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Electric Airplanes, contra-rotating propellers


Northstar1989
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Apparently, use of electric motors allows you to design aircraft with contra-rotating propellers- as is demonstrated in these electric racing aircraft designs:

I had no idea this was possible, and this massively changes the game in terms of electric vs. internal combustion aircraft.

For one, with contra-rotating electric propellers it's easily possible to pack twice as much Thrust into a given airframe as with an ICE design, allowing for higher speeds (in racing designs) ot larger wings and more space/weight for batteries (in actual utilitarian planes).

It's also possible to achieve a much higher ratio of propeller-area to Thrust with the same Thrust- allowing for better performance at higher altitudes (electric engines also operate better than ICE ones at higher altitudes, due to not requiring Oxygen..)

 

I could easily imagine an Electric passenger aircraft being designed someday with 5 sets of contra-rating propellers (1 on the nose and 2 on each wing), each with 3-4 blades rotating in each direction with a rather large propeller diameter, being used to support very high-altitude flight with electric aircraft for the inherent advantages this provides (lower drag for a given speed, less turbulence, more glide time/distance in case of engine-failure).

At high altitudes electric propellers eventually reach their maximum torque and rotation-speed, and start to consume less and less electrical power as the Thrust produced and air resistance to rotation starts to fall off with even greater gains in altitude.

So, if you climbed high enough you could eventually reach the point where 10 contra-rotating propellers (5 sets of 2) with large diameter blades spans and 3-4 blades on each propeller only draws as much electrical power, and produces as much Thrust, as a single propeller at sea level.  However at this altitude, Drag would be greatly reduced- meaning you could (and would need to in order to generate adequate Lift to maintain altitude) fly substantially faster than that amount of Thrust would allow you to fly at sea-level.

Even accounting for decreasing Lift/Drag at higher speeds, you could still cover far more miles of distance per kilowatt-hour of electrical power consumed with a design optimized for high-altitude flight and cruising at high altitude...  This would lead to substantially extended max range compared to electric planes with lower cruising-altitudes, and might (just barely) allow for practical electric Transatlantic flights on a large plane (larger planes encounter less parasitic drag relative to their volume, and have less mass dedicated to cockpit computers, landing gear, and pilots compared to their payload) with a substantial portion of volume (maybe 50% of the plane's internal space) dedicated to batteries, for instance...

 

Other videos worth watching on the topics of electric aircraft and high-altitude flight:

(Above: a general overview of efforts at small electric aircraft...)

(Above: a video on the development of "Alice", a proposed 800 km range passenger aircraft by Eviation...)

(Above: Airbus and Rolls Royce' efforts at developing Distributed Electric Propulsion for large passenger jetliners- with company estimates putting a rollout date of about 2030-2050, target of 2042)

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24 minutes ago, Northstar1989 said:

Apparently, use of electric motors allows you to design aircraft with contra-rotating propellers- as is demonstrated in these electric racing aircraft designs...

 

Contra-rotating props are nothing new, they've been around for a century or so by now. Also, there is nothing inherently special that allows electric propulsion to have contra-rotating props, you can just as easily do it with ICEs.

Edited by Steel
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Contra-rotating propellers have been around for longer in helicopters I think ?

... Also in the war ...

One of the thing which "plagues" batteries are that they stay as deadweights - an empty battery weighs almost the same as a charged one - unlike fuel, which when empty simply leaves a hollow tank.

 

In any case, I'm actually more interested in how would they replace jets. Would we just see high-velocity electric fans, or would be see propellers ?

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1 hour ago, Steel said:

Contra-rotating props are nothing new, they've been around for a century or so by now. Also, there is nothing inherently special that allows electric propulsion to have contra-rotating props, you can just as easily do it with ICEs.

Electric engines are smaller, lighter, and easier to miniaturize- which means you can more easily have one engine for each direction of propeller rotation- reducing the mrchanical complexity of contra-rotating propeller designs (with just one engine, complex gear arrangements are necessary).

Otherwise, you're right, there's nothing new about contra-rotating propellers- they're just infinitely more practical with electric propulsion systems...

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On 10/26/2017 at 1:55 PM, YNM said:

Contra-rotating propellers have been around for longer in helicopters I think ?

... Also in the war ...

One of the thing which "plagues" batteries are that they stay as deadweights - an empty battery weighs almost the same as a charged one - unlike fuel, which when empty simply leaves a hollow tank.

 

In any case, I'm actually more interested in how would they replace jets. Would we just see high-velocity electric fans, or would be see propellers ?

Electric engines are better-suited for contra-rotating propeller designs as they are easier to miniaturize and you can have one engine for each direction.

