# Airplane with full regen!



## brian_ (Feb 7, 2017)

While I'm sure that regen during descent works, normal descent and approach are not done at idle - engine power is still used, and drag is added with a greater angle of flaps than needed for the lowest stall speed. I doubt that much energy will be recovered during descent, given how little "braking" is required.

Also, on any reasonable length of flight, descent is very little of the flight duration. It's like a long highway trip on a freeway without traffic - there's no braking until one trivial stop at the end.

Regenerative braking in a car is worth doing, but only adds a few percent to range because it isn't a lot of the drive and the recovery of energy isn't highly efficient. It will likely be even less significant in an aircraft... but every little bit helps.


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## brian_ (Feb 7, 2017)

Pipistel does build good aircraft; I've only seen the Virus up close - the Alpha Electro appears to be a variant of the Alpha Trainer, which in turn is very close to the Virus (Rotax-powered) and Sinus (motorglider).

The Virus is pretty slick (aerodynamically) and so actually uses air brakes, which is unusual for powered light aircraft. Perhaps this is a particularly good basis for an electric aircraft that can regenerate on descent.

Pipistrel's page for the aircraft says


> The Alpha Electro is optimized for traffic-pattern operations, where 13% of energy is recuperated on every approach, increasing endurance and at the same time enabling short-field landings.


Yeah, this is not normal aircraft operation; it never leaves the pattern while training students specifically on take-off and landing, and yet still only recuperates 13% of the energy used. Much better than nothing...

I note that in the video they talk about how it can regenerate, but in the flight segment they didn't mention it.


Unrelated to the regeneration, I see that the charging connector appears to take three-phase power (has L1, L2, L3, plus N and ground pins) at 10 kW.


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## Sunking (Aug 10, 2009)

Last thing you want when landing is the your propeller to add drag. That would cause your plane to fall out of the sky. Bad things happen when you run out of air speed and altitude. Only after the plane is landed with all wheels on the ground do you want drag from your power source, and propellers designed to do that go into reverse thrust at full power. 

Planes with Constant Speed Props (prop and engine RPM remains constant) vary the pitch angle of the propellers to control thrust. In the event of an engine failure (multi-engine and some single) the props are designed to Feather or turn to a very high angle into the wind to prevent Windmilling. If a Prop Windmills creates a lot of Drag, and on a two or more engines can throw the plane out of control into a spin. It can also be disastrous on a single engine plane making the plane have to descend on a very steep angle to hold enough airspeed to remain flying. That means you cannot glide very far. 

Planes land and with power on. There are very few times a pilot wants to add drag to slow the plane down very quickly. Some planes like Jets and Turbo Props have spoilers used to slow a descending aircraft and decrease lift. Just about all planes use Flaps to add drag and add lift so they can touch town at the lowest speed possible with power on. You cut power when you flare for landing to let the plane sit down gently, not fall out of the sky.


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## brian_ (Feb 7, 2017)

Sunking said:


> Last thing you want when landing is the your propeller to add drag. That would cause your plane to fall out of the sky. Bad things happen when you run out of air speed and altitude.


No - while I don't see a lot of benefit to regenerative operation for most aircraft, the added drag certainly won't cause an aircraft to simply fall out the sky; that only happens in a horribly bad spin or when a wing is lost. In this case, we're not talking about a wild theory; the Alpha Electro is a operational aircraft which executes approaches with regeneration.

Added drag makes the glide path steeper. The glide path of the non-electric Virus is apparently not steep enough for some approaches, so they fitted air brakes; the regen mode is essentially a more productive air brake. Most of my flying experience was in gliders - they have substantial air brakes (and spoilers, typically) to provide glide path control, since their glide path is very flat (high lift-to-drag ratio) and they don't have engine power for control.



Sunking said:


> ...
> Planes with Constant Speed Props (prop and engine RPM remains constant) vary the pitch angle of the propellers to control thrust. In the event of an engine failure (multi-engine and some single) the props are designed to Feather or turn to a very high angle into the wind to prevent Windmilling. If a Prop Windmills creates a lot of Drag, and on a two or more engines can throw the plane out of control into a spin. It can also be disastrous on a single engine plane making the plane have to descend on a very steep angle to hold enough airspeed to remain flying. That means you cannot glide very far.


