Applying Tesla technology to aviation - a 100 seat aircraft with propellers driven by electric motor

[Peter Egan]

While the aviation industry is starting with 1 to 4 seat electric aircraft to mature technology, an indicative specification for a plug-in hybrid version of an industry standard 100 seat regional propeller powered aircraft could look as follows below. Telsa technology would be used in the battery system and in the ground 'drone' used to power the aircraft while taxiing to the runway.

In the attached image of a modified CRJ1000, I have left the turbofans in place to compare with the new electric drive layout.

The aircraft has a 100 km electric range. However, the batteries are mainly used on take-off with the turboshaft-generator APU contributing power on take-off and fully powering cruise flight, aircraft services and battery recharging.

The 100 km electric range, along with aircraft glide capacity, is kept in reserve in the event of turboshaft engine failure. The aircraft has one jet fuel burning turbine compared to three for conventional 100 seat aircraft - a large cost saving and a big start on the journey to fully electric aircraft. 800 kWh of usable battery storage for the 100 km range is expected to weigh 4 tonne. An electric range of 500 km for a 100 seat aircraft is likely some decades away.

This aircraft would compete with the 78 seat ATR72-600 and 86 seat Q400 turboprops and 100 seat CRJ100 on short hall routes.

100 seat plug-in hybrid electric aircraft – indicative specification
Cabin – 26 seat rows with two 2 toilets, two front doors, 4 emergency exits over wings
Cabin – 24 m long, 2.65 m wide, aisle headroom 1.90 m
Cargo hold – 20 cu.m, 4 m long, behind cabin
Cockpit – 4 m long
Tail – 4m long – houses propulsion equipment
– elevators/horizontal stabilisers mounted on propeller motors frame
Fuselage – 2.8 m wide, 3.0 m high, 32 m long
Wing area – 70 sq.m
Wing span – 27 m
Payload – 10.0 tonne including 3 tonne Cargo
Fuel – 4.0 tonne, 5000 litres
Typical operating empty weight – 20 tonne including 4 tonne flight power batteries
Maximum take-off weight (MTOW) – 32 tonne (2 tonne less aircraft with max. payload and max. fuel)
Flight battery – 800 kWh (2880 MJ), 4.0 T, with 40 x 100 kg modules each with 1152 x 70200 NCA Tesla cells
– module – 1152 cells in series, 1 cell parallel, each cell 18.36 W.hr, 3.6 V, 5.1 Amp.hr
– module – 4147 V (nominal)
– module – 1152 x 18.36 W.h = 21.15 kW.hour (nominal)
– battery – 21.15 kW.h x 40 = 846 kW.h (3046 MJ, 3GJ) – 5.75% above nominal capacity
Turboshaft APU – 1 tonne (including 483 kg PW127TS):
– Pratt & Witney PW127TS – 1864 kW class with 2386 kW take-off power ((813 H, 686 W, 1626 L, mm))
– 2 x 1000 kW generators, each with 1200 kW take-off capability
2 x motor/counter-rotating propeller units, each with:
– 2 x permanent magnet electric motors, 1200 kW continuous (1400 kWx4 = 5600kW for take-off),
weight 4x280 kg, 1120 kg, 4147 Volt (nominal), 337.6 Amp (likely rated to 400 amp)
All wheel drive (in-wheel motors) for taxiing and regenerative braking
Ceiling – 25,000 ft, 7620 m
Range at MTOW – 2500 km at cruise 301 kt, 558 kmh, 155 m/s
Range - battery (APU fails) – 100 km plus 10 km reserve plus altitude glide reserve plus battery reserve
Flight time without APU – 12 minutes at cruise plus reserves
Fuel economy Jet A-1 – 1.6kg/km average, 1.5kg/km cruise – 37% energy conversion to thrust
Fuel economy - Battery – 7.27 kWh/km (26.2 MJ/km) – 90% energy conversion to thrust
Take-off run – 1400 m
Price – US$30 m

At large airports with long taxi paths, a ground drone (cart) with Tesla driverless technology, and 200 kWh battery capacity (for numerous taxi runs before recharge), will power aircraft out to runway, disconnect and return to terminal. Planes will generally land with enough electricity stored to reach terminal. If needed, the plane could restart its APU.

