Electric Aircraft - The Thread

[Divalent 142]

On 4/9/2021 at 12:23 PM, nwt said:

If you have enough extra power from the combusting engine to recharge the batteries, then

• You could just use that power to climb directly instead of losing energy in conversion, adding an entire second motor, prop, generator, etc.
• Now in cruise you're either burning extra fuel to charge the batteries, or you're running substantially less than WOT, both scenarios defeating the entire point of the idea

I think you are strawmanning what is being proposed. The proposal is that an engine that provides less than the needed power to take off (say, 80%) is combined with an electric with batteries that can provide the needed 20% additional for takeoff climb. Then when cruising, where you need (say) 75% power, the remaining 5% of combustion power charges the batteries. So you don't need a 100% combustion engine to take off and charge.

~20 minutes of charging while cruising would then recover the initial battery power, which then gives you your reserve. Further, when the flight is descending and using only a fraction of the combustion engine output, you could charge the batteries even faster (meaning you might not need less than 5% excess capacity to charge your reserve).

Yes, the economics will depend on whether the actual combination of weights and power make sense. But the scheme can't be dismissed on a logical basis by assuming you absolutely need a 100% combustion engine at some point in the flight, as that is not what is being proposed.

 

[billvon 3,063]

38 minutes ago, olofscience said:

We do, but the problem with power-to-weight ratio is it's a feedback cycle. Unlike cars, the engine(s), quite literally, have to lift their own weight.

So if you make the engine heavier for the same amount of power, you'll need more power to climb, making it heavier, and so on.

Yep.  But people still make aircraft heavier for all sorts of reasons, all of which (of course) trace back to $$$.

This is going to evolve over time.  The first thing that will happen are electric aircraft; there have already been several designs flown, and at least one company planning on going commercial with them.

The second thing that will happen is that the turbine will move inside the aircraft, and the fans will be electrically instead of shaft driven.  The big advantages here are NOx reduction in the exhaust and greater efficiency via better RPM control.  (Note that in current aircraft the APU's are already inside.)

After that happens, some designer is going to say "we could increase MTOW by 10,000 lbs if we had 100 kilowatts more power for 30 seconds."  And the bean counters will do the math and decide they'll be able to make $150 more an hour by adding a 500 lb battery.

[olofscience 489]

3 hours ago, billvon said:

This is going to evolve over time.  The first thing that will happen are electric aircraft; there have already been several designs flown, and at least one company planning on going commercial with them.

Agreed.

3 hours ago, billvon said:

The second thing that will happen is that the turbine will move inside the aircraft, and the fans will be electrically instead of shaft driven.

Agreed, eliminating the gearbox will save massively on MRO. The benefits of distributed electric propulsion are also very compelling.

3 hours ago, billvon said:

The big advantages here are NOx reduction in the exhaust

Wait, what? How would NOx be reduced by moving the location of the turbine? Genuinely curious.

[Divalent 142]

1 hour ago, olofscience said:

Wait, what? How would NOx be reduced by moving the location of the turbine? Genuinely curious.

The idea is that with the engine inside the hull, the exhaust can be treated before exiting. (Something that can't be done now with the engine out of the wing, as that exhaust it the thing that is giving the airliner the thrust.)  See a link I posted a page or two back on the concept. (https://www.dropzone.com/forums/topic/266444-electric-aircraft-the-thread/?do=findComment&comment=4936934)

Edited April 10, 2021 by Divalent

[nwt 131]

On 4/10/2021 at 12:00 PM, Divalent said:

I think you are strawmanning what is being proposed. The proposal is that an engine that provides less than the needed power to take off (say, 80%) is combined with an electric with batteries that can provide the needed 20% additional for takeoff climb. Then when cruising, where you need (say) 75% power, the remaining 5% of combustion power charges the batteries. So you don't need a 100% combustion engine to take off and charge.

~20 minutes of charging while cruising would then recover the initial battery power, which then gives you your reserve. Further, when the flight is descending and using only a fraction of the combustion engine output, you could charge the batteries even faster (meaning you might not need less than 5% excess capacity to charge your reserve).

Yes, the economics will depend on whether the actual combination of weights and power make sense. But the scheme can't be dismissed on a logical basis by assuming you absolutely need a 100% combustion engine at some point in the flight, as that is not what is being proposed.

 

I'm not straw-manning and I'm not saying logically you must have 100% combustion. I'm saying (1) the numbers will never work out, and (2) you absolutely must be able to go around, climb to avoid weather/traffic, etc.

Cruising at 75-80% power already negates the original idea, which was to cruise at 100% power where engines are most efficient.

Where did your numbers come from? Did you just make up numbers that you thought sounded good? We can make any idea sound good if there's no requirement for the engineering to be based in reality.

[BMAC615 209]

@nwt & @Divalent Pipistrel worked on what you are talking about w/ Rotax & Siemens motors.https://www.youtube.com/watch?v=P840YDo2d6k

Edited April 12, 2021 by BMAC615

[billvon 3,063]

On 4/10/2021 at 2:01 PM, olofscience said:

Wait, what? How would NOx be reduced by moving the location of the turbine? Genuinely curious.

