Why are GE90 blades curved?

[mcleanr]

I'm working on a project developing a turbofan for a 1/2 scale model hyperloop pod. Our faculty advisor has always pushed us to mimic the GE90 engine for our blade design, however, I am not sure why the blades are curved. I'm not talking about the twist of the blade rather the curving that occurs along the length of the blade if that makes sense. As in as if you move along the blade from the root radially outwards, why are there curves that make the blade depart a straight radial line? Sorry, this is difficult to explain but I hope I was able to get the point across.

Our group is unsure whether to include this in our design or not because we want to make sure it will benefit our design and how to approximate how much curving we need.

Any help would be greatly appreciated,

[pattern_is_full]

Similar to "scimitar" turboprop blades.

http://www.cap-ny153.org/E2%20Hawkeye%20Prop%2002.jpg

The outer ends of the blades travel farther per rotation (2pi*(r)) and thus approach Mach 1 at high rpms. Close to Mach 1, supersonic shockwaves start to form. Which 1) interfere with efficiency and 2) produce noise (that "growling sound" heard from many turbofans and props.)

Just as with swept wings, "sweeping" the blades delays the onset of the supersonic shocks.

But with a cowled fan, once the blades are very close to the cowling, the cowling itself reduces/delays shock onset, so the leading edge can re-curve "forward" again, grabbing just a bit more air without producing more shock.

Not new ideas aerodynamically, but the ability to produce the complex blade curvatures with formed composites (instead of machining chunks of titanium) makes it now economical to "tune" the blade shape for each radius, producing the "wavy" leading edge.

[lomapaseo]

What Pattern said is true

But I'm not sure that this directly causes them to be curved.

I would also suggest (let the reader beware) that since there is a natural untwist moment that to counteract this the blade shape is offset from the blade stacking line. Where blades have low hub-to tip ratios (low by-pass) this offset is not easily seen. However in the giant machines the amount of twist is much greater and the stacking line tracks along a curve rather than straight to accommodate the untwist per-span with speed.

[vapilot2004]

I was once told by a GE tech that they are curved "because we can do that now".

Apparently, the use of CF for the core material not only allows for a very wide chord without the weight (and centrifugal force) penalty, the scimitar shape comes from advances in computer modeling of fluid dynamics.

Alongside of mass concerns, efficiency, noise, were among the primary design drivers with FOD core bypass for a bonus. RR, first to use wide-chord blades, are supposed to be working on their own CF fan blades.

[TURIN]

I think Rolls pioneered the wide chord fan. Not sure about the swept shape though.

https://www.rolls-royce.com/about/our-technology/gas-turbine-technology/fans.aspx

Fans
The fan in a gas turbine draws air into the engine, compressing the bypass stream to produce 80 per cent of the engine's thrust, and feeding air to the gas turbine core.
The hollow, titanium wide-chord fan blade, pioneered by Rolls-Royce and introduced into airline service in the 1980s, set new standards in aerodynamic efficiency and resistance to foreign object damage. Since that time we have continued to innovate and improve on our design of wide-chord fan blades. Designed specifically for high-bypass turbofans, the breadth of these blades sets them apart from the narrow and less efficient earlier equivalents.
Fan efficiency
Fan efficiency is an increasingly important contributor to overall improvements in engine efficiencies.
The biggest improvements in fan efficiency have resulted from the analytical techniques that have evolved as a result of the revolution in computing power.
Hollow fan blades
Rolls-Royce has designed wide-chord fan blades to be hollow in order to save weight. This employs a unique manufacturing process.
Hollow design allows significant weight savings to be made in the fan blade, especially at larger sizes, and a follow-on weight saving in the fan disc, structure and containment features.
New fan technologies
New concepts and technology in fans are constantly being considered as the benefits can be very powerful. Examples of new fan concepts include fan blades made from composite material, where the move to ever larger and slower fans together with advances in manufacturing technology could provide significant benefits for some engine designs.
Open rotor concepts can offer a step change in efficiency. The larger diameter unducted fans improve propulsive efficiency, resulting in a potential to reduce fuel burn by up to 30 per cent.

[EstorilM]

Might as well get some extra credit and go for the GE9X design. ;)

If the GE90 blades were a work of art, the new design (especially with fewer blades) is a masterpiece.

http://www.aerospacemanufacturinganddesign.com/FileUploads/image/Capture-gen9x-620x.jpg

[msbbarratt]

A feature of GE's CF blades is that they're reportedly quite thick at the root. They have to be like this so that they can stand the radial forces trying to pull the blade apart.

This thickness hinders airflow into the core; it is aerodynamically unsatisfactory. In effect GE have to strike a balance between aerodynamic freedom (they can make the blade any shape they want) and structural requirements (there is a limit to what can be achieved with the CF technology they use).

RR's point of view with hollow Ti blades is that they can be thin at the root. This improves core air flow, and (if the PR blurb is to be believed) the improved core efficiency makes up for any losses caused by additional weight, or the fan blades not being the ideal aerodyanic shape. Clever though they are, there are almost certainly some limitations on just how curvy RR can make a hollow Ti blade (though looking at various examples shows that they're pretty good!).

And, casting an eye round the Internet, I think we have to conclude that both GE and RR's engine performances are remarkably similar considering the quite significant differences in architecture, materials, etc.

The thing that GE might be worried about is that RR now seem intent on doing a CF blade that is thin at the root. Not only might this be lighter than GE's blade, but it would have better aerodynamic performance especially with regard to core air flow. They've been flying these in the Advance demonstrator engines. And they've apparently done a lot of work in automated layup processes, saves a lot of manpower in manufacturing.

If RR can make those blades work, we might see some interesting upgrade options; want a CF fan blade set on one's old Trent 900s? Could almost be a ramp upgrade...

[EstorilM]

GE has been quoted as having developed new technologies for the 9X blades which allow them to be thinner than previous designs; 4th generation fibers and resins, with blade-thickness similar to conventional metals.

Honestly GE's experience and success with their composite blades seems to be absolutely dominating in the current market. I'm sure RR will begin work on studies for production and such, but to completely master the processes required is unlikely for a while - and they won't be able to leverage an already mature, reliable product and production method like GE seems to have done for the 9X after the 90. GE's scale (not only in manufacturing / quantity) but SIZE is vastly different as well, the -115 and 9X are no "demonstrator"-sized fans. They also have locked-down, flight-ready engines.

I imagine we're watching the same scenario initiate all over again with their use of ceramics - bite the bullet now when you have a massive infusion of R&D cash for a huge contract like the 777X. Again, in a few years, other companies will be forced to study these materials to remain competitive (I realize many already are, but again - GE's use has resulted in essentially a certification-ready design already.)

Personally I think they have essentially locked-down the 'very large' engine/thrust portion of the market, to a certain degree - everyone seems happy with this as well. ;)