Propellers set high on tail to be well out of the way in the event of tail strike on take-off.

Aircraft will recharge its batteries by plugging-in while being serviced at terminal gate – 1000 kW charger needed.

Aircraft accelerates faster down the runway as electric motors reach full power much faster than jet engine.

APU started at push-back, or at ground drone detachment where they are available. APU has sufficient power to recharge battery while at cruise.

Aerodynamic drag, rather than aerodynamic lift to carry aircraft weight, is the prime user of power at cruise. The weight of the batteries is mainly a concern at take-off.

25% improvement in fuel economy expected compared to 86 seat Bombardier Q400 turboprop.

Aircraft construction has ‘armoured cockpit’ and ‘propulsion unit tail’ constructed separately and bolted to hull. Fuel lines and power cables run in separate ducts on underside of hull for ease of construction and maintenance. Fuel stored in wings, flight batteries under floor ahead of the wings.

Plug-in hybrid systems could be trialled on a Bombardier Q400 with the electric motors/propellers mounted on the wing in place of the PW150A engines, with the PW127TS turboshaft engine and generators replacing the UTC Aerospace Systems APS 1000 in the APU bay.

 

[Peter Egan]

On review, it would be better to use a pair of PWC 980A APUs from the A380 as power generators. These can each generate 1350 kW, and would be paired with a single electric motor. Two fully independent jet-fuel powered electrical electrical circuits, that can feed power to the batteries and the electric motor powered propellers, gives better safety and therefore better emergency range and energy use in flight. The 40 flight battery modules would be grouped into 4 independent batteries with a capacity to isolate any module with thermal or other issues. Thus, their will be six separate sources of power. The APUs would need to sit in separate fire-insulated compartments on either side of the tail. This would cut costs.

This means the batteries can be run down on take-off and climb to cruise, partially recharge during cruise, and possibly regen charge when descending to land. The plane would use battery power to taxi back to the terminal. With two independent APUs, battery power can also be used to taxi from terminal to take-off, but it would be better use an autonomous ground drone to power the aeroplane out to near the runway, to ensure the plane takes off with its electrical system fully charged.

Tesla has announced its new cells are 21700 in size, not 20700 as speculated by industry, and may have 5.5 Amp.hr capacity. However, as the above is a first-pass specification, its not a significant change to warrant changes to other elements of the specification.

 

[wdolson]

We will have experimental electric aircraft here and there, but a practical electric, or even hybrid electric aircraft is a long ways away.

In aviation weight is critical to everything. It's vastly more important than with cars. Jet fuel has about 34X the energy density that batteries currently do and the jet fuel goes out the tailpipe as you fly so the plane gets lighter. Fully charged batteries weigh the same and empty batteries.

On commercial aircraft, payload is everything. If the airline isn't moving payload in the form of passengers and cargo, it isn't interested. 100Km range on an aircraft is pretty minimal. The CRJ100 today has a range of 3000Km, the LR has a range of 3710 Km. With the batteries taking up the front cargo hold, and the short electric range no airline would have any interest in it.

Carrying the extra weight and complexity of 4 engines is another no go for airlines. To get FAA certification, aircraft engines need to go through some of the most stringent tests in any industry. Because of this, they are very expensive. The engines on a 747 cost $10 mil USD a piece 20 years ago. I don't know what they cost now. Carrying 4 engines on such a small plane adds weight and complexity airlines don't want to mess with.

It's a novel idea and maybe OK as an experiment/proof of concept kind of thing, but electric aircraft are a long ways from commercial viability. I worked as a design engineer at Boeing and saw the sausage being made for the 777 program. I was a very tiny cog in a very big project, but on a program like that you hear all sorts of stories from other departments.

The FAA throws disaster scenarios at the engineers and if they can't prove that disaster is a low enough probability event (winning the lottery odds are "too high" for the FAA), the engineers need to go back the drawing board. One scenario they threw at the engine design guys was an engine fan explodes on one side of the plane and one of the blades goes through the passenger cabin and takes out the engine on the other side. This is something that hasn't remotely happened in the entire history of aviation (though a TWA DC-10 had a fan explode on the center engine which took out the hydraulic system for the tail controls.)