A central location for the turbine(s) allows treatment of the exhaust - for example with a catalytic converter.  It would be there primarily to reduce NOx, but modern three-way catalysts also reduce CO and unburned hydrocarbon exhaust.  Converters have been used on gas turbines since the 1970's, so this is a fairly mature technology.

[olofscience 489]

32 minutes ago, billvon said:

A central location for the turbine(s) allows treatment of the exhaust - for example with a catalytic converter.  It would be there primarily to reduce NOx, but modern three-way catalysts also reduce CO and unburned hydrocarbon exhaust.  Converters have been used on gas turbines since the 1970's, so this is a fairly mature technology.

But if so they should already be on some turboprops, no? A C208 has the PT6 mounted on the fuselage, so what's stopping them from putting NOx scrubbers in? Also APUs for airliners, do any of them have NOx scrubbers? My searches have so far only turned up scrubbers for stationary gas turbine installations.

[wolfriverjoe 1,523]

57 minutes ago, olofscience said:

But if so they should already be on some turboprops, no? A C208 has the PT6 mounted on the fuselage, so what's stopping them from putting NOx scrubbers in? Also APUs for airliners, do any of them have NOx scrubbers? My searches have so far only turned up scrubbers for stationary gas turbine installations.

Even though they are mounted to the fuselage, the exhaust pipes come directly out of the cowl, very similar to the wing mounted ones.

I don't think the APUs mounted inside the fuselages have any sort of emissions controls on them.

It's not required...
Yet.

[Divalent 142]

On 4/12/2021 at 1:56 PM, nwt said:

I'm not straw-manning and I'm not saying logically you must have 100% combustion. I'm saying (1) the numbers will never work out, and (2) you absolutely must be able to go around, climb to avoid weather/traffic, etc.

Cruising at 75-80% power already negates the original idea, which was to cruise at 100% power where engines are most efficient.

Where did your numbers come from? Did you just make up numbers that you thought sounded good? We can make any idea sound good if there's no requirement for the engineering to be based in reality.

"80%" is WOT for that combustion engine. (80% of the total max power, with the other 20% coming from the electric engine). So cruising at 75% of total power (combustion plus electric) for an engine where WOT can contribute only 80% means that engine would be at 75/80 = ~94% of WOT.

So in this scenario, you'd always have an available 5% of max power as a reserve, plus whatever the status of the batteries are, up to 20% (maybe none very early in flight; full 20% but for only a short burst until batteries are replenished thereafter).

 

Edited April 15, 2021 by Divalent

[Divalent 142]

12 hours ago, Divalent said:

"80%" is WOT for that combustion engine. (80% of the total max power, with the other 20% coming from the electric engine). So cruising at 75% of total power (combustion plus electric) for an engine where WOT can contribute only 80% means that engine would be at 75/80 = ~94% of WOT.

Just to be clear, I'm assuming the 100% total maximum power the aircraft can produce consists of 80% from the combustion engine, and 20% from the electric. I'm also assuming that cruising power needed will be 75% of that total maximum power, and in the example above, the batteries are sufficient to provide 5 minutes of sustained "reserve" capacity. These #s are variables you can adjust to fit reality for a particular aircraft and situation.

Regardless of the numbers, as long as the combustion engine power exceeds what is needed to cruise by some amount, there will be excess power to recharge the batteries and restore reserve power during the cruise portion of the flight (and the descent portion as well). So the combustion engine will be running much closer to WOT than it would be if it was the only engine.

[billvon 3,063]

2 hours ago, Divalent said:

Regardless of the numbers, as long as the combustion engine power exceeds what is needed to cruise by some amount, there will be excess power to recharge the batteries and restore reserve power during the cruise portion of the flight (and the descent portion as well). So the combustion engine will be running much closer to WOT than it would be if it was the only engine.

I'd say you can be arbitrarily close to that limit.  Even if you fly a profile where cruise is at 100% turbine power, you'll be able to recharge on descent, providing you with that "contingency" power for a go-around (for example.)

[billvon 3,063]

On 4/12/2021 at 4:55 PM, wolfriverjoe said:

It's not required...
Yet.

Exactly.  But it is likely coming.  As other sources of pollutants (power plants, industrial processes, vehicles) are cleaned up, aviation becomes a larger and larger percentage of the remaining pollution emissions.

[nwt 131]

15 hours ago, Divalent said:

"80%" is WOT for that combustion engine. (80% of the total max power, with the other 20% coming from the electric engine). So cruising at 75% of total power (combustion plus electric) for an engine where WOT can contribute only 80% means that engine would be at 75/80 = ~94% of WOT.

I'll concede that I misunderstood this point. I don't have much of an understanding of what the efficiency gains really are at 100% WOT or at 94% WOT compared to a typical cruise setting, but I can give the idea the benefit of the doubt. The idea overall is so far beyond what makes sense that these small details won't make the difference.

3 hours ago, Divalent said:

These #s are variables you can adjust to fit reality for a particular aircraft and situation.

I understand what you mean here. My point is that you can turn these knobs around however you want, and the engineering will never work out. As you change these parameters, you're not seeing the penalties of cost, weight, complexity, etc.