Sure, but a windmilling engine is just like engine braking in a car... that can add a lot of drag. Regeneration would be controlled at a suitable level, and obviously is in this case. The pilot would not use regeneration if needing to extend the glide.



Sunking said:


> Planes land and with power on. There are very few times a pilot wants to add drag to slow the plane down very quickly. Some planes like Jets and Turbo Props have spoilers used to slow a descending aircraft and decrease lift. Just about all planes use Flaps to add drag and add lift so they can touch town at the lowest speed possible with power on. You cut power when you flare for landing to let the plane sit down gently, not fall out of the sky.


I think the thing to keep in mind that aircraft vary in their drag characteristics, and that this is a low-drag aircraft. I suppose it's likely that most aircraft which are seriously considered for electric operation would be have relatively high lift-to-drag ratios, for efficiency... since energy is a precious commodity when it comes from a battery!


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## dcb (Dec 5, 2009)

has up to 65kw on takeoff. recovers 2-5kw on landing. what exactly do you consider "full regen"?!?


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## brian_ (Feb 7, 2017)

dcb said:


> has up to 65kw on takeoff. recovers 2-5kw on landing. what exactly do you consider "full regen"?!?


I don't know what the original poster meant, and I thought the term "full regen" was strange, too. I would consider "full regen" to be operation of the motor-generator as a generator at the same power level as it runs as a motor.

Of course in this case power will be limited by the effectiveness of the prop in reverse to its normal mode... they're not symmetric. There's also the question of how much drag is desired on descent - the pilot certainly wouldn't want nearly as much drag to descend in a normal approach (even steeply) as thrust to climb. Even if the motor can regen at 65 kW, that condition would never occur in routine operation of the aircraft.


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## nucleus (May 18, 2012)

*braking an aircraft*

I like the braking effect of a constant speed prop, super effective for getting the plane down and slowing down on short final, but you sure aren't going to regen much.


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## aeroscott (Jan 5, 2008)

they can shorten the landing run out by reversing the motor , great for the breaks and tires.
Google is funding a electric aircraft that will run on a tether controlled by AI. they say a 35 ft wing will generate 250kw making 9g circles about 500 ft. diameter about 1000ft. high.
So if you had electric seaplane with short range, you could land every so often ,auto takeoff ( without pilot)with the tether attached to a tree or hydrofoil recharging the batteries , land get in and fly on. 
Similarly a off road rv or car could be charged with a electric kite/ aircraft . Even while being driven as long as the wind holds up.
There is another type of kite generator that relies on the tether pull to generate the power. They change the angle of attack to real in the tether then power up the kite with higher angle of attack and spool out the tether generating power.


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## Karter2 (Nov 17, 2011)

aeroscott said:


> ...... Google is funding a electric aircraft that will run on a tether controlled by AI. they say a 35 ft wing will generate 250kw making 9g circles about 500 ft. diameter about 1000ft. high.."........


... Can you explain what that is doing ???


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## brian_ (Feb 7, 2017)

aeroscott said:


> ... Google is funding a electric aircraft that will run on a tether controlled by AI. they say a 35 ft wing will generate 250kw making 9g circles about 500 ft. diameter about 1000ft. high.





Karter2 said:


> ... Can you explain what that is doing ???


It apparently "flies" on the tether like a kite on a string, but running propellers on generators (wind turbines) to harvest power. It would function as a wind power generator, with a tether instead of a tower.

The acceleration is so high because it is maintaining high speed (apparently about 100 km/h or 60 mph) in a tight circle.

I just found the company's web site: Makani


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## Karter2 (Nov 17, 2011)

I can understand a tethered "glider" with prop driven generators..
But what is the flying in circles at 9g got to do with it ?
Or am i to imagine it like a stunt kite looping madly to increase the apparent speed over the props ?
Google is my friend !
https://youtu.be/GSYMHzgLLn8


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## aeroscott (Jan 5, 2008)

I was reading a book on designing a wind generator and was supersized at the low force that the wind imparted on mast.I think this is do to the lift ratio of the blades say 20 to 1 .
This would make tethering easier


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## aeroscott (Jan 5, 2008)

Karter2 said:


> I can understand a tethered "glider" with prop driven generators..
> But what is the flying in circles at 9g got to do with it ?
> Or am i to imagine it like a stunt kite looping madly to increase the apparent speed over the props ?
> Google is my friend !
> https://youtu.be/GSYMHzgLLn8


 that's it. 
At 60mph I think it would take a large prop even with the cub power of wind speed.


