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Electric Aircraft - The Thread

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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.

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6 minutes ago, nwt said:

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

Nope.  Then you wouldn't have enough power to take off, which takes significantly more power than cruise.  You could take off and then shut down one engine completely, which has been done.  But you don't get the benefits of reduced fuel usage in that case.

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1 hour ago, nwt said:

Sorry, you've lost me here.

Aircraft engines need to produce varying amounts of power.  High for takeoff, somewhat less for climb, moderate for cruise, minimum to negative for descent.  With a standard reciprocating engine, that means that you have to design for WOT (wide open throttle) happening at takeoff, and significantly less (say half throttle) at cruise.  Half throttle means you are pumping a lot of air past a partially closed throttle plate, which are where the pumping losses come from.

One way to overcome that is to operate at or close to WOT all the time.  But if you are at or near WOT at cruise, you need a way to get more power for takeoff/climb.  That's where the second engine comes in.

The second engine can, of course, be another recip engine, and you can shut it down completely during cruise. (That's what Rutan's Voyager did.)  Or it could be electric which was my proposal.

(There are a million variables that go into this, of course.  On normally aspirated engines you lose power anyway as you climb, so at higher altitudes you may be close to WOT for cruise.  With variable pitch propellers you can use the prop RPM control to control power and operate at very high manifold pressures, but engine manufacturers say to not do that.)

 

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17 minutes ago, billvon said:

Aircraft engines need to produce varying amounts of power.  High for takeoff, somewhat less for climb, moderate for cruise, minimum to negative for descent.  With a standard reciprocating engine, that means that you have to design for WOT (wide open throttle) happening at takeoff, and significantly less (say half throttle) at cruise.  Half throttle means you are pumping a lot of air past a partially closed throttle plate, which are where the pumping losses come from.

One way to overcome that is to operate at or close to WOT all the time.  But if you are at or near WOT at cruise, you need a way to get more power for takeoff/climb.  That's where the second engine comes in.

The second engine can, of course, be another recip engine, and you can shut it down completely during cruise. (That's what Rutan's Voyager did.)  Or it could be electric which was my proposal.

(There are a million variables that go into this, of course.  On normally aspirated engines you lose power anyway as you climb, so at higher altitudes you may be close to WOT for cruise.  With variable pitch propellers you can use the prop RPM control to control power and operate at very high manifold pressures, but engine manufacturers say to not do that.)

 

You said with two gas engines there would not be enough power for takeoff, but with a singe gas engine and single electric engine there would be. This is where you lost me.

You have also not addressed my comments on the utility and danger of having only 5 minutes of power on the second engine.

If the gas engine only has enough power that it cruises at WOT, having only 5 minutes of additional power is absurdly far outside any kind of acceptable realm. Are you a pilot?

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I can foresee using an electric starter-generator to boost the power output of a stock gasoline engine. When the gas engine is producing full power for take-off, the electric motor supplies additional torque spin the prop shaft even faster. If prop tips are almost supersonic, then the prop governor steepens prop pitch to absorb the extra horsepower.

Alternately, the electric motor could turn an additional propeller.

If one gasoline engine quits, then you should have enough reserve electrical power for level cruise and return to the departure runway.

As for asymmetric thrust - after an engine quits on a twin - remember that piston engines more than 300 horsepower and all turboprops don't actually turn props at the same speeds as the crankshaft. Larger piston engines and all turboprops turn their crankshafts at the most efficient speed then use propeller speed reduction units to slow propeller rotation to a speed that keeps prop tips sub-sonic. To change prop speeds or direction of rotation, you just rearrange gears inside the PSRU.

In the not-too-distant-future, we will see electric (trickle charge) stations at airports, similar to the block-heaters already used in colder climates. These will re-charge airplane batteries over-night so that they depart with "full" batteries next morning.

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11 minutes ago, riggerrob said:

I can foresee using an electric starter-generator to boost the power output of a stock gasoline engine.

Guys... All this talk of hybrids using both fossil fuels AND batteries is a weird, fantastical tangent that will never make sense. If someday it ever did sort of make sense, then fully electric would make much more sense. I struggle to see what problem you are trying to overcome with this train of thought, and how you're for some reason suddenly willing to ignore the problem of energy density.

