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Jay Kempf

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Regenerating power with an electric engine with a freewheeling prop has been suggested.

The mission profile for this is climb, cruise, then on the descent you would freewheel the prop at some constant speed and at the cost of some drag you would be able to generate power to put some energy back in the batteries.

I've been in a bunch of conversations with people of all levels in the industry and no one seems to ever get to the end of the conversation with any conclusion.

So the question is is there any meaningful ability to generate power during normal flight ops with an all electric airplane. And if so how would one configure the system to best take advantage of it?

... one data point... I had a Prius to use for a while and sort of tested this out. Seemed to be a trivial subsystem with little actual contribution to overall efficiency.
 

Vigilant1

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So the question is is there any meaningful ability to generate power during normal flight ops with an all electric airplane. And if so how would one configure the system to best take advantage of it?
Two major questions to answer I think:
1) How often would "extra" energy really be available? All that potential energy available at cruise altitude can be put to very good use with a well planned descent. If we are "harvesting free energy" with a later, draggier descent, then we stayed at cruise power longer than we would have otherwise.
2) How much are you willing to spend on $$ and weight? It seems unlikely that the best prop pitch for cruise and climb would be ideal for wind generation. Gonna fit controllable pitch props to get this "free' energy? And, the airfoil itself won't be optimum, either.

I don't think there's much/anything net to be gained in most cases. But, folks will keep proposing it.
 
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TFF

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I would think with an aircraft that is designed to travel, it would have no consequence. It might be nice to use the braking for decent profiles if high.

Why? Until the regeneration can be a significant charge, it will not help. The local buses are hybrid. Regenerative braking gives them .5 miles. For them it’s huge. As a hybrid, the bus gets 5-6 miles to the gallon. An over the road truck going the distance might only hit the brakes 5 times in 300 miles and maybe once to a stop. He will barely generate any power.

Now if decent of 10,000 ft gave you enough power for a free go-around, that might be enough reason to call it a safety item. It might be a way for some battery life management, to maybe prebalance cells. Right now if you have flown somewhere, you will be charging with a high power charger or swapping batteries. Your recovered energy will not translate into distance.
 

Jay Kempf

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I guess then a mission has to be defined to see if there is any appreciable benefit. So let's use your FL10 mission. Accelerate and takeoff climb at 750FPM (13.33 minutes) call it 15 minutes from stop to FL10. 1 hour of cruise and the same 15 minutes to descend at 750FPM and and stop. Throw another 5 minutes of taxi at each end for a total of 1 hour 40 minutes. The normal power profile would be full power to lift off, 75% climb, 50-60% cruise power and 25-30% power on the descent.

The only way I think you can look at this is you would have to maintain the same descent rate but slower so steeper to overcome the drag of the prop freewheeling. Or maybe a custom prop with some position near feathered where you set the descent rate and RPM of the generator upon descent.

To me it seems that you would need to extend cruise and head downhill faster and steeper so it would just negate any benefit. But I can't come up with a way to prove that. It does seem that the size of the motor and the size of the plane doesn't matter because the benefit would scale. It's all about whether there would be any benefit at all.

I guess I am not looking to see if there is any useful energy for some other use like a go around. I am looking to draw a line under this and say there is no reason to pursue it as part of a design because there is no net benefit and why.
 

Hot Wings

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Where is the 'spare' energy?
We have some potential energy due to altitude and a little spare kinetic energy from the delta between cruise and best L/D or 1.3 Vs. That is the only energy in the system at the end of the flight to recover - and pretty insignificant compared to the overall flight energy expended.
There might also be some energy to recover during the flight from updrafts - but that too is going to be limited for most power plane mission profiles.
 

Vigilant1

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To me it seems that you would need to extend cruise and head downhill faster and steeper so it would just negate any benefit. But I can't come up with a way to prove that.
That's it. Proving it is more trouble than it is worth, IMO. But, you could spend a day in research to figure out the two-way total efficiency of this wind generator-into-battery-back-to-prop setup (expect a very dismal number). Figuring out the conversation efficiency of the designed-to-pull-an-airplane prop in this role is the toughest part.

The biggest value of this endeavor, IMO, might be in marketing. "And so green! This new plane makes its own power on descent with a wind turbine!! Other old planes just waste this energy. It is the Prius of the skies! ”
 

Hephaestus

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Don't forget the fun involved in putting that charge back in...