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## brian_ (Feb 7, 2017)

Karter2 said:


> I can understand a tethered "glider" with prop driven generators..
> But what is the flying in circles at 9g got to do with it ?
> Or am i to imagine it like a stunt kite looping madly to increase the apparent speed over the props ?


Yes 
A usefully large stationary turbine, held up by a wind and tethered by a cable, would be very large - this just appears to be a strategy to keep the size down.


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## aeroscott (Jan 5, 2008)

I looked up wind power per square meter 60 mph is 10kw and 120 mph is 1000kw X 40% efficiency. 
So you would need to fly fast in order to use a aircraft size prop.


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## piotrsko (Dec 9, 2007)

Fwiw: Back in the 70's we used to fly tethered U Control on windy days with the motor stopped. Might be the same process.


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## aeroscott (Jan 5, 2008)

I watched the video of Makani's flight . If the path is about 500 meters long
(circumference of 145 meter dia.) It looks like about 5sec. /per circle or 80-100 meters per sec. Thats well over the 1000kw per sq. meter theoretical 
times 40% prop eff.=400kw.
I want to check this with the 9g force.
CEO dies at office in 2012 of heart insufficiency , age 38, world class fitness.
300 meter dia. circle at 2rpm = 1gforce , so a person could be on board 314 meters/per sec. air speed Oh! that's 700 miles per hr.
the 9 g's works out also at 5 rpm , 12 sec./revolution 
how much drag does the tether cause.
rechecked 31 meters/ sec. 70mph


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## aeroscott (Jan 5, 2008)

Just watched a Mankni vid , they say 150mph or 67m/s. They feel noise levels are to high.Just reduce tip seed with slower larger props or duct the props with a little more loss. thats about the speed that ducts start adding more drag.
added Makani power


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## aeroscott (Jan 5, 2008)

I misspelled Makani. been thinking about the kite as a replacement for the sailing rig on a catamaran. I know how to build sailing rigs at low cost , but this would be automated great power and charge batteries and be able to run a large electric prop between the hulls and have a electric airplane to boot.


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## duncgyro (Aug 22, 2017)

Sunking said:


> Last thing you want when landing is the your propeller to add drag. That would cause your plane to fall out of the sky. Bad things happen when you run out of air speed and altitude. Only after the plane is landed with all wheels on the ground do you want drag from your power source, and propellers designed to do that go into reverse thrust at full power.


Not true, pilots always reduce power when landing. If above the glide slope, then they reduce power to add drag in order to descend to the desired path. Pitch is used to control airspeed. The engine is generally producing less "thrust" than the forward speed. Airplanes do not "fall out of the sky" unless structural failure or a uncontrollable spin has occurred.



Sunking said:


> Planes with Constant Speed Props (prop and engine RPM remains constant) vary the pitch angle of the propellers to control thrust.


No. A constant-speed propeller is a variable-pitch aircraft propeller that generally uses oil pressure to automatically change blade pitch in order to maintain a chosen rotational speed. The thrust delivered is proportional to the arithmetic product of rotational speed and torque (radians/second × torque), and the propeller operation places emphasis on torque. Engines produces varying amount of torque as the throttle is varied, but you don't want the propeller RPM to get out of the "sweet spot" where it has best efficiency, depending on airspeed. Thus its RPM is maintained. In multi-engine propeller aircraft, training is usually done with one engine configured for "zero thrust" as a safer method producing the desired training scenario as the retarded engine is still running and can be quickly brought online if needed.



Sunking said:


> In the event of an engine failure (multi-engine and some single) the props are designed to Feather or turn to a very high angle into the wind to prevent Windmilling. If a Prop Windmills creates a lot of Drag, and on a two or more engines can throw the plane out of control into a spin. It can also be disastrous on a single engine plane making the plane have to descend on a very steep angle to hold enough airspeed to remain flying. That means you cannot glide very far.


No. Other than motor-gliders, there are no common certified single engine aircraft built that have feathering propellers. Most, but not all, multi-engine aircraft are built with props that will auto-feather only if oil pressure is fully lost, but the engine will continue to spin and thus still have oil pressure even if the ignition is turned off. The pilot must move the propeller control into "feather" or else press a switch that will electrically pump oil pressure into the feathering mechanism. The latter type is used on most WWII aircraft and that oil is pumped from a lower reserve sump, that is not used as an engine oil source. (I have flown many including the B-17, since 1985)



Sunking said:


> Planes land and with power on.


