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Is it sensible to harness wind energy on an airplane?

Actually the idea is to build an electric aircraft whose power comes from solar and from wind while the plane is landing. instead of using spoilers as airbrakes, these wind turbines separately mounted could add drag and harness energy at the same time . In other words, regenerative braking.

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Normally all aircraft harness wind energy when flying with the wind, and not against it. In such situation, the energy of the wind contributes to kinetic energy of an aircraft directly and with astounding near-100% effectiveness. But usually such energy input alone is insufficient to keep flying (except kites; e.g. gliders need 'meteorological' vertical lift). – kubanczyk Mar 8 at 11:57
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It helps to realize that the wind turbine provides drag that the engines have to compensate. So you are delivering mechanical energy from the engines to the turbine, with air as the intermediate medium. Ais is not a good medium for that. It's a lot more efficient to not have any intermediate at all, and stick the generator straight on the engine. – MSalters Mar 8 at 13:20
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@kubanczyk: Planes harness wind power in both directions. When flying with the wind, they get increased ground-speed for the same power. When flying against the wind, they get the same lift for less power, but with less ground speed. – abelenky Mar 8 at 14:02
    
@abelenky There are physical definitions of what words like energy and power mean. In level flight, lift needs zero energy input (as does any force contributing to zero position change). – kubanczyk Mar 8 at 15:02
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One situation where it makes sense is a helicopter with engine failure. In autorotation, energy can be harnessed during descent and stored in the blades' momentum. It comes in useful at sufficiently low altitude. – Brian Drummond Mar 8 at 18:10

Normally not: Extracting energy from the airflow produces drag, which must be overcome by added thrust. Since every form of energy conversion produces losses, more thrust energy must be added than can be gained from the airflow.

Only when the engines fail and the generators stop running does it make sense to extract energy from the airflow. In airplanes there are two applications which are driven by "wind energy":

  • Older airplanes use gyros for the artificial horizon which are powered by ram air. This way the gyros work even after an engine failure.
  • Jets use Ram Air Turbines (RAT), propeller-driven generators which are moved into the airflow when all other means of generating electricity and hydraulic power have failed. Note that this is done in unpowered flight and increases the sink rate.

Deployed RAT

Deployed RAT (picture source)

Rocket-powered airplanes have no easy way to generate electricity, so the Me-163 B used a small windmill at the tip of the fuselage to drive a generator.

Me-163 B

Me-163 B (picture source)


EDIT: With your question's new focus on electric propulsion the answer will become different. Now you most likely will have propellers which are powered by electric motors. During landing, those could run inverted and charge a battery, which will most likely be empty by the time the aircraft lands. This can be done during final approach until the end of the runout after touchdown. It can be expected that any electric airplane will have a high L/D, so deploying speed brakes to allow for a steeper approach makes sense.

I would be surprised if an additional device would be economical, however. This recharging must be done by the regular propulsion system, or it would add dead weight during most of the flight.

You asked for formulas, but all I can provide here are some back-of-the-envelope calculations. First, it must be said that variable-pitch propellers will be lousy as windmills, because their camber and twist are wrong for windmilling mode. I would expect that their efficiency is around 30%, which means that only 30% of the energy extracted by drag will be converted into mechanical energy which drives the electric motor.

Next, running an electric motor as a generator will again require compromises. Good motors make poor generators, and re-wiring the motor for better generator performance will degrade its efficiency in normal use. You will quickly lose more than you gain from the short flight phase when running the motor inverted makes sense.

Now let's assume that you keep your propulsion system at top efficiency (say, 90%) and accept that it will only convert 10% of the drag energy into electric energy. Let's also assume that your prospective solar airplane has an L/D of 30 which needs to be reduced to 10 for a practical approach. You do this from 1000 ft downwards and use the windmilling propeller also during the runout. Approach speed is $v$, mass is $m$ and the initial energy of the airplane is $305\cdot m\cdot 9.81 + \frac{m}{2} \cdot v^2$. Two thirds of the potential energy go into propeller drag, and to be generous we assume that 100% of the kinetic energy will also go into propeller drag, even though the braking force of the propellers at low speed is really lousy and needs support from wheel brakes.

Now it is important how fast your airplane flies, because this will shift the ratio between potential and kinetic energy. To keep things simple, I will relate both to the energy required for the next flight. 10% of the full kinetic energy will accelerate the aircraft to less than one third of its flight speed - after this, the remaining 91% of the energy to reach $v$ must be added by charging the batteries between flights.