They couldn't prove this was a low enough probability event so the engine design team had to line the inside of the engine nacelles that face the passenger cabin with armor plate. That added weight and reduced the range of the aircraft but the FAA didn't care.

In the conceptual stages are lithium-air batteries that have the potential to be very light as well as very high density. They are experimenting with the concept in labs now, but they are a decade from production at minimum, more likely 2-3 decades. The materials would work well together, but nobody has a clue how to hold them in any controlled space to do the job. Gasses are rather hard to keep in one place for a long period of time. A liquid or a solid is much easier to keep in one place.

 

[Saghost]

I'm skeptical - you'd have to be enough more efficient in converting jet fuel into thrust to offset the extra weight of the new drivetrain - even after the double conversion to/from electricity (though you may need a little less thrust from reduced drag, too.)

Also, if I'm reading it right, you're proposing to use ~4kV DC? What insulation do you have planned? I'm not an expert on the subject, but I thought voltages in that range were mostly dealt with by big air gaps and stacks of ceramic insulators.

More electrification and some hybridization is clearly coming - the 787 is the first step in that. It has a multiple kWh lithium battery on board, and uses high voltage power for things that have historically been done with engine bleed air (deicing, cabin pressurization, and HVAC,) which Boeing says saves both weight and fuel in cruise, while giving higher quality air.

The main engines have high power motor-generators in them to feed the high voltage system, which are also used for starting.

There has been a lot of work on wheel motors for the front landing gear, which would tie in to this architecture well and allow the aircraft to push itself back and taxi without engines

 

[Peter Egan]

It is obviously a tough ask to make a hybrid aircraft stack up economically. But countries looking a technological lead, like China, might be prepared to invest in it. It has a government subsidised 100 seat aircraft development program.

Weight is an issue at take-off and landing. At cruise, drag is the issue. Due to short battery range, the aim is to compete with the turboprop aircraft on regional short haul routes where payload weight capacity is not critical.

The Bombardier Q400 has a significant speed advantage over the smaller ATR 72 but payload is similar. Cargo capacity is not that important for short haul. Thus cabin size is important, but some payload can be converted to batteries and electric motors.

Bombardier does comparisons with other aircraft over a 300 nm (555 km range). Over this range, a hybrid might be able to cut fuel costs, and fuel pollution, by one-third, and cut air pollution at the airport by over 90%. There will be a cost for electricity. To generate some ball park figures, if 500 kW.h are used, it would have a cost of about $100 during a 555 km flight. Jet fuel costs would be about $400 (assuming US$0.60/litre).

As wdolson says in the above comment, engines are expensive. An aim is to reduce reduce the size if engine required. I thought to reduce to one turbine, but range is significantly reduced if the single turbine fails. With battery/motor capability for takeoff, the turbines only need to be large enough to maintain cruise and provide some recharge capability.

The turbines will be in the rear of the aircraft. As they are smaller than standard turbines, it is easier to built a blade containment structure around them. The propellers are separated from one another by the tail - the tail can be reinforced to prevent a lost blade taking out the other set of propellers and their motor.

A turboprop engine with a small turbine, electric motor/generator and transmission, is likely a better package than the one proposed, and can be supplied by the engine suppliers. But the turboprop engine is larger than an electric fan and thus has more drag. An APU style arrangement for the turbine and therefore separate motors to transfer power from the turbine to the fan may have a better outcome.

In Australia, 1,100 Volt and 11,000 Volt insulated cables are common. As the body of the aircraft will be composite, and the high-voltage cables will be in duct outside the airframe, high-voltage safety will be simplified. High voltage will only be in the flight battery bay and in the tail. The flight battery exclusively serves the electric fans and the drive motors in the wheels for which the voltage will need to be reduced to about 350 Volts. Separate batteries are used to power the avionics and controls. These issues are similar to those for electric and hybrid vehicles.

 

[Saghost]

Maybe what you need is a Honda Accord type approach - put the turbine in the nacelle in front of the propfan, with the propfan driven by one motor, and a second motor attached to the turbine shaft - and a dog clutch between them.

That way you can have all the options - straight mechanical for ideal conditions with or without power boost, series hybrid to run the engines and props separately for best efficiency, and run only the props for pushback/taxiing and possibly the whole descent and landing phase.