On a long descent - how much can you return into to hot batteries? At what charge rate? Are the risks even worth it with the current generation of batteries?
 

Jay Kempf

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Well, that's where all this started. Marketing wanting to sell "regeneration." That might or might not be there.

Would be good to have a no based in reality.
 

wsimpso1

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You want to generate power during the descent? Noble but... The energy is not free - it comes from someplace and it MUST change your descent profile. Remember that rules on the energy game are:
  1. You can not win - you will not get more energy out than you put in;
  2. You will lose - some of the energy put in and some more that you take out will be lost;
  3. You must play the game anyway.
Let's look at where that power would come from.

You start the descent at altitude with both airspeed (kinetic energy) and altitude (potential energy). If you do the descent normally, you dissipate some of this energy as drag through a distance while arriving at a new airspeed and altitude.

Now imagine you have a turbine taking energy from your energy stores (that airstream came because you had airspeed and altitude and you are using it up) to charge a battery. If you use the same descent profile with the same drag and distance, and some of the combined kinetic and potential energy was diverted to the battery, you will end the descent at lower total energy (kinetic and potential) than in the non-generating descent. Yeah, you will finish the descent either slow or low or both. Another possibility is that you maintain same descent rate, but the energy pulled results in a lower airspeed during the descent, meaning you finish the descent further from the destination with a now longer low cruise portion, or you have to fly further at your high cruise before starting the descent - take your pick. In all cases, the best you can hope for is no actual gain in block-to-block fuel use. That is rule 1 above.

Since there are energy losses in converting engine power to a moving plane and energy losses in converting moving air back to power to run a battery, You are actually guaranteed to lose a minimum of about 24% of the energy through aero losses (before you get to electrical losses), while just running your alternator only loses a couple percent mechanically. Rule 2 above.

So, if the other guys were not definitive, I will be. In normal use I fully expect an external turbine driven battery charger to cost you fuel over using an engine driven alternator.

The only places where using an external turbine driven generator will benefit you are:
  • If you were high and/or hot before starting your descent (by ATC or poor planning on your part), then engaging the turbine might allow you to still make your target altitude/airspeed/position without resorting to S-turns, speed brakes, etc. Remember though that you got in that position by flying longer than optimal at high cruise;
  • Your engine failed and your battery is insufficient to complete the flight without some additional power;
  • If you avoid having to put in ADS-B and a transponder.
The Prius, other Toyota hybrids, Ford hybrids, Chrysler hybrids, etc all use a powertrain of the same type. The management want these cars to behave as much like the conventional automatic transmission equipped cars the customers are used to. So, when you let go of the gas pedal, they coast with only basic drag slowing the car - no regeneration going on there. They do regen braking when you push on the brake pedal - the motors in the transmission generate power and charge the batteries then. They will capture braking energy up to about 25% of one g decel. All braking above 0.25 g is still going out in foundation brakes. This is kinetic energy that is normally dumped to the atmosphere from brakes, but some of it is captured to be used in the next accel. This improves fuel economy on the EPA city cycle and for many of its users. The other big fuel save is to turn off the engine while in coast, while stopped, while reversing, and anytime road loads and battery state will allow electric only (golf cart mode) operation.

So, yeah, no or very little regen on hill descents unless you are pushing on the brake pedal. Most of these vehicles have hill mode to reduce speed increase on downhill runs. While you might expect that this mode would store energy in the battery, most hybrids only have enough battery to allow best fuel economy on a city cycle. The battery quickly becomes full on a long downgrade, so they adjust effective speed ratio to raise engine speed for engine braking, same as selecting a lower gear in other types of transmissions. Now maybe the hybrids with the big batteries and capable of long drives electric only will do some significant generation to the battery while in hill mode.

Billski
 
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Dan Thomas

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If one plans on using the drive propeller and motor to generate electricity, one has to remember that the propeller is a terribly inefficient thing with the airflow at negative angles of attack over it. It will generate way more drag than power, and drag just steepens the descent so much that you'd have to come in unusually high.
 

pictsidhe

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The optimum electric descent involves no drag from the prop at best glide speed. If you regenerate, you will come down steeper. But, you have had to fly further under power to do that. Once you count losses, you lose range.

