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Electric Aircraft - The Thread

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I was looking at wind turbine power charts elsewhere and noticed that because of minimum wind speed (before the blades spin), it gets a little jumpstart on the power of cube early in the curve.  The formulas are fairly complex though and I certainly could have overlooked some variables such as:

2 hours ago, nwt said:

Another realization: A good chunk of gravitational potential energy is going out the door on jump run.

Touché.  That's a good observation!

Still, even 5-10% regen return is still free cash.  Resistance on the propeller will cause some weird flight dynamics but nothing an approved ePlane jump pilot couldn't handle since they already do frequent dives and well versed on the dynamics of an open jump door and sudden weight shifts.  Regen ops will be much tamer than those mudane plane-bouncy factors...

Some tests and training might be needed for FAA approval of regen, which might later have rules about regen approvals (like no regen below 1000 feet or such). 

Edited by mdrejhon

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I think you are overthinking it.

First off, a variable pitch propeller would probably do best for regen at low pitch. The relative wind would spin it the fastest there. 
I don't think any exotic stuff would have to be done with the prop to get it to work better for regen. There might be some gains, but the 'cost to gain' would probably be not worth it. The primary function of the prop will still be moving the plane. Regen would be secondary.

The loss of weight (gravitational potential energy) would also not be a huge factor. 
I think the limiting factor would be the size of the prop. It's only going to produce so much drag, and so much energy.

This wouldn't be a big thing no matter what.

You aren't going to regen a significant amount of energy. Not in a 5-10 minute descent. But that's not the point. 

It's far more that some energy can be recovered. That would reduce the charge time on the ground. 

It would take a bit of experimenting to find out how much regen could be achieved and how beneficial it is. 

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55 minutes ago, mdrejhon said:

I was looking at wind turbine power charts elsewhere and noticed that because of minimum wind speed (before the blades spin), it gets a little jumpstart on the power of cube early in the curve.  The formulas are fairly complex though and I certainly could have overlooked some variables such as:

Touché.  That's a good observation!

Still, even 5-10% regen return is still free cash.  Resistance on the propeller will cause some weird flight dynamics but nothing an approved ePlane jump pilot couldn't handle since they already do frequent dives and well versed on the dynamics of an open jump door and sudden weight shifts.  Regen ops will be much tamer than those mudane plane-bouncy factors...

Some tests and training might be needed for FAA approval of regen, which might later have rules about regen approvals (like no regen below 1000 feet or such). 

Small propellers will have high drag losses for regen, so actually electric helicopter ops for skydiving will actually be more optimized - electric helicopter climbs to altitude, then does autorotation descent which will (hopefully) recover a significant amount of the energy used to climb (minus the skydivers' gravitational potential energy and drag losses of course).

19 minutes ago, wolfriverjoe said:

This wouldn't be a big thing no matter what.

Well *theoretically*, in an aircraft that climbs to altitude then descends to the same point (no one jumps out), the net gravitational potential energy change is zero, so you should be able to get 100% of the energy of the climb back, minus drag and engine losses. Unfortunately those losses will be pretty high.

Edited by olofscience

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17 hours ago, mdrejhon said:

Resistance on the propeller will cause some weird flight dynamics

Nah, pilots are already used to using propellers as speed brakes, so nothing changes much there.

 

16 hours ago, wolfriverjoe said:

First off, a variable pitch propeller would probably do best for regen at low pitch. The relative wind would spin it the fastest there. 

I think you might be missing the fact that during regeneration, you are going to need significant torque on the prop--it isn't going to be freewheeling. I'd expect the prop settings to be closer to that of a climb than anything else. To put it another way: intuitively, I'd expect the prop settings to be similar when you are trying to transfer the maximum amount of energy between the motor and the air, regardless of the direction of that transfer. Though we do care about efficiency, so maybe a cruise setting would be better... but anyway, this is a minor point as it doesn't affect feasibility--If we ever get to that point, we'll figure it out just fine either way.

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I don't think any exotic stuff would have to be done with the prop to get it to work better for regen. There might be some gains, but the 'cost to gain' would probably be not worth it. The primary function of the prop will still be moving the plane. Regen would be secondary.

Agreed. There should be no changes that affect powered flight in any way and no additional systems apart from the electronics required for harvesting the energy.

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The loss of weight (gravitational potential energy) would also not be a huge factor. 

The energy loss here will be exactly equal to the amount of weight lost in proportion to the total weight--no more, no less. For example, if an otter is fully loaded at max gross weight (12,500 lbs) and drops 23 jumpers at 200 lbs each (4,600 lbs), you've lost 37% of your gravitational potential energy. That's pretty significant, and my gut says jump otters don't operate anywhere near max gross.

Quote

You aren't going to regen a significant amount of energy. Not in a 5-10 minute descent. But that's not the point. 

It's far more that some energy can be recovered. That would reduce the charge time on the ground. 

It would take a bit of experimenting to find out how much regen could be achieved and how beneficial it is. 

I agree wholeheartedly with this sentiment, except for the reference to time. Stop thinking about time and instead think about gravitational potential energy. Time isn't relevant in the way you're thinking, as I've explained previously.

 

16 hours ago, olofscience said:

Small propellers will have high drag losses for regen, so actually electric helicopter ops for skydiving will actually be more optimized - electric helicopter climbs to altitude, then does autorotation descent which will (hopefully) recover a significant amount of the energy used to climb (minus the skydivers' gravitational potential energy and drag losses of course).

Haha that's a really interesting point, but using a helicopter because it will recover energy better would be like cutting off your nose to spite your face. You are going to spend so much more energy to recover a couple more peanuts!

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46 minutes ago, nwt said:

Haha that's a really interesting point, but using a helicopter because it will recover energy better would be like cutting off your nose to spite your face. You are going to spend so much more energy to recover a couple more peanuts!

Yeah, I realised that after I posted :rofl:

But the maintenance and fuel savings for electric ops will already be massive, so maybe regen is just a tiny bit of icing on the cake.

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1 minute ago, olofscience said:

Yeah, I realised that after I posted :rofl:

But the maintenance and fuel savings for electric ops will already be massive, so maybe regen is just a tiny bit of icing on the cake.

Yes, I think regen would just be a bit of icing. It might end up being useful, but I don't think the success of electric aircraft will depend on it.

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4 minutes ago, nwt said:

Yes, I think regen would just be a bit of icing. It might end up being useful, but I don't think the success of electric aircraft will depend on it.

I'm also not quite sure what drives prop sizing on C208s for example, but my guess would be the gearbox loading and max engine power - so I'd imagine with electric, having a direct electric drive and the power advantages you mentioned earlier, you can mount a much bigger prop than before. If you mount the biggest prop that's practical (for ground handling, etc) then you'll get the most benefit out of regen.

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