much all modern turbofans run with a blade tip speed at full throttle of mach 1.5 or so.

Latest jet engines have a reducing gear from the compressor shaft to the fan ( just like turboprops).An electric motor will spin the fan ,with practically 100% efficiency, without any gear.A Dreamliner burns 1.3 Kg/sec.That is 50 Mj/sec ( or Mw). My guess is that 4 Mw electric will be needed operating the fan from an electric motor from a battery. weighing 120 Tons (6 hours flight). The plane will burn 28 tons of fuel.280 passengers @60 Kg/passenger is 17 Tons. Conclusions: 1)I am not suggesting to convert a Dreamliner to e drive,2)batteries are no more than an order of magnitude away from intermediate range, subsonic aircraft for passenger flight.
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.

#### gmax137

Pratt & Whitney said:
Overall efficiency here refers to the efficiency with which the engine converts the power in the fuel flow to propulsive power. It is the product of thermodynamic efficiency of the process that converts fuel flow power to shaft power (herein called motor thermodynamic efficiency) and propulsive efficiency (the conversion of shaft power to propulsive power).

The most efficient commercial aircraft gas turbines in service or entering service in this decade have takeoff thrusts of 20,000 lb and above. These turbines operate at cruise, with motor thermodynamic efficiencies of up to 55 percent and propulsive efficiencies of well over 70 percent, yielding an overall efficiency (the product of the two) of about 40 percent

#### essenmein

Just for reference we work with reasonably state of the art electric machines (automotive), so a stake in the ground:
55kW peak, 35kW continuous liquid cooled PM 6ph machine, 22k rpm max, 135mm stator OD, 120mm stator stack length, weight about 12-15kg (estimated weight, too much other stuff connected to it to measure machine on its own...). This is at a reasonable limit for air gap, air gap flux and demag on relatively cost effective PM material.

If a number mentioned earlier is correct (100k Hp for dream liner turbo fan), then this is about 75MW e machine, and if built using similar tech as the PM machine mentioned, then you're looking at about 22500kg machine. I don't know what a core of a turbofan weighs, but I somehow doubt its 22 tones... Thats using the 55kW peak number not the 35kw continuous.

In cars you can play games with big short term numbers, unlikely for example you'd sit at full throttle for more than 10-20sec continuously, compared to a plane or boat, the power is needed for much longer periods (eg climbing to altitude) so relying on thermal mass is not really going to work, at least the air is cold up there...

#### cjl

You could probably get away with more like 50MW, and you only really need about a minute of full throttle capability, so you might be able to pull that mass down a bit. That having been said, an entire GEnx engine (including the fan and nacelle, which you'd still need for the electric) only weighs about 6 tons, so you're still at a massive weight disadvantage there.

#### essenmein

Yeah and thats just the metal in the machine, stator steel, PM, wire, bearings etc, you'd still need to add the control electronics etc, not huge numbers there, but they are not zero weight.

#### essenmein

And when people say "the technology needs to catch up", a lot of the time that is just wishful thinking, the laws of physics just get in the way.

If we are talking electric machines, then the tech is pretty stable, they are basically determined by F=BIL, B is fundamentally limited to about 1T in the air gap due to the magnetic material limits (B sat of stator steel, demag of the PM material, bearing tolerance etc), I is limited by the capacity to cool a wire and if PM then demag also will limit current, then L is a physical dimension (length), so if B and I are limited by existing materials/physics then all you can do is change L, ie larger size. I think its highly unlikely we'll discover a magic bullet around the B sat limit and the demag issue with permanent magnets...

#### merriam

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.

Electric aircraft exist, generally as small, experimental, short range aircraft. However, the Solar Impulse 2 has circumnavigated the globe (powered by solar cells). A number of projects are exploring hybrid aircraft, using the electric motors to increase the takeoff thrust, but turning them off in flight.

The power required for supersonic flight more or less precludes electric supersonic aircraft irrespective of the jet/propeller question.

Regarding jets and flight speed, the key is to look at the diameter of the column of gas coming out. You can have a large diameter at low speed or a small diameter at high speed. For the same level of thrust, low speed/large diameter takes less power. Example: A Harrier jump jet (hovering) vs any helicopter (hovering). The former has (4) small diameter jets of gas and requires a lot of power to hover. The latter has a big blade circle (and a big jet of air going down) and requires a lot less power. Top speed of helicopters is under 200 mph though, while the Harrier is over 600 mph.

That's why commercial aircraft have high bypass engines. Effectively they increase the diameter of the blade circle. This improves the gas mileage and lowers the noise. However, the top speed in level flight is limited to the speed at which the air comes out the back - the higher the bypass, the lower the top speed. Since all commercial aircraft (since Concorde) are subsonic (M = 0.85), the bypass is optimized for this speed. The fans are powered by the middle of the jet (so called hot section) where all the fuel burning happens. This de-energizes the exhaust from that part and makes it a lot quieter.

In summary, there is a long way to go in electric batteries and motors, before they enter commercial service (let alone military service).

#### OCR

Top speed of helicopters is under 200 mph. . .

You are real close, and for all practical purposes I agree. . . .

But, to get every thing right on the money. . . lets say 249.09 mph . .

It's rather amazing. . . the record still stands as of this year!

.

#### merriam

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.
Lift is about 1 gram/liter. Lets call it 100,000 metric tons or 100 million Kg = 100 billion grams = 100 billion liters. Roughly speaking, a cube that is 1500 feet on a side.

#### merriam

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.

Electric aircraft exist, generally as small, experimental, short range aircraft. However, the Solar Impulse 2 has circumnavigated the globe (powered by solar cells). A number of projects are exploring hybrid aircraft, using the electric motors to increase the takeoff thrust, but turning them off in flight.

The power required for supersonic flight more or less precludes electric supersonic aircraft irrespective of the jet/propeller question.

Regarding jets and flight speed, the key is to look at the diameter of the column of gas coming out. You can have a large diameter at low speed or a small diameter at high speed. For the same level of thrust, low speed/large diameter takes less power. Example: A Harrier jump jet (hovering) vs any helicopter (hovering). The former has (4) small diameter jets of gas and requires a lot of power to hover. The latter has a big blade circle (and a big jet of air going down) and requires a lot less power. Top speed of helicopters is under 200 mph though, while the Harrier is over 600 mph.

That's why commercial aircraft have high bypass engines. Effectively they increase the diameter of the blade circle. This improves the gas mileage and lowers the noise. However, the top speed in level flight is limited to the speed at which the air comes out the back - the higher the bypass, the lower the top speed. Since all commercial aircraft (since Concorde) are subsonic (M = 0.85), the bypass is optimized for this speed. The fans are powered by the middle of the jet (so called hot section) where all the fuel burning happens. This de-energizes the exhaust from that part and makes it a lot quieter.

In summary, there is a long way to go in electric batteries and motors, before they enter commercial service (let alone military service).
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. #### russ_watters Mentor 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.

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cjl

#### 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.
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.

Gold Member
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.

Gold Member
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.

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#### sophiecentaur

Gold Member
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

Gold Member
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.

Gold Member
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

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

#### 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

Gold Member
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.

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