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I am a student carrying out a project focused on efficiency on aircraft. I have arrived at an idea I believe to be feasible. I am looking for suggestions, specification of what I would need, and any general tips and advice if this idea is actually reasonable.

I have found, using another page on this forum, the amount of fuel used by the engines on electrical generation. I am proposing to relieve this duty from the engines while in flight by opening large air intakes embedded into the rear of the fuselage on both left and right sides of the aircraft. These would guide air in and split into two convergent ducts that will spin a turbine that spins a generator. This power will be AC. It will be directly linked to the busbar system as a power source and feed into the battery after being converted to DC.

From an electrical and aerodynamic standpoint, is this feasible? Be as critical as possible. It is not a perfect idea, but the point is to save fuel by not generating power from the engines while in flight. This is aimed towards commercial airliners.

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    $\begingroup$ The approach is widely used in another transportation context - the (older) railroad rolling stock. The passenger cabin's lighting is powered by a generator rotated by the wheels. While it does not involve aerodynamics and is much more effective, it is still less effective than using wires between the carts. It is used because it reduces the complexity. $\endgroup$
    – fraxinus
    Commented Feb 6, 2023 at 13:35
  • $\begingroup$ @MaxCremona its not clear from the question or your profile as to what level you're at in your studies. One thing I'd suggest, is to do a literature review, and find known inefficiencies and attempted solutions before getting attached to a given solution idea $\endgroup$ Commented Feb 7, 2023 at 15:23
  • $\begingroup$ This is a gentle reminder not to post (pseudo) answers in the comments. Please see this meta post. $\endgroup$
    – DeltaLima
    Commented Feb 8, 2023 at 19:56
  • $\begingroup$ @MaxCremona welcome to aviation.stackexchange.com $\endgroup$
    – DeltaLima
    Commented Feb 8, 2023 at 19:58
  • $\begingroup$ Just now found this highly related question -- aviation.stackexchange.com/questions/25933/… -- also this one aviation.stackexchange.com/questions/32669/…? $\endgroup$ Commented Feb 27, 2023 at 14:59

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No, because large intake ducts would greatly increase drag. The propulsion system moves the aircraft, so you are only indirectly and inefficiently drawing power from the engine/aircraft instead of using a simpler and lighter fan belt.

But all is not lost because internal combustion engines are hugely inefficient converting combustion energy into mechanical force. Much heat is simply wasted in exhaust or radiated through cooling systems.

This waste heat energy could be recovered (as it is in steam boilers) by boiling expansion pressure --> turbine or piston motor --> condensation --> reboiling.

Recently a cascading boiling system with 2 or 3 lower temperature boilers recovering the heat from the previous condenser came to mind.

These would add weight and complexity, perhaps better suited for trains. But the amount of heat loss is huge. You may find success there.

Here is some information on power savings by removing an alternator from the engine. Now one must see if additional weight of an energy recovery system adding to power requirements is greater or less than that saved from detaching the alternator belt.

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  • $\begingroup$ The cascading boiler system sounds cool! Can you please provide a link? $\endgroup$ Commented Feb 6, 2023 at 8:17
  • $\begingroup$ @GeorgeMenoutis these were utilized on large ocean liners of the previous century. $\endgroup$ Commented Feb 6, 2023 at 8:25
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    $\begingroup$ @GeorgeMenoutis it is called recuperator and is normally used on micro-turbojets or on turbojets used to produce electricity (on ground). On aircraft turbojets it's not used due to the higher weight and complexity. $\endgroup$
    – sophit
    Commented Feb 6, 2023 at 9:22
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    $\begingroup$ Engine efficiency, complexity, and weight are pretty closely linked. Weight directly impacts fuel consumption on an aircraft. More stages on the turbine means more efficient, but more cost/maintenance/weight. Bigger fan means more efficient, but more frontal area/drag/weight, and a slower turbine speed (unless you had a (heavy,expensive) gearbox). $\endgroup$ Commented Feb 6, 2023 at 11:25
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opening large air intakes... These would guide air... that will spin a turbine that spins a generator.

You basically want to install a "ducted wind turbine" in the airplane.

Off the top of my head I see the following limitations/criticality:

  • Wind turbines are not as efficient as one might suspect: considering the aerodynamic and the electrical losses, an efficiency of 50% might be already a very good value. This value might be a bit higher since your turbine would be enclosed inside the fuselage i.e. it would be ducted. But, on the other side, since it would be located by the aft fuselage, it would work with highly turbolent airflow severely reducing its efficiency.

  • It would use up space and weight which could be used for the payload instead.

  • Something rotating is always imbalanced and therefore always producing vibrations; an even higher weight might be expected in order to cure potential resonance phenoma.