There's a lot to be said for the instant response of the electric motors and the ability to drag the propellers any time you want to slow down and recover energy in the process.

I'm not sure any electric range beyond the energy needed for takeoff/climb and recovered on approach/landing makes sense yet.

 

[jaguar36]

At cruise, drag is the issue.

I think you're missing the physics of how an airplane flies. To counteract the weight you need more lift, more lift = more drag. Weight is hugely important in aircraft design. Weight is also compounding as for every pound of batteries you need more structure to support it, more wing to lift it, more motors to power it, which then take more batteries and so on. Right now batteries just aren't good enough for a commercially successful aircraft (even with government funding).

 

[Saghost]

I think you're missing the physics of how an airplane flies. To counteract the weight you need more lift, more lift = more drag. Weight is hugely important in aircraft design. Weight is also compounding as for every pound of batteries you need more structure to support it, more wing to lift it, more motors to power it, which then take more batteries and so on. Right now batteries just aren't good enough for a commercially successful aircraft (even with government funding).

Everything you said is true, but he's also not wrong. We generally break drag into two pieces for analytical purposes: induced drag and parasitic drag.

Induced drag is what you're talking about - drag produced as a byproduct of generating lift.

Parasitic drag is what he's talking about - resistance of the air to moving something through it.

Weight and induced drag are critically important for takeoff and landing. But the high subsonic cruise of a typical airliner, the parasitic form drag dominates, which means that a more efficient form can reduce the thrust requirement.

Of course, if that more efficient form reduces the lift on takeoff or increases the weight, then some analysis is needed to decide what is important.

Balancing all of the performance and cost tradeoffs is what makes or breaks an aircraft design. No one promised it would be easy...

 

[Peter Egan]

Everything you said is true, but he's also not wrong. We generally break drag into two pieces for analytical purposes: induced drag and parasitic drag.

Induced drag is what you're talking about - drag produced as a byproduct of generating lift.

Parasitic drag is what he's talking about - resistance of the air to moving something through it.

Weight and induced drag are critically important for takeoff and landing. But the high subsonic cruise of a typical airliner, the parasitic form drag dominates, which means that a more efficient form can reduce the thrust requirement.

Of course, if that more efficient form reduces the lift on takeoff or increases the weight, then some analysis is needed to decide what is important.

Balancing all of the performance and cost tradeoffs is what makes or breaks an aircraft design. No one promised it would be easy...

Thankyou. That says it so much better.

Electric fans spool-up to speed faster than propellers. An electric aircraft will likely accelerate faster and reach a higher take-off speed for the same runway length.

They will be much quieter and will be able to operate at night at airports with noise restrictions.

With the fans at the back, the noise cancelling technology used on turboprop aircraft will not be needed.

It would be nice to see NASA do trade-off exercises for electric and hybrid aircraft to give industry guidance on the likely benefits and costs, and to indicate where technology investment is needed and where existing technologies are adequate to the task.

 

[wdolson]

I can say right from the get go that the performance is not going to make it with current battery tech.

Turboprops are also not very popular with airlines anymore. Most commercial airports have noise ordinances that get tougher at night and turboprops can't be made quiet enough to pass.

Before the 777 program began I worked on a project that was called the 7J7. Boeing was working with Mitsubishi to develop a turboprop regional airliner, but the whole program got dropped because of the noise issues.

 

[Saghost]

I can say right from the get go that the performance is not going to make it with current battery tech.

Turboprops are also not very popular with airlines anymore. Most commercial airports have noise ordinances that get tougher at night and turboprops can't be made quiet enough to pass.

Before the 777 program began I worked on a project that was called the 7J7. Boeing was working with Mitsubishi to develop a turboprop regional airliner, but the whole program got dropped because of the noise issues.

As I mentioned up above, I tend to agree that battery for primary power is probably impractical for the time being. However, Boeing is proving that very mild hybridization is a significant benefit with 787, and I think that Peter may be correct that there's room for somewhat more hybridization with continued gains - especially if we can recapture energy from slowing the aircraft both in flight and on the ground.

I'm not an expert on propeller technology, but what Peter's proposing isn't the typical propeller driven aircraft, either.

