# Would “wind turbine generators” in the bypass stream save fuel for jetliners?

What is the location of an aircraft's alternator, in a large airliner like a Boeing 747 or Airbus A380?

How much power does it produce in kilowatts?

And what is the possibility of installing it with fan blades? To act as a windmill/wind generator at the point where air is slowed down in a bypass turbo fan engine. I mean, incorporate it into the engine design so that it slows down the air to generate electricity, rather than using the motion of the engine.

The aim would be to reduce the amount of fuel required to generate electricity.

• You mean something like a "Ram Air Turbine"? I have a feeling that using the residual power inside the engine is a lot more fuel efficient than the added drag from using a RAT all the time. – Ron Beyer May 1 '17 at 15:07
• You don't get anything for nothing. Since taking energy from the airflow to spin the turbine cannot be 100% efficient, then you lose more energy than you gain as you reduce the energy going into the engine from the airflow. A direct geared drive is much more efficient (better than 99%) and uses fuel to generate the energy. – Simon May 1 '17 at 15:27
• Related: Is it sensible to harness wind energy on an airplane?. The second paragraph of this question has been largely (if not exhaustively) addressed in the related question linked above. – J Walters May 1 '17 at 16:41
• When you write "so that it slows down the air to generate electricity, rather than using the motion of the engine", you mean converting airflow kinetic energy into electricity rather than converting mechanical energy of the engine. But you miss the airflow is itself created by the engine. So to use airflow, you need first to burn (additional) fuel to create a force, then convert this force into motion (relative airflow), then airflow energy into mechanical rotation using blades. These three steps are all imperfect. Using a shaft requires only the first step, so is more efficient. – mins May 1 '17 at 18:01
• The normal power generation is on the engines or the APU. It may just be the light metal I fly, but normally I have starter-generators, with ratings like 28V -200ADC or 28V -300ADC. Typically the APUs have starter/generators. One issue with alternators is the turbine rotational velocity is difficult to manage for frequency control. AC cannot be paralleled. There are numerous configurations, and they all seem different. Generally not driven with bleed air, which is used for starting. – mongo May 1 '17 at 20:02

By Olivier Cleynen (Own work) [CC BY-SA 3.0], via Wikimedia Commons

The vertical shaft shown above runs the generator, which is housed near the engine (usually just under it). See: Dave's answer

### Power output

The maximum load on an IDG on a Boeing 737 is 90 kVA, or 72 kW. Those usually run at 0.3 load but let's say it's full load. (Source)

### Fuel consumption

The shaft power off-take in a GE CF6 jet engine found on a Boeing 747 for example, consumes 0.125 kg of fuel for every kWh. (Source)

The shaft power factor for the CF6 and the CFM56 on the 737 are very similar. In an hour that's 9 kg of fuel, for a 6 hour flight that's 54 kg of fuel.

In cruise, that's about 0.45% of what the engine is already burning at 2,000 kg/hr per engine. As you can see, it's not much, especially that I've used an unrealistic 100% load.

### Windmills in the bypass flow

If the turbines you want to add into the stream of the bypass are realized, they too will reduce the thrust of the engine as they'll take from it, add more weight and complexity, disrupt the airflow, and very possibly end up eating more fuel.

• Another way for the B747-400: 72 kW during an hour is 72kWh, or 259 MJ. Considering the energy density of Jet-A (43.3 MJ/kg), this requires 6 kg of fuel per hour, at 100% efficiency. As a turbofan is about 35% efficient, this means 17 kg / h, ignoring IDG efficiency (which should be for 400 Hz around 90%). Now there are 4 IDG on a B747, so 68 kg / h. – mins May 31 '17 at 18:24

There are a few Alternators on big planes.

Most large aircraft have an APU which is often in the back of the aircraft. It is effectively a small turbine directly bolted to an alternator. It provides power while the plane is on the ground (and the engines are off) and can also be used in flight if need be to supplement the main system or in the event of a main engine failure.

The engine alternators are generally mounted and driven directly by the turbine (via a transmission) in this case near the compressor stage. You can read more in this thread and check out this video for a nice overview/explanation of jet accessory drives. You can also see where in this picture,

(source)

As mentioned in the comments you can have a Ram Air Turbine driven by airflow but these are presumably not as efficient as directly mounting it to the engine.

Large airlines DO have a wind vane turbine driven alternator that is deployed under emergency conditions, i.e. total engine / normal electrical system failure. The device also generates hydraulic pressure.

They are called Ram air turbines, or RAT for short.

Video: 747 RAT Test bed

These devices have saved many a flight. Most noticeably the Gimli Glider. A Boeing 767-233 that ran out of fuel, due to a math error, mid-flight over Canada in 1983.

• Math error. That's what I'll start telling the AAA guy when I run my car out of gas... – Harper - Reinstate Monica Apr 4 '19 at 15:12