When people talk about hybrid aircraft, they're talking about using fossil fuels to generate electrical energy for propulsion. There are real advantages to this *today* for some niche applications like intracity VTOL.

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20 minutes ago, nwt said:

When people talk about hybrid aircraft, they're talking about using fossil fuels to generate electrical energy for propulsion.

Who needs a flying car when they can have a diesel electric flying train?

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7 hours ago, nwt said:

You said with two gas engines there would not be enough power for takeoff, but with a singe gas engine and single electric engine there would be.

No I didn't.  I said that if you designed the system (whether it had one or two engines) to run WOT at cruise you would not have any additional power available for takeoff.

Quote

You have also not addressed my comments on the utility and danger of having only 5 minutes of power on the second engine.

There are a great many aircraft powerplants now that have 1 (or 5 or 10) minute power ratings, allowing for more power when urgently needed.  Some even break it down further - they rate for continuous power, takeoff power (up to one hour) maximum contingency power (three minutes) and emergency (30 second) power.

They seem to do OK with that.

Quote

If the gas engine only has enough power that it cruises at WOT, having only 5 minutes of additional power is absurdly far outside any kind of acceptable realm.

Apparently not for many aircraft.

Quote

Are you a pilot?

Yes, I am.  And I can think of perhaps two instances while flying that I had a need for more power than I expected to (and that the engine could produce.)  Having even a 30 second 'emergency power' throttle position available would have been useful.

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Cranfield Aerospace Solutions (CAeS)—the UK SME leading the Project Fressonconsortium—will exploit recent advances in hydrogen fuel cell technology to develop a commercially viable, retrofit powertrain solution for the nine-passenger Britten-Norman Islander aircraft.
https://www.greencarcongress.com/2021/03/20210330-fresson.html

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10 hours ago, BMAC615 said:

Cranfield Aerospace Solutions (CAeS)—the UK SME leading the Project Fressonconsortium—will exploit recent advances in hydrogen fuel cell technology to develop a commercially viable, retrofit powertrain solution for the nine-passenger Britten-Norman Islander aircraft.
https://www.greencarcongress.com/2021/03/20210330-fresson.html

Cool, will that leave a vapour trail in the sky? Maybe the government could sponsor it as a way to spread contrail chemicals.

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On 3/3/2021 at 10:35 PM, billvon said:

There are a great many aircraft powerplants now that have 1 (or 5 or 10) minute power ratings, allowing for more power when urgently needed.  Some even break it down further - they rate for continuous power, takeoff power (up to one hour) maximum contingency power (three minutes) and emergency (30 second) power.

They seem to do OK with that.

Yes, I have a plane with a Rotax 912ULS in it that has two different power ratings for max and max continuous. When I get to my destination, that max power is available for as many go-arounds as I need. I'm not cruising at WOT. I'm really not seeing the comparison here.

On 3/3/2021 at 10:35 PM, billvon said:

Apparently not for many aircraft.

5 minutes at a time is completely different from 5 minutes total for the entire flight. I'm honestly confused why you're trying to present this comparison.

On 3/3/2021 at 10:35 PM, billvon said:

Yes, I am.  And I can think of perhaps two instances while flying that I had a need for more power than I expected to (and that the engine could produce.)  Having even a 30 second 'emergency power' throttle position available would have been useful.

Sure, who wouldn't a magic handle that gives you 30 seconds of extra power? Nobody would turn that down. All you have to do to sell that idea is ignore the tradeoffs you've made to provide that handle.

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On 4/1/2021 at 12:58 PM, nwt said:

5 minutes at a time is completely different from 5 minutes total for the entire flight. I'm honestly confused why you're trying to present this comparison.

Couldn't they recharge the batteries in flight? Maybe design for cruising at a tad less than 100% WOT, with the remaining % available to run a generator. Then you'd be able to call on that reserve later in the flight.

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7 hours ago, Divalent said:

Couldn't they recharge the batteries in flight? Maybe design for cruising at a tad less than 100% WOT, with the remaining % available to run a generator. Then you'd be able to call on that reserve later in the flight.