The electric energy taken from the potential energy will help you climb to 60ft or sustain level flight at $v$ for a distance of 1800ft. At an L/D of 30 the airplane will fly a distance of 30,000ft without thrust, and by braking you will extract the energy to cover 20,000ft, which at 10% conversion efficiency (and 90% propulsion efficiency!) will carry you over just 1800 ft.

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Perhaps in the future lightweight RATs could be deployed when speedbrakes are normally used. The electricity could then be used to charge on-board batteries. Electricity could then be used to taxi with the engines of. An airbus A320 RAT provides about 40kW of electrical power at 130kts. Running 4 of them for 2 minutes, would add to about 5kWh. Probably 100kWh would be needed to taxi a couple kilometers, so it doesn't seem that feasible. Might be a better idea to just put bigger generators on engines and let them generate more electricity when the engines are on idle.Might save fuel, might not – rkantos Mar 8 at 8:16
    
Yeah, it appears that wind turbines can act as better airbrakes compared to spoilers. In other words, it is called regenerative braking in electrical science. But this must have got some negatives and hence has not been in practical. Please consider this and let me know why it hasn't been yet implemented. – B Uttej Mar 8 at 8:56
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Among other things, the general goal pilots of airliners tend to shoot for is an idle power descent, where the engines are at idle from top of descent to some reasonable point on final approach (in reality, ATC considerations mean you won't always be so perfect). In other words, the aim is to avoid using speedbrakes unless necessary. With your plan, you'd have to use the engines more, consuming more fuel, so that you'd be able to use your turbines to charge a battery. That would be horribly inefficient. – Zach Lipton Mar 8 at 9:57
    
Another way to think about this is from the perspective of energy management. An aircraft at cruising altitude has a certain amount of potential energy. It can set the engines to idle (because regulators and passengers frown on turning them off completely in flight) and glide down to the ground (or to glideslope intercept anyway), just as a car can eventually coast to a stop if nothing is in the way. Your turbine would take energy out of the system, but that energy has to come from somewhere. The only source of more energy is the engines and fuel already on board. – Zach Lipton Mar 8 at 10:05
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Cars may use regenerative breaking because roads aren't designed to allow everyone to coast to a stop without crashing all the time, but cars are not airplanes and the sky is not a road. – Zach Lipton Mar 8 at 10:05

Most other answers focus on normal flight; your (updated) question specifically asks about regenerative braking. In theory, yes, it's possible, in practice, no, it's really not a practical idea.

Let's first focus on the descent from cruise altitude to final approach. Ideally, this descent is done with engines on their lowest thrust setting (flight idle), which means the engines are providing power to the various electric and hydraulic systems, and pressurization, and a little bit of thrust since you can't really avoid that on a jet. Theoretically, this could be done by a Ram Air Turbine (let's for now assume that our hypothetical airplane is equipped with a rather big version that comfortably powers all aforementioned systems, since a typical RAT can only provide limited emergency power). However, this will greatly increase the drag, and to maintain speed, the descent profile will be steeper. This in turn means that the airplane must spend a longer time at cruise altitude, which requires energy to maintain.

Another way to look at that is by a simple energy balance: there is a finite amount of potential energy, and it doesn't really matter whether you use it to maintain speed only, or whether you use it to maintain speed and for regenerative braking, since the total amount of energy you can extract from it is always the same.

Of course, there is one moment that airplanes must brake, and this is after touchdown. Let's for now assume that we are very environmentally aware, and do not use any other means of braking than a regenerative braking system. The airplane is still going at about, say, 130kts, 240km/h or 150mph, so surely that will give us a lot of energy? Let's make a back-of-the-envelope calculation for some figures I found for a Boeing 737-300.

Let's say we land weighing 60 000kgs (close to MLW) with flaps reduced to 15, which gives us a landing speed of 158kts=81,3m/s. The amount of kinetic energy is then $\frac{1}{2}mv^2\approx200 MJ$ (yes, that's mega-joules!). That's a lot of energy, right?! Well... not really. Kerosene (which is approximately the same as jet fuel) comes at an energy density of around 46MJ/kg. We're talking about the equivalent of a little over 4kg of kerosene, on an aircraft that carries about 16,000kg of the stuff. That means we're regenerating about 0.025% of the fuel capacity.

I will leave it as an exercise to the reader to think whether a regenerative braking system could be designed, such that the extra fuel used due to its weight and size is under 0.025%.

