Electric or Bleed Air start?

[jhurditch]

Just a quick question.

What are the reasons that a manufacturer will opt for a bleed air crank/start system over an electriclly run crank in a Jet engine. Both will achieve the max motoring needed for start so why is there variation?

Cheers

[leewan]

I've never seen an electric starter on a commercial jet engine( except APU) before although I think the 787 will be the first with it.
My guess will be the weight savings. Modern jet engines with their huge size will need electric starters of a few hundred kilos compared to air starters which only weigh abt 20 kgs max. And i believe that the 787 has an electric starter that can also function as an IDG. Could be wrong there, but remembered reading somewhere abt that.

[BarbiesBoyfriend]

The BAe 146/ RJ 85 RJ 100 use electric start (LF 507 engines). It;s not done via the IDGs (as only two of the engines are fitted with IDGs).

It's actually quite handy compared with air starting because when the APU is u/s, it's a lot easier to get a ground power cart than a huffer.

Also huffers tend to vary a great deal in efficacy.

There might be a weight penalty but really the whole a/c is a bit of a weight penalty!:ooh:

[dhc83driver]

146 / RJ uses DC electric motors to start. Fairly small engines but means that you can start from APU Gen, Ground AC (Thru TRU), Ground DC, Cross GEN start or even from BATT (with a second batt option). Having said all that an air starter would save weight.

[Swedish Steve]

Before the B787 it was for weight.
Go and look at the size of the electric starters on the old engines, the Tyne Dart and early Avons all had electric starters. The starters on the Tyne were massive, and the electric supply cables were the size of your thumb!
I expect that everyone will have to buy new GPUs when they get their B787 to handle the load.

[muduckace]

2 issues,

The 787 is supposed to be a bleedless aircraft, now take out all the engine and apu valves and ducting as well as the ducting in the wings to the cross/bleed valves and back to the APU. Put it all on a scale and I wonder if a couple of electric starters that weigh a bit more than the existing IDG's would not weigh a whole lot less than a pile of scrapmetal that would fill a U-haul?

I understand the IDG's of the 787 are to be rated at 120kva, common volt/amp rating to atleast the 777. gear ratio has IDG's rotating at 400hz. I believe it quite possible to get an estimated 20% n2 or n3 out of as little as 60kva or less, regulate enough amperage to get her rolling and accelerating at a demand within spec. I am interested to find out more but do not believe the electric starter demand will be a factor.

[411A]

...but do not believe the electric starter demand will be a factor.



Neither do I.

The 787 is so far ahead, design-wise, of any other jet transport airplane, Boeing has absolutely nothing to worry about...late first flight, or not.

Lets face facts here.
Boeing (and Lockheed, on a smaller scale) have absolutely led the design criteria as regards large jet transport airplanes.

Comet excepted...but then they had their problems as well, as we all know, now.:rolleyes:

[repariit]

Convential electric start systems use power from a battery as the source of energy. A battery with sufficient energy for starting large engines would be excessively heavy. The 787 uses new technology, not available years ago. The engine mounted generators can also perform start motor operation. They are variable frequency machines that can be driven by the generators on the APU, or the other engine. The starting operation would be similar to a bleed air system except that AC power through wires is used in place of bleed air through ducts. The initial start power would come from a battery to light off the APU, followed by much higher capacity electrical power from the APU to turn the engines.

[TURIN]

Lets face facts here.
Boeing (and Lockheed, on a smaller scale) have absolutely led the design criteria as regards large jet transport airplanes.


This should be good, let me get a brew on, sit back and wait for the inevitable Airbus V Boeing slanging match!


Back to the thread.

On an associated topic, does the 787 APU also have air start facility like the 777?

Just seems like all the electric eggs are in one basket otherwise. :suspect:

[BAe146s make me cry]

Repariit

I'm with Swedish on this one, will all B787 operators/line stations require >140KVA GPUs?

BAe146

[Swedish Steve]

I'm with Swedish on this one, will all B787 operators/line stations require >140KVA GPUs?

No because there are FOUR GPU sockets.
It really depends on how frequency sensitive they are. We have big problems here with old GPUs on the B777. A GPU that can easily handle any narrow body has problems with the B777 because of the high loads, and narrow frequency range the aircraft will accept. We always use two GPUs, and have no problems with modern FEP. But most of our stands have only a single FEP, so the second ground power must be a mobile diesel one. Of the ten that our handling agent has, only one will stay on line reliably. But they all work on other older aircraft ( and A320s B737NG etc.)

[muduckace]

On an associated topic, does the 787 APU also have air start facility like the 777?


I have had to divert into airports that had very limited GSE, one time we had our APU out and the airport could not provide the 2 air start machines needed to crank a PW4000. I am sure multiple GPU's were available as they are common in demand for normal usage.

It took me 30 minutes to convince the people to do a hot refule with #2 running on the MD-11, thinking back I should have given in and gone to the hotel as we were in the Dominican Republic. There is nothing like an ice cold Presedente Beer to set the mood right.

Screw the APU, worthless as an operational requirement when you can crank engines with electrial power.

On the gate at a hub even, no need to drag a huffer cart out for an inop APU. The theory is sound.

[Capt Claret]

One could easily start all 4 BAe146 engines quicker during pushback, than 2 B717 turbofans. Still rather fly a 71 though. :}

[muduckace]

Prefer 2 717 turbofans to a dozen BAE146 motors myself. B.E.A. is well known around the world for Bring Another Engine. Forgive me for having a little fun here.

[Denti]

According to Boeing (http://www.boeing.com/commercial/787family/programfacts.html) the 787 will have four 250kVA starter generators (two per engine) and two 225 kVA generators on the APU. That is some serious generator potential and it will be interesting to see the GPU demands with an inop APU.

[TURIN]

Screw the APU, worthless as an operational requirement when you can crank engines with electrial power.


What I was trying to say was,
Is the 787 APU fitted with an air start motor or just electric?

[gas path]

TURIN
Don't think there is any bleed air after all they call it a bleedless engine! BUT I'm not so sure about NAI though. Even the aircon packs for pressurisation are electric motor driven... 400VDC IIRC:8

edit due to poor description!!!!

[leewan]

UT I'm not so sure about NAI though. Even the aircon packs for pressurisation are electric motors... 400VDC IIRChttp://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/nerd.gif

If I'm not mistaken, the NAI systems will be using electrical heating strips. The brakes on the B787 will also be electrical. Almost all APU starts nowadays are by electrical start, and i think the "electrical" revolution B787 will be no exception.

Even the aircon packs for pressurisation are electric motors


WTH ? I am waiting with anticipation to see how some of these new age technologies will cope with real life demand. This is one of them.

[SMOC]

Some additional info

The power source for APU starting may be the airplane battery, a ground power source, or an engine-driven generator. The power source for engine starting may be the APU generators, engine-driven generators on the opposite side engine, or two forward 115 VAC ground power sources. The aft external power receptacles may be used for a faster start, if desired.

I'd agree the Hamilton Sundstrand APS 5000 APU for the 787 does not have an air starter like the Honeywell 331-500 Series APU for the 777 as there is no bleed system to start it, the 787 APU does have two starter/generators so does have redundancy.

[TURIN]

Thanks for the replies, I forgot about the bleedless engines . :O