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dog

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Breaking on landing? Have you calculated that?
As well as a propeller that can transition to a windmill.


BJC
Have not calculated the braking force of an
electric aircraft useing the generator function
,bet its small, but if used as a mater of course
might pay off over time in tires and brakes, or not.
Thinking the electric power control through in the
proposed configuration and it could have three zones, generator in the middle,power in thrust/flight mode and then the back side of the
envelope would be reverse thrust for active braking,or to make a decent even steeper.
The windmill idea is total greenwashing gold.
Brilliant for marketing,and in a few very windy spots might even work,sortoff,still chirpy save the
planet and eat cake headline idea.
 

Dan Thomas

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Have not calculated the braking force of an
electric aircraft useing the generator function
,bet its small, but if used as a mater of course
might pay off over time in tires and brakes, or not.
Thinking the electric power control through in the
proposed configuration and it could have three zones, generator in the middle,power in thrust/flight mode and then the back side of the
envelope would be reverse thrust for active braking,or to make a decent even steeper.
The windmill idea is total greenwashing gold.
Brilliant for marketing,and in a few very windy spots might even work,sortoff,still chirpy save the
planet and eat cake headline idea.
You still have to address the huge losses created by airflow striking the prop at a negative angle of attack. Ever seen an airplane with a cambered wing fly upside down during an aerobatic show? See the very nose-high attitude required to maintain flight, the full power, and the mushy performance? That's what a windmilling propeller faces. Windmills (wind turbines) have a completely different blade geometry and twist to extract power from the wind. Regeneration from a windmilling prop will cause so much drag that you'll have to add power to reach your destination, and there's goes what little you generated plus a whole bunch more. Low power, basically zero-thrust, would let you glide there much more efficiently.
 

BJC

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The windmill idea is total greenwashing gold.
I think that you are referring to my comment, “as well as a propeller that can transition to a windmill.” What I mean by that is that to recover even a small percentage of the theoretical energy available when descending or breaking, a propeller that is designed to act as a windmill, rather than a (very, very inefficient) propeller being turned by the wind, is needed.


BJC
 

dog

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301
I see the point, when I thought it through its likely
going to generate just enough power to not quite
run the electronic controls,though another poster
reports some power in a ridge soaring glider.
"Windmills (wind turbines) have a completely different blade geometry and twist to extract power", yes and no, there are wind turbines with
constant cord,no twist,no taper, blades that do
just fine, and construction is vastly simpler.
Wind turbines are not(or shoud not) be designed
like airplanes,duty cycle is 100%,and weight is only
a secondary issue.
The idea of an electric aircraft generating some of its own power is and will continue to be so compelling that as long as there is no weight,or other penalty then best bet for a manufacturer would be to include it as a "feature" and have the
alternative press run with it,have to consider the risk of an obsesive compulsive type getting in trouble trying to get "free energy"
Big birds yes,small ones no,gliders are going to do
anything they feel like anyway so go go go.
 

Aerowerx

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Don't forget about having to divert to an alternate field. What would that do to your scenario?

Approach your destination, start your descent. Almost down and then have to divert to the alternate (even if that means turning around and going back home). Climb back up, fly for another half hour, and then descend again. Will you have enough battery charge left to do that, even with regeneration?
 

Vigilant1

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Don't forget about having to divert to an alternate field. What would that do to your scenario?

Approach your destination, start your descent. Almost down and then have to divert to the alternate (even if that means turning around and going back home). Climb back up, fly for another half hour, and then descend again. Will you have enough battery charge left to do that, even with regeneration?
"Even" with regeneration? Especially with regeneration.
Hopefully any regeneration option will also come with a time machine capability for instances like the one you describe--so you can rewind to the beginning of your descent and get a "do over" where you do the smart thing and come out of cruise power earlier and avoid the "feels good but wastes energy" steeper windmilling regeneration descent. Instead you keep that extra (wasted) cruise-time juice in the battery and end up way ahead.
 

Dan Thomas

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I see the point, when I thought it through its likely
going to generate just enough power to not quite
run the electronic controls,though another poster
reports some power in a ridge soaring glider.
"Windmills (wind turbines) have a completely different blade geometry and twist to extract power", yes and no, there are wind turbines with
constant cord,no twist,no taper, blades that do
just fine, and construction is vastly simpler.
Wind turbines are not(or shoud not) be designed
like airplanes,duty cycle is 100%,and weight is only
a secondary issue.
The idea of an electric aircraft generating some of its own power is and will continue to be so compelling that as long as there is no weight,or other penalty then best bet for a manufacturer would be to include it as a "feature" and have the
alternative press run with it,have to consider the risk of an obsesive compulsive type getting in trouble trying to get "free energy"
Big birds yes,small ones no,gliders are going to do
anything they feel like anyway so go go go.
We need someone to build it and prove it, someone really sure that it will work. You're appointed.
 

BBerson

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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.
That's because gravity as a thrust source is 100% efficient.
The Grob prop can easily be feathered quickly with a handle in the cockpit. I have many times "pumped" the stopped prop back and forth from feather to get the stopped engine started in flight. It doesn't work that well. For regen, the prop would need to go beyond feather and into a position where the flat bottom is facing forward.
 