Edit Let's redo the calculation on an airplane that actually has most of the necessary equipment (batteries and electric motors that could perhaps double as alternators or dynamos) on board: the Solar Impulse 2. It has a whopping 4x41kWh battery capacity (590MJ). Assuming the takeoff and landing speed are the same (20kts=36km/h=10m/s) and with a loaded weight of 2300kg, the kinetic energy upon landing is 115kJ. This is 0.0195% of the battery capacity - about the same as our B733 example! And note that this is again assuming that the propellers recover 100% of the kinetic energy... This idea is never going to work. (For comparison, it's less than two seconds of energy generated by the solar panels at their respective peak rating)

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It's also good to realize that we're talking about $2 or so in fuel, per landing. How much is a certified system going to cost? – MSalters Mar 8 at 13:16
    
That means we're regenerating about 0.025% of the fuel capacity. That right there is the #1 reason this hasn't and won't be implemented. It's simply not worth it in terms of dollars, economics, or environmental concerns (don't forget it takes natural resources to produce all those regenerative braking components & especially batteries). – FreeMan Mar 8 at 13:37
    
@FreeMan Especially since kinetic energy is quadratic in speed. For a more realistic landing (40 000kg, 115kts@flaps 40) we would have a kinetic energy of only 70MJ. That's the equivalent of serving tea on a British Airways flight (i.e., 1 liter of tea per passenger) – sanchises Mar 8 at 14:06
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@sanchises even with a BA flight full of Brits, I somehow doubt they'd get through an average of 1 litre of tea per passenger. I would do my bit to help though. – abligh Mar 8 at 20:49
    
Sure, equipping a jet with such a system seems inefficient. But what about propellers? They seems much more appropriate for the job – they can be powered by electricity alone, and a dynamo is basically an electric motor working in reverse. – Édouard Mar 9 at 9:03

No, because, from the point of view of a flying aircraft, wind does not have any energy.

Wind turbines stand on the ground and the air mass is moving at some velocity past them, so it has kinetic energy. But aircraft is moving relative to the wind, so the wind is the rest frame and has no energy. So when the aircraft is using the air flow and/or ram air pressure, it is using its energy, not the wind's¹.

Now of course a turbine mounted on aircraft will produce energy. But it will be at expense of the aircraft energy. When flying level under power, the power comes from the engines, so it is more efficient to extract it directly via generator mounted on the accessory drive. But even when the aircraft is descending on idle, it will be at expense of its potential energy and that was originally provided by the engines during climb. Shutting down the engine earlier and using all of it to compensate for drag during the glide is more efficient.

This applies to a solar powered motor glider just like any other plane. It is more efficient to stop the engines earlier and glide at around best glide angle than running the engines longer and then regenerating the energy, because neither conversion from electric energy to potential energy via engine and propeller, nor the conversion from potential energy to electric energy via turbine and generator is particularly efficient.

And it applies even when using thermals or other raising air to gain potential energy. It is again more efficient to avoid converting the energy and simply use the thermal to gain altitude and extend the glide while taking electric energy directly from the solar panels.

Also, weight will be important limiting factor for solar motor glider. This means you won't be able to put many batteries on board and that in turn means that regenerative braking won't be very useful as you won't have much capacity to store the energy. It also means you might want to avoid dedicated turbine to save weight; you'll still be able to regenerate some energy from windmilling propellers at somewhat lower efficiency. But as explained, it does not make all that much sense.


The only time aircraft use turbines is during emergency. When all engines fail, ram air turbine is used to power the essential electric and hydraulic systems. It shortens the gliding distance a bit, but it is worth it if no other power sources are available.

As far as wind goes, the only practical use of wind is choosing route so that there is as much tail wind during cruise as possible. For example the North Atlantic Tracks are periodically adjusted to allow the eastbound flights to utilize the Jet Stream. In a sense, this utilizes the wind energy, because the aircraft thus burns less fuel to get to its destination.


¹ Energy is a weird quantity. It is conserved in all inertial reference frames, but some of its forms will have different values in each. You can choose a reference frame where wind does have energy, but it will make less sense.

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@mins, well, yes. It would still be at the expense of potential energy and therefore gliding distance though. – Jan Hudec Mar 8 at 8:23
    
@mins, of course it does. But if you have a wind turbine and extract energy, it increases more slowly than if you didn't. – Jan Hudec Mar 8 at 8:28
    
@mins, it is always more efficient to use the energy from thermal to gain altitude and thus save fuel than to extract energy from it via RAT. – Jan Hudec Mar 8 at 8:44
    
Actually the idea is to build an electric aircraft whose power comes from solar and from wind while the plane is landing. instead of using spoilers as airbrakes, these wind turbines separately mounted could add drag and harness energy at the same time . In other words, regenerative braking. Please help with this – B Uttej Mar 8 at 8:49
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@BUttej - your comment about regenerative braking really should be in the question, to make it clear what you're talking about. I think at the moment nobody is answering the question you really meant to ask. – Andy Mar 8 at 9:05

For the reasons other answerers have said, it isn't normally worthwhile, because it isn't really wind energy, it's the aircraft's energy.