  • Something rotating can break and release debris possessing very high energy; the whole assembly would need to be enclosed in a solid casing to shield the structure; this obviously would add again weight.

  • It would work only when the aircraft is already moving with enough speed i.e. not on ground, nor at takeoff, nor at landing.

  • Everything on an airplane has to be regularly tested and inspected; the relevant cost would have to be added to the bill.

  • For sure there's something else that I'm forgetting.

Anyway there's a basic misunderstanding here:

I am proposing to relieve this duty from the engines while in flight... the point is to save fuel by not generating power from the engines while in flight.

Obviously the power extracted from the airflow by the turbine has been given to the airflow by the engines in the first place: so the wind turbine wouldn't work "for free" but it would anyhow consume power from the main engines possibly consuming even more power due to its drag and weight.

As also pointed out in other answers, something similar is actually already installed on aircrafts and is called ram air turbine. Anyway it is used only as a "last-chance" after a major power issue in order to get some electric or hydraulic energy to power the bare minimum systems to be able to land.

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Small antique aircraft which did not possess electrical systems originally do this already: on the bottom of the fuselage the owner can mount a little "windmill" propeller connected to a small generator. When flying, the windmill spins the generator and electrical power is carried into the fuselage with wires that are connected to a battery, which hence remains charged whenever the plane is in the air even though the engine does not possess a generator. Radios, lights, etc. are then connected to the battery.

Using a ram air turbine to spin a generator would be complicated and not very efficient. The right way is by gearing a generator right onto one of the plane's engines.

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  • $\begingroup$ and why do they do this? is it for added safety so that in case the generator seizes up, the propeller will still turn? $\endgroup$ Commented Feb 7, 2023 at 8:36
  • $\begingroup$ @user253751 For example Airbus are completely Fly-by-Wire so a loss of electrical power would be catastrophic (there is mechanical backup of flight controls, but...), so in case of dual generator failure (like dual engine failure), a Ram Air Turbine pops up to power the instruments and flight controls. $\endgroup$ Commented Feb 7, 2023 at 13:22
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    $\begingroup$ @progmagor the answer talks about "small antique aircraft" $\endgroup$ Commented Feb 7, 2023 at 13:25
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    $\begingroup$ @user253751 The type of antique aircraft that neils is referring to didn't originally come with any kind of electrical generator at all. So the engine wouldn't have a convenient way to connect to an aftermarket generator, which means an external turbine is the only way to get things like lights, radio, transponder, etc. working without major engine modifications. $\endgroup$ Commented Feb 7, 2023 at 14:55
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Today's airliners have a small emergency generator that is airflow driven. It pops into action automatically when the last of the powerplants fails. This electicity is needed to operate the hydraulics that operate the control surfaces, and the computers that control the hydraulics. The plane is flown by wire and cannot be flown any other way.

The concern here is not efficiency. The concern is the absolute need for power to fly the plane as a glider. At best, this will result in a controlled descent ending in a very hard landing. This is not good, but it's better than a crash.

There are a few cases where an airliner had to be glided to an airfield and landed without jet power. These ram air generators worked.

Here are two accounts of airliners gliding to a landing. One of them is at an airfield.

https://timeline.com/in-1983-two-pilots-miraculously-landed-a-jumbo-jet-with-no-fuel-from-40-000-feet-e51782deb01d

https://en.wikipedia.org/wiki/Air_Transat_Flight_236

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  • $\begingroup$ I think this is an important piece of the answer: Yes, ram air turbines are totally doable and implemented on a regular basis but only as an emergency measure because they are totally inefficient (why add a lossy indirection if you can power the generator directly from an exiting engine!?). From a physics perspective one might add that they essentially convert "potential" (i.e., gravitational) energy of the airplane's mass at the given altitude into electric (or mechanical, via pressure) energy. $\endgroup$ Commented Feb 7, 2023 at 8:35
  • $\begingroup$ This is tangential to the question, but there's normally not any reason why an engine-out landing needs to be an unusually hard one. You can land more or less normally without engine power, aside from the obvious lack of ability to maintain altitude and airspeed at the same time and the inability to go around for another attempt. Of course, this assumes that you're in gliding range of a maintained runway, which the Gimli Glider and U.S. Airways 1549 were not. If you land somewhere other than at an airport, then, yeah, you'll probably have a hard landing. $\endgroup$
    – reirab
    Commented Feb 7, 2023 at 17:02
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It's an over-unity/"perpetual motion" myth

Something you regularly have to confront as a starting engineer.

Sort of like putting a windmill on an electric car to generate power to recharge the batteries so you arrive at your destination at 100%.