Dan Thomas

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For regen, the prop would need to go beyond feather and into a position where the flat bottom is facing forward.
That's true, but at that point the twist of the propeller is all wrong. The root areas will be flat to the wind while the outboard sections will be trying to generate power. Horrible drag.
 

dog

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Dec 29, 2019
Messages
301
That's true, but at that point the twist of the propeller is all wrong. The root areas will be flat to the wind while the outboard sections will be trying to generate power. Horrible drag.
Horrible drag,or excellent braking?
plus didnt know that a variable pitch prop could
start a stopped engine,thats something like
5hp,so in a scenario that involves a steep decent
from altitude into a short strip, might get back
enough juice to taxi in and out.
Electric aviation is going to need every last little
bit of help in order to avoid having people running
out of juice ,pv on the whole aircraft is currently
possible,though very pricy.
Once the time in the air gets to 2hrs, and the planes are generating some,or all(if you can wait)
of there own power ,electric sport aviation will be
viable and atractive to the public at large.
They just launched two all electric cruise ships for
the norwiegen fiords,plus they have installed charging stations up to and past the arctic circle,
and there is sustained interest and developement
of electric aviation.
Cant wait to see an electric unlimeted airobatic
plane,contra rotating,reversable rotation motors
with variable pitch props, torque rolls anyone?
 

John.Roo

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Oct 8, 2013
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Location
Letohrad / Czech Republic
Hello!
Windmilling prop is really efficient "brake" but you must be very carefull to do not exceed max. RPM. Electric motor / generator will "survive" high RPM, but max. RPM of prop is limited. Electric motor has no compression like combustion engine so prop RPM increase really quickly (personal experience). As already mentionned - you definitelly need adjustable pitch prop + I would personally prefer some system of automatic pitch (RPM) control. Something like "intensity of regeneration" in cars.

In this file you can read about theory of Regenerative Flight + expected efficiency of windmilling prop:
DEVELOPMENT OF THE REGENERATIVE SOARER: THEORETICAL AND PRACTICAL ASPECTS
(PDF attached)

Best regards!
Martin
 

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Dana

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Some totally made up numbers, theoretical extreme case:

Say you have a 1500# aircraft with a 10:1 glide ratio (not unreasonable, a Cessna is around 9:1 but I like round numbers) cruising at 5000'. The maximum energy you could regain is the aircraft's potential energy due to altitude, e.g. 1500*5000 or 7,500,000 lb-ft. That assumes a constant velocity vertical dive where all of the aircraft's weight is resisted by the propeller.

If the L/D is 10:1 that means it takes 150# of thrust for level flight. A 10:1 glide from 5000' will take you 50,000' or about 9-1/2 miles. The energy needed to cruise that 9-1/2 miles to where you start the vertical dive is 50,000 * 150, or... the same 7,500,000 lb-ft. No advantage, and a 10:1 glide is much less alarming to the passengers than a terminal velocity vertical dive.
 

BJC

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.... less alarming to the passengers than a terminal velocity vertical dive.
BTDT, from 10,500 to 500, at 180 MPH.

Not alarming to the pilot, but after the first 6 or 8 seconds, the sustained down line was unusual.


BJC
 

jedi

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Like in all aircraft design the mission defines the aircraft. The OP proposal would be best applied to a skydiving transporter or a glider aerotow tug.

The glider tug optimization would likely morph into the electric self launch motor glider and we already have that so the answer is known.

For the skydiving plane the optimum solution would be to eliminate the plane and power the individual. That would be powered paragliding an that has also been covered.
 

Tiger Tim

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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?
While regen in descent may be feasible, I’m not convinced it would be practical or useable in any current production aircraft (or anything similar to current offerings) converted to electric or hybrid electric. The main issue I see is actually a human factor, that being the effect of a steep descent on the occupants of the plane.

Generally speaking the further beyond about 500fpm cabin descent you go, the more it starts to be an issue. Consider the power-off descent rate of whatever you fly then think about how much steeper it would be if you were trying to use the propeller as a windmill. I forget the formula but you ought to be able to take how many kilowatts you want to produce, convert to horsepower, then using the weight of the plane find the descent rate... I think. It’s been a while since I last wrapped my head around this and I’m assuming that excess hp required to climb is functionally the same as braking force required to steepen a descent.

A better case can probably be made for an efficient airplane (regardless of power source) to effectively be a sort of touring motor glider that just starts descent early and coasts down at a couple hundred feet per minute at low power. This, however, doesn’t sound new or ‘disruptive’ and probably won’t get you grant money and other backing when you say you’re working on it.
 

BJC

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I forget the formula but you ought to be able to take how many kilowatts you want to produce, convert to horsepower, then using the weight of the plane find the descent rate... I think.
Don't forget the losses due to the inefficiencies of the propeller operating at a negative angle of attack. It will be significant, even if using a symmetric airfoil, due to the twist distribution.


BJC
 

Tiger Tim

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Don't forget the losses due to the inefficiencies of the propeller operating at a negative angle of attack. It will be significant, even if using a symmetric airfoil, due to the twist distribution.
You’re right of course, I rattled off my post above while waiting to catch a flight this morning so I omitted the efficiency losses. Taking that into account only makes the amount of pilot/passenger discomfort per unit of energy produced even worse. Before someone says “but if you had a pressurized cabin” it would need to run a very high differential pressure to keep from catching on the way down (which brings a weight penalty) and also the deck angle inside the cabin on a regenerative descent would be extremely steep. To steep to routinely do with passengers.

Possibly the most successful ‘charge-in-descent’ setup I can think of would be a turbine electric parallel hybrid. The idea being that the turbine always runs at its design temp and RPM (that gives the best specific fuel consumption) and provides some constant fraction of takeoff power while coupled to a motor/generator that either adds horsepower for takeoff and climb or generates electricity for battery charging in descent. There are inefficiencies in throttling back a turbine, which this seeks to avoid, but I don’t know if you really could make up the difference by adding all this complexity. In any case the energy generated in descent wouldn’t be critical to making it to your alternate, all it could reasonably do is shorten your turnaround time on the ground by giving the battery a head start on recharging. You would of course never take off in the first place without adequate reserves.
 
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