I believe that typically most of the sharp noises from aircraft come from pressure waves impinging on air going in another direction - as McDonell Douglas graphically proved with NOTAR (the helicopter developed to not have a tail rotor is much, much quieter than those with tail rotors, because the main and tail rotor wakes don't intersect with high relative air speeds.)

On a typical propeller aircraft, the propellers are right in front of the wing - good for short field performance, since the propwash rolls over the wing and generates lift. Bad for noise, with those high energy wakes slapping against the airframe.

I haven't seen a study on it, but I suspect that a many blade propeller designed with modern aerodynamics (including tip devices to minimize the vortex like both modern airliner wings and modern rotor blades have) located as Peter proposes at the rear of the aircraft with nothing for the wake to hit would produce minimal noise.

If the noise levels still proved to be excessive, nothing prevents the design approach suggested from using the duct and fan section of a conventional high bypass turbofan with a hybrid electric drive system. In principle ducts located here with proper control surfaces could even replace much of the tail surfaces of the aircraft.

 

[wdolson]

As I mentioned up above, I tend to agree that battery for primary power is probably impractical for the time being. However, Boeing is proving that very mild hybridization is a significant benefit with 787, and I think that Peter may be correct that there's room for somewhat more hybridization with continued gains - especially if we can recapture energy from slowing the aircraft both in flight and on the ground.

The systems on the 787 are not really hybirdization. The new electrical systems replace systems run from heat from the engines or hydraulics in other planes. This continues a trend all aircraft manufacturers have been doing for years. The 757/767 had more electronics than any previous aircraft, the 777 was Boeing's first fly-by-wire aircraft, and Airbus has been doing fly-by-wire for some time.

Calling the 787 a hybrid is stretching the term to the breaking point.

I'm not an expert on propeller technology, but what Peter's proposing isn't the typical propeller driven aircraft, either.

I believe that typically most of the sharp noises from aircraft come from pressure waves impinging on air going in another direction - as McDonell Douglas graphically proved with NOTAR (the helicopter developed to not have a tail rotor is much, much quieter than those with tail rotors, because the main and tail rotor wakes don't intersect with high relative air speeds.)

On a typical propeller aircraft, the propellers are right in front of the wing - good for short field performance, since the propwash rolls over the wing and generates lift. Bad for noise, with those high energy wakes slapping against the airframe.

I haven't seen a study on it, but I suspect that a many blade propeller designed with modern aerodynamics (including tip devices to minimize the vortex like both modern airliner wings and modern rotor blades have) located as Peter proposes at the rear of the aircraft with nothing for the wake to hit would produce minimal noise.

If the noise levels still proved to be excessive, nothing prevents the design approach suggested from using the duct and fan section of a conventional high bypass turbofan with a hybrid electric drive system. In principle ducts located here with proper control surfaces could even replace much of the tail surfaces of the aircraft.

The 7J7 had the props on the tail in the same place as the DC-9 with the blades pointing backwards.

Vintage Magazine Mania: A Collection: Boeing 7J7...

 

[jkn]

Electric motors ability to go from idle to full power very quickly would give very large safety benefit while landing.

 

[Jdcorbitt3]

I like the concept. Turbine engines are most efficient at high power settings. If the drive, like a locomotive is the electric motors, the batteries could provide the additional power required for the takeoff and climb phase of flight. The turbine APU running at near 100% high altitude load would provide cruise power. Decent, from altitude can be 150 miles and is virtually a glide. The decent, landing, and taxi could all be on electric as there would be little or no draw.
As for acceleration on takeoff, if the takeoff is runway limited, the turbine engines on a conventional aircraft are at 100% power prior to brake release. The acceleration would be the same.
The electric motors and batteries need to be as close together as possible to reduce wire size.
I like the concept of wheel motor generators as well. They could be used to pre spin the mains prior to touchdown, greatly increasing the life of tires, and then used on the ground for taxi up to the gate.
The theory is lower fuel consumption. Therefore less fuel for a specific range, resulting in a lower net takeoff weight, but a higher landing weight as the battery don't loose weight.
The largest problem I see is having such a large class D fire source in a crash and getting that past certification. One of the planes I fly started life with a LI ion battery. After Cessna used a maintenance battery for multiple starts, it experienced a thermal runaway and caught the plane on fire. An AD came out, and all operators had to remove the battery from their aircraft. Initial start on the engines can draw 1100 amps. The LI ion batteries maintained voltage during the start and resulted in much cooler starts than we are getting now. It is a shame, but it looks like the CJ4 is not getting a LI ion battery, at least from Cessna again.

