Electric Aircraft - The Thread

Another article that gives more specific details on the Ampaire hybrid version of the Cessna Skymaster (named the Electric EEL), which has two engines in a front-and-back orientation (push-pull). The front engine has been replaced with an electric, so it is configured with a 300 hp piston in the back, and a ~200 hp electric in front.

Mokulele Airlines is current doing a month long series of trial flights on a 20 min route between two airports on Maui. Apparently it does the whole round trip (40 min) set on just one electric charge. Should provide good data on reliability and performance under simulated real route use.

https://www.flyingmag.com/story/aircraft/ampaire-hybrid-electric-trials-begin/

Here's the Wikipedia page for the Electric EEL:
https://en.wikipedia.org/wiki/Ampaire_Electric_EEL
 

An interesting variant is hybrid electric planes, even the size of big airliners. The idea is to have fuel burning engines inside the plane, which generate electricity to drive the electric motors. The benefit is that the exhaust can be passed through a pollution control system before exiting the plane, greatly reducing (90%+) nitrogen oxide emissions, which are apparently are a major source of harmful pollutants from airliners. But it's not a CO2 saver (would somewhat increase CO2 due to additional weight).

https://scitechdaily.com/hybrid-electric-plane-concept-may-reduce-aviations-air-pollution-problem/

Another hybrid variant I ran across for smaller planes would include some batteries to provide additional power for takeoff. It's utility would depend on whether the weight saving of a smaller generator motor would offset the additional static weight of the batteries.

15 hours ago, Divalent said:

But it's not a CO2 saver (would somewhat increase CO2 due to additional weight).

Pratt&Whitney spent more than 20 years developing the GTF - geared turbofan - because it was a big improvement in fuel efficiency, by removing the requirement that the compressor run at the exact same RPM as the turbine. So a hybrid aircraft design could optimise its turbine for generating electricity, because electric motors can then easily rotate at any RPM needed by the aircraft, and batteries could buffer the variation in power needed. Whether a hybrid has an overall increased weight is to be determined - there are too many factors to determine right now, but hybrids will probably be more feasible with larger aircraft, and smaller aircraft will probably be more suited to full electric.

CO2 is only one reason for going electric. The operations and maintenance savings are potentially massive - so if hybrids improve this, even if not as good as full electric, the market will adopt it. (Which is why Airbus is investigating hybrids right now)

3 hours ago, olofscience said:

Pratt&Whitney spent more than 20 years developing the GTF - geared turbofan - because it was a big improvement in fuel efficiency, by removing the requirement that the compressor run at the exact same RPM as the turbine. So a hybrid aircraft design could optimise its turbine for generating electricity, because electric motors can then easily rotate at any RPM needed by the aircraft, and batteries could buffer the variation in power needed. Whether a hybrid has an overall increased weight is to be determined - there are too many factors to determine right now, but hybrids will probably be more feasible with larger aircraft, and smaller aircraft will probably be more suited to full electric.

CO2 is only one reason for going electric. The operations and maintenance savings are potentially massive - so if hybrids improve this, even if not as good as full electric, the market will adopt it. (Which is why Airbus is investigating hybrids right now)

Hybrids are a good option during the transition away from fossil fuels to whatever the final platform will be. Can existing turbines be retrofitted to hybrid or is it a whole new platform?

3 hours ago, BMAC615 said:

Can existing turbines be retrofitted to hybrid or is it a whole new platform?

Aircraft, and their structures are usually designed for a particular engine. Since certification depends so much on that, it's pretty difficult to retrofit. The GTF I talked about took 20 years to develop, then Airbus spent about $1 billion and approximately 4 years from announcement to first flight to put it on the A320 NEO (new engine option). There had to be some structural reinforcements for the wing and changes to the pylon to accommodate it.

But as billvon said, hybrids probably don't offer that much advantage for skydiving ops, especially for the smaller cessna operations I think those will go straight to full electric.

On 2/26/2021 at 10:12 AM, billvon said:

The place that will be useful is during normal operations, where a battery assists during takeoff, then shuts down while the combustion engine handles cruise. That would allow smaller and more efficient engines to handle the bulk of propulsion while still having enough power for takeoff/go arounds.

I don't get it. Unless you jettison the batteries after takeoff, I'm lost to the advantages. Yes, you have a smaller generator... But now you're back to heavy batteries.

Hybrid would be a huge advantage for intracity VTOL transport where noise is the limiting factor, by allowing slower, higher torque blades. Outside of that specific application, the advantages are less obvious and more speculative.

4 hours ago, nwt said:

Outside of that specific application, the advantages are less obvious and more speculative.

Certification costs and MRO.

Right now one of the certification requirements for turbofans is UERF - uncontained engine rotor failure, where a blade of the high-pressure turbine flies off. The structure has to be reinforced where the blade could possibly go, making the aircraft heavier.