One situation where it is worthwhile is to generate electricity when it is difficult to take energy from the engine directly. For example, the vintage aircraft I fly didn't originally have an electrical system. To power their radios and transponders (essential for practical flight nowadays), they've been retrofitted with a small turbine under the nose. It increases the drag of the aircraft slightly, but the alternative would be trying to fit an alternator to a vintage engine, or replacing the engine completely, both much bigger modifications.

I've also seen touring aircraft which use small wind generators when they're tied down on the airfield, presumably to keep the battery topped up. The generator in that case is a small wind turbine atop a vertical pole, similar to what you might see on a houseboat or a caravan. They're removed and stowed before flight, so they're not quite what you're thinking of, but they're still aircraft using wind power.

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I think no one really adressed your question. I'll try to, as far as I understood. (I still wonder why you need a plane landed with non-empty batteries, but let's assume it's for faster refuel & takeoff?)

First of all, it is only possible to harvest wind energy (of air moving w.r.t. ground) if you have access to the ground.(You need to be a the interface between the two moving objects). A wind turbine must be grounded, a sail boat is in contact with the water, etc. For an aircraft this is not the case obviously, so as said in other answers it is impossible to gather free wind energy mid-flight.

However, it is possible to use windspeed to harvest some of the aircraft's energy. Let us recall there are three main energy tanks in an aircraft (i'll use A320 and TB20 (sorry french link for numbers) both in cruise in a regular mission, it would scale nicely for any e-aircraft):

  • Kinetic energy (momentum) - A320=2.2GJ / TB20=3.5MJ
  • Potential energy (its height in the earth gravity field) - A320=7.7GJ / TB20=66MJ
  • Fuel energy (required to cover least 1000 n.m. against the drag force, plus the above) - A320=180GJ. / TB20=1000MJ

As you can see, fuel energy in a classic airplane really dwarfs potential energy about 20-to-1, and potential surpasses kinetic about 10-to-1. It's the travelling part that costs the most, and unfortunately, airplanes are kinda designed expressedly for travel :D

Since what you used to travel (against the drag) can never be recovered, only the surplus kinetic and potential at the end of the mission may be. You propose to perform the descent and landing with the wind turbines. Let's suppose you have the wind turbine for the job. You may only re-gather kinetic energy during approach and landing, and potential energy during descent at constant-speed. Even if you manage to recover the whole energy, you only get ~5% of what you used in your whole mission! (and i used 100% efficiency everywhere, full system should be close to 20% efficient including turbine, generators, power unit, batteries etc. so we're really speaking about 1% net energy return).


That said, there might be a use for a windspeed turbine in very specific missions, but you would prob. drop the solar panels. I'm thinking parachuting planes. Mission: go high very quickly, drop your buddies, descend, repeat.

For this the energy demand isn't very high since you're not travelling. I'm thinking a mission requirement of 2x pot. energy to:

  • Get to speed
  • Climb to altitude
  • Beat the drag while climbing but not travelling Then you'll have 1x pot. energy + 1x kinetic energy to recover, and the numbers change a bit: there is 50% of energy available to recover, so with 20% efficiency, about 10% of the whole input can be brought back down.

I'm saying drop the solar panels, because this will be very power-intensive, and solar panels just have too low energy density

Note: energy density of a system is energy packed/mass on board. For batteries and fuel this is straight forward, for solar panels it's different: the longer the mission, the more energy produced, the higher the power density. This mission is so short it's not worth it.


TL;DR : Unless you have a very weird mission, there just isn't enough energy (~1%) to collect to be worth bringing a few wind turbines for a whole travelling mission.

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Airplanes move pretty fast (in some cases very fast) and have lots of drag. So they use lots of energy just to cruise.

Regenerative braking allows you to capture, as an absolute upper limit, all the kinetic energy and gravitational potential energy that the plane has when it starts its descent (obviously there's then an efficiency consideration too, so it will actually be less than this). The work you've done during the flight just to overcome drag at cruising speed is gone no matter what. So first thing: any drag that your doohickey adds to the plane while it's not braking, is costing energy through the whole flight. Let's assume that it's somehow stowed away, just like landing gear can be, and figure that it's going to bulk the plane up a bit but it isn't going to totally ruin the aerodynamics.

Now, on a typical flight what proportion of the fuel used, is used to reach cruising altitude and speed? I don't actually know, and of course it depends on the length of the flight among other things, but I'm sure some actual pilots could chip in with rough figures.