That way they could stay airborne perpetually! 

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On 4/3/2021 at 11:56 PM, Divalent said:

Couldn't they recharge the batteries in flight? Maybe design for cruising at a tad less than 100% WOT, with the remaining % available to run a generator. Then you'd be able to call on that reserve later in the flight.

Depending on what you mean by "a tad", maybe. And you would lose all of the alleged advantages of this nonsense.

 

On 4/4/2021 at 7:19 AM, gowlerk said:

That way they could stay airborne perpetually! 

Couldn't have said it better myself!

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On 4/1/2021 at 9:58 AM, nwt said:

5 minutes at a time is completely different from 5 minutes total for the entire flight. I'm honestly confused why you're trying to present this comparison.

Sure, who wouldn't a magic handle that gives you 30 seconds of extra power? Nobody would turn that down. All you have to do to sell that idea is ignore the tradeoffs you've made to provide that handle.

You referred to the danger of having only 5 minutes of additional power from the second engine, as if having that would be a bad thing.  You now say people would want 30 seconds of extra power, which is 10 times shorter.

I agree.

 

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2 minutes ago, billvon said:

You referred to the danger of having only 5 minutes of additional power from the second engine, as if having that would be a bad thing.  You now say people would want 30 seconds of extra power, which is 10 times shorter.

I agree.

 

Are you trying to troll me?

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On 4/6/2021 at 1:14 PM, nwt said:

Are you trying to troll me?

I'm not seeing it that way.

He's just 'poking' a bit at your objections.

I can see a lot of what's being suggested being fairly practical. Maybe.

Look at how hybrid cars operate.

Small combustion engine (C/E) and battery powering electric motor (B/E).

For acceleration, like from a dead stop, both the C/E & B/E give the car pretty good pickup. A Prius can take off from a stoplight faster than a lot of people understand.

When cruising, the car decides when to run the C/E to drive the car and charge the battery and when to shut the C/E down and run the car off the B/E.

For planes, having a similar hybrid setup would allow both the C/E & B/E for takeoff and climb, as long as the battery lasts. Once at cruise, the C/E would then be used to propel the plane and charge the battery that was depleted from the climb.

It's NOT anything close to a 'perpetual motion' machine (or even claimed to be) because the charging system is run from the C/E motor, dependent on the fuel supply.

It wouldn't be too farfetched to see a system where the C/E runs only a charger/generator and the propulsion motors are only electric. This is analogous to a diesel/electric locomotive or 'old school' diesel electric submarine.

As with anything in airplanes, the biggest deciding factor is weight. 
Can a hybrid setup be put in a plane at the same or less weight as a piston motor? Maybe.
Can it be made lighter than a turbine?
That I doubt. Not yet anyway.

But, given the advances in Li-Po batteries, a turbine powered generator and battery/electric propulsion motor setup may not be as unrealistic as it seems to be today (totally unrealistic with today's technology).

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8 hours ago, wolfriverjoe said:

Look at how hybrid cars operate.

Small combustion engine (C/E) and battery powering electric motor (B/E).

For acceleration, like from a dead stop, both the C/E & B/E give the car pretty good pickup. A Prius can take off from a stoplight faster than a lot of people understand.

Yep.  I had a Prius for almost 10 years and 160,000 miles.  During that time I never noticed the "lack" of engine power - except for once.

I was climbing the Grapevine with a load of solar panels on the roof (about 200 lbs) and a lot of beer in the back.  I was maintaining about 70mph.  At one point I pulled off because I heard something rattling.  I tried to get back on the freeway - and had almost no acceleration.  The 1500 foot ascent with a heavy load and a draggy roof load had used up the battery completely.  I was still able to maintain 70mph (until I stopped) on the gas engine alone.  But when I tried to get back on the road and accelerate uphill - it had nothing left.  I had to wait for 10 minutes for the gas engine to recharge the battery, then it was fine.

I figure if I had to abuse it that hard to see the loss of overall power, it was a pretty well balanced system.

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On 4/8/2021 at 8:20 AM, wolfriverjoe said:

I'm not seeing it that way.

