Airbus to Proceed With New A350 Jet

[EagleRJ]

Anything we say negative will probably have had a billion spent on it to make it better. I'm willing to say they've taken care of all these negtive aspects. It could be possible they could just groundstart each engine so it isn't requiring the planes electrical system to do so.

But that would make a windmilling start the only option for inflight restart. That's a Part 25 no-no.

It's not that difficult to design a starter-generator that is able to start a turbofan in the 60,000# thrust class. It's just that it hasn't been done before. If there's any problem getting that amount of power, it might be with the batteries. I'd guess that the engines will need to be started with either the APU or a GPU. The batteries alone won't hack it.

 

[mzaharis]

Anything we say negative will probably have had a billion spent on it to make it better. I'm willing to say they've taken care of all these negtive aspects. It could be possible they could just groundstart each engine so it isn't requiring the planes electrical system to do so.

Two things:

1. Groundstarts - one of the problems is that an electrical motor startup takes more current than the average groundstart cart has - they may need to use 2 groundstart carts. They're counting on the APU to supply juice for startups, even on the ground. (Source - same AvWeek article as in my previous post)

2. I have every confidence that Boeing will solve the challenges ahead of them; it's just interesting to realize how radical doing a nearly "all-electric" jet is from a design perspective, and how much it's exercising the engineers' frontal lobes.

I'm sorry if my post came off as a pessimistic assessment of the 787 - I think it'll be a very successful product. But, Boeing definitely hasn't taken the "easy way out", and I don't think some of the people over at a.net who say "just make airplane x (747-ADV, A350, etc.) bleedless and it'll be tons better" realize what's involved in doing that.

 

[Pedro]

Will the APU still have bleed air? If so that would be a way to start the engines, no bleeds required in the engine for it to have an air turbine starter...

I haven't read much about the 787, so I'm just asking.

 

[mzaharis]

Will the APU still have bleed air? If so that would be a way to start the engines, no bleeds required in the engine for it to have an air turbine starter...

I haven't read much about the 787, so I'm just asking.

The 787's "bleedless". The only thing that bleed air is used for is engine inlet deicing - everything else (cabin pressurization, engine starts, wing deicing) is electric, based on some MASSIVE generators on the engines. Engine starting is via the electric generators on the engines, which double as starter motors, nominally powered by electricity from the APU. The engines have no provision for pneumatic start. Pressurization is driven by electrically-driven air compressors. Deicing/anti-icing is performed with electric heat. From a systems standpoint, the aircraft is a really radical shift from prior, bleed-driven, airliner designs.

 

[mzaharis]

It's not that difficult to design a starter-generator that is able to start a turbofan in the 60,000# thrust class. It's just that it hasn't been done before. If there's any problem getting that amount of power, it might be with the batteries. I'd guess that the engines will need to be started with either the APU or a GPU. The batteries alone won't hack it.

Here's the quote from the AvWeek article:

"Making an electric starter replace the proven pneumatic starter is a top concern. If the generators weren't so large, it wouldn't make sense to use them as starters, but as it is, their size is driven by both roles. The problem is accelerating the large inertia and drag of a big turbofan in a competitive time. The typical Trent pneumatic starter motor makes 200 hp., Booth said. Both electric motors turn the engine and for a quick start are driven at 1.2 times the current rated for continuous generating. "Before, overload [was] rare on aircraft. Here we do it every start," says Robert W. Guirl, the 787 program director in Rockford. "