P.s. the RE220 in the GV is rated for starts up to 30,000ft and Certified to operate at 41,000 ft. It can provide 40KVA at 41,000 ft. That would not be enough power for cruise as well as pressurizing the plane. It would have to be considerably larger.

John

 

[deonb]

Cool... now can you make that work on a Eclipse 550 please .

 

[Peter Egan]

I like the concept. Turbine engines are most efficient at high power settings. If the drive, like a locomotive is the electric motors, the batteries could provide the additional power required for the takeoff and climb phase of flight. The turbine APU running at near 100% high altitude load would provide cruise power. Decent, from altitude can be 150 miles and is virtually a glide. The decent, landing, and taxi could all be on electric as there would be little or no draw.
As for acceleration on takeoff, if the takeoff is runway limited, the turbine engines on a conventional aircraft are at 100% power prior to brake release. The acceleration would be the same.
The electric motors and batteries need to be as close together as possible to reduce wire size.
I like the concept of wheel motor generators as well. They could be used to pre spin the mains prior to touchdown, greatly increasing the life of tires, and then used on the ground for taxi up to the gate.
The theory is lower fuel consumption. Therefore less fuel for a specific range, resulting in a lower net takeoff weight, but a higher landing weight as the battery don't loose weight.
The largest problem I see is having such a large class D fire source in a crash and getting that past certification. One of the planes I fly started life with a LI ion battery. After Cessna used a maintenance battery for multiple starts, it experienced a thermal runaway and caught the plane on fire. An AD came out, and all operators had to remove the battery from their aircraft. Initial start on the engines can draw 1100 amps. The LI ion batteries maintained voltage during the start and resulted in much cooler starts than we are getting now. It is a shame, but it looks like the CJ4 is not getting a LI ion battery, at least from Cessna again.

P.s. the RE220 in the GV is rated for starts up to 30,000ft and Certified to operate at 41,000 ft. It can provide 40KVA at 41,000 ft. That would not be enough power for cruise as well as pressurizing the plane. It would have to be considerably larger.

John

Many issues to be taken into account I see.

In a crash, the sparks from friction of aircraft parts hitting the ground, and each other, ignite spilt fuel. The batteries will contribute to an already burning fire. New FAA certification rules for high-voltage circuits will be required. The high voltage will be a concern to first responders. The aircraft will need external high-voltage isolation switches. The battery modules may need internal isolation switches, operated by rapid deacceleration, to reduce the voltage found in a battery module, or part battery module, when a plane crashes. The high-voltage circuits could have isolation switches operable from the cockpit for crash landings.

The turbines would need to be similar in size to that in the A380 - PW 980A - which is apparently rated for 1350 kW. Obviously, it would need to be modified for the extra demands of producing near max power on demand at any time during the flight.

 

[wdolson]

Another problem with a crash landing would be the battery compartment getting damaged, cells shorting out which leads to thermal runaway and a battery fire. With tons of aluminum in an airplane, shorted cells in a bad accident are a real possibility.

 

[Peter Egan]

This is where I think the Tesla approach to cells is very good. The 21700 cells would get scattered in a bad crash, but don't have too much energy to be a major concern. The fuel and the aircraft body are the main fuel in a fire. The body hitting the ground is the main spark that sets off a fire.

 

[Jdcorbitt3]

Cool... now can you make that work on a Eclipse 550 please .

The cruise fuel burn on the Eclipes is less than the RE 220 APU fuel burn. The Eclipes is an amazing aircraft. It just had a few upsets.

John

 

[wdolson]

This is where I think the Tesla approach to cells is very good. The 21700 cells would get scattered in a bad crash, but don't have too much energy to be a major concern. The fuel and the aircraft body are the main fuel in a fire. The body hitting the ground is the main spark that sets off a fire.

We had sort of a preview of what to expect this week with the fatal Tesla crash in the Netherlands. The battery pack split open and shorting batteries caught fire on the road.

This would be an issue with a 787 crash today, but more batteries would mean a more serious danger.