No. Unless you mean the engine is effectively idling. Then this is true. 



Sunking said:


> There are very few times a pilot wants to add drag to slow the plane down very quickly.


No. Besides when actually the landing approach, there are situations requiring that air speed be reduced in order to be below the landing gear extension speed. What to do? Retard the throttle!



Sunking said:


> Some planes like Jets and Turbo Props have spoilers used to slow a descending aircraft and decrease lift.


Many piston powered aircraft are also equipped with spoilers. 



Sunking said:


> Just about all planes use Flaps to add drag and add lift so they can touch town at the lowest speed possible with power on.


This is "almost" right, except the pesky "power on" part. All lift will generate drag. They are indivisible. However is the lift to drag ratio that is being manipulated, and the drag portion becomes exceedingly greater as the flap angle exceeds the full deflection aileron angle. Aircraft equipped with flaps that extend beyond 40 degrees can achieve extremely high descent angles while still at approach speed, and can be safely landed in a shorter distance due to the increased drag.



Sunking said:


> You cut power when you flare for landing to let the plane sit down gently, not fall out of the sky.


No. I don't know who taught you to fly, but you certainly did not learn anything about the aerodynamics. I for one would never declare you ready for the FAA exam. ALL student pilots must demonstrate proficiency with engine out landings.

In closing I offer just one word that fully rebuts all that you originally said; Gliders

Dunc - CFI-ASMEL-G-IA-A&P


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## brian_ (Feb 7, 2017)

duncgyro said:


> The engine is generally producing less "thrust" than the forward speed.


This is nonsensical, so I assume it's just not what you intended to say, dunc. Thrust is a force; it cannot be more or less than a speed. Even if "thrust" was intended to mean "power", it still doesn't make sense as-is. 

My guess is that the intent was to say that the engine is producing less thrust than would be required to maintain altitude at the current forward speed... and that, of course, is why the aircraft is descending. If the configuration (flaps, etc) and speed are not changed, then reducing thrust further increases the rate of descent (steeper path); increasing thrust reduces the rate of descent (shallower path).

Adjusting the amount of regenerative braking is simply motor thrust adjustment (but the thrust is rearward instead of forward).


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## brian_ (Feb 7, 2017)

Dunc, your statements about the use of power during descent and approach are inconsistent. You insist that power is not used, and that engines are merely idling; at the same time, you refer to reducing power to steepen descent. You can't reduce power further from idle.

The solution, for a powered fixed-wing aircraft not in engine failure, is that low power is used in combination with a high-drag configuration; zero power can only be used if the aircraft has spoilers or comparable devices available because otherwise there would be no way to control the angle of descent. I don't think tweaking the flap setting for fine control is a viable option as the only path control on typical light singles, and in a quick online search for references to approach power settings in light aircraft I didn't see anyone saying that they used idle.

In gliders, spoilers and/or dive brakes are used... as already discussed a couple of months ago.


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## duncgyro (Aug 22, 2017)

brian_ said:


> This is nonsensical, so I assume it's just not what you intended to say, dunc. Thrust is a force; it cannot be more or less than a speed. Even if "thrust" was intended to mean "power", it still doesn't make sense as-is.
> 
> My guess is that the intent was to say that the engine is producing less thrust than would be required to maintain altitude at the current forward speed... and that, of course, is why the aircraft is descending. If the configuration (flaps, etc) and speed are not changed, then reducing thrust further increases the rate of descent (steeper path); increasing thrust reduces the rate of descent (shallower path).


Take statement to limits. How would you define when the engine power setting is producing less thrust, as in the speed of accelerated air molecules, vs the airspeed? Example, lets say an idling engine produces a 10 mph wind when at rest on the ground. Now use the same idle power setting while cruising along at 100 mph. You must admit that the propeller is then actually driving the engine, thus adds its drag to the induced drag of the lift-producing wing, and the parasitic drag of the entire airframe.

It is exactly this drag by the windmilling propeller that causes multi-engine aircraft to be immediately very dangerous when an engine is lost. If the aircraft is below a certain airspeed, then the pilot will be unable to control the aircraft due to insufficient airflow over the control surfaces. Usually this airspeed is well above the usual airfoil stall speed, but decreases with altitude (among other factors). 