As they say, "there's no free lunch". Your ram air turbine will increase drag on the aircraft, which will require more engine power to overcome. So it is a net lose compared to a gear driven generator.

However, it is widely used when a module needs to be added to an aircraft that needs a lot of energy of one kind or another, and they don't want to enlarge the plane's electrical or hydraulic system to avoid a long re-certification process.

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  • $\begingroup$ Intresting. Is there an example where this method was used to avoid re-certification process? $\endgroup$ Commented Feb 8, 2023 at 6:08
  • $\begingroup$ Finally a nice analogy with the car/windmill, I usually use a ship and a propeller. I've had many disappointing "inventors" at my desk when I explain them the physics of it. The answer to the OPs question is "yes, but not for free". $\endgroup$
    – 0scar
    Commented Feb 8, 2023 at 12:22
  • $\begingroup$ But actually a propeller on a car can add energy. See the whole downwind-faster-than-the-wind paradox (google it, for example en.wikipedia.org/wiki/Blackbird_(wind-powered_vehicle). Adding an electric motor and batteries would reduce efficiency hugely but in theory it would still work. (Kidding, sort of. It would only work when the wind was blowing. But indefinitely sustained travel would theoretically be possible downwind as well as upwind. The whole downwind-faster-than-the wind paradox is rather mind-bending to most of us... (hint: it can't work in an airplane though!) $\endgroup$ Commented Feb 8, 2023 at 13:06
  • $\begingroup$ @quietflyer I found that whole "Woo woo, wind powered car exceeds speed of wind" thing to be a case study in the importance of viewing a problem from the proper perspective/POV. From the car's frame it is obvious lol. But good point, ambient wind breaks my metaphor. $\endgroup$ Commented Feb 8, 2023 at 20:34
  • $\begingroup$ @AdityaSharma ECM jamming pods on combat aircraft. $\endgroup$ Commented Feb 8, 2023 at 20:36
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Yes. Early F-4 aircraft had Ram Air Turbines, (we called them RATs), that provided emergency electrical and hydraulic power in the event of system failures. They were small propellor driven hydraulic pumps and electrical generators/(or alternators, not sure)

The RAT would deploy, when you actuated it, into the slipstream from a compartment in the lower fuselage

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    $\begingroup$ +1 for mentioning existing RATs, but you might want to add that these do not save fuel. Also, since the question was about commercial airliners, RATs are quite common on airliners as well (Boeing: since 757, Airbus: all). $\endgroup$
    – Bianfable
    Commented Feb 6, 2023 at 7:36
  • $\begingroup$ Bianfable.. not sure what you are saying (about saving fuel). In the F-4, they were there to provide hydraulic (& electric) power in emergency situations. $\endgroup$ Commented Feb 6, 2023 at 23:12
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    $\begingroup$ @CharlesBretana Consider reading the question :-) $\endgroup$ Commented Feb 7, 2023 at 2:17
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The only situation where this could make sense is when the plane descends from travel altitude as some kind of recuperative "braking". But the weight of the extra gear needed would make this viable only for very short flights, on most the time spent in descent is simply too short to have any impact on overall energy usage. Additionally, it wouldn't be enough to eliminate the need for thrust reversal.

This could, however, get interesting for electrically powered planes where the engines' power flow could simply be reversed, needing no extra parts, while also being able to store the energy in the - at that point - mostly depleted batteries, making every bit of generation a net gain.

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  • $\begingroup$ Good point about electrically powered airplanes. Anyway I'm not that sure that the same circuitry can be used both ways $\endgroup$
    – sophit
    Commented Feb 7, 2023 at 14:35
  • $\begingroup$ @sophit: The power electronics would have to be designed for it, but that's relatively light-weight. I think most electric motor designs can Just Work as generators, converting kinetic energy to electrical instead of the reverse. It's well worth it in cars where stop-and-go is common, and any extra weight that is required doesn't cost cruise efficiency, except a tiny bit of extra rolling resistance from the tires. Neither of those things are true in airplanes. So it would only be worth it if the extra weight is minimal. (And cost of engineering design, maintenance, and reliability). $\endgroup$ Commented Feb 9, 2023 at 4:55
  • $\begingroup$ @PeterCordes: thanks for your interesting answer. I know that in real-size electric aircrafts inverters+relevant electronics need massive heat exchangers. Can those inverters be used both ways or do we need another set? $\endgroup$
    – sophit
    Commented Feb 9, 2023 at 5:31
  • $\begingroup$ @sophit: You don't need a (true RMS) inverter, that produces AC voltage. To charge a battery, you just need to rectify and filter to DC. Also keep in mind the duty-cycle involved: occasional bursts of energy recovery in cars give plenty of time for things to cool down. (A temp sensor could disable energy recovery if things are getting too hot, but I expect it's fairly efficient so not a lot of energy lost as heat). In a plane you might be doing energy-recovery for a longer descent, but probably at a lower power level. And air cooling is easily available; it can only operate while moving. $\endgroup$ Commented Feb 9, 2023 at 5:35
  • $\begingroup$ @PeterCordes 👍 $\endgroup$
    – sophit
    Commented Feb 9, 2023 at 5:38
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Yes. Ram Air Turbines (RATs) are very common in aviation for applications where equipment needs electrical or hydraulic power that aircraft engines cannot provide.

