Hi Folks,

I was watching a video about the new GEnx engine that is on the 747-8 and the narrator stated the the low pressure and high pressure sections rotate in opposite directions and that helped with efficiency. Does anyone know why that would be true?

I can't think of why reversing the rotational inertia of the fluid stream would be inherently more efficient...the opposite in fact, without a mathematically perfect process you will lose at least some energy just to achieve the reversal.

But there must be a reason, maybe it enables some other design feature.

Maybe it's not related to that, maybe the only way to make a compact gear housing is to have the different sections spin in opposite directions? The whole point of the geared turbofan is to have different speeds and increase the efficiency due to that fact, but maybe a byproduct is given the space constraint and gearing, it makes sense to have it spin the opposite direction?

From the GE site:

"When it comes to the low-pressure turbine, we decided to put a new twist on things by incorporating a counter-rotating design. Rigorously tested in other engine programs, this technique provides numerous benefits, including fewer parts, less weight and improved efficiency -- with no additional architectural complexity. Also, this turbine is specifically designed and shaped for maximum performance and achieves great performance retention."

Which doesn't explain much.

I didn't think the GEnx was a geared turbofan. As far as I knew (and it's not very far), the GTF from P&W, first used on the Bombardier "C", was the first geared fan to be used on a production airplane....the GTF will also be used on the Mitsubishi RJ, and one of two engine options on the A-320NEO family.

Oops, I was thinking too far ahead!