Bottom line the drag vs thrust value is somewhere between full throttle and idle, and also depends on the airspeed and the pitch of the propeller.


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## Ivansgarage (Sep 3, 2011)

WOW 24 post all about aerodynamics from one simple post on regen.
Bean flying 40 years 20 years in my Titan.
There is only two things to remember.
Throttle forward go fast, Throttle back go slow
Stick back go up, stick forward go down.

Patty Wagstaff said, there is *one thing* to remember "when flying upside down 10 feet off the runway 150 MPH, DON'T pull the stick back"


--


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## piotrsko (Dec 9, 2007)

Minor modification: stick forward go down, stick back go up until go down.

Why do you need a throttle?


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## Ivansgarage (Sep 3, 2011)

piotrsko said:


> Minor modification: stick forward go down, stick back go up until go down.
> 
> Why do you need a throttle?


*when flying upside down *10 feet off the runway 150 MPH, DON'T pull the stick back


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## brian_ (Feb 7, 2017)

In this electric vehicle forum, many people go very wrong in their plans by not understanding, or disregarding, basic physics. Misunderstandings are common even when people know the science, but don't communicate it. "Force" and "speed" are different concepts, so even if you know what you mean, equating them doesn't work.



duncgyro said:


> How would you define when the engine power setting is producing less thrust, as in the speed of accelerated air molecules, vs the airspeed?


The reaction force on the aircraft by the prop is due to the change in speed of the air as it passes through the propeller disk. The power required to produce that difference - and resulting force - increases with airspeed.

It does make sense that the engine speed which produces some speed difference (acceleration of the airstream through the propeller disk) when stationary may produce no difference or even retard the airflow at a higher airspeed.



duncgyro said:


> Example, lets say an idling engine produces a 10 mph wind when at rest on the ground. Now use the same idle power setting while cruising along at 100 mph. You must admit that the propeller is then actually driving the engine, thus adds its drag to the induced drag of the lift-producing wing, and the parasitic drag of the entire airframe.


Sure, the prop speed which produces some thrust at zero airspeed would likely produce net drag at flying speed.

The relevance to regeneration is that any amount of regeneration likely corresponds to substantial drag. That makes sense, and would be a reason for an aircraft using regeneration to be configured with less drag from other sources (e.g. less flap).



duncgyro said:


> It is exactly this drag by the windmilling propeller that causes multi-engine aircraft to be immediately very dangerous when an engine is lost. If the aircraft is below a certain airspeed, then the pilot will be unable to control the aircraft due to insufficient airflow over the control surfaces. Usually this airspeed is well above the usual airfoil stall speed, but decreases with altitude (among other factors).


I think you're getting at the idea that the power setting used is whatever produces zero net thrust, because actual idle would be almost as bad as engine shutdown, producing excessive drag. That would make sense.



duncgyro said:


> Bottom line the drag vs thrust value is somewhere between full throttle and idle, and also depends on the airspeed and the pitch of the propeller.


If that is supposed to mean that the power setting is somewhere between full throttle and idle, and that developed thrust depends on the airspeed and the pitch of the propeller... then sure, those seem like reasonable statements to me. 


None of this addresses the issue that if the engine is at the minimum usable power setting during descent, it can't be reduced to adjust the angle of approach. 

Regeneration for descent could mean replacing other drag sources (e.g. high flap angles) with a fixed regen setting, or - more sensibly - using control of regen level to control rate/angle of descent.


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## duncgyro (Aug 22, 2017)

brian_ said:


> I think you're getting at the idea that the power setting used is whatever produces zero net thrust, because actual idle would be almost as bad as engine shutdown, producing excessive drag. That would make sense.


Retarding the throttle to idle position, with the engine running in flight, will never yield a static idle RPM, as the relative wind will spin (windmill) the propeller. Constant speed propellers are commonly used on multi-engine aircraft, and they will maintain the last RPM setting as long as possible. So you will not see an RPM difference, but will notice a drag difference. 

Although a full idle method can be used for training, and would produce a more realistic fully blown engine drag scenario, it is simply not good for the engine. One does not want to induce a true engine failure while practicing for engine failures. Granted the full drag of a fully blown, windmilling engine is worse, the aircraft will not fall out of the sky assuming you are above Vmc, so it is aerodynamically safe. Zero thrust is just more safe. Besides most engine failures still produce partial power in real life.


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