The most common application of a RAT is an emergency module, usually stored retracted and faired, but can be extended into the airstream. They are fitted with a RAT which drives an electrical generator and/or hydraulic pump to provide emergency electrical or hydraulic power for actuation of primary flight controls in the event of a major systems failure.

enter image description here

Another application for RATs is to deliver electrical power to mission system modules independent of ship’s power. A common specific application of this is the ALQ-99/E jammer pods carried by both the EA-6B and EA-18G aircraft.

enter image description here

You can see the RATs in operation on the EA-18G aircraft below.

enter image description here

The RAT on the nose of the pod drives a generator providing electrical power to the pod’s systems. Advantages to the system are the ability to carry and use these pods without a beefed up ships electrical system and additional power distribution wiring in the airframe as well as flexibility in mission configuration. The downsides are drag, generally resulting in a 2-3% range reduction per RAT in operation. And it’s isn’t a very stealthy option, either.

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Here's a concept inspired by this related answer -- imagine an airplane that is essentially a sailplane with an electric motor for added assistance. Normally, once a sailplane has climbed to the top of a thermal updraft, there is no way to extract any more energy from that updraft. However, our new concept motorglider will use the propeller and motor as a generator to charge a battery while climbing, accepting a lower climb rate for the sake of extracting more energy from the thermal updraft. The same can be done in wave lift, etc. Wave lift in particular would be conducive to simply remaining at a fixed altitude where the updraft is strongest, and extracting energy with the generator at a very high rate, until the batteries are fully charged.

The purpose of our new concept aircraft will be to make very long-range flights-- potentially trans-continental or trans-global, and potentially spanning multiple days-- using no energy other than atmospheric energy. This could be done for the sake of competition (flying against other similar aircraft), or to attract publicity to various causes, or simply to advance the state of aeronautical technology. A new category of FAI record might be appropriate.

In order to meet the stated goal, take-off would be conducted with the batteries containing only enough charge to climb to a limited altitude representative of a normal aerotow, or else the batteries would be drained after the initial climb before proceeding with the rest of the flight.

A logical extension of the concept would be to also include solar panels, in which case the goal would be to fly using only external, natural energy harvested during the course of the flight, be it "direct" solar, or "indirect" solar (i.e. atmospheric). Or if the "using... only energy harvested during the course of the flight" portion of the concept were dropped, the batteries could be fully charged (via solar energy) before takeoff and freely used as needed before in-flight recharging by both available methods. Another variation of the concept would omit the solar panels, but would fully charge the batteries before takeoff using energy harvested from wind turbines.

This thought experiment should make it clear that the answer to the original question--

Can an aircraft use ram air to generate power while flying?

as well as the implied question "would it ever make any sense to do so?"

Is "yes".

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    $\begingroup$ It could work, but you need to carefully estimate the impact of increased drag. In majority of motorgliders, the whole engine block or at least propeller blades have to be folded when not in use - otherwise they hardly can climb. $\endgroup$ Commented Feb 8, 2023 at 11:11
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As some of the commentators stated - theoretical answer is "yes", but practical is "it will be less effective". If we compare two systems - main engine(s) pulling the air through and causing this air to rotate the turbine vs main engine(s) rotating generator(s) via mechanical gears - the losses in the former one will be much higher, due to "openness" of the system, turbulence etc.

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    $\begingroup$ Welcome to aviation.SE. I fail to see how your answer is adding anything that was not said in the other answers already. Would you mind expanding it? $\endgroup$
    – Federico
    Commented Feb 6, 2023 at 9:49
  • $\begingroup$ Some citations etc needed here - there is plenty of reason to think that a well-designed RAT could be slightly more efficient than an engine-driven generator, since it eliminates mechanical losses. $\endgroup$
    – MikeB
    Commented Feb 8, 2023 at 9:54
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    $\begingroup$ @MikeB it eliminates minor mechanical losses and introduces major aerodynamic ones. You can do a simple test - put electrical motor with propeller against the same-size propeller that can spin freely. Then measure the rotation speed of both - the difference will tell you how much energy is lost in transmission of the power. $\endgroup$ Commented Feb 8, 2023 at 10:23

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