The batteries stay on as "deadweight", but this is also an advantage of sorts- the Center of Gravity doesn't shift due to fuel-consumption, which allows for some interesting new design choices...

 

As for replacing jet engines- that's what the Rolls Royce "E-Fan" is designed to accomplish.  It's essentially an electric turbofan capable of operating up to higher exhaust velocities than propellers, allowing for higher speeds.  However It's also heavier and less energy-efficient than propellers at low speeds: much like jet engines are heavier and less fuel-efficient than ICE propellers at low speeds...

I suspect that due to the limitations of battery technology, propeller planes might dominate the electric aircraft markets for some time before electric turbofans eventually take over.  That's because they can travel farther on a kilogram of batteries than can electric turbofans: at the expense of longer flight-time.

 

The designs of the past may be the designs of the future with electric aircraft.  Biplanes, for instance, are useful for increasing your range without exceeding airport size-limitations: a biplane with twice the wing-area and the same wing-chord as a monoplane produces 20% more Lift for 2x the subsonic Drag from the wings (or about a 60% increase in overall drag on the entire airframe), a better ratio (maybe 10%/40% or 5%/32% instead of 20%/100%) if the wing-chord is reduced on each wing for a smaller overall increase in wing-area but a better aspect-ratio...  

At these ratios, the increased Lift is useful for increasing your range by allowing you to pack in more batteries: for instance in a plane that is 12% batteries by mass to start with, a 20% increase in Lift might allow for a 130% increase in battery-mass, allowing for a 43.75% increase in maximum flight-time assuming constant throttle (or a roughly 50% increase in range, assuming substantial time spent to reach Cruise Altitude and speed, and higher Takeoff and Climb Thrust than Cruise Thrust in the shorter-range monoplane...)

In fact, some of the Rolls Royce/Airbus designs for a Distributed Electric Propulsion plane even incorporate a short biplane span beneath the electric turbofans- though presumably this is for structural reasons to support 3-4 electric turbofans on each wing than for any aerodynamic reason...

With electric aircraft, raw performance is far more important than energy-efficiency.  Batteries and electricity are substantially cheaper than jet fuel when amortized over thousands of flights (assuming they can be made sufficiently reliable- and designed such that individual batteries in massive arrays can be replaced when they fail, rather than the entire array), whereas the range-limitations impose a massive economic cost.  So one would expect that modern biplanes would be initially desirable to increase the range- even if the extra batteries and wings drove up initial construction costs of the planes significantly...

 

So, in short, I predict a progression of electric planes similar to ICE planes:

Propeller Biplanes --> Propeller Monoplanes--> Turbofan Monoplanes

The advancement will be limited largely by the energy-density of batteries: as it improves, designs will follow a similar progression to ICE designs to chase shorter flight-times and higher cruise-speeds...

 

Right now we're still in the Wright Brothers-like era of experimental electric aircraft.  But as electrics progress, I predict we'll see the first commercial electric passenger planes in a generation or two.  Once again, the first ones will be biplanes- because they can either carry more passengers or stay in the air longer for the same wingspan, and achieve potentially longer ranges.

These initial flights will be slow and relatively shorter-range (the best designs *might* manage a Transatlantic flight-time but probably won't be used this way due to the very long flight-times) but cheaper than ICE flights due to their much lower fuel-costs (automation will also help reduce the higher labor-costs of slower flights: the days of robotic flight-attendants and pilotless airplanes are just around the corner, in my opinion- and will follow in the wake of driverless trucks and buses).  They will mainly be used for shorter-range flights between nearby cities, while ICE jets continue to service long-distance flights.  However as battery technology progresses, faster, longer-range electrics will become a reality...

Alternatively, electric blimps (filled with Helium) with heavier-than-air lifting-body designs might also become the first generation of commercial electric aircraft.  Blimps require far less energy expenditure per ton-mile of cargo, due to their use of buoyancy to support most of their weight, and heavier-than-air lifting-body designs can achieve some respectable flight speeds.

Thus blimps have the capability to provide an alternative form of long-distance passenger transport (*especially* if the piloting of the blimps is automated) which may be more comfortable for passengers due to the inclusion of gambling areas, restaurants, etc. on board to take advantage of the much lower cost per ton-mile than ICE planes.  These blimps might in turn give way to the first generation of electric passenger planes- likely biplanes for some of the reasons above...

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1 hour ago, YNM said:

Contra-rotating propellers have been around for longer in helicopters I think ?

... Also in the war ...

One of the thing which "plagues" batteries are that they stay as deadweights - an empty battery weighs almost the same as a charged one - unlike fuel, which when empty simply leaves a hollow tank.

