Open rotor aircraft

[bascule]

 

Open rotor designs are being pushed as a more energy efficient alternative to jet engines. The design uses two counterrotating propellers which push the aircraft along.

 

What do you think about this approach?

[phin1ght]

Open rotor aircraft are a great step for experimental aircraft, but it seems to me that prop balance issues would be a big safety issue.

[CaptainPanic]

Is this just a standard jet engine (turbofan type) with it's outer casing removed? I'd say a little more info could be useful. A turbofan is a jet engine with the shaft connected to a fan in the front that creates additional thrust (through a bypass flow that does not go through the combustion). I cannot see from the picture what is actually driving the propellers in this case.

 

I think normally propellers are good for speeds up to 600-700 km/h, while jet engines are used up to 800-900 km/h.

[Pantaz]

Is this just a standard jet engine (turbofan type) with it's outer casing removed?

 

From what I have read, "open rotor" is just a new name for "turboprop".

 

I found this: http://www.aviationweek.com/aw/generic/story_generic.jsp?channel=awst&id=news/aw051208p2.xml

General Electric is joining with NASA to revive studies of its long-abandoned GE36 unducted fan, or "open rotor" and is simultaneously launching a next-generation CF34 technology effort as part of a pressing drive to develop families of fuel-saving engines.

[CaptainPanic]

This is just speculation, but it might be that the "fuel savings" in this case originate from the fact that turboprops generally fly at lower speeds... therefore saving fuel.

[cjohnso0]

From my understanding, the engine in the OP is an open rotor design, off of an Antonov.

 

Open rotor is not another name for turboprop. Turboprops have the propellor connected to the turbine engine via a gearbox, while the open rotor design has a direct connection. Kind of like removing the cowling from a more traditional jet engine and extending the fan blades.

 

Turboprops are efficient to about 450 mph, after that speed props are not as efficient. Open rotor designs are trying to solve this problem with the counter rotating blades and new swept blade designs.

 

disclaimer: some of this came from wikipedia, but it can't be too far off. They have good articles on all jet engine derivative there.

 

Also check out the new Airbus A400M, real nice military transport to replace the C130's and such. It's a turboprop design.

[bascule]

It's my understanding that open rotor aircraft differ from both turboprops and turbofans, and are best compared to a propfan:

 

http://en.wikipedia.org/wiki/Propfan

 

Here's an article:

 

http://www.guardian.co.uk/environment/2008/oct/20/travelandtransport-rollsroyce

 

Here's a snippet comparing open rotors to turbofans:

 

Turbofan engines work by sucking in air with a enclosed fan at the front of the engine. Most of this air is pushed out of the back to produce the thrust needed, with the rest used to burn fuel to drive the fan. The more air that is pushed out rather than burned, known as the bypass ratio, the more efficient the engine is. Put simply, open-rotor engines have a higher bypass ratio than turbofans or turbojets for an equivalent-sized device.

 

Another reason for the higher efficiency of open-rotor engines is that, unlike traditional engines, they do not have a casing around the propeller. The casing increases weight and drag. "Because you've removed the [casing], you're able to go to much bigger fan diameters and not incur the weight and drag penalties," said Taylor.

 

Here's a cutaway:

 

[Flashman]

Hmmm, I had thought that large diameter turbofans minimised the tip losses that would otherwise be seen with propeller blades. I also assumed that the cowlings would be taking advantage of Coanda effect forces.