3 hours ago, Divalent said:

Regardless of the numbers, as long as the combustion engine power exceeds what is needed to cruise by some amount, there will be excess power to recharge the batteries and restore reserve power during the cruise portion of the flight

Correct. And once you choose the amount of excess power that "makes sense" and try to design the entire system, you'll realize you're ending up with something worth less than the sum of its parts.

55 minutes ago, billvon said:

you'll be able to recharge on descent, providing you with that "contingency" power for a go-around (for example.)

The overlying problem is this thread has seemingly changed from a place where we try to apply math and engineering to come up with things that might actually work, to a place where we just spout out things that sound good.

Yeah, harvesting wasted energy during descent to provide power for a go-around sounds great! Let's not bother with silly little details like... How much power will be recovered, how much is required for a go-around, how much it costs, and how much it weighs. The burden of evidence is on the person proposing the idea, and none has been presented.

[olofscience 489]

1 hour ago, billvon said:

I'd say you can be arbitrarily close to that limit.  Even if you fly a profile where cruise is at 100% turbine power, you'll be able to recharge on descent, providing you with that "contingency" power for a go-around (for example.)

I don't think that's going to fly at all (pun not intended) with the civil aviation authorities.

The contingency power will probably have to be available ALL the time, what happens if you encounter an emergency situation and your contingency is still recharging?

[billvon 3,063]

8 hours ago, olofscience said:

I don't think that's going to fly at all (pun not intended) with the civil aviation authorities.

The contingency power will probably have to be available ALL the time, what happens if you encounter an emergency situation and your contingency is still recharging?

But contingency power is not available all the time; it's only available for a short time and under specific conditions.  For example, for any normally aspirated recip engine with a contingency power rating, you won't be able to get it at altitude at all for any amount of time.  That's generally fine because for non-military aircraft, the times you will really need it are close to the ground.

Look at density altitude calculations.  There are all sorts of conditions where the power you get is going to be severely restricted by weather conditions.  (And it should be noted that electric or hybrid aircraft will be much less limited by density altitude restrictions.)

[wolfriverjoe 1,523]

9 hours ago, billvon said:

...Look at density altitude calculations.  There are all sorts of conditions where the power you get is going to be severely restricted by weather conditions.  (And it should be noted that electric or hybrid aircraft will be much less limited by density altitude restrictions.)

I would call it 'somewhat less', not 'much less'.

The prop & wings lose thrust & lift, along with the engine producing less power.

Why else would skydivers be warned about how their canopy will fly at higher altitude DZs?

[nwt 131]

10 hours ago, billvon said:

But

Keep dreaming

[BMAC615 209]

https://www.yahoo.com/entertainment/why-electric-planes-havent-taken-171500303.html

[billvon 3,063]

3 hours ago, wolfriverjoe said:

I would call it 'somewhat less', not 'much less'.

The prop & wings lose thrust & lift, along with the engine producing less power.

Right.  So you have to fly faster to get the same lift and drag.  (Which is a net plus for aircraft - and which is why they generally fly as high as possible during cruise.)

This is already being seen in EV vs hybrid vs gas engine performance.  EV power is not affected by density altitude, but drag is decreased - which is why most EV speed records are run in Leadville, CO.

 

[BMAC615 209]

Bye Electric eFlyer 800 seeks to end King Air’s Reign

[IanHarrop 42]

31 minutes ago, BMAC615 said:

Bye Electric eFlyer 800 seeks to end King Air’s Reign

Better link  https://www.ainonline.com/aviation-news/business-aviation/2021-04-22/bye-electric-eflyer-800-seeks-end-king-airs-reign

[BMAC615 209]

5 hours ago, IanHarrop said:

Better link  https://www.ainonline.com/aviation-news/business-aviation/2021-04-22/bye-electric-eflyer-800-seeks-end-king-airs-reign

Yep, not sure how I broke the link - thanks for fixin’ it!

[Divalent 142]

On 4/22/2021 at 2:36 PM, IanHarrop said:

Better link  https://www.ainonline.com/aviation-news/business-aviation/2021-04-22/bye-electric-eflyer-800-seeks-end-king-airs-reign

Interesting quote from this article on the energy density of the batteries they intend to use:

Quote

According to Oxis, the first-generation of these batteries have a specific energy of 450 Wh/kg and an energy density of 550 Wh/L. The company believes these levels could be boosted to 550 Wh/kg and 700 Wh/L by late 2023, and then to 600 Wh/kg and 900 Wh/L by 2026.

That's about double the numbers others used earlier in this thread when assessing the feasibility of electric planes. Admittedly, although they refer to this 1st generation in the present tense, I suspect they are still in development.

[BMAC615 209]

On 4/25/2021 at 12:23 AM, Divalent said:

Interesting quote from this article on the energy density of the batteries they intend to use:

That's about double the numbers others used earlier in this thread when assessing the feasibility of electric planes. Admittedly, although they refer to this 1st generation in the present tense, I suspect they are still in development.

450 Wh/kg -> 600 Wh/kg by 2026 doesn’t seem too unreasonable.