Where regen would be useful is as a 'drag brake' for those times where you need to come down fast rather than efficiently. It is easy to implement with motor controllers, but the gains won't be large.

The altitude potential energy of something is mgh, if you use metric units, you get joules.
 

TFF

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IFR controllers push descent till the last minute. Sometimes you have to tell them, you need to start coming down; I don’t want to dive at the runway. Because of that, using the prop as a descent control like a spoiler could be put to use. The power generated would be small. Controllers could also start using it for a crutch knowing they can bring you in closer before dropping you down so they might expect it down the road.
 

John.Roo

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Here you can find large and detailed article about regen. soaring...

On our first electric prototype we had prop. optimalized for "windmilling" (in flight adjustable up to feathering position + modified profile of blade). We have been able to see "some" generated energy during descent, but at that time (year 2010-2011) our controller did´t allowed us recharge battery.

I am sure there is energy available in thermals (or during descent etc.) but problem is.... efficiency.
In simple words - how much drag you add vs how much energy you get :)
 

wsimpso1

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Where regen would be useful is as a 'drag brake' for those times where you need to come down fast rather than efficiently. It is easy to implement with motor controllers, but the gains won't be large.

The altitude potential energy of something is mgh, if you use metric units, you get joules.
Agreed. It can work, but let's also remember some things needed to make t work...

You also must have sufficent headroom in the battery. If you do not have enough spare capacity, part way down the descent, the battery is "full" and you must stop using the regen. If you have always got enough headroom in the battery, and this descent ends up not doing any regen, the remaining battery capacity must be adequate for ground activities on the next flight. The result is you must specify a battery with significantly more capacity than you will use on many flights and specify a battery that works fine when significantly down on stored charge. We know about this in cars - the battery is not maintained at full, but with enough spare capacity so that the largest energy stop on the EPA, Euro, and Asian fuel economy cycles can be done entirely on regen, but only barely. And a 3000 pound car decel from 35 mph is the same energy as a 2000 pound airplane descending 61 feet. The energy storage for a 10,000 pound descent , even if only a modest amount of the potential energy is to be stored, is a huge amount of battery capacity.

OK, so we have figured out that it can not reduce fuel block-to-block and it will be impractical as a dive brake, and we have not even gotten into adding cost and weight and bulk to the airplane yet. Sounds like air driven generation is a bad thing to do to an airplane.

Billski
 

dog

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There will always be more than enough charge capacity for decent charging and useing the
generator feature as a brake on landing, as the power used in getting airborne will always be larger that any that is recoverable.
And as having the feature is mostly programing , a real question is ,what are the possible failure
modes , such as "charging" into the ground.
And the general tendency of "high teck" gadgety
stuff to suffer from feature bloat.
The big birds will have it,and for the few doing steep decents from 10k+ then the best bet would be have it as a feature that can be "enabled" and the rest of us can mabey spend more time piloting
and less time pressing buttons
 

WonderousMountain

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Supposing your motor and Prop could be adjusted to a similar
rpm - if different from cruise - it ought to provide use over 1/2
your potential descent energy.
(Real descent - natural descent) x Efficiency.
Negative angle should remain within you blades
low drag range. With a good glide ratio, there's
a usefull amount of power.

What you really lack is accessible examples.

THX all,
luPii
 

BJC

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Assuming that the objective is to go somewhere, the least total energy required per mile through the air while descending (or powered in level flight) is achieved at the best L/D speed. Any driving of the propeller by the airstream will reduce the L/D. Likewise, an automobile will, from, for example, highway cruise speed, cover a greater distance simply by coasting to a stop than by extracting energy from the momentum to charge the energy source for use in propulsion.

Regeneration makes sense when the vehicle needs to be braked and the energy recovery mechanism is sufficiently efficient to justify the added weight and complexity.


BJC
 

Hot Wings

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Regeneration makes sense when the vehicle needs to be braked and the energy recovery mechanism is sufficiently efficient to justify the added weight and complexity.
Even the movie versions of the future highway in the sky don't have stoplights.
No need to brake = no need to regenerate.

Imagine a world where the swarm of BlackFlys all have to wait at stop lights for the other half to go by. 🤔
 
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