He's just 'poking' a bit at your objections.

I can't take him seriously anymore. The idea of a combusting engine running WOT at cruise, plus a few minutes of extra power by battery just isn't even near the ballpark of what makes sense, for the reasons I've stated.

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

And apparently I've contradicted myself by agreeing a magic free power boost with no engineering tradeoffs would be nice to have? That's an obvious troll, and I think he knows how absurd the whole idea is.

On 4/8/2021 at 8:20 AM, wolfriverjoe said:

Look at how hybrid cars operate.

The practicality of hybrid cars is debatable. Giving them the benefit of the doubt: What's practical on the ground is often not practical in the sky.

On 4/8/2021 at 8:20 AM, wolfriverjoe said:

It's NOT anything close to a 'perpetual motion' machine (or even claimed to be) because the charging system is run from the C/E motor, dependent on the fuel supply.

Right, it wouldn't literally be a perpetual motion machine, but it's closer to that than it is to reality. The reason you think this idea makes sense is that you're falling for the same mental traps that lead people to chase perpetual motion. You want to convert energy forms multiple times without any understanding of the cost, and for no good reason because the power was perfectly usable in its original state in the combusting engine with propeller. If you come to understand this, I expect you will understand the analogy to perpetual motion.

On 4/8/2021 at 8:20 AM, wolfriverjoe said:

But, given the advances in Li-Po batteries, a turbine powered generator and battery/electric propulsion motor setup may not be as unrealistic as it seems to be today (totally unrealistic with today's technology).

Generator-motor hybrid may be viable today for some applications--batteries are not required. I actually saw a pretty compelling presentation on this topic from a company developing such a power system and I've posted about it before. And no, Bill, I'm not contradicting myself by saying this.

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(edited)
19 hours ago, billvon said:

Yep.  I had a Prius for almost 10 years and 160,000 miles.  During that time I never noticed the "lack" of engine power - except for once.

For a car, power at cruising speed might be around 20% of maximum engine power, but for aircraft this number is 70-80%.

The most important number will be total power-to-weight ratio of the combined system (combustion + electric) and this is almost always lower for hybrids.

So sorry Bill but I'll have to side with nwt here - power-to-weight ratio of piston engines are just too low to make hybrids practical. Turbines are probably OK and there are actually several studies being done now by the big aircraft manufacturers about turbine-electric hybrids.

On 4/8/2021 at 2:20 PM, wolfriverjoe said:

(totally unrealistic with today's technology)

Not unrealistic - it just takes time to develop, certify, and sell, especially in this economic climate. All the ingredients are already here, but it takes time to put them all together. Kind of like baking - you can't assume you'll instantly have a full loaf of bread as soon as you obtain all the ingredients.

Edited by olofscience

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3 hours ago, olofscience said:

For a car, power at cruising speed might be around 20% of maximum engine power, but for aircraft this number is 70-80%.

The most important number will be total power-to-weight ratio of the combined system (combustion + electric) and this is almost always lower for hybrids.

Right.  Hence battery size is critical.  Sizing it such that it gets you from takeoff to clean climb configuration will be key there.

Right now the 80-year-old jet engine technology will beat everything else hands-down for power-to-weight ratio.  But we already have considerations other than power-to-weight ratio in aircraft engines, like noise, fuel efficiency and greenhouse gas emissions.  And if a hybrid significantly improves any of those metrics, it will be attractive for some operators.

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12 minutes ago, billvon said:

But we already have considerations other than power-to-weight ratio

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.

Sure, you can sacrifice some performance, but you can't go below legal and practical minimums on things like climb rate, runway length, etc. and the margin isn't very big.

I'm very supportive of fully electric aircraft (C208 size and below) and turbine-electric hybrids above, but I just don't see any space in between them for piston-electric hybrids. The big efficiency win for automotive hybrids was stop-start traffic, and you don't really get that in aircraft (but regen is quite an intriguing possibility for skydiving ops).

So I don't think there will be a big improvement in the metrics you've mentioned. I'd be happy to be wrong though, and it would be interesting to see a piston-electric hybrid for novelty's sake, but for now I don't think the numbers quite work out in its favour.

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