If you move the turbine to the rear of the aircraft - actually, just enlarge the existing APU on a single-aisle aircraft and pipe electrical power to the wings, you can:

• stop worrying about UERF and make your wing lighter
• distribute the electric motors along the wings, especially the wingtip to reduce the wingtip vortex (save fuel)
  • you can then use distributed thrust to reduce the stall speed, so you can optimise wing area for cruise - 40% reduction in wing area could mean a huge reduction in weight;
• redundancy - if you have 2 turbines driving 10 electric motors, if one turbine fails you can still power the 10 motors at 50%
  • this will reduce certification costs for the one engine-out scenario, which dictates the size of the tail fin. Reducing this would save some weight too.

This is for a theoretical design which doesn't have any batteries at all - just turbines generating electricity to directly drive motors on the wing. Mission requirements will probably dictate if designers decide to put batteries in or not. But the weight savings could be significant, and airlines will jump at single-digit % fuel savings, which is why Airbus came up with the A320 NEO and Boeing had to follow with the 737 MAX.

13 minutes ago, olofscience said:

Certification costs and MRO.

Right now one of the certification requirements for turbofans is UERF - uncontained engine rotor failure, where a blade of the high-pressure turbine flies off. The structure has to be reinforced where the blade could possibly go, making the aircraft heavier.

If you move the turbine to the rear of the aircraft - actually, just enlarge the existing APU on a single-aisle aircraft and pipe electrical power to the wings, you can:

• stop worrying about UERF and make your wing lighter

 

We already have jet aircraft with all the engines in the back.

When I say advantages are less obvious and more speculative, I'm not suggesting there wouldn't be any, just that you'd have to look really close to determine if they outweigh the costs. The costs seem much more obvious.

2 hours ago, olofscience said:

 

• redundancy - if you have 2 turbines driving 10 electric motors, if one turbine fails you can still power the 10 motors at 50%
  • this will reduce certification costs for the one engine-out scenario, which dictates the size of the tail fin. Reducing this would save some weight too.

 

 

Wow. I never considered the reduction in asymmetrical thrust. 

That could be huge. 

3 minutes ago, billvon said:

You end up with a plane that's about the same weight (remove a gas engine replace with electric motor and small battery) but considerably more efficient (throttle losses reduced)

I'd like to know why you're convinced these things are true. I'm not convinced they aren't, but I'm highly skeptical, and it really seems hypothetical on your part.

8 minutes ago, billvon said:

I would also point out that it's a real PITA to make gas engines that rotate in opposite directions; you basically need two different engines.

Yet it's been done, and a commercially-viable hybrid electric hasn't. Would the latter not also be a real PITA? Pretty ironic that you criticize the need for two different engines, but then your solution involves multiple engines that are orders of magnitude more different from each other.

6 minutes ago, billvon said:

Nope.  Two identical engines.  Electrically commutated motors don't care which direction they spin in.  That's the advantage.

Are you saying that the electric engines can safely be mover further outboard disregarding the effect of asymmetrical thrust? I would point out that although the failure rate of these motors would be far lower, it would not be zero.

20 minutes ago, billvon said:

But you also pay a price in design, since the engines have to be as close as possible to midline to reduce those tendencies. 

5 minutes ago, billvon said:

Let's compare two engines.

The Continental IO-520, a common light aircraft engine.  200hp nominal, 406lbs.  Add another 100lbs for a moderate amount of fuel.

The Hypstair, a Siemens electric motor for aviation.  260hp, 110lbs.  Now we add 396lbs of batteries to make the weights equal.  That's 42kwhr of energy using best available cells (21700 Panasonic-Sanyo cells) which come in at 237 wh/kg.

42kwhr gives you 16.8 minutes of operation at 150kw (200hp.)  Let's call it 5 minutes to account for the batteries being cold, or a little worn out, with the rest in reserve.

So your electric system is the same weight, and very conservatively gives you 5 minutes of 200hp operation.  It has power in reserve if you need it (will go to 260hp) and has extra capacity if you need it.  The gas engine will run for far longer, of course, on those 14 gallons of fuel.  But you have a second one of those to give you the range.

Why are you convinced this will be more efficient? You're going to burn through that 5 minutes before you're done with your initial climb out and then it's dead weight. Forget about climbing to your cruise altitude or the ability to go-around at your destination. Is it even usual for a twin to cruise at 50% or less power?

12 minutes ago, billvon said:

(Note - I am talking about a pure electric in the reply to WRJ above, not a hybrid.)

I misunderstood.

5 minutes ago, gowlerk said:

Are you saying that the electric engines can safely be mover further outboard disregarding the effect of asymmetrical thrust? I would point out that although the failure rate of these motors would be far lower, it would not be zero.

He's not saying that--he's saying unfavorable torque effects of the critical engine can be eliminated.

e: or I guess it would be mostly the slipstream effects. But, all of the adverse yaw effects apart from asymmetrical thrust

1 minute ago, billvon said:

Because the single gas engine will be operating at or close to WOT for most of the flight, which significantly reduces pumping losses.

You could do the exact same thing with the secondary motor burning fuel.

1 minute ago, billvon said:

Nope.  Then you wouldn't have enough power to take off

Sorry, you've lost me here.