 

In any case, I'm actually more interested in how would they replace jets. Would we just see high-velocity electric fans, or would be see propellers ?

Most of the concepts I've seen use electric ducted fans. Which are almost the same as a high-bypass turbofan anyway. You lose some of the thrust from the turbine, but most of the thrust comes from the bypass flow anyway.

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57 minutes ago, Northstar1989 said:

Electric engines are better-suited for contra-rotating propeller designs as they are easier to miniaturize and you can have one engine for each direction.

I could imagine an electric-transmission thing. But I think it's basically how far you want the whole thing to be. After all, electric motors are just a hunk of winding metal...

59 minutes ago, Northstar1989 said:

The batteries stay on as "deadweighr", but this is also an advantage of sorts- the Center of Gravity doesn't shift due to fuel-consumption, which allows for some interesting new design chpices...

I thought that moveable CG would be better !

--------

1 hour ago, Northstar1989 said:

As for replacing jet engines- that's what the Rolls Royce "E-Fan" is designed to accomplish.  It's essentially am electric turbofan capable of operating up to higher exhaust velocities than propellers, allowing for higher speeds.  However It's also heavier and less energy-efficient than propellers at low speeds: much like jet engines are heavier and less fuel-efficient than ICE propellers at low speeds...

 

25 minutes ago, peadar1987 said:

Most of the concepts I've seen use electric ducted fans. Which are almost the same as a high-bypass turbofan anyway. You lose some of the thrust from the turbine, but most of the thrust comes from the bypass flow anyway.

Thrust generation comes best when the exhaust comes close to the airspeed. It's just a fun thought seeing future "jets" aircraft basically being a lifting gas - less airships !

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As with any other electric mode of transportation, except trains, the stumbling block is still the battery. Even in applications where mass of the battery pack is of secondary importance, such as cars, the manufacturers are struggling to reach a meaningful autonomy, with customers needing to live with the need to remember to charge their car every night so they can get to work tomorrow.

I have no idea how this system could possibly work on an airliner that needs to be in air as much as possible to be profitable, and not sitting on tarmac for hours waiting to be recharged for the next flight.

"Good afternoon, this is your Captain speaking with just a little bit of information. On your left we can see the servicemen just plugging in the charging cable, which means that we should be on our way rather soonish; an hour or two perhaps. If you could please refrain from using the onboard outlets for charging your own mobile devices, it would speed up the whole process. Thank you for waiting with Spark Airlines."

This is not happening without a revolution in battery tech.

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48 minutes ago, Shpaget said:

As with any other electric mode of transportation, except trains, the stumbling block is still the battery. Even in applications where mass of the battery pack is of secondary importance, such as cars, the manufacturers are struggling to reach a meaningful autonomy, with customers needing to live with the need to remember to charge their car every night so they can get to work tomorrow.

I have no idea how this system could possibly work on an airliner that needs to be in air as much as possible to be profitable, and not sitting on tarmac for hours waiting to be recharged for the next flight.

"Good afternoon, this is your Captain speaking with just a little bit of information. On your left we can see the servicemen just plugging in the charging cable, which means that we should be on our way rather soonish; an hour or two perhaps. If you could please refrain from using the onboard outlets for charging your own mobile devices, it would speed up the whole process. Thank you for waiting with Spark Airlines."

This is not happening without a revolution in battery tech.

This, note that batteries has tow horrible disadvantages over fuel. 
The small one is that storage is inefficient, an fuel tank is just an container holding the fuel 
Second and worse is that fuel uses the oxygen in the air, batteries has to do without, yes this is an benefit underwater or in space but limit batteries a lot. 
An battery who used say lithium / air would change everything. 
Electrical has the benefit of high efficiency and no local pollution. 

Electrical cars work because you only use your 150 hp to accelerate once up to speed  you use 15 or something to cruise. Electrical engines are high trust to weight. 
This is not the case of planes who will load so much fuel the can not land without dumping for long distance flights. 

Note that hybrid planes has two huge benefits, first it increase single engine safety, it makes vtol / stol much easier and you can do with an smaller engine using battery for extra trust. An pure electrical plane might work for some stuff like an small island hopper or personal planes but very specialized.


 

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The problem with batteries is also their safety. Because they don't undergo a reaction with air, yet they definititely will react with air, this means they need a lot of insulation and casing.

Also, regarding air batteries, do they get heavier the more used (less charged) they are ? I mean, they get oxygen binded to them as they get used doesn't it ?

