Flight Power Profile

[glum]

I'm conducting a study into alternative electrical power sources in flight.

Can anyone provide a flight-power profile for their aircraft? What I'm interested in is the amount of power and the timing, for example:

Main galley operates from wheels up for two hours at 50kW.

Anti icing operates from top-of-drop until touchdown (1 hour) at 60kW.

The idea being that will a little ingenuity, the power needs can be managed to reduce the peak loading and reduce the size of the generators / decrease fuel burn.

Thanks in advance for any replies... :ok:

[Checkboard]

The main power draw in the galley is the oven. On long haul it operates at meal times, and that depends on the time of the day the flight is taking place. On low cost airlines it operates when someone orders a hot food product - ie. randomly.

Electric anti-ice (apart from the windows) is rare, it's normally pneumatic (pressure or hot bleed). Power requirements are thus fairly constant throughout the flight.

[glum]

Thanks Checkboard,

What I'm aiming at is that all systems require power. Anti ice does take power - albeit from bleed air for the most part on current aircraft.

Since we're looking at alternative power so that main engines are used purely for propulsion, it is important to know what the other aircrat loads are.

[Checkboard]

The big users are the pumps - fuel and hydraulic (in the airbus the blue electric hydraulic pump runs continuously), probably followed by the flight deck instruments and computers - and these all run constantly. As a pilot, I look at the electrical load every now and then, and never see much change from 30% on each generator.

I guess you should do a little research on the 787, as that uses electrickery for everything - no engine bleed for pressurisation or anti-ice for example ...

[glum]

Thanks again, the 787 is a very interesting look at the future for sure.

[GlueBall]

The short answer is that the gas turbine energy required to turn high capacity electric generators is significantly less than collectively powering direct drive hydraulic pumps and pneumatic systems from engine bleed air extraction.

[FullWings]

Don't forget the IFE - that can suck up a lot of juice.

I suppose you could theoretically reduce the size of the generators but on a twin, just one of them needs to be able to run all essential services in the case of engine and/or generator failure. You don't want to load-shed the FBW to get the anti-icing to work.

I'm not an aircraft systems engineer but even I can see that it's better to over-specify the electrical power sources as you don't know what might be fitted to the airframe in the future. Passengers won't be impressed by the lights going out every time someone flushes the toilet!

[Landroger]

I'm not an aircraft systems engineer either, but surely we have missed the air conditioning packs? On the Qantas London-Sydney non stop 744 flight, they only had about 23 or 24 people on board and ran only one of the four A/C Packs, thus saving a measurable amount of fuel.

Roger.

[glum]

We definitely don't want to upset or scare the pax - it's bad for repeat business!

What we do need to do though, is try and generate power more efficiently, and using the propulsion engines to do that isn't the best way. Turbines are at best about 35% efficient, and with modern developements in fuel cells, it's possible to get efficiencies as high as 55%.

A further problem with using jet engine mounted generators, is that they have to be sized for the peak load expected which makes them bigger than they need to be if only we could manage the loads more effectively.

As Already pointed out, the genny's are mostly running at 30% load, yet the engine is sized for the full power output and could therefor be smaller giving drag benefits and efficiencies.

As I understand it, some aircraft have to maintain an artificially high flight idle during descent purely to keep the generators from stalling them!

Oil is running out, so the quicker we find a way to get our power from something else the better.

Can any of you gents put numbers on the loads you're highlighting? (in kW if you know them).

Thanks all. ;)

[exeng]

As I understand it, some aircraft have to maintain an artificially high flight idle during descent purely to keep the generators from stalling them!

Very Wrong

[As Already pointed out, the genny's are mostly running at 30% load, yet the engine is sized for the full power output and could therefor be smaller giving drag benefits and efficiencies. /QUOTE]

The fuel flow is reduced for a given gen load. The gen load is a very small factor.

[QUOTE]Oil is running out, so the quicker we find a way to get our power from something else the better

Glum - you just got glummer

Regards
Exeng

[glum]

I did? How so - are you saying oil isn't running out?

[Checkboard]

The A320 engine driven and APU generators are 90 kVA, so if you are running two gens at 30% for normal operations, you are looking at 55 odd kVa for all usual in-flight systems. The emergency generator is 5kVa.

[exeng]

My response could be seen as a bit unkind - I apologise.

I do agree that we should be looking at alternative fuel sources for aviation and other things. This would be for my great granchildren as I don't really believe we are close to the point of running out of oil anytime soon (another discussion if we could leave it there)

Any increased efficiency in electrical power generation would be a good thing I agree. However I do still believe that you have some incorrect assumptions in regard to engine operation.


Kind regards
Exeng

[glum]

Agreed, peak oil is a discussion for another forum entirely!

Also agreed, my engine knowledge is limited, hence coming on here to ask for information from those better placed to know!

Do you have any more specifics regarding power use you can share?

[FullWings]

B777 main generators 120kVA x 3 (engines & APU), backup generators 20kVA x 2 and at least four PMGs (permanent magnet generators) for the FBW, FADEC and other essential stuff, don't know the power but would guess 1-3kVA.