The energy to get up is an upper limit (again, there's inefficiency in the engines) on how much energy you can extract from the process of getting down. So, that proportion of fuel used to get up puts an absolute upper limit on the proportion by which regenerative braking can possibly reduce the total energy requirements of your solar-powered plane. And that's before considering inefficiencies of both processes (the engines and the regenerative brakes). So to completely invent some figures, supposing take-off is 25% of fuel for some particular journey, and combined efficiency is 50%, then regenerative braking could perhaps reduce energy requirements by 12.5%. This seems worthwhile on the face of it, but (a) I've picked what I think are overly-large numbers, and (b) we haven't yet paid for the mechanism that does it.

Can the batteries (or whatever) that store the energy from the brakes, store more energy than it costs to add them to the plane, thereby hauling a larger weight up to altitude in the first place, and the energy lost in cruise to the extra drag imposed by incorporating the whole system? In the worst case where they can't, you've made a net loss by adding regenerative braking.

Regenerative braking works reasonably well for cars, especially in city driving, because they slow down frequently and so they're otherwise dispersing a lot of unwanted kinetic energy as heat. To a rough approximation, planes only slow down once per trip. And I believe it's harder to efficiently capture the energy when you're braking against air rather than against static friction on the road, and so the efficiency of the system on the plane will be much less than that for a car. So I don't think your plan is in good shape right now :-)

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The normal way to harness the energy of the air in an aircraft is a glider.

The wikipedia article covers it pretty well: find an area where the air is rising, and use it to lift the aircraft up. There is no fancy energy conversion on the aircraft, it's all done directly by the lifting surfaces.

Solar-powered planes have been built, although the necessary compromises mean they're not yet popular. Some tech companies are looking at the possibility of high-endurance automated solar aircraft as radio relays for connecting remote areas to the Internet.

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Downvote because the information provided in this answer—while interesting—does not address the question: how to harness the energy of the aircraft's forward motion through the air. – Jonathan Walters Mar 8 at 11:58
    
Upvoted for thinking outside the box. Gliders do indeed harvest energy from wind. Vertical wind, that is. Any electrical airplane could do worse than using this energy. Electric airplanes have low energy density, so they fly slowly and have a glider-like shape. Flying like a glider would make most sense for them. – Peter Kämpf Mar 9 at 18:55

Very Short Answer

Is it sensible to harness wind energy on an airplane?

I assume as "wind energy" you mean, extracting energy from the "airflow". In theory yes, only in descent, if you can prove a design which brings benefits in terms of energy, complexity, economics. In practice, you have very slim margins with your solution, and only "one case" when harvesting energy makes sense.

Little Longer Answer

Let's analyze the idea:

... to build an electric aircraft whose power comes from solar and from wind while the plane is landing.

While the airplane is landing it is descending from "cruise" altitude to the airport. In theory it would need no "extra power", since you have a lot of potential energy, you could just use this potential energy to "glide" to the airport, similar on what sailplanes do. An important note the energy to get to "cruise" altitude has already been spent, for example, what Solar Impulse 2 do is to climb and recharge the batteries during the day, and glide and use the energy-from-the-batteries during night. Don't underestimate the efficiency of the gliding maneuver over night, with altitude you gain a lot of potential energy! You would not gain the same energy by harvesting it to from the airflow because you are transforming this potential energy in electrical energy with a transformation chain that bring you some losses.

That said, the one case when makes sense to harvest energy in descent is the moment that you need to loose some altitude in a "short" period of time, (not glide to the ground, which would be the most efficient solution). In this case it is desirable and possible to "harvest" some energy, what you are proposing is:

instead of using spoilers as airbrakes, these wind turbines separately mounted could add drag and harness energy at the same time . In other words, regenerative braking.

This is correct, it is done in a similar way with Ram Air turbines in emergency situations (no power) on larger jets. On a variable-pitch propeller aircraft theoretically you could change the pitch of the propeller to make them work as windmills, so that you would not need to add extra complexity and weight. If you have an electric motor driving the propellers you could potentially using it as a generator. In case you add "wind turbines", or any extra system, you should make sure that:

  • The energy to fly the extra weight of the system during all the mission is less that the energy that the system helps you harvesting
  • The cost of the harvested energy helps you breaking even the cost of the system in a reasonable amount of time.

A last note: Spoilers are use not only to add drag but even to reduce lift, the main use case here is descent (emergency or not)!

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Aircraft generate lift by increasing airspeed, while wind harvesters generate power by decreasing airspeed, so they'd be competing with each other.

Assuming that there was a way to avoid that problem, it'd also need to store this energy, and large capacity batteries are incompatible with an aircraft's requirements for both high specific energy and high specific power.

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