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https://en.wikipedia.org/wiki/Contra-rotating_propellers

Contra-rotating propellers on a Rolls-Royce–Griffon–powered P-51

220px-Contrarotating.jpg

Contra-rotating propellers of a Spitfire Mk XIX

220px-Supermarine_Spitfire_XIX_vr.jpg

One of the quartet of contra-rotating propellers on a Tu-95 Russian strategic bomber

220px-Kuznetsov_NK-12M_turboprop_on_Tu-9

XB-35 Flying Wing shown its quartet of pusher contra-rotating propellers.

220px-YB-35_42-13603_on_the_ramp.jpg

https://en.wikipedia.org/wiki/Coaxial_rotors

Kamov Ka-32A-12

300px-KamowK32A.jpg

Russian Air Force Ka-52

220px-Ka-52_at_MAKS-2009.jpg

Sikorsky S-69/XH-59A

220px-XH-59A_helicopter_in_1981_%283%29.

 

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11 hours ago, Northstar1989 said:

Electric engines are smaller, lighter, and easier to miniaturize- which means you can more easily have one engine for each direction of propeller rotation- reducing the mrchanical complexity of contra-rotating propeller designs (with just one engine, complex gear arrangements are necessary).

Otherwise, you're right, there's nothing new about contra-rotating propellers- they're just infinitely more practical with electric propulsion systems...

They are not "infinitely more practical" for generating counter-rotating forces. In fact, it's actually quite synergistic for turbine engines to drive counter-rotating props. Usually every turbine stage needs to be "de-swirled" before more energy can be extracted from it, but a counter-rotating turbine automatically de-swirls itself.

The TU-95 has been in service with counter-rotating props since 1952. I'm not sure exactly how those work -- probably just by reversing the gears in the gearbox. But fanjet designs with direct connection between the props and the turbine shafts are typically designed as counter-rotating turbines.

The thing is, it turns out that for most propeller applications, it's not actually worth the weight and complexity of de-swirling the prop exhaust. It's only worth it on very high-speed designs (like the TU-95).

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Was about to post a picture of the Tu-95, but @mrfox ninja'd me. Oh well, here it is.

1024px-Russian_Bear_'H'_Aircraft_MOD_451

@Northstar1989 The basic idea is sound, but the biggest hurdle is battery capacity - current state-of-the-art battery tech can carry nowhere as much energy to a fuel tank of comparable size and mass. It's possible to sidestep the problem by using an onboard fuel cell or generator, but then someone would point out that it's much more straightforward to use the fuel these devices use to power a conventional turboprop/turbofan engine.

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16 hours ago, Shpaget said:

"Good afternoon, this is your Captain speaking with just a little bit of information. On your left we can see the servicemen just plugging in the charging cable, which means that we should be on our way rather soonish; an hour or two perhaps. If you could please refrain from using the onboard outlets for charging your own mobile devices, it would speed up the whole process. Thank you for waiting with Spark Airlines."

On hub-spoke operations, such waiting time isn't exactly unnormal I think ?

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1 minute ago, YNM said:

On hub-spoke operations, such waiting time isn't exactly unnormal I think ?

It is normal for the passengers to wait, it is not normal for the airplanes to spend much time on the ground.

Planes are expensive, and constant usage gives quicker return on investment.

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1 minute ago, Terwin said:

It is normal for the passengers to wait, it is not normal for the airplanes to spend much time on the ground.

Planes are expensive, and constant usage gives quicker return on investment.

I'm not talking about LCC - yes they need to use their planes ASAP, but for bigger, more "established" airlines, surely the planes can end up waiting so long on the ground ?

 

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7 hours ago, mikegarrison said:

They are not "infinitely more practical" for generating counter-rotating forces. In fact, it's actually quite synergistic for turbine engines to drive counter-rotating props. Usually every turbine stage needs to be "de-swirled" before more energy can be extracted from it, but a counter-rotating turbine automatically de-swirls itself.

The TU-95 has been in service with counter-rotating props since 1952. I'm not sure exactly how those work -- probably just by reversing the gears in the gearbox. But fanjet designs with direct connection between the props and the turbine shafts are typically designed as counter-rotating turbines.

The thing is, it turns out that for most propeller applications, it's not actually worth the weight and complexity of de-swirling the prop exhaust. It's only worth it on very high-speed designs (like the TU-95).

You use gearing for counter rotating propellers. 
the main reason to use two engines on an small plane is dual engine safety. another option would be to go for push and pull propeller or 3 there you folded or pulled  the extra ones into wing after takeoff. 

Many large quadcopter designs uses separate engine on the upper and lower propeller for engine out safety. If you lose an engine in an quadcopter you will crash. 

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1 hour ago, YNM said:

I'm not talking about LCC - yes they need to use their planes ASAP, but for bigger, more "established" airlines, surely the planes can end up waiting so long on the ground ?

 

The more they sit on the ground, the less they earn money. All airlines worry about this issue, not just LCCs.

1 hour ago, magnemoe said:

You use gearing for counter rotating propellers.

As I said, several of the propfan designs used direct-drive from counter-rotating turbines. https://en.wikipedia.org/wiki/General_Electric_GE36

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The problem of downtime can be solved by keeping charged battery packs in reserve, then just switching them in when they're needed, which could in theory take a matter of minutes. If we end up solving the energy density problem, this problem is pretty trivial in comparison.

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2 hours ago, YNM said:

for bigger, more "established" airlines, surely the planes can end up waiting so long on the ground ?

No more than absolutely necessary.  A plane on the ground is a plane not earning money - and margins are extraordinarily thin in the airline industry.  Net revenue can be as little $3/passenger/flight.

 

2 minutes ago, peadar1987 said:

The problem of downtime can be solved by keeping charged battery packs in reserve, then just switching them in when they're needed, which could in theory take a matter of minutes.


Things are always simple seen from the armchair.   The labor costs alone make this a nonstarter.

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Just now, peadar1987 said:

The problem of downtime can be solved by keeping charged battery packs in reserve, then just switching them in when they're needed, which could in theory take a matter of minutes. If we end up solving the energy density problem, this problem is pretty trivial in comparison.

Few aspects of airplane design end up being "trivial". Such a design would severely constrain where the batteries could be located, because they would have to able to accessed from outside the airplane and most likely slid in and out using heavy equipment. In all sorts of weather. Within the gate box space constraints for service vehicles. They would have to be 100% reliably secure when you installed them, and yet reasonable easy to detach and reattach. I think this would be a bigger chore than you anticipate.

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21 hours ago, Northstar1989 said:

As for replacing jet engines- that's what the Rolls Royce "E-Fan" is designed to accomplish.  It's essentially am electric turbofan capable of operating up to higher exhaust velocities than propellers, allowing for higher speeds.  However It's also heavier and less energy-efficient than propellers at low speeds: much like jet engines are heavier and less fuel-efficient than ICE propellers at low speeds...

If that relies on superconductors as shown in the last video of the OP, we have a while to wait yet. Yes, the superconductor cables themselves are only grams compared to KG's for copper, but even "warm" superconductors are still running at -250C or thereabouts (there are warmer examples, but rather esoteric and not really practical in an engineering solution), so you need heavy and power-hungry cooling equipment.

Room-temperature superconductors (ie, > 0C, not even 20C!!) are the holy grail, whoever invents or discovers a usable one will change the world.

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19 hours ago, Shpaget said:

"Good afternoon, this is your Captain speaking with just a little bit of information. On your left we can see the servicemen just plugging in the charging cable, which means that we should be on our way rather soonish; an hour or two perhaps. If you could please refrain from using the onboard outlets for charging your own mobile devices, it would speed up the whole process. Thank you for waiting with Spark Airlines."

This is not happening without a revolution in battery tech.

I'd expect that "revolution" to be a flow-battery.  These can be thought of as "reversible" fuel cells.  Unfortunately, I don't think that there are terribly useful in cell phones (so the primary driver of battery technology isn't driving them) but would be extremely useful for cars.  While their primary interest is low-cost capacity (the idea is to increase the chemical parts storing the energy and decreasing the parts/catalysts that do the chemical process), it should be extremely likely that you could have multiple/larger "battery heads" recharging the chemicals much faster than the plane's equipment possibly can.

This type of thing would essentially make electric cars replace gas engines in most situations (although I wouldn't count on recharging stations that directly charged the chemicals like suggested above).  No idea on much more research it takes to get there (there doesn't appear to be a workable set of chemicals), but it is a potential game changer.
- Note: as electric car companies work closer to battery manufacturers (like the gigafactory), there is much less willingness to try this type of thing.  Don't count on it happening.

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54 minutes ago, mikegarrison said:

The more they sit on the ground, the less they earn money. All airlines worry about this issue, not just LCCs.

 

47 minutes ago, DerekL1963 said:

No more than absolutely necessary.  A plane on the ground is a plane not earning money - and margins are extraordinarily thin in the airline industry.  Net revenue can be as little $3/passenger/flight.

Yeah, I see. Still often the case though, hence why I mention it.

I mean, LCCs barely keep their planes on the ground if not for overnight storage. Some airlines can afford having them standby-ready instead. It really depends on what you're